key: cord-021453-vf8xbaug
authors: Dysko, Robert C.; Nemzek, Jean A.; Levin, Stephen I.; DeMarco, George J.; Moalli, Maria R.
title: Biology and Diseases of Dogs
date: 2007-09-02
journal: Laboratory Animal Medicine
DOI: 10.1016/b978-012263951-7/50014-4
sha: 
doc_id: 21453
cord_uid: vf8xbaug

nan

status as a cooperative companion animal of reasonable size. Dogs were used in the mid-1600s by William Harvey to study cardiac movement, by Marcello Malpighi to understand basic lung anatomy and function, and by Sir Christopher Wren to demonstrate the feasibility of intravenous delivery of medications (Gay, 1984) . The use of dogs continued as biomedical research advanced, and they were featured in many noteworthy studies, including those by Pavlov to observe and document the conditioned reflex response and by Banting and Best to identify the role of insulin in diabetes mellitus. For a comprehensive but concise review of the use of the dog as a research subject, the readers are directed to the manuscript by Gay (1984) .

The breed of dog most commonly bred for use in biomedical research is the beagle. Some commercial facilities also breed foxhounds or other larger-breed dogs for use in surgical research studies. Some specific breeds with congenital or spontaneous disorders are also maintained by research institutions (see specific examples below). Random-source dogs used in research are most frequently mongrels or larger-breed dogs (e.g., German shepherd, Doberman pinscher, Labrador and golden retrievers) that are used for surgical research and/or training.

According to a computerized literature search for beagle for the years 1998-1999, approximately 40% of the biomedical scientific publications identified were in the fields of pharmacology or toxicology. Especially common were studies focusing on pharmacokinetics, alternative drug delivery systems, and cardiovascular pharmacology. The next most common areas of research using beagles were dental and periodontal disease and surgery (12% of publications), orthopedic surgery and skeletal physiology (7%), and radiation oncology (4%). Other research areas that utilized beagles included canine infectious disease, surgery, imaging, prostatic urology, and ophthalmology.

Most large-sized dogs (either purpose-bred or randomsource) are used in biomedical research because of their suitability for surgical procedures. Anesthetic protocols and systems for dogs are well established, and the organs of larger-breed dogs are often an appropriate size for trials of potential pediatric surgical procedures. Surgical canine models have been used extensively in cardiovascular, orthopedic, and transplantation research.

There are also some unique spontaneous conditions for which dogs have proven to be valuable animal models. A colony of gray collies is maintained at the University of Washington (Seattle) for the study of cyclic hematopoiesis. This condition is manifested by periodic fluctuations of the cellular components of blood, most notably the neutrophil population. These dogs are used to study the basic regulatory mechanisms involved with hematopoiesis, as well as possible treatments for both the human and the canine conditions (Brabb et al., 1995) . Golden retrievers affected with muscular dystrophy have been used as models of Duchenne muscular dystrophy in human children. Duchenne muscular dystrophy is caused by an absence of the muscle protein dystrophin, inherited in an X-linked recessive manner. The dystrophy in golden retrievers is caused by absence of the same protein and is inherited in the same way. The clinical signs (such as debilitating limb contracture) are also similar between the canine and human conditions (Kornegay et al., 1994) . Bedlington terriers have been used to study copper storage diseases (such as Wilson's disease), and the development of spontaneous diabetes mellitus and hypothyroidism in a variety of dogs has also been studied for comparisons with the human conditions.

Although historically the dog has been a common laboratory animal, the use of dogs in research has been waning over the past few years. According to the U.S. Department of Agriculture (1998) , the number of dogs used in research has declined from a high use of 211,104 in 1979 211,104 in to only 75,429 in 1997 . This decrease was caused by a variety of factors, including (but not limited to) increased cost, decreased availability, local restrictive regulations, conversion to other animal models (such as livestock or rodents), and shift in scientific interest from pathophysiology to molecular biology and genetics.

Dogs used for research are generally segregated into two classes: purpose-bred and random-source. Purpose-bred dogs are those produced specifically for use in biomedical research; they are intended for use in long-term research projects and/or pharmacologic studies in which illness or medication would require removal from the study. Usually these dogs are either beagles or mongrel foxhounds, although other breeds may be available. Purpose-bred dogs typically receive veterinary care throughout their stay at the breeding facility. They are usually vaccinated against canine distemper virus, parvovirus, adenovirus type 2, parainfluenza virus, Leptospira serovars canicola and icterohemorrhagiae, and Bordetella bronchiseptica. Rabies virus vaccination may also be included. Purpose-bred dogs are also usually treated prophylactically for helminths and ectoparasites, intestinal coccidia, and bacterial ear infections (R. Scipioni and J. Ball, personal communication, 1999) .

Random-source dogs are not bred specifically for use in research. They may be dogs bred for another purpose (e.g., hunting), retired racing dogs, or stray dogs collected at pounds or shelters. The health status of these dogs can be the same quality as purpose-bred dogs, or it can be an unknown entity. Randomsource dogs that have been treated and vaccinated in preparation for use in research are termed conditioned dogs. These dogs are then suitable for long-term studies or terminal preparations that require unperturbed physiologic parameters. Conditioned dogs are often tested for heartworm antigen because of the implications that infestations can have on cardiovascular status and surgical risk. Nonconditioned random-source dogs are useful only in a limited number of research studies, such as nonsurvival surgical training preparations.

Options for procurement of dogs for biomedical research typically include purchase from a U.S. Department of Agriculturedesignated Class A or Class B licensed dealer or directly from a municipal pound. The requirements for USDA licensure are detailed in Code of Federal Regulations (CFR), Title 9, Chapter 1 (1-1-92 edition), Subchapter A, Animal Welfare, 1.1, Definitions, and 2.1, Requirements and Application. Briefly, Class A licensees are breeders who raise all animals on their premises from a closed colony (suppliers of purpose-bred dogs are typically Class A dealers). Class B licensees purchase the dogs from other individuals (including unadopted animals from municipal pounds) and then resell them to research facilities. There are additional regulations that apply to Class B dealers (such as holding periods and recordkeeping documentation) because of the public concern that stolen pets could enter biomedical research facilities in this manner. Regulations regarding the sale of pound dogs to research facilities or Class B dealers vary from state to state and include some bans on this practice.

The best resource for identification of possible vendors is the "Buyer's Guide" issue of the periodical Lab Animal. Typically the last issue of each year, the "Buyer's Guide" lists sources for both purpose-bred and random-source dogs and denotes such features as pathogen-free status, documentation of health status, and availability of specific breeds and timed pregnant females. Some suppliers also have separate advertisements within that issue of the journal.

Welfare Act (7 CFR 2.17, 2.51, and 371.2[g] ) are described in 9 CFR Chapter 1 (1-1-92 edition), Subchapter A, Animal Welfare. Regulations pertaining specifically to the care of dogs used in research are found in Subpart A, Specifications for the Humane Handling, Care, Treatment, and Transportation of Dogs and Cats, of Part 3 (Standards) of Subchapter A. Particular attention should be paid to Section 3.6c (Primary Enclosures--Additional Requirements for Dogs), because the space required for housing dogs is calculated using the length of the dog rather than the body weight (which is used for other species and also for dogs, according to National Research Council (NRC) guidelines). Section 3.8 (Exercise for Dogs) describes the requirements that dealers, exhibitors, and facilities must follow in order to provide dogs with sufficient exercise.

The Institute of Laboratory Animal Research (ILAR) has written the "Guide for the Care and Use of Laboratory Animals" (Seventh edition, 1996) . The "Guide" is the primary document used by institutional animal research programs to develop and design their programs, as well as by the Association for Assessment and Accreditation of Laboratory Animal Care International (AAALAC International) and other animal care evaluation groups to facilitate site visits and inspections. The ILAR committee on dogs has also written "Dogs: Laboratory Animal Management " (1994) . This publication describes "features of housing, management, and care that are related to the expanded use of dogs as models of human diseases" and includes "an interpretive summary of the Animal Welfare Regulations and the requirements of the Public Health Service Policy on Humane Care and Use of Laboratory Animals." The reader is encouraged to use these publications to obtain further information on care and husbandry of dogs in the biomedical research setting.

Growth data for beagles from a purpose-bred dog breeding facility are provided in Table I . Table II features hematology data from beagles from the same commercial facility. Table III lists serum and urine chemical data for beagles. Normal physiologic data for dogs (no breed specified) are provided in Table IV . The information presented in the tables represents a range of normal values that can vary, depending on the analytical method and equipment used as well as the age, breed, gender, and reproductive status of the animal.

Federal regulations promulgated by the Animal and Plant Health Inspection Service, USDA, in response to the Animal Good nutrition and a sound, balanced diet are essential to the health, performance, and well-being of the animal. The basic nutrient requirements for dogs have been compiled by the NRC and represent the average amounts of nutrients that a group of animals should consume over time to maintain growth and prevent deficiencies (National Research Council, 1985) . The reader is referred to these guidelines for useful reference points for management of an animal's diet during various physiologic states (e.g., gestation, lactation, maturational age). Most commercially available balanced dog diets are "closedformula" diets, in which the labeled specific minimum requirements for protein and fat, and the maximum values for ash and fiber, are met. These diets do not necessarily provide the identical composition of ingredients from batch to batch. Ingredient composition varies, depending on the cost relationships of the various ingredients as the manufacturer attempts to achieve the label requirements at the lowest ingredient cost. An "openformula" (or "fixed-formula") diet provides more precise dietary control. In these diets the ingredients are specified, and the percentage of each ingredient is kept constant from batch to batch. "Semipurified" diets provide for the strictest control of ingredients and are formulated from the purified components: amino acids, lipids, carbohydrates, vitamins, and minerals.

The animal care provider should be aware of the manufacture date of the diet, which should be clearly visible on the bag. As a general rule, diets are generally safe for consumption up to 6 months following the manufacture date when stored at room temperature. Refrigeration may prolong the shelf life, but the best strategy is to use each lot based on the date of manufacture in order to prevent food from expiring and to ensure that only fresh diets are fed. Specifications for feeding and watering of dogs are provided in the regulations of the Animal Welfare Act.

Recommendations for feeding the appropriate amount of diet are determined by the dog's metabolic requirements. The basal metabolic rate, or basal energy requirement (BER), refers to the amount of energy expended following sleep, 12-18 hours after food consumption, and during thermoneutral conditions (Kleiber, 1975; Lewis et al., 1987) . The maintenance energy requirement (MER) is the amount of energy used by a moderately active adult animal in a thermoneutral environment, which in the dog is approximately twice the BER (Lewis et al., 1987) . For dogs weighing greater than 2 kg, the MER may be calculated using this simplified linear equation: MER (metabolizable kcal/day) = 2(30 weightkg + 70) (National Research Council, 1985; Lewis et al., 1987) . The quantity of a correctly balanced diet to be fed to each dog can then be determined by dividing the MER by the energy density of the diet.

Fat provides three major dietary functions, including absorption of fat-soluble vitamins (A, D, E, and K), enhancement of palatability, and provision of essential (unsaturated) fatty acids. Dietary fat is an excellent, highly digestible energy source, providing 2.25 times more energy on a per weight basis than either soluble carbohydrates or proteins (Lewis et al., 1987) . However, fats are not needed for this purpose when adequate carbohydrate and protein are present. Consumption of fat in excess of an animal's ability to metabolize it results in steatorrhea and has been related to the development of acute pancreatitis, whereas lack of dietary fat may lead to a fatty acid/energy deficiency. Fatty acid deficiency is associated with poor growth, poor physical performance, reduced reproductive performance, and weight loss.

Dogs are considered to be "easy keepers," because they do not have as many absolute nutritional requirements as their domestic counterpart, the cat. However, they do possess a unique requirement for certain polyunsaturated fatty acids, a deficiency of which may predispose them to decreased growth rates and dermatologic abnormalities, such as "hot spots." Dogs require linoleic (f2-6) acid, an essential fatty acid (National Research Council, 1985) , and more recently it has been demonstrated that the f2-3 fatty acids may play a role in maintaining healthy skin (Logas and Kunkle, 1994) . Supplementation with a balanced essential fatty acid product (e.g., Derm Caps) may alleviate allergy-related dermatoses such as flea-bite dermatitis and pyoderma (Logas and Kunkle, 1994; Miller, 1989) . Essential fatty acid deficiency can occur in dogs receiving low-fat dry dog food that has been stored too long, particularly under warm, humid conditions (Lewis et al., 1987) .

There are 22 a-amino acids, 10 of which cannot be synthesized in sufficient quantity to meet a dog's normal metabolic demands for growth and maintenance. Hence, as their name implies, these essential amino acids are required by all dogs and must be provided in the diet. The essential amino acids and the minimal requirements for growth are listed elsewhere (Lewis et al., 1987) .

Chronic excessive protein intake may be detrimental to the kidney by contributing to accelerated renal aging and subsequent glomerulosclerosis (Lewis et al., 1987) . Conversely, inadequate protein intake results in retardation of growth and aData graciously provided by R. Scipioni and J. Ball of Marshall Farms USA, Inc., North Rose, New York. Beagles tested for period 2/28/99-9/01/99. b S.D., standard deviation; WBC, white blood cells; RBC, red blood cells; HGB, hemoglobin; HCT, hematocrit; MCV, mean corpuscular volume; MCH, mean corpuscular hemoglobin; MCHC, mean corpuscular hemoglobin concentration; RDW, red cell distribution width; HDW, hemoglobin distribution width; PLT, platelets; MPV, mean platelet volume; NEUT, neutrophils; LYMP, lymphocytes; MONO, monocytes; EOS, eosinophils; BASO, basophils; LUC, large unstained cells; LI, lobularity index; MPXI, mean peroxidase activity index reduction in production and/or performance. Protein deficiency, a potential consequence of decreased food intake, results in decreased energy intake. As a compensatory mechanism for a lack of fat or carbohydrate, body protein catabolism ensues in order to meet energy demands, thus exacerbating the negative protein balance and contributing to the clinical signs of edema/ascites, unkempt appearance, lethargy, and weight loss. Thus, caloric needs must be met before protein needs (Lewis et al., 1987) , an important concept to bear in mind in the event of research experiments that may predispose to anorexia. In general, providing a good quality commercial diet that supplies the required amount of amino acids and caloric requirements of the animal, while avoiding excess protein, will ensure nutritional stability and promote longevity.

Appropriate mineral balance in the diet is very important. The best approach in the laboratory setting is to feed a commercial diet that has been formulated with the proper amount and balance of minerals for normal growth. The recommended amount of dietary minerals and the major causes and clinical signs of deficiencies are published elsewhere (Lewis, 1987) . Determining the specific mineral involved in an imbalance can be a diagnostic challenge, because the clinical signs for several excesses/ 2.0 (basal) 12.6 (anestrus) 2.0 c <0.5 c 20-100 c 4-73 r (continues) deficiencies are similar and nonspecific. A definitive diagnosis is often made only after the diet has undergone analysis of the mineral components. Once the imbalance has been identified, the safest resolution to the problem is to discard the entire lot of misformulated diet. Attempting to correct the imbalance through oral supplementation is likely to be more harmful than beneficial, and it risks intensifying the problem by creating additional mineral imbalances.

Vitamins function as enzymes that regulate a wide variety of physiologic processes. They are divided into two groups based on their solubility. The fat-soluble vitamins include A, D, E, and K, whereas the rest are water-soluble. A list of the vitamins, their requirements, and clinical signs associated with deficiencies and toxicities is published elsewhere (Lewis et al., 1987) . Cases of dietary deficiency are rarely encountered in the research setting, because laboratory dog chows are fortified with vitamins. Additional vitamin supplementation may occasionally be required during prolonged clinical illnesses, such as polyuria or diarrhea, which predispose to loss of water-soluble vitamins (B complex and C) (Lewis et al., 1987) . However, as with minerals, routine supplementation of vitamins may induce inadvertent toxicity and exacerbation of an imbalance. 

Management of a breeding colony requires broad knowledge of the dog's anatomy, reproductive physiology, and behavioral needs during breeding, gestation, and parturition. Although a comprehensive discussion of the biology of canine reproduction is beyond the scope of this chapter, essential features of the broad topics noted above are presented. This section is largely based on information assimilated from texts such as "Miller's Anatomy of the Dog" (Evans and Christensen, 1993) , "Veterinary Reproduction and Obstetrics" (Arthur et al., 1989) , and an issue of Veterinary Clinics of North America: Small Animal Practice devoted to pediatrics of puppies and kittens (Hoskins, 1999) .

The ovaries of the bitch are attached to the dorsolateral walls of the abdominal cavity caudal to the kidneys by the broad ligaments and are not palpable abdominally. The uterus consists of the cervix, uterine body, and uterine horns. The cervix is an abdominal organ, located approximately halfway between the Birchard and Sherding (1994) .

ovaries and the vulva. When the bitch is in proestrus and estrus, the cervix can be distinguished during abdominal palpation as an enlarged, turgid, walnut-shaped structure. Catheterization of the cervix is usually not possible in the normal bitch at any stage of the reproductive cycle, except during or immediately following parturition. Thus, semen is deposited at the external cervical os during natural or artificial insemination. The vagina is a long musculomembranous canal that extends from the uterus to the vulva. When the vagina is examined, the gloved finger or examination instrument should be introduced through the dorsal commissure of the vulva so as to avoid the deep ventral clitoral fossa. Examination should proceed at an angle of approximately 60 ~ until the instrument or fingertip has passed over the ischial arch, after which it can be directed further craniad toward the cervix. The bitch has a monoestrous cycle, with clinical estrus occurring predominantly in January or February and again in July or August (although it can occur at any time of year). The estrous cycle consists of four stages: proestrus, estrus, diestrus, and anestrus. The average duration of proestrus is 9 days. During this stage the vulva is enlarged, turgid, and firm, and a sanguinous vaginal discharge is present. Endocrinologically, proestrus is the follicular stage of the cycle, and estrogen levels peak at this time. Estrus generally lasts 9 days, and the vulva is softer and smaller than in proestrus. A vaginal discharge persists during estrus and may remain serosanguinous or become straw-colored. The endocrine feature of estrus is the luteinizing hormone (LH) surge, followed by ovulation within 24-72 hours. Diestrus begins approximately 9 days after the onset of standing heat. The end of this stage is 60 days later, which would be coincident with whelping if the bitch had become pregnant. Serum progesterone levels peak during diestrus. The duration of anestrus is approximately 4 months. Anestrus is the stage of reproductive quiescence, characterized by an absence of ovarian activity and serum progesterone levels of less than 1 ng/ml.

Components of the canine spermatic cord include the ductus deferens, the testicular artery and vein, the lymphatics and nerves, and the cremaster muscle. The cremaster muscle and pampiniform plexus aid in thermoregulation of the testicles, which are maintained at 2~176 lower than basal body temperature. Sweat glands in the scrotum assist in lowering the scrotal temperature through evaporation. The penis is a continuation of the muscular pelvic urethra and is attached to the ischiatic arch by two fibrous crura. It is composed of fibrous tissue and three cavernous sinuses: corpus cavernosum, corpus spongiosum penis, and corpus spongiosum glandis. The accessory sex glands of the dog consist of only a well-encapsulated prostate gland that surrounds the pelvic urethra, and ampullary glands at the termination of the vas deferens in the urethra. The dog does not have seminal vesicles or bulbourethral glands.

The onset of puberty ranges from 5 to 12 months of age and is affected by breed, season, and nutritional and disease status. Testicular growth is rapid at this time, and the seminiferous tubules begin to differentiate. The Sertoli cells form the bloodtestis barrier, the tubules become hollow, and spermatogenesis commences. This process is initiated by the secretion of LH from the anterior pituitary, which stimulates the production of testosterone by the interstitial, or Leydig's, cells. Secretion of follicle-stimulating hormone (FSH) by the anterior pituitary stimulates the production of other key hormones by the Sertoli cells, including inhibin, androgen binding protein, and estrogen. FSH stimulates spermatogenesis in the presence of testosterone, while inhibin and estrogen play a role in a feedback loop on the pituitary gland to decrease FSH production. Spermatogenesis in the dog is completed in 45 days, with subsequent maturation of sperm occurring in the epididymis for approximately 15 days. Thus, the entire process from initiation of spermatogonial mitosis to delivery of mature sperm to the ejaculate is 60 days.

A breeding soundness exam should be conducted to assess the probability of a male dog's successful production of offspring. Factors affecting male fertility include libido, ability to copulate, testicular size, and quality and number of sperm produced. Problems with libido may occur in dogs due to early weaning, isolation, or inherited abnormalities that suppress sexual behavior. Animals with poor hindlimb conformation or with trauma to the back or hindlimbs may be unable to properly mount the female. There is a positive correlation with the size of the testicles as measured by scrotal circumference and the number of sperm produced. Finally, parameters used to assess the quality of sperm include motility, morphology, volume, and concentration. An ejaculate (5 ml) that contains approximately 500 million progressively motile sperm without significant morphological abnormalities (such as a kinked tail) is a good indicator of normal male fertility.

In general, erection, which involves muscular contractions and increased arterial blood flow to the penis, is controlled by the parasympathetic nervous system, whereas ejaculation is under sympathetic control. On mounting, the initial thrusting and ejaculation of semen last about 1 minute. The bulbus glandis becomes enlarged, which lodges the penis in the female reproductive tract. The male then dismounts and brings one hindleg over the female, and the two continue to be joined "rear to rear," a position classically termed "the tie." Ejaculation of the accessory gland fluid continues for 5-30 minutes. The continued expulsion of prostatic fluid during the "tie" may serve to propel the semen from the vagina through the cervix into the uterus.

Fertilization occurs in the oviduct and may occur as late as 8 days after coitus, because of the long life span of sperm in the dog. However, once ovulated, oocytes generally remain viable for only 12-24 hours. Therefore, the bitch should be bred prior to ovulation to ensure the presence of sperm for fertilization of live oocytes.

Cells of the vaginal epithelium mature to keratinized squamous epithelium under the influence of estrogen. Because of the rise in estrogen throughout proestrus, with peak levels occurring just prior to the onset of standing heat, the vaginal smear can be used as an indicator of the bitch's readiness for breeding. The smear will not confirm the presence of ovulation, nor is it of prognostic value in normal bitches during anestrus.

The percentage of vaginal epithelial cell cornification is an index of estrogen secretion by the ovarian follicles. As cornification of vaginal epithelial cells proceeds, the cells become larger, with more angular borders. The nuclear-cytoplasmic ratio decreases until the nuclei reach a point where they no longer take up stain (coincident with the onset of estrus). The cells appear "anuclear" and are classified as "cornified" or "anuclear squames." Cornification occurs approximately 2 days prior to the estrogen peak and 4 days prior to standing heat. The percentage of cornified cells (of the total number of epithelial cells) decreases gradually to zero after the onset of diestrus. The vaginal cytology smear of the bitch changes from predominantly cornified to noncornified 6 days after ovulation. The day of this change is the first day of diestrus. Other epithelial cell types noted on vaginal cytology include superficial cells (large, angular cells with small nuclei); intermediate cells (round or oval cells with abundant cytoplasm and large, vesicular nuclei); and parabasal cells (small round or elongated cells with large, well-stained nuclei, and a high nuclear-cytoplasmic ratio). Based on vaginal cytology, the estrous cycle is classified as follows: Although vaginal cytology is a useful tool, it is not a substitute for observation of behavioral estrus, which is the best criterion to use in breeding management. During proestrus the male is attracted to the bitch and will investigate her hindquarters, but she will not accept breeding. The behavioral hallmark of estrus is standing receptivity toward the male. During this stage the bitch will exhibit "flagging," or elevation of her tail with muscular elevation of the vulva to facilitate penetration by the male. In order to maximize the conception rate, and the number of pups whelped per egg ovulated, it is recommended to breed the bitch on days 1, 3, and 5 of the standing heat.

Fertilization is completed in the mid-to distal oviduct. Implantation is evident by areas of local endometrial edema 17-18 days after breeding. There is no correlation between the number of corpora lutea and the number of fetuses in the corresponding uterine horn, suggesting transuterine migration of embryos.

The dog has endotheliochorial placentation. The endothelium of uterine vessels lies adjacent to the fetal chorion, mesenchymal, and endothelial tissues, so that maternal and fetal blood are separated by four layers. The canine placenta is also classified as zonary and deciduate, indicating that the placental villi are arranged in a belt and that maternal decidual cells are shed with fetal placentas at parturition. The length of gestation is 59-63 days. Luteal progesterone is responsible for maintaining pregnancy, and canine corpora lutea retain their structural development throughout gestation. Serum progesterone rises from less than 1 ng/ml in late proestrus to a peak of 30-60 ng/ml during gestation, then declines to 4-5 ng/ml just prior to parturition. Progesterone is essential for endometrial gland growth, secretion of uterine milk, attachment of the placentas, and inhibition of uterine motility.

Pregnancy detection can be performed by abdominal palpation of the uterus 28 days after breeding. The embryos and chorioallantoic vesicles form a series of ovoid swellings in the early gravid uterus. They are approximately 2 inches in length at 28-30 days, the time at which pregnancy is most easily and accurately diagnosed. By day 35 the uterus begins to enlarge diffusely, so that the vesicles (and, therefore, pregnancy) are difficult to identify by palpation. Fetal skeletons become calcified and are radiographically evident by day 42. Bitches in which a difficult whelping is anticipated should be radiographed in late pregnancy to determine the litter size and to evaluate the size of the fetal skulls in relation to the bony maternal birth canal. Real-time ultrasound can be utilized for pregnancy detection of vesicles as early as 25-28 days.

An abrupt drop in body temperature to less than 100~ indicates impending parturition within 18-24 hours. The process of parturition has been divided into three stages, Stage 1 of labor lasts 6-12 hours and is characterized by uterine contractions and cervical dilation. During this stage, the bitch may appear restless, nervous, and anorexic. Other common clinical signs include hard panting and increased pulse and respiration rates.

Fetal expulsion occurs during stage 2, which lasts approximately 3-6 hours. As the fetus engages the cervix, the neuroendocrine system induces the release of oxytocin; this is referred to as the Ferguson reflex. Oxytocin strengthens the uterine contractions and may elicit voluntary abdominal contractions as well. The bitch is usually recumbent during stage 2 but is able to inhibit this stage if labor if disturbed. The chorioallantois ruptures either during passage of each neonate through the birth canal or by the bitch's teeth at birth. Interestingly, posterior presentation is common in dogs but does not predispose to dystocia. The time interval between delivery of each pup is irregular, but the average time lapse is less than 1 hour between pups until parturition is complete. Veterinary assistance is necessary if the bitch remains in stage 2 for more than 5 hours without delivering the first pup, or for more than 2 hours before delivering subsequent pups.

The placentas are expelled during stage 3 of labor, immediately following delivery of a pup, or up to 15 minutes thereafter. If two pups are delivered from alternate uterine horns, then the birth of both puppies may precede expulsion of the respective placentas. The bitch will lick the newborn vigorously to remove the membranes from its head and to promote respiration. She will also sever the umbilical cord. The bitch may ingest the placentas, although they confer no known nutritional benefit and may induce a transient diarrhea.

Thermal support should be provided prior to parturition. Dogs housed on grated flooring should be provided with mats, and those on solid floors would benefit from blankets placed in a corner of the primary enclosure. Shavings are discouraged as they have the potential to coat the umbilical cord, which may predispose to ascending infections. Heat lamps may be placed 24 hours prior to parturition and remain until all neonates dem-onstrate vigorous and successful suckling behavior. However, the use of heat lamps necessitates strict supervision in order to prevent thermal burns. If possible, whelping bitches should be housed in a quiet corridor in order to decrease periparturient stress, especially in primiparous or young mothers. Thus, monitoring of parturition is important, but human intervention should be minimal in order to prevent stress-induced cannibalism. Weak or debilitated puppies may be cannibalized by the bitch before the research staff recognizes the need for veterinary attention.

The postpartum use of oxytocin is required only in the event of uterine inertia, stillbirths, or agalactia. In these cases, 5-20 units of oxytocin may be administered intramuscularly. Uterine involution occurs during anestrus within 4-5 weeks of parturition. During this time a greenish to red-brown vaginal discharge, or lochia, may be noted. Although lochia is normal, the presence of an odiferous, purulent discharge, accompanied by systemic signs of illness, indicates metritis or pyometra. Desquamation of the endometrium begins by the sixth postpartum week, with complete repair by 3 months.

Newborn puppies are easily sexed by examination of the anogenital distance. In female puppies the vulva is evident a short distance from the anus, whereas the prepuce of male puppies is nearly adjacent to the umbilicus. Eyes are open at approximately 12 days, and ears are patent at approximately 12-20 days. Solid food can be introduced between 4.5 and 6 weeks of age, and puppies can be weaned at 6-8 weeks.

Artificial insemination (AI) is indicated when the male is physically incapable of mounting or penetrating the bitch, when there are vaginal abnormalities such as strictures, or when the bitch refuses to stand for breeding. Semen for AI is collected using a plastic centrifuge tube and rubber latex artificial vagina. The male is introduced to the bitch's scent and manually stimulated. After collection of the first two fractions, a sufficient amount of the third fraction, which consists predominantly of prostatic fluid, is collected to bring the total semen volume to 4-6 ml. The semen is then drawn into a sterile 10 or 12 ml syringe attached to a sterile disposable insemination pipette.

The bitch is inseminated either standing or with raised hindquarters. A gloved index finger is inserted into the dorsal commissure of the vulva and directed craniodorsally until it is over the ischial arch. The tip of the insemination pipette is introduced and guided by the gloved finger toward the external cervical os. The semen is injected, and 2-3 ml of air are then flushed through the syringe and pipette. The pipette is withdrawn, and the gloved finger is used to feather the ceiling of the vagina until contractions of the vaginal musculature are palpable. The bitch's hindquarters are subsequently elevated to promote pooling of semen around the external cervical os.

As with natural breeding, AI should be performed on days 1, 3, and 5 of standing heat, or on the days of maximal vaginal cornification. The bitch should be palpated for pregnancy approximately 4 weeks after the first insemination.

False pregnancy (pseudocyesis), a stage of mammary gland development and lactation associated with nesting or mothering behavior, is common in the bitch. The condition occurs after the decline in serum progesterone toward the end of diestrus. There is no age or breed predisposition. Pseudopregnancy does not predispose the bitch to reproductive disease or infertility. However, in the event of extreme discomfort due to mammary gland enlargement, bitches may be treated with mibolerone (Cheque Drops) at an oral dose of 16 ~tg/kg q24 hr for 3-5 days (Brown, 1984) .

Reproductive performance in the bitch is optimal prior to 4 years of age. Although normal cycle lengths are reported to occur up to the ages of 5-7 years, the interestrous interval tends to increase by 4 years of age. Cycling does not completely cease; however, after 8 years of age, bitches demonstrate significant decreases in conception rate and number of live pups whelped. By 8-9 years of age, pathologic conditions of the uterus, such as cysts, hyperplasia, atrophy, and neoplasia are extremely common.

Beagles have been a popular animal model because of their docile nature. They are easily handled and for the most part respond favorably to repetitive manipulations such as body weight measurements, physical examination, electrocardiogram (ECG) recordings, oral gavage, and venipuncture.

Dogs are sexually mature by 6-9 months of age, but they are not socially mature until 18-36 months of age. The socialization process should begin early during development, when puppies are receptive to conspecific and human contact. For example, from 3-8 weeks of age, puppies are most capable of learning about how to interact with other dogs. Between weeks 5 and 12, puppies are most capable of learning how to interact with people. By 10-12 weeks of age dogs voluntarily wander and explore new environments. Thus, early handling and mild stress (such as vaccination) appear to be extremely beneficial components of a dog's social exposure.

The extent to which breed affects behavior has been the subject of popular speculation but is difficult to prove. In general, breed-specific patterns do tend to emerge. For example, it appears that beagle pups are very motivated by food reward (Overall, 1997 ). This is not surprising, because the breed was selected to work with its nose, and this may be a useful attribute for laboratory investigations that are predicated on food restriction.

Canid social systems use signals and displays that minimize the probability of outright aggression. These behavior patterns are most likely elicited during distressful situations, such as strange environments, being handled by strange people, or encountering new animals. An excellent, illustrated discussion of normal canine behavior patterns can be found in the third chapter of "Clinical Behavioral Medicine for Small Animals" (Overall, 1997).

By virtue of the dog's status as a companion animal, there are many veterinary publications and reference texts on the diagnosis, medical management, pathology, and epidemiology of the disorders that can affect this species. The authors of this chapter have chosen to emphasize those diseases that are more frequently encountered in the research setting. Especially noted in this chapter are infectious diseases associated with the use of random-source dogs that have unknown vaccination history and have had intensive contact with other similar animals at pounds and/or shelters, or conditions seen frequently in the beagle, the most common breed used in biomedical research. For more thorough and detailed discussion of these diseases, as well as those not discussed in this chapter, the reader should consult standard veterinary textbooks, such as the "Current Veterinary Therapy" series (J. D. Bonagura and R. W. Kirk, eds.), "Veterinary Internal Medicine" (S. J. Ettinger and E. C. Feldman, eds.), and "Infectious Diseases of the Dog and Cat" (C. E. Greene, ed.) . Full citations of some chapters from these texts are listed in the references (W. B. Saunders Co. of Philadelphia publishes all three texts.)

Canine Infectious Tracheobronchitis (Kennel Cough Complex) Etiology. Infectious tracheobronchitis (ITB) is a highly contagious illness of the canine respiratory tract that usually manifests as an acute but self-limiting disease. Several organisms have been incriminated as causative for this condition: Bordetella bronchiseptica; canine parainfluenza virus (CPIV); canine adenovirus types 1 and 2 (CAV-1, CAV-2); canine herpesvirus; canine reovirus types 1, 2, and 3; and mycoplasms and ureaplasms.

Clinical signs. Clinical infectious tracheobronchitis can be subdivided into mild or severe forms. The mild form is the more common presentation and is characterized by an acute onset of a loud, dry, hacking cough. Increased formation of mucus sometimes results in a productive cough, followed by gagging or retching motions. Cough is easily elicited by tracheal palpation and may be more frequent with excitement or exercise. Otherwise the dog is typically asymptomatic, with normal body temperature, attitude, and appetite. Mild tracheobronchitis usually lasts 7-14 days, even if left untreated.

The severe form of tracheobronchitis generally results from mixed infections complicated by poor general health, immunosuppression, or lack of vaccination. Secondary bronchopneumonia can occur and may be the determinant of severity (Sherding, 1994) . Animals are clinically ill and may be febrile, anorexic, and depressed. Productive cough and mucopurulent naso-ocular discharge are more common than in the mild form. These cases require more aggressive treatment and may be fatal.

Bordetella bronchiseptica is considered to be the respiratory tract of infected animals (Bemis, 1992) . This bacterium is very easily spread by aerosol and direct contact, and fomite transmission is also possible (Bemis, 1992) . Transmission is favored by confined housing of multiple animals. In experimental studies, B. bronchiseptica transmission to susceptible individuals was 100% (Thompson et al., 1976; McCandlish et al., 1978) .

The incubation period is 3-10 days. CPIV and CAV-2 are also spread by aerosols. Of these two viruses, CAV-2 is the most persistent, lasting for up to several months in the environment, whereas CPIV is fairly labile (Hoskins, 2000a) . Both viruses can be destroyed by quaternary ammonium disenfectants.

Pathogenesis. The most common clinical isolates are CPIV and Bordetella bronchiseptica. However, B. bronchiseptica may be a commensal organism, and it is often recovered from asymptomatic animals. In cases of clinical infection, B. bronchiseptica attaches to the cilia on the mucosal surface of the upper airway epithelium, causing suppurative tracheobronchitis and bronchiolitis. Infections with CPIV or CAV-2 alone are usually subclinical; coinfections with B. bronchiseptica or other microbes may result in clinical ITB (Keil and Fenwick, 1998; Wagener et al., 1994) . The characteristic lesion from CPIV or CAV-2 infection is necrotizing tracheobronchiolitis (Dungworth, 1985) . Pathogenic infection of the upper airways typically results in inflammation and ciliary dysfunction.

Diagnosis and differential diagnosis. Diagnosis of infectious tracheobronchitis is often based on clinical signs. Isolation of Bordetella bronchiseptica or mycoplasma by nasal swabs allows only a presumptive diagnosis. Viral isolation or paired serology can be done but is often impractical and expensive. If cough persists for more than 14 days, other disease conditions should be considered. Canine distemper virus infection, pneumonia, heartworm disease, tracheal collapse, and mycotic infections are differential diagnoses for dogs with similar signs. Bronchial compression as a result of left atrial enlargement, hilar lymphadenopathy, or neoplasia may also elicit a nonproductive cough (Johnson, 2000) and should be considered as a differential for ITB.

Prevention. Prevention is best achieved by avoiding exposure to infected animals, but this is oftentimes not practical. Dogs should be vaccinated prior to, or at the time of, admission to the animal research facility. Intranasal vaccine combinations for Bordetella bronchiseptica and CPIV are preferred. Intranasal vaccines protect against both infection and disease, can be given to dogs as young as 2 weeks of age, and can produce immunity within 4 days.

Control. Sanitation and ventilation are critical for control. The animal care staff must practice proper hygiene to prevent fomite transmission. Symptomatic animals should be isolated, and animal-to-animal contact avoided. Kennels should be disinfected with agents such as bleach, chlorhexidine (Nolvasan) or quaternary ammonium chloride (Roccal-D). Proper ventilation and humidity are important in controlling spread of these infectious agents; 15-20 air changes per hour at 50% relative humidity are recommended (Sherding, 1994) .

No specific treatment is available for viral infections. Bordetella bronchiseptica is typically sensitive to potentiated sulfas, chloramphenicol, quinolones, tetracyclines, gentamicin, and kanamycin. Use of antibiotics is indicated when severe or persistent clinical signs occur, and it should be continued for 14 days. Use of empirical antibiotic treatment in mild cases may hasten the resolution of clinical signs. For severe or unresponsive infection, treatment should be based on bacterial culture sensitivity patterns; nebulized gentamicin may be helpful. Cough suppressants (e.g., dextromethorphan) should be avoided if the cough is bringing up mucus (productive); however, their use is indicated if coughing is causing discomfort or interfering with sleep. Bronchodilators such as aminophylline, theophylline, or terbutaline can be helpful in reducing reflex bronchoconstriction and minimizing discomfort.

tis results in altered respiratory tract histology and impaired mucociliary clearance, infected animals should not be used for pulmonary studies. Animals with clinical disease would also be poor surgical candidates.

Etiology. [3-Hemolytic Lancefield's group C streptococcus (Streptococcus zooepidemicus) is a gram-positive non-spore-forming coccus and an etiologic agent for pneumonia and septicemia in dogs.

Clinical signs. Clinical signs vary based on the organ system affected. Pneumonic disease is typically associated with coughing, weakness, fever, dyspnea, and hematemesis. Peracute death without clinical signs has been reported in a previously healthy research dog (Bergdall et al., 1996) , and conjunctivitis can also be caused by this organism (Murphy et al., 1978) .

Epizootiology and transmission. Lancefield's group C streptococci have been isolated as commensal flora in the upper respiratory tract and the vagina of clinically normal dogs (Olson et al., 1973) . Epizootics have been reported in both racing greyhounds and research colonies (Sundberg et al., 1981; Garnett et al., 1982) . In these epizootics, and in the reported case of peracute death (Bergdall et al., 1996) , recent transportation (within 7 days) was associated with the disease. As such, Lancefield's group C streptococcus may be an opportunistic pathogen in dogs.

Pathologic findings. In the peracute case reported (Bergdall et al., 1996) , hemorrhage from the mouth and nose and within the pleural cavity was the most striking lesion. Ecchymotic and petechial hemorrhages were seen on other organ surfaces. The lungs were heavy and wet, and blood oozed from cut surfaces. "Bull's-eye" lesions were observed on the pleural surface of affected lung lobes, similar to ischemic lesions seen with fungal infections (Fig. 2) . Histologically, the lungs were characterized by areas of hemorrhage surrounding foci of degenerative neutrophils, blood, and necrotic debris. Gram-positive cocci were seen in both the lung and the tonsils.

Pathogenesis. The pathogenesis for disease caused by Lancefield's group C streptococcus is unclear. Strain variation with respect to virulence and host immune factors is probably significant.

Diagnosis and differential diagnosis. Definitive diagnosis is made based on bacterial culture and identification. Any cause of pneumonia and/or peracute death in dogs needs to be considered as a differential diagnosis. Bacterial pneumonias or septicemias can be caused by other pathogenic Streptococcus spp., Staphylococcus spp., Escherichia coli, Pasteurella multocida, Pseudomonas spp., Klebsiella pneumoniae, and Bordetella bronchiseptica. Nonbacterial causes include rodenticide intoxication, coagulopathies, heartworm disease, pulmonary thromboembolism, ruptured aneurysm, and left-sided congestive heart failure.

Prevention and control. Too little is known about the pathogenesis of Lancefield's group C streptococcus to make any recommendations about prevention and control. Treatment. Antibiotic therapy should be provided, based on culture and sensitivity. Intravenous fluids are indicated for febrile or systemically ill patients. For dyspneic patients, oxygen therapy and strict activity restriction are required.

Research complications. Clearly, dogs with severe hemorrhagic pneumonia or septicemia are not appropriate for any research study. The association between epizootics of this disease and transportation shipment supports the philosophy of providing acclimation periods to animals upon arrival at research facilities to evaluate health status and enable the animals to normalize physiologically.

Etiology. Serovars of the spirochete Leptospira interrogans sensu lato cause canine leptospirosis. Disease in dogs is primarily due to serovars canicola, icterohemorrhagiae, grippotyphosa, pomona, and bratislava.

Clinical signs. Leptospirosis may present as either an acute or a chronic problem. Clinical signs are nonspecific and include lethargy, depression, abdominal discomfort, stiffness, anorexia, and vomiting. Animals may be febrile and may be reluctant to move, because of muscle or renal pain or meningitis. Icterus, congested mucous membranes, or signs referable to disseminated intravascular coagulation (petechial/ecchymotic hemorrhages, melena, epistaxis, or hematemesis) are also possible. Animals with peracute leptospirosis are characterized by septicemia, shock, vascular collapse, andrapid death. Uveitis, abortions, and stillbirths have also been associated with leptospirosis.

Epizootiology and transmission. Vaccination and reduced exposure to reservoir hosts have markedly decreased the prevalence of leptospirosis over the past 30 years. Wild animals, cattle, and rodents are reservoirs for Leptospira. The epidemiology of the disease is not static, and recent changes have been observed. Serovars pomona, grippotyphosa, and bratislava are becoming more common causes of canine disease, with canicola and icterohemorrhagiae becoming less common. This may be due to vaccination practices and increased movement of wildlife reservoirs (raccoons, skunks, and opossums) into urban/suburban areas. Rats have been implicated as important in the transmission of serovars canicola and icterohemorrhagiae (Rentko et al., 1992; Brown et al., 1996; Kalin et al., 1999) .

Transmission occurs primarily by environmental contact, and not directly from animal to animal. Infected hosts shed leptospires in urine, thereby contaminating the environment; naive animals are infected when the organisms contact mucous membranes or abraded skin. Recovered animals may shed organisms in their urine for months to years. The organisms are actually labile in the environment; moisture, moderate temperatures, and alkaline soil favor survival and subsequent transmission. Close contact, bites, ingestion of infected meat, and transplacental and venereal transmission are also possible. Leptospirosis is a zoonotic disease.

Pathologic findings. The kidneys consistently have gross and microscopic lesions. In the acute phase of the infection, kidneys are swollen and have subcapsular and cortical ecchymotic hemorrhages. Petechial or ecchymotic hemorrhages and swelling of the lungs and liver may also be noted. Hepatic lesions during the acute phase consist of diffuse hemorrhage and focal areas of necrosis (Searcy, 1995) . In chronic stages of leptospirosis the kidneys become small and fibrotic. Endothelial cell degeneration and focal to diffuse lymphocytic-plasmacytic interstitial nephritis are the characteristic histopathological findings.

Pathogenesis. Infection occurs after the leptospires penetrate a mucous membrane or abraded skin. The organisms then invade the vascular space and multiply rapidly. Several days postinfection the renal tubular epithelium (and, to a variable extent, the liver) is colonized. The hematogenous phase lasts 4-14 days. Acute renal failure or progressive renal failure leading to oliguria or anuria may occur. The most common clinical syndrome is chronic or subclinical infections after recovery from the acute phase (Greene, 2000) . The nephritis may or may not be accompanied by hepatitis, uveitis, and meningitis. Icterus, if it develops, is most common in the acute phase. The combination of azotemia and icterus should alert the clinician to the possibility of leptospirosis. Disseminated intravascular coagulation is often a secondary complication. The severity and course of leptospirosis depend on the causative serovar and the age and immune status of the patient.

Diagnosis and differential diagnosis. Zinc toxicity in dogs most closely mimics the clinical syndrome of leptospirosis. Other causes of acute and chronic renal failure, icterus, and acute hepatic failure must also be considered. Paired serology is the most reliable means of definitive diagnosis; however, seroconversion may not occur until after the first week of infection.

Prevention and control. Vaccination for leptospirosis is standard veterinary practice. Bivalent inactivated bacterins for serovars of L. interrogans canicola and serovars of L. interrogans icterohemorrhagiae are commercially available. However, immunization does not prevent development of the carrier state or protect against other serovars. For outdoor-housed dogs, an effective program to prevent contact with wildlife reservoirs is important. Control requires identification and either treatment or elimination of carrier animals.

Treatment. Penicillins are the drugs of choice for treating leptospiremia, and prompt use reduces fatal complications. Aggressive fluid therapy and supportive care may also be needed. Elimination of renal colonization and the carrier state can be accomplished with dihydrostreptomycin or doxycycline administration.

should not be used in research studies because of the effects of the disease on renal and hepatic function.

Etiology. Campylobacteriosis in dogs is caused by Campylobacter jejuni, a thin, curved or spiral, microaerophilic, thermophilic motile gram-negative rod.

Clinical signs. Most adult animals infected with C. jejuni are asymptomatic carriers; clinical signs are most commonly noted in dogs that are less than 6 months of age (Greene, 2000; Burnens et al., 1992) . In cases of clinical illness, small volumes of mucoid or watery diarrhea, with or without frank blood, are most commonly noted. These signs are usually mild, may be intermittent, and typically last 5-21 days. Tenesmus, inappetance, vomiting, and a mild fever may accompany the diarrhea.

Epizootiology and transmission. The role of C. jejuni as a primary pathogen has been questioned; it may require a coenteropathy to produce disease (Sherding and Johnson, 1994) .

Clinical signs of disease most often occur in dogs less than 6 months of age, although any age may be affected. Stress or immunosuppression may make animals more susceptible to clinical disease. Pound and shelter populations have the highest rates of fecal excretion of C. jejuni (Sherding and Johnson, 1994) .

Transmission is via the fecal-oral route, mostly through fecally contaminated food or water. Unpasteurized milk, poultry, and meat are other sources of infection. Campylobacter jejuni can be zoonotic; children and immunocompromised individuals are at the greatest risk.

Pathologic findings. The actual lesions observed depend upon the mechanism of the enteropathy (Van Kruiningen, 1995) . Enterotoxin production results in dilated fluid-filled bowel loops, with little or no histopathologic alteration. In cytotoxin-mediated disease, hyperemia and a friable, hemorrhagic mucosal surface are noted. On histopathology the mucosal surface is irregular and ulcerated, and a lymphocytic-plasmacytic ileitis or colitis may be seen. When translocation occurs, the lamina propria becomes edematous and congested, with focal accumulation of granulocytes in the crypts and lamina propria. Focal areas of epithelial hyperplasia and decreased numbers of goblet cells are also noted. With Warthin-Starry silver staining, C. jejuni may be seen between enterocytes but only rarely inside them.

Pathogenesis. Clinical disease may be produced by several different mechanisms after the Campylobacter has populated the intestinal tract (Van Kruiningen, 1995) . After colonization of the enterocyte surface, C. jejuni can produce an enterotoxin that causes a secretory diarrhea. Campylobacterjejuni can also cause an erosive enterocolitis by invasion of the ileal and colonic epithelium along with production of a cytotoxic agent; this may be the mechanism that causes hematochezia. In addition, C. jejuni can produce illness by translocation, i.e., multiplication in the lamina propria and transportation to regional lymph nodes by macrophages. This causes mesenteric lymphadenitis.

Diagnosis and differential diagnosis. Fresh feces (per rectum) are best for ensuring an adequate diagnostic sample. Presumptive diagnosis may be made by demonstration of highly motile curved or spiral organisms with dark-field or phase-contrast microscopy. Gram-stained C. jejuni appear as gull-winged rods.

Definitive diagnosis requires isolation of the organism (Sherding and . Culture requires selective isolation media, and growth is favored by reduced oxygen tension and a temperature of 42~ Any disorder that can cause diarrhea in dogs should be considered as a differential diagnosis, including canine parvovirus, coronavirus, distemper virus, Giardia, and Salmonella infections; helminth infestations; and hemorrhagic gastroenteritis. Clinical signs. Based on experimental infections in dogs, three phases to the disease have been described: acute, subclinical, and chronic. Clinical signs observed vary with the phase of the disease, and the acute and subclinical phases are often missed or misdiagnosed (C. G. Couto, personal communication, 1993; Waddle and Littman, 1988; Woody and McDonald, 1985) . A history of tick exposure may be noted prior to onset of signs.

In the acute phase, clinical signs range from mild to severe and may last 1-2 weeks. They include inappetance, lethargy, fever, generalized lymphadenopathy, hepatosplenomegaly, exercise intolerance or dyspnea, petechial or ecchymotic hemorrhages, and peripheral edema. Central nervous system (CNS) signs may also be present such as hyperaesthesia, myoclonus, and cranial nerve deficits. Clinical laboratory abnormalities noted during the acute phase include thrombocytopenia, anemia, neutropenia or neutrophilia, and bicytopenia or pancytopenia. Hyperplastic bone marrow, mild hyperglobulinemia, and elevated hepatic enzymes may be noted during this phase (Kuehn and Gaunt, 1985) .

Clinical signs are generally absent during the subclinical phase. Mild thrombocytopenia, anemia, or leukopenia may be seen. The chronic phase develops 1-4 months after the initial infection, and signs may be subclinical to severe. An extremely varied clinical picture can emerge during this time and can mimic several other clinical syndromes. The following constellation of clinical signs may be observed: chronic lethargy, weight loss, inappetance or anorexia, fever, generalized lymphadenopathy, hepatosplenomegaly, petechial or ecchymotic hemorrhages, epistaxis, hematuria, melena, pallor, anterior or

posterior uveitis, chorioretinitis, peripheral edema, ataxia, upper and lower motor neuron deficits, altered mentation, cranial nerve deficits, and seizures. Persistent thrombocytopenia is the most consistent laboratory abnormality noted for all three stages. Many other hematologic abnormalites may be found, such as regenerative or nonregenerative anemia (more frequently the latter), positive Coombs' test, bicytopenia or pancytopenia, and splenic plasmacytosis or lymphocytosis. On bone marrow evaluation, plasmacytosis along with hypoplasia of erythroid, myeloid, and/or megakaryocyte lines may be seen. Hyperglobulinemia as a result of polyclonal or occasionally monoclonal gammopathy has been noted in 50-100% of E. canis seropositive or infected dogs (Kuehn and Gaunt, 1985; Breitschwerdt et al., 1987; Shimon et al., 1996) . Proteinuria and/or hypoalbuminemia have also been seen.

Epizootiology and transmission. Ehrlichia canis is an obligate intracellular parasite that infects mononuclear cells. The definitive hosts are arthropods; domestic and wild canids are parasitized secondarily. The primary vector and reservoir is the brown dog tick, Rhipicephalus sanguineus. Ehrlichia canis is found worldwide and follows the distribution of the vector. Infection in dogs is most prevalent in tropical and subtropical areas (Greene, 1991) . In the United States, cases are concentrated in the southeastern and southwestern states but have been reported in almost every state (Breitschwerdt, 2000) . Transmission is primarily by tick bites, but it can also occur via blood transfusions from dogs infected for as long as 5 years. Ticks become infected by feeding on an infected dog that is in the first 10-15 days of an acute infection (Lewis et al., 1977) , and ticks can shed the organisms for up to 5 months. Within the tick population, E. canis is transmitted transstadially (within developmental stages) but not transovarially (from female to offspring) (Groves et al., 1977) .

Pathogenesis. In experimental infections, the incubation period prior to the onset of the acute phase is 7-21 days. During the acute phase, which can last from 2-4 weeks, the bacteria replicate within circulating and tissue monocytes, resulting in lymphoreticular hyperplasia in affected tissues. Infected monocytes then spread hematogenously to other organs in the body, in particular the lungs, kidney, and meninges. Infected cells adhere to the vascular endothelium and induce vasculitis, which is the primary mechanism whereby the organism causes disease. The thrombocytopenia during the acute phase is due to both sequestration and destruction, and the development of anemia is a result of red blood cell destruction and suppression of erythrocyte production.

The subclinical phase of the disease occurs 6-9 weeks after initial infection. During this stage, dogs that can mount an effective immune response clear the infection. Those that cannot mount such a response progress to the chronic stage. Infection does not confer protective immunity in dogs that recover. German shepherds and Doberman pinschers seem to be more severely affected than other breeds.

Pathologic findings. Gross lesions are varied and change, depending on the phase of the disease. The most common findings are petechial and ecchymotic hemorrhages and edema of dependent tissues (Woody and Hoskins, 2000) . The most common histologic abnormality noted is lymphocytic-plasmacytic inflammation of numerous organs. Mononuclear phagocytic system hyperplasia, extramedullary hematopoiesis, and splenic erythrophagocytosis may also be seen.

Diagnosis and differential diagnosis. The most sensitive, specific, and commonly employed method for diagnosing E. canis infections is the indirect fluorescent antibody (IFA) test. Antibodies can be detected as early as 7 days postinfection, although some dogs may not seroconvert until 28 days postinfection (Buhles et al., 1974) . Cross-reaction may occur between E. canis, E. chaffeensis, and E. ewingii. Titers greater than 1:10 are considered positive and indicative of infection and may persist for up to 1 year. Effective treatment typically produces seronegative results in 6-9 months. In some cases, asymptomatic dogs may remain seropositive for years after treatment or may be seropositive with a persistent hematologic abnormality (Bartsch and Greene, 1996) . The exact mechanism for this finding has not been elucidated. Ehrlichia canis morulae can be demonstrated in circulating monocytes of Giemsa-stained blood smears. However, this method is labor-intensive and has low sensitivity, as morulae are present transiently and in low numbers. Using buffy coat smears from capillary blood may increase the diagnostic yield. Polymerase chain reaction (PCR) assays are also available to identify E. canis. Differential diagnoses include immune-mediated hemolytic anemia/thrombocytopenia, multiple myeloma, chronic lymphocytic leukemia, and lymphoma.

Prevention. Preventing laboratory animals from contacting ticks is the primary means to avoid monocytic ehrlichiosis in research dogs. Avoid exercising dogs in areas infested with ticks. Use topical acaricides to prevent tick infestations. Keep kennel areas tick-free. Dogs used as blood donors and dogs from unproven sources should be tested for E. canis.

Treatment. Doxycycline is the drug of choice for treating monocytic ehrlichiosis. Oral doses of either 2.5-5 mg/kg q12 hr or 10 mg/kg q24 hr for 21 days are very effective at eliminating the organism. Tetracycline, chloramphenicol, and enrofloxacin are also effective antibiotics; however, chloramphenicol should not be used in animals with cytopenias. In chronic cases, antibiotic treatment should be extended for an additional 1-2 weeks.

Research complications. The most significant research complication is the thrombocytopenia that persists for all stages of the disease. Additionally, there is probable alteration in immune function and increased susceptibility to infectious agents. For these reasons, dogs positive for antibodies to E. canis should not be used in research.

Etiology. This disease, caused by Ehrlichia platys, was first described as cyclic thrombocytopenia by Harvey et al. in 1978 .

Clinical signs. In most cases, infection with E. platys results in subclinical disease. A generalized lymphadenopathy may be noted.

Epizootiology and transmission. The vector for E. platys is assumed to be a tick; however, this mode of transmission has not been established. Experimental studies by Simpson et al. ( 1991) failed to demonstrate Rhipicephalus sanguineus as a vector for E. platys. Coinfection with E. canis has been reported, which suggests a common vector for both organisms (French and Harvey, 1983; Kordick et al., 1999) . Dogs have been experimentally infected by inoculation with infected blood or infected platelets from other dogs (Harvey et al., 1978; Gaunt et al., 1990) . The geographic distribution of thrombocytic ehrlichiosis is assumed to follow that of other Ehrlichia organisms. The highest concentration of cases seems to be in southeastern states, but isolated cases have been reported as far north as Michigan and as far west as Oklahoma (Wilson, 1992; Mathew et aL, 1997) . The prevalence of seropositive dogs can be high in some parts of the country. A study by Bradfield et al. (1996) reported that 74% of the dogs entering a research institute's quarantine facility from sources in eastern North Carolina were seropositive for E. platys. Hoskins et al. (1988) reported a 54.2% seropositive prevalence in healthy dogs from kennels in Louisiana.

Pathologic findings. Gross and histopathologic findings during experimental E. platys infection in dogs have been described by Baker et al. (1987) . Generalized lymphadenopathy was the only gross lesion noted. Follicular hyperplasia and plasmacytosis were the predominate findings in lymphoreticular tissues. All dogs also had extramedullary hematopoiesis, erythrophagocytosis, and crescent-shaped hemorrhages in the spleen. Multifocal Kupffer's cell hyperplasia was noted in the liver, and mild multifocal lymphocytic-plasmacytic interstitial inflammation was seen in the kidneys.

Pathogenesis. The pathogenesis of E. platys in dogs has primarily been determined through experimental infection (Harvey et al., 1978) . After inoculation the organism directly infects platelets. Thrombocytopenia occurs by day 10-14 and fluctu-ates, along with parasitemia, at 10 to 14 day intervals. In some cases the rebound may be within the normal range for thrombocyte counts. The nadir can be lower than 20,000 platelets/~d. Concurrent with low platelet counts is the development of megakaryocytic hyperplasia in the bone marrow. Interestingly, despite extremely low platelet counts, spontaneous bleeding has not been reported in cases of E. platys infection. The mechanism responsible for the cyclic nature of the infection has not been elucidated.

Diagnosis and differential diagnosis. Ehrlichia platys infection may be diagnosed on stained blood smears by visualization of the organisms within platelets. However, this method is very unreliable due to the cyclic nature of the parasitemia and the low numbers of infected thrombocytes. Available IFA assays are much more sensitive and specific, and there is reportedly no serologic cross-reaction with other Ehrlichia species. Dogs usually develop detectable titers 2-3 weeks postinfection. PCR assays for E. platys have now been developed as well (Chang and Pan, 1996; Mathew et al., 1997) . Differential diagnoses for thrombocytic ehrlichiosis include E. canis infection, immunemediated thrombocytopenia, and disseminated intravascular coagulation (DIC).

platys is the same as described for E. canis, above.

Research complications. Ehrlichia platys infection may increase the risk of bleeding during surgical or traumatic procedures. Coinfection with E. platys may potentiate the pathogenicity of other infectious agents, in particular E. canis (Breitschwerdt, 2000) .

Etiology. Lyme disease is caused by Borrelia burgdorferi sensu lato, a microaerophilic spirochete that is primarily an extracellular pathogen.

Clinical signs. Clinical signs may be highly variable; lameness due to polyarthritis has been reported as the most common sign. The onset of lameness may be acute or chronic, shift from limb to limb, and be accompanied by swelling and joint pain. Synovial fluid analysis from affected joints is consistent with a diagnosis of suppurative arthritis. Other clinical signs include fever, anorexia, lethargy, lymphadenopathy, and weight loss. Over the course of the disease, signs may wax and wane over a period of weeks to months. Dogs rarely develop erythema chronicum migrans (the characteristic rash seen in infected people) and do not exhibit the severe arthritis and neurologic sequelae seen in human beings (Greene, 1991; Manley, 1994) . Hematologic and biochemical profiles are generally unremarkable.

Lyme disease is thought to be the most common arthropod-borne disease of human beings (and possibly of dogs) in the United States. It affects humans and dogs worldwide. The geographic distribution of canine borreliosis is assumed to follow that of the human disease and is related to the range of the arthropod vectors. Three major endemic foci that have been identified in the United States account for 94% of reported human cases (Appel and Jacobson, 1995). The distribution of these cases is as follows: Northeast/mid-Atlantic focus, 85%; midwestern focus (Michigan, Wisconsin, Minnesota, Iowa, Illinois, and Missouri), 10%; and California and Oregon, 4%. For the most part, dogs in the remainder of the country are not at risk for contracting Lyme disease. Borrelia burgdorferi is transmitted exclusively by Ixodes ticks. Other arthropod hosts may carry the organism but have not as yet been implicated in the transmission of disease. Ixodes scapularis, a three-host tick with a 2 to 3 year life cycle, is the prototypical vector for North America. The spirochetes are spread by tick bites from both nymphs and adults. Ticks become infected by feeding on an infected mammal and by transstadial transmission (transovarial passage is rare). In endemic areas, 50-80% of adult ticks may be infected (Appel and Jacobson, 1995) . The primary reservoir for the organism is the whitefooted deer mouse, Peromyscus ieucopus, which can carry spirochetes for its life span without becoming ill. Evidence also indicates that the eastern chipmunk, Tamias striatus, is an important reservoir (Slajchert et al., 1997) , and birds may also be a significant reservoir. Deer, however, serve only as hosts for the tick vectors and not as a reservoir for the spirochete.

Pathogenesis. The pathogenesis of Lyme disease is poorly understood, primarily because of a lack of good animal models and the chronic nature of the disease. Infection can be induced experimentally by the bite of a single infected tick. Clinical signs develop 60-90 days postinfection. Some evidence points to the host's inflammatory response to the organism as etiologic for disease (Pershing et aL, 1994; Greene, 1991) . Seroconversion in dogs occurs 4-6 weeks after infection with B. burgdorferi. Antibody titers may remain extremely elevated for at least 18 months. IgM titers also remain elevated for several months and are indicative of neither acute nor active infection (Appel and Jacobson, 1995) . Because antibiotic treatment may not eliminate the organism, persistent infections in dogs (treated for 30 days with antibiotics) can be reactivated by steroid treatment up to 420 days postinfection (Straubinger et al., 1998) .

Diagnosis and differential diagnosis. Appel and Jacobson (1995) recommend that three of the following four criteria be met to establish a diagnosis of Lyme disease in dogs: (1) history of exposure to Ixodes ticks in an endemic area, (2) characteristic clinical signs, (3) positive serology, and (4) rapid resolution of clinical signs with antibiotic therapy. IFA or ELISA tests for Borrelia antibodies are the assays of choice. It should be re-membered, however, that a positive titer in an endemic area indicates exposure and not necessarily disease and that vaccinated dogs will also have a positive titer. Responses to vaccine versus infection may be distinguished by Western blot. Culture or identification of the organism provides a definitive diagnosis but is very difficult to perform. Differential diagnoses include immune-mediated polyarthritis and septic arthritis from other etiologic agents.

Prevention and control. Prevention and control are the same as for the other tick-borne diseases (see discussion of monocytic ehrlichiosis, Section III,A,l,e above). A vaccine against B. burgdorferi is available but should not be necessary in a research setting.

Treatment. Doxycycline is the drug of choice for treating Lyme borelliosis. A typical dosing regimen is 10 mg/kg q12 hr for 3-4 weeks. Amoxicillin, tetracycline, and the quinolones are also effective. Of significant note is that antibiotic treatment results in resolution of clinical signs but may not result in elimination of the organism. (Fox and Lee, 1997) . "Helicobacter heilmannii" and H. bizzozeronii are thought be the same species, with the latter being the updated nomenclature. This species, as well as H. rappini and H. canis, is considered to be zoonotic (Fox and Lee, 1997) .

Clinical infections may present with vomiting, diarrhea, fever, and anorexia, pica, or polyphagia.

Epizootiology and transmission. The epizootiology and transmission of Helicobacter spp. in the dog remains to be elucidated. The prevalence of canine Helicobacter infections in colony or shelter situations has been reported to range from 82% to almost 100% (Fox, 1995; Hermanns et al., 1995) . Both oral-oral and fecal-oral routes for transmission have been suggested.

Pathologic findings. No gross lesions are noted; the primary lesion is that of histologic gastritis. This is typically characterized by reduced mucus content of the surface epithelium; vacu-olation, swelling, karyolysis, and karyorrhexis of parietal cells; and multifocal infiltrates of plasma cells and neutrophils into the subepithelium, primarily around blood vessels and between the gastric pits (Hermanns et al., 1995) . Focal areas of lymphocytic inflammation and lymphoid follicles may also be seen.

Pathogenesis. Some Helicobacter spp. colonize the gastric epithelium exclusively and other species colonize lower parts of the gastrointestinal tract. Helicobacter felis and "H. heilmannii" infections have been linked to gastric lesions in laboratoryraised beagles (Fox and Lee, 1997) . The mechanism by which these organisms cause disease may be related to the host's inflammatory response to colonization and the Helicobacter's ability to produce urease. Urease splits urea into ammonia and bicarbonate; ammonia is toxic for the epithelial cells, and bicarbonate may help the organism survive the acidic environment (Marshall et al., 1990; Shimoyama and Crabtree, 1998 ).

Diagnosis and differential diagnosis. Any of the numerous causes of acute or chronic vomiting and diarrhea in the dog (including canine distemper, viral or bacterial gastroenteritis, and ingested toxicants) should be considered as differential diagnoses. Definitive diagnosis for dogs requires either endoscopic or surgical biopsy. Confirmation of infection with Helicobacter spp. requires demonstration of the organism in biopsy samples by histopathology, culture, or recognition by PCR. A positive urease test on a biopsy sample may give a presumptive diagnosis, but only for those species that produce urease. The use of Warthin-Starry silver stain may increase the sensitivity for histopathologic diagnosis.

Prevention and control. Until more is known about the epizootiology and transmission of Helicobacter spp. in the dog, specific recommendations cannot be made about prevention and control in this species.

Treatment. Combination therapy has proven to be the most effective method for treating Helicobacter spp. infections in dogs. Combination therapy of amoxicillin (10 mg/kg q12 hr), metronidazole (30 mg/kg q24 hr), and sucralfate (0.25-0.5 mg/kg q8 hr) for 21 days has been suggested for dogs (Hall and Simpson, 2000) . Replacing the sucralfate with famotidine (0.5 mg/kg q24 hr), omeprazole (0.3 mg/kg q24 hr), or bismuth subsalicylate (0.2 ml/kg q4-6 hr) may also be effective (Marks, 1997; Jenkins and Bassett, 1997; DeNovo and Magne, 1995) . The benefits of antimicrobial therapy in dogs still need to be established by controlled therapeutic studies.

Research complications. Helicobacter spp. infections could result in altered gastrointestinal responses to drugs and toxic or carcinogenic compounds. Therefore, dogs used in gastric physiology or oral pharmacology studies should be free from helicobacteriosis.

Clinical signs. Clinical signs of canine parvovirus usually appear 5 days after inoculation by the fecal-oral route and are characterized by anorexia, fever, depression, and vomiting. Profuse, intractable diarrhea ensues, which may become hemorrhagic. Approximately 85% of affected dogs develop severe leukopenia, with a total granulocyte/lymphocyte count ranging from 500-2000 WBC/~d or less. Repeated hemograms may provide prognostic value, because rebounds in leukocyte counts are indicative of impending recovery. Terminally ill dogs may develop hypothermia, icterus, or disseminated intravascular coagulation due to endotoxemia.

Parvovirus can infect dogs of any age, but puppies between 6 and 20 weeks of age appear to be particularly susceptible. Puppies less than 6 weeks of age are generally protected from infection by passive maternal antibody. Adult dogs probably incur mild or inapparent infections that result in seroconversion.

Pathogenesis. Canine parvovirus has an affinity for rapidly dividing cells of the intestine and causes an acute, highly contagious enteritis with intestinal crypt necrosis and villus atrophy. The virus also has tropism for the bone marrow and lymphoid tissues; thus leukopenia and lymphoid depletion accompany the intestinal destruction.

Diagnosis and differential diagnosis. Parvovirus can be detected in fecal samples with a commercially available ELISA from CITE. At necropsy, diagnosis is based on gross and histopathologic evidence of necrosis and dilatation of intestinal crypt cells with secondary villous collapse. Other lesions include myeloid degeneration and widespread lymphoid depletion. Parvovirus can also be demonstrated in frozen sections by fluorescent antibody techniques. Differential diagnoses should include other viral enteritides, salmonellosis, and small intestinal obstruction.

Prevention and control. Prevention of transmission begins with isolation of affected animals and quarantine for 1 week after full recovery. Disinfection of potentially infected kennel and diagnostic areas with diluted bleach (1:30) or commercially prepared disinfectant (such as Kennesol, available from AlphaTech, Lexington, Massachusetts) is essential for elimination of the virus. Six-week-old puppies should be vaccinated every 2-4 weeks with a commercially available modified live vaccine until 16-18 weeks of age. Young Rottweilers and Doberman pinschers appear to be predisposed to parvoviral enteritis and should be vaccinated every 3 weeks (5 times) from 6-18 weeks of age.

Treatment. Treatment is largely supportive and is aimed primarily at restoring fluid and electrolyte balance.

Research complications. Infection with parvovirus obviously precludes the use of a particular dog in an experimental protocol. Given the potential for significant discomfort of the affected animal, and the cost of therapy, humane euthanasia is usually the option chosen in a research setting.

Canine coronavirus infection is usually inapparent or causes minimal illness. This epitheliotropic virus preferentially invades the enterocytes of the villous tips, resulting in destruction, atrophy, and fusion and subsequent diarrhea of varying severity. Subclinical infections are most common, but abrupt gastrointestinal upset accompanied by soft to watery, yelloworange feces is possible. Definitive diagnosis by virus isolation or paired sera is usually not made, because supportive therapy generally results in rapid resolution of the diarrhea. Inactivated coronavirus is present in commercially available combination vaccines, which are administered immunoprophylactically at 6 -8, 10 -12, and 12-14 weeks of age and then annually thereafter. The role of these vaccines in protection from coronaviral infection is unknown, because the virus typically causes inapparent or mild illness (Hoskins, 1998) .

Etiology. Canine distemper virus (CDV) belongs to the family Paramyxoviridae, within the genus Morbillivirus, which includes human measles virus and rinderpest virus of ruminants.

Although there is only one serotype of CDV, there is a wide difference in strain virulence and tissue tropism. Some strains produce mild clinical signs that are similar to tracheobronchitis, whereas other strains cause generalized infections of the gastrointestinal tract, integument, and central nervous system, resulting in enteritis, digital hyperkeratosis, and encephalitis, respectively. Other factors contributing to the severity and progression of clinical signs include environmental conditions, immune status, and age of the host. A transient subclinical fever and leukopenia occur 4-7 days after exposure, with a subsequent fever spike 7-14 days later, accompanied by conjunctivitis and rhinitis. Other clinical signs associated with acute distemper include coughing, diarrhea, vomiting, anorexia, dehydration, and weight loss. Secondary bacterial infections can cause progression to mucopurulent oculonasal discharge and pneumonia. An immune-mediated pustular dermatitis may develop on the abdomen; this is usually a favorable prognostic sign (Greene and Appel, 1998) , because dogs that develop skin lesions often recover.

Neurologic complications of distemper infection may occur weeks to months after recovery from an acute infection. Dogs that develop late-onset disease are usually immunocompetent hosts, suggesting that the virus may have escaped complete elimination by the immune system, possibly because of protective effects by the blood-brain barrier. Classic neurologic signs that may occur in acute or chronic CDV infection include ataxia, incoordination, vocalization, "chewing gum" seizures, and myoclonus with or without paresis of the affected limb. Canine distemper is the most common cause of seizures in dogs less than 6 months of age. Dogs with extensive neurologic involvement often have residual clinical deficits, including flexor spasm and olfactory dysfunction. CDV has also been associated with two forms of chronic encephalitis in mature dogs: multifocal encephalitis and "old dog encephalitis."

Epizootiology and transmission. The virus is highly prevalent and contagious to dogs and other carnivores, especially at the age of 3-6 months, coincident with the waning of maternal antibody. Transmission is primarily by aerosolization of infective droplets from body secretions of infected animals.

Pathologic findings. The predominant histopathologic lesion in neurologic forms of distemper is demyelination, which may .. be accompanied by gliosis, necrosis, edema, and macrophage infiltration. Acidophilic cytoplasmic inclusions can be found in epithelial cells of mucous membranes, reticulum cells, leukocytes, glia, and neurons, while intranuclear inclusions are often present in lining or glandular epithelium and ganglion cells.

Diagnosis and differential diagnosis. Diagnosis Of CDV is based on history of exposure and clinical signs. Young dogs who have not received routine immunoprophylaxis (or similarly, mature dogs with a questionable vaccination history) and present with rhinitis, mucopurulent oculonasal discharge, plus or minus hyperkeratosis of the footpads and neurologic signs, are highly likely to have CDV. Ophthalmologic examination may reveal chorioretinitis with acute disease or retinal atrophy in chronic cases. Definitive diagnosis of acute infection can be made by fluorescent antibody testing of intact epithelial cells from conjunctival and mucous membranes. Attenuated strains of CDV, found in modified live vaccines, are not disseminated from lymphoid tissue to epithelial cells and thus are not detected by the fluorescent antibody. Serologic testing is usually not useful, because dogs frequently fail to mount a measurable immunologic response. Because of the variety of clinical signs, there are many differential diagnoses for canine distemper. An important differential diagnosis for respiratory illness is infectious tracheobronchitis (kennel cough). Bacterial, viral, and protozoal causes of gastroenteritis must be considered for cases presenting with vomiting and diarrhea, and rabies, pseudorabies, bacterial meningitis, and poisonings are differential diagnoses for dogs with central nervous system disorder.

Prevention and treatment. A series of three immunizations from 6 to 14 weeks of age, followed by yearly boosters, is a recommended preventative. Treatment is largely supportive, but because of the profound immunologic effects and significant morbidity of CDV, humane euthanasia is usually undertaken in the research setting.

Etiology. Canine herpesvirus (CHV) infection causes a generalized hemorrhagic disease with a high mortality rate in newborn puppies less than 2 weeks of age. In adult dogs, CHV causes a persistent, latent infection of the reproductive tract with recrudescence and shedding during periods of physiologic stress.

Clinical signs. Clinically affected puppies do not suckle, cry persistently, become depressed and weak, and fail to thrive. Petechial hemorrhages of the mucous membranes and erythema of sparsely haired regions such as the caudal abdomen and inguinal area are evident. Older puppies, aged 3-5 weeks, develop less severe clinical signs and are likely to survive with neurologic sequelae such as ataxia and blindness resulting from reactivation of latent infection. Infection in adult dogs may result in stillbirths, abortions, and infertility. Lesions in adult bitches include raised vesicular foci in the vaginal mucosa, accompanied by mild vaginitis. Adult males have preputial discharge due to vesicular lesions at the base of the penis and on the preputial mucosa.

passage of puppies through the birth canal or venereally in adult dogs. Puppies can also be horizontally infected by littermates. Entire primiparous litters may be lost, with subsequent litters protected by colostral antibody.

Pathologic findings. Pathologic findings include multifocal ecchymotic hemorrhages of the kidneys, liver, lungs, and gastrointestinal tract. Basophilic intranuclear inclusions in necrotic areas of parenchymal organs are characteristic findings.

Diagnosis and differential diagnosis. Diagnosis of canine herpesvirus infection in adult dogs is based on a history of reproductive infertility and the presence of genital vesicular lesions. Differential diagnoses for stillbirths, abortions, and infertility include canine brucellosis, canine distemper virus and parvovirus infections, and pyometra. The diagnosis in infected puppies is usually made based on clinical history and characteristic lesions (multifocal systemic hemorrhages) (Carmichael and Greene, 1998) . Differential diagnoses for the disease in neonates would include canine ehrlichiosis and causes of disseminated intravascular coagulation, including bacterial endotoxemia.

there is no effective curative treatment. Supportive therapy is unrewarding, and death usually ensues within 48 hours in in-fected neonates. In general, adult bitches that have multiple abortions, stillbirths, or persistent infertility should be culled from the breeding colony. Examination of these animals may reveal raised vesicular lesions on the vaginal mucosa. Adult male dogs that have vesicular lesions on the base of the penis and preputial mucosa should be similarly culled.

adult dogs would obviously interfere with production operations, and affected animals should be culled based on the criteria noted above in the discussion of prevention and treatment. Because of the severity of clinical illness in puppies, such animals should be humanely euthanatized.

Etiology. Rabies virus is a member of the rhabdovirus family and is essentially contagious to all species of warm-blooded animals.

Clinical signs. Clinical progression of neurologic disease occurs in three stages. The first, or prodromal, stage is characterized by a change in species-typical behavior. The loss of the instinctive fear of humans by a wild animal is a classic sign of impending rabies. In the second, or furious, stage animals are easily excited or hyperreactive to external stimuli and will readily snap at inanimate objects. The third, or paralytic, stage is characterized by incoordination and ascending ataxia of the hindlimbs due to viral-induced damage of motor neurons. Death usually occurs within 2-7 days of the onset of clinical signs, due to respiratory failure.

Epizootiology and transmission. Wild animals such as raccoons, skunks, and bats are common reservoirs of infection for domestic animals, which in turn are the principal source of infection for humans. Transmission occurs primarily by contact of infected saliva from a rabid to a naive animal (or human), usually via bite wounds.

Pathogenesis. The incubation period for rabies is generally 3-8 weeks from the time of exposure to the onset of clinical signs but can range from 1 week to 1 year. Bites of the head and neck typically result in shorter incubation periods because of the proximity to the brain. Following infection, the virus migrates centripetally via peripheral nerve fibers to the central nervous system and eventually to neurons within the brain, resuiting in neurologic dysfunction. On reaching the brain, the virus migrates centrifugally to the salivary glands, thus enabling shedding and subsequent transmission.

Diagnosis and differential diagnosis. Diagnosis of rabies is based on clinical signs; differential diagnoses include pseudorabies, canine distemper, bacterial meningitis, and toxicants that affect neurologic function. Definitive diagnosis is based on fluorescent antibody demonstration of the virus in Negri bodies of hippocampal cells.

Prevention and treatment. Puppies should be vaccinated at 4-6 months of age, "boostered" in 1 year, then vaccinated annually or triennially, depending on state and local laws and which vaccine product is used. Treatment of rabies is not recommended, because of the risk of human exposure.

Research complications. In a research setting, dogs are often not vaccinated for rabies, because of the low incidence of exposure to wild-animal reservoirs. A healthy, purpose-bred dog that bites a human in a research facility should be quarantined for 10 days and observed for signs of rabies. This quarantine interval is based on the knowledge that dogs do not shed rabies in the saliva for more than a few days before the onset of neurologic disease. A random-source dog with an unknown vaccination history that bites a human should be immediately euthanized. The brain should be examined for rabies virus to determine if the dog was infected, and if the test is positive, postexposure immunization should be initiated for the human patient. A rabies vaccine licensed for use in humans is available, and immunoprophylaxis is recommended for animal care and research personnel who may have high work-related risks of exposure.

a. Protozoa i. Giardiasis Etiology. Giardiasis is a small-intestinal disease of the dog caused by Giardia duodenalis (lamblia), a binucleate flagellate protozoan.

Clinical signs. Most Giardia infections are subclinical. When dogs are clinically affected, diarrhea is the most prominent sign. The diarrhea is a result of intestinal malabsorption and is often characterized as voluminous, light-colored, foul-smelling, and soft to watery. Weight loss has also been associated with clinical infection. Clinical illness is more often seen in young animals.

Epizootiology and transmission. Giardia has a direct life cycle. Dogs (and people) typically become infected when they consume water (or food) contaminated with Giardia cysts. The pH change from the stomach (acid) to duodenum (neutral) causes excystation. Trophozoites migrate to the distal duodenum and proximal jejunum and attach to the villus surface. Eventually the trophozoites encyst and pass in the feces to perpetuate the life cycle.

Pathologic findings. Giardiasis is rarely fatal. On histopathology of duodenal or jejunal specimens, Giardia trophozoites can be seen attached to enterocytes. Mucosal inflammation and ulceration, and villous atrophy, have been observed.

Pathogenesis. The exact pathogenesis of Giardia-induced illness is unknown. It is thought that tissue invasion, although occasionally observed, is unimportant for pathogenesis. It is suspected that illness is caused by physical obstruction of enteric absorption, enterotoxicity, competition for nutrients, excess mucus production, and/or secondary bacterial overgrowth.

Diagnosis and differential diagnosis. Definitive diagnosis requires observation of the organism in fecal or intestinal samples. Direct fecal smears are considered best for observing trophozoites, and zinc sulfate flotation is preferred for detection of cysts. Commercial ELISA kits and direct immunofluorescent tests are available to detect fecal Giardia antigens, but the diagnostic specificity and/or sensitivity of these tests may not be sufficient to warrant substitution for the less expensive direct fecal examination or zinc sulfate preparation (Barr, 1998) . Differential diagnoses for giardiasis include bacterial and protozoal enteritis, coccidiosis, and whipworm infestation.

Prevention. High-quality water sources will eliminate the possibility of infection developing within an animal research facility. Use of dogs with a known husbandry and medical background will minimize the chances of giardiasis developing in a research colony.

Control. Once giardiasis has been diagnosed in a canine population, segregation of infected animals will help to reduce further infection (provided other dogs were not preinfected at the same source location as the signal case). Disinfection with quaternary ammonium compounds, bleach, or steam is usually successful in eradication of Giardia cysts.

Treatment. The most common treatment for giardiasis is metronidazole (Flagyl) at 25-30 mg/kg per os twice per day for 5-10 days. Quinacrine hydrochloride (Atabrine) at 9 mg/kg per os once per day for 6 days, furazolidone (Furoxone) at 4 mg/kg per os twice per day for 7-10 days, and the anthelmintics albendazole and fenbendazole have been proposed for use against metronidazole-resistant strains of Giardia. A1bendazole is recommended at 25 mg/kg per os q12 hr for 2 days, and fenbendazole at 50 mg/kg per os q24 hr for 3 days. Fenbendazole was thought to be safer for both puppies and pregnant females (nonteratogenic) (Barr, 1998) .

Research complications. Typical asymptomatic infections probably have no consequence on research protocols, with the exception of intestinal physiology or immunology studies.

Clinical diarrhea would clearly need to be treated before a dog could be used as a research subject.

ii. Coccidiosis Etiology. Intestinal coccidia that have been associated with enteropathy in dogs include Cystoisospora canis, C. ohioensis, C. burrowsi, and C. neorivolta.

Clinical signs. Dogs are typically asymptomatic when infected with intestinal coccidia, and oocysts are an incidental finding on fecal flotation or direct smear. Dogs that are clinically infected usually develop diarrhea, which can vary from soft to watery and may contain blood or mucus. Vomiting, dehydration, lethargy, and weight loss can also be seen.

Epizootiology and transmission. Cystoisospora oocysts are typically spread by fecal-oral transmission, usually by ingestion of fecal-contaminated food or other objects in the environment. An indirect form of transmission is also possible, whereby the dog consumes a rodent or other animal that is serving as a transport host. Once inside the small intestine, the cyst releases sporozoites that infect enteric epithelium. Several generations of asexual reproduction can occur in the enterocyte before sexual reproduction produces gamonts. The gamonts fuse to become a zygote, which encysts, ruptures the enterocyte, and passes in the feces. Once in the environment the cyst sporulates and is now an infective stage for ingestion by another host.

Pathologic findings. Dogs with coccidiosis may have hyperemia or fluid retention at affected intestinal segments. The mucosa may appear normal, raised, or ulcerated. Histologically, there may be necrosis of enterocytes, hyperemia, and submucosal inflammation. The oocysts are usually readily apparent within the epithelial cells (Van Kruiningen, 1995) .

Pathogenesis. Intestinal coccidia are opportunistic organisms; they do not typically cause illness unless other predisposing factors are present. Such factors include immunodeficiency, malnutrition, and/or concurrent disease. Overcrowding and unsanitary conditions can also promote clinical coccidiosis by providing a high population of infective oocysts to stressed animals.

Diagnosis and differential diagnosis. Diagnosis is somewhat difficult, as coccidian oocysts (of both Cystoisospora and non-Cystoisospora spp.) can be seen on fecal examinations of clinically healthy dogs, as well as animals with diarrhea. Other causes for diarrhea (e.g., parvovirus, roundworms, Giardia spp., Campylobacter jejuni, and inflammatory bowel disease) should be excluded before a coccidial etiology is implicated.

Prevention. Clinical coccidiosis can be readily prevented by adhering to proper sanitation guidelines, reducing any over-crowding, and providing as stress-free an environment as possible.

Treatment. Treatment for the presence of coccidial oocysts may often not be necessary, because Cystoisospora infections are typically self-limiting and clinically insignificant. Treatment may, however, help to limit the number of oocysts shed in a kennel housing situation and may be necessary in cases of protracted clinical illness. Possible choices for treatment include daily administration of sulfadimethoxine (25-30 mg/lb per os for 10 days), trimethoprim sulfa (15 mg/lb per os for 10 days), or quinacrine (5 mg/lb per os for 5 days). Amprolium, which is not labeled for dogs, can also be used as a coccidiostat. It can be given in gelatin capsules for 7-12 days at a daily dose of 100 mg for small-breed pups and 200 mg for larger breeds.

Research complications. As with any enteric disease, the presence of clinical coccidiosis can cause aberrations in gastrointestinal physiological parameters. Dogs used in intestinal pharmacokinetic studies should be confirmed to be free of Cystoisospora infections.

b. Nematodes i. Ascarids Etiology. The most common ascarid of dogs is Toxocara canis. Toxascaris leonina can also infect both dogs and cats.

Clinical signs. Ascarid infestations are most commonly subclinical. However, large worm burdens can cause diarrhea, vomiting, dehydration, and abdominal discomfort with vocalization. Puppies may have a classical "potbellied" appearance and dull hair coat. Heavy infestations can cause intussusception and/or intestinal obstruction, in which case the young dogs may be found dead. Visceral larval migrans caused by Toxocara canis can cause pneumonia.

Epizootiology and transmission. Toxocara canis typically infects puppies. In fact, a unique characteristic of T. canis is its ability to infect prenatal puppies by transplacental migration, and neonatal puppies by transmammary migration. Ingestion of infective eggs that have been shed in the feces is another common route of transmission, and infection by ingestion of a transport or intermediate host is also possible.

Pathologic findings. Puppies that die from ascarid infestations typically have large worm populations in the lumen of the small intestine. Such populations can cause intestinal obstruction and may also result in intussusception or intestinal perforation. Puppies that experience lung migrations of large larval worm populations can have severe pulmonary parenchymal damage and develop fatal pneumonia.

Pathogenesis. The infective stage of T. canis is the third-stage larva (L3). Infections initiated by ingestion of infective eggs have three possibilities for larval migration: liver-lung migration (which leads to intestinal infection), somatic tissue migration, and intestinal wall migration. Older dogs that become infected typically have an age-related resistance to liver-lung migration and instead experience the other two migratory patterns. These larval migrations are often asymptomatic, and progression of the L3 larvae is arrested in the tissues. It is these larvae that become reactivated in a pregnant bitch, thus establishing the transplacental and transmammary routes of transmission. If the source of infection is transplacental, puppies may be born with L3 larvae in their lungs, because larval migration is already in progress (Sherding, 1989 ).

Diagnosis and differential diagnosis. The characteristic large (70-85 ~tm in diameter) and relatively round ascarid eggs can be readily diagnosed by standard fecal flotation methods.

Prevention and control. Monthly administration of milbemycin or ivermectin plus pyrantel pamoate (Heartgard Plus) is recommended for prevention and control of canine ascarid infestation (Hall and Simpson, 2000) .

Treatment. Most anthelmintics are effective for treatment of ascariasis. Pyrantel pamoate (Nemex) and fenbendazole (Panacur) are commonly used. Treatment should be started early in puppies (2, 4, 6, and 8 weeks) because of the possibility of prenatal or neonatal infection. Pyrantel pamoate, dosed at 5 mg/kg per os, is safe for puppies and is also effective in treatment of hookworms (see Section III,A,3,b,ii). In breeding colonies in which ascarid infestation is a known problem, treatment of the pregnant and nursing bitch may be advantageous. Extended fenbendazole therapy (50 mg/kg per os twice per day for 14 days or once per day from day 40 of gestation through day 14 of lactation) has been shown to be experimentally safe and effective in decreasing ascarid burdens in puppies.

Research complications. Puppies with large worm burdens make poor research subjects and should be treated aggressively before placement on an experimental study.

ii. Hookworms Etiology. The most common and most pathogenic hookworm of dogs is Ancylostoma caninum. Other, less pathogenic canine hookworms found in North America are A. braziliense, which can be found in the American tropics and southern United States, and Uncinaria stenocephala, which is distributed in the northern United States and Canada.

Clinical signs. Only A. caninum infestation typically results in clinical illness, because of the amount of blood that it con-sumes. Puppies with A. caninum infestations are typically pale and weak (from anemia), with bloody diarrhea or melena. Other clinical signs include lethargy, anorexia, dehydration, vomiting, and poor weight gain.

Epizootiology and transmission. Infective larvae (L3) are typically ingested by puppies and develop directly in the intestinal tract. Ingestion can be from the bitch's milk (transmammary migration occurs with A. caninum), from food or objects contaminated with infective larvae, or from ingestion of a paratenic host. Transplacental migration does occur with A. caninum, but to a much lesser extent than is seen with Toxocara canis. Larvae can also penetrate intact skin, migrate to the lung via somatic or circulatory routes, and be coughed and swallowed to reach the intestine. The prepatent period is 3 weeks.

Pathologic findings. Infected puppies often have severe anemia and eosinophilia. The anemia can be from acute blood loss or can also be an iron-deficiency anemia caused by chronic blood loss coupled with limited iron reserves. On gross necropsy, the small-intestinal tract contains worms admixed with intestinal contents containing fresh or digested blood (Fig. 3a) . Ulcerative enteritis caused by hookworm attachment is evident on histopathologic examination, and worms with mouthparts embedded in the mucosa can be identified in some sections (Fig. 3b) .

Pathogenesis. The severe pathogenicity of A. caninum is a direct result of its voracious consumption of blood and body fluids. Each adult hookworm can consume 0.01-0.2 ml of blood; thus an extensive infection could deplete a puppy of 20 ml of blood per day, which is approximately 15% of the blood volume of a 2.0 kg animal. In contrast, A. braziliense and U. stenocephala consume 0.001 and 0.0003 ml per worm, respectively.

Diagnosis and differential diagnosis. Diagnosis of ancylostomiasis is made by identification of eggs or larvae from fecal samples by either flotation or direct smear. Parvovirus should be considered for puppies with bloody diarrhea, and autoimmune hemolytic anemia should be considered in the diagnosis of a young dog with anemia.

Prevention and control. Purchase of purpose-bred animals will limit the exposure to hookworm larvae, and effective sanitation programs will easily eradicate the infective larvae. Unlike ascarid eggs, hookworm eggs are readily killed by drying, sunlight, or cold; however, they do survive readily in warm, moist environments. Monthly administration of milbemycin or ivermectin plus pyrantel pamoate (Heartgard Plus) is recommended for prevention and control of canine ascarid infestation (Hall and Simpson, 2000) . Treatment. Pyrantel pamoate (Nemex) is the anthelmintic of choice because it is safest in young ill animals and is also effective against ascarids and other enteric helminths. Because of the possibility of transplacental or milk-borne infection, puppies should be treated every 2 weeks from weeks 2-8. A follow-up treatment at 11 weeks is recommended to kill any larvae that have migrated and matured since the initial therapy. Severely ill puppies may require supportive fluid therapy and possibly whole blood transfusions and iron supplementation.

Research complications. Anemic puppies with large worm burdens make poor research subjects and should be treated aggressively before placement on an experimental study.

iii. Strongyloides Etiology. Strongyloides stercoralis is a small strongyle that can cause hemorrhagic enteritis in puppies. It is found in warm, humid climates such as the southeastern United States.

fects dogs and other animals by third-stage larval penetration of the skin or mucous membranes. Larvae migrate via the circulatory system to the lung and then are coughed and swallowed to initiate the intestinal parasitism. The eggs of S. stercoralis hatch within the gut lumen, and so it is the first-stage larvae that pass in the feces and need to be identified by diagnostic examination. Once passed, the larvae can either develop into the infectious third-stage larvae or mature into free-living, nonparasitic adults.

Diagnosis and differential diagnosis. The Baermann procedure is usually performed on fresh feces in order to detect the motile first-stage larva (280-310 ~tm x 30-80 ~tm). The larvae must be distinguished from larva of Filaroides hirthi and hatched Ancylostoma caninum.

Treatment. The usual treatment for S. stercoralis is fenbendazole (Panacur) at 50 mg/kg per day for 5 days.

iv. Whipworms Etiology. Trichuris vulpis, the canine whipworm, can cause acute or chronic large-intestinal diarrhea. The adult whipworm typically resides in the cecum or ascending colon.

Clinical signs. Most whipworm infections are subclinical. In symptomatic cases, the typical clinical sign is diarrhea with blood and/or mucus. Abdominal pain, anorexia, and weight loss are also seen. Dogs may have eosinophilia, anemia, and/or hypoproteinemia on clinical hematology. Severe dehydration with electrolyte imbalance has occurred occasionally as an acute crisis episode.

life cycle. Adult worms residing in the canine large intestine intermittently release eggs that pass in the feces. The eggs are very hardy and can persist for years. In optimal conditions, the eggs develop into an infective embryo within 10 days. After ingestion by a dog, the larvae hatch in the small intestine, burrow into the small-intestinal mucosa, and then reemerge several days later to travel and burrow into the cecal and colonic mucosa. The prepatent period is typically 2-3 months long.

Pathologic findings. Dogs do not typically die from whipworm infestations. Lesions seen as incidental findings feature adult worms embedded into the colonic and cecal mucosae, causing local granulomatous inflammatory reactions and mucosal hyperplasia.

Pathogenesis. The penetration of the adult worm into the enteric mucosa, and the associated inflammation, can lead to the clinical development of diarrhea. Factors that influence the possible.development of clinical symptoms are the number and location of adult whipworms; the severity of inflammation, anemia, or hypoproteinemia in the host; and the overall condition of the host.

Diagnosis and differential diagnosis. Whipworm infestation is diagnosed by the presence of characteristic trichurid eggs on fecal flotation. These eggs are barrel-shaped, with thick walls and bipolar plugs. Because of the intermittent release of eggs by the adult female worms, negative fecal flotation does not exclude the possibility of clinical whipworm infection. Adult worms can be seen on colonoscopy (Jergens and Willard, 2000) . Differential diagnoses for whipworm infestation include giardiasis, coccidiosis, and bacterial enteritis.

Prevention and control. Trichuris eggs are resistant to disinfection, making control difficult. Dessication or incineration is the only completely effective means to eradicate whipworm eggs from the environment.

Treatment. Fenbendazole, oxibendazole, and milbemycin have all been recommended for treatment of whipworms. Treatment for whipworm infestation should be at monthly intervals for 3 months (Jergens and Willard, 2000) . Treatment is also suggested in cases wherein whipworm infestation is suspected but not confirmed by multiple fecal flotation. Rapid response to treatment would be indicative of a correct diagnosis; lack of response should prompt further diagnostic efforts.

Research complications. Whipworm infestation has not been documented to interfere with research protocols, although one would anticpate that aberrations in local enteric immune function and absorptive functions of the large intestine could result from trichuriasis.

Etiology. Heartworm disease of dogs is caused by the filarial worm, Dirofilaria immitis. Adult heartworms reside in the pulmonary artery; severe infestations can result in the presence of worms in the right ventricle and atrium. Microfilariae, the immature worms produced by the adults, circulate in the bloodstream until a mosquito (intermediate host) ingests them.

Clinical signs. Most heartworm infestations are asymptomatic. The most common clinical signs observed are coughing and dyspnea. Clinical signs of exercise intolerance and rightsided heart failure can be seen in severe infestations.

Epizootiology and transmission. Successful heartworm transmission requires the presence of mosquitoes. For this reason, random-source dogs or dogs housed in outdoor kennels are much more likely to have heartworm infestations than indoor, purpose-bred dogs. Mosquitoes become infested with heartworm microfilariae when they take a blood meal from the dog. The microfilaria progress through several larval stages within the mosquito, eventually terminating at the third stage. This stage is then returned to the canine bloodstream during feeding. This stage matures within the dog's circulatory system, and the adults reside in the pulmonary artery. Male and female heartworms will then sexually reproduce to create more microfilariae and propagate the parasitic life cycle. In the United States, transmission of heartworm by mosquitoes occurs over a 6month or shorter period, except for the southeastern and Gulf Coast states. Here, climatic conditions enable longer survival of the mosquitoes (possibly year-round), thus resulting in the highest prevalence of heartworm infestation (Knight, 2000) .

Pathologic findings. On necropsy, the small, slender worms can be seen in the pulmonary artery, right ventricle, and/or right atrium (Fig. 4a ). There may be no histologic abnormalities associated with a minor worm burden, although typically the arterial endothelium in these areas is hyperplastic (Fig. 4b) . Endothelial cell hyperplasia, vascular smooth muscle hyperplasia, inflammation, and thrombosis of the pulmonary arteries and arterioles characterize more significant infestations. Severe infestations can lead to right-sided heart failure and its pathologic sequelae of ascites, pleural effusion, hepatomegaly, and right heart and pulmonary artery enlargement. Verminous pulmonary embolism can result from treatment of dogs with anthelmintics when a worm burden is present. Immune responses to circulating microfilariae can cause pathologic lesions, most commonly glomerulonephritis.

Pathogenesis. The physical presence of the worms in the pulmonary artery is partially responsible for clinical signs observed in severe cases. However, the host immunologic response to this infestation, coupled with secretion by the heart-worms of physiomodulative factors, contributes significantly to the complications seen with this disease. Endothelial cell proliferation, damage, and sloughing stimulates periarteritis and proliferation of the vascular media of pulmonary arteries and arterioles. These changes lead to thrombosis of these vessels and the arterial truncation that can be seen radiographically in severe infestations. The heartworms also release circulating factors that affect vascular tone and can promote bronchoconstriction (Dillon, 2000) . These factors are discussed in more detail below, under "Research complications."

Diagnosis and differential diagnosis. For dogs used in biomedical research, diagnosis of asymptomatic heartworm disease is important, especially if the dogs are used in cardiovascular, pulmonary, or long-term studies. A diagnosis of dirofilariasis is typically made by detection of adult heartworm antigens in a blood sample. Use of adult heartworm antigen tests has virtually eliminated the historical status of "occult" heartworm disease, which was caused by infestation of adult worms without corresponding microfilarial circulation. Commercial test kits that assay for the presence of adult heartworm antigens, and designed for use by veterinary practitioners, are readily available. False-negative results can occur during the prepatent period after initial infection (first 6-7 months), and when the adult worm burden is light or predominantly male. Infections consisting of more than three mature female worms are usually detected by antigenic serology (Knight, 2000) . A significant feature of these tests for circulating antigen is that they have a very high specificity (low rate of false-positive resuits). If a dog were negative on initial testing because of prepatency or small worm burden, it will more than likely be detected on a follow-up test 7 months later.

Examination for circulating microfilariae could be used to confirm an antigenic diagnosis of dirofilariasis or to establish that microfilarial production had occurred. Microfilarial detection can be done by microscopic examination of the buffy coat of a microhematocrit tube or by concentration techniques, such as the modified Knott test and filter tests. Tests that examine for microfilariae have the inherent problem of false positives caused by microfilariae of Dipetalonema reconditum, a nonpathogenic filarial worm. Other serologic diagnostic tests that were more common historically, and that may still be useful, include detection of antibodies to either adult heartworm antigens or microfilarial antigens.

These same techniques can be used to diagnose clinical heartworm disease. Additional diagnostic tests that can augment a diagnosis of clinical heartworm disease include thoracic radiography (pulmonary artery and right-heart enlargement), electrocardiography (right-heart enlargement), and hematology (eosinophilia). Differential diagnoses for symptomatic heartworm disease (coughing, dyspnea, and exercise intolerance) include canine distemper, canine infectious tracheobronchitis (complicated), streptococcal or other bacterial pneumonia, nocardiosis, and congestive heart failure.

Prevention and control. For dogs used in biomedical research, prevention is primarily via insect control and housing of the dogs in a controlled, indoor environment. Purpose-bred dogs reared in such an environment are usually free from dirofilariasis. However, any dog (random-source or purposebred) exposed to mosquitoes could become inoculated with infective larvae and, if untreated, could develop adult heartworm disease. There are many commercial anthelmintic preparations used to prevent heartworm infestation by killing the larval stages in the canine bloodstream before they become adult worms (e.g., ivermectin, milbemycin, and diethylcarbamazine). These could be used in a research setting in which heartwormnegative dogs are housed outdoors and thus could potentially be infected through mosquito bites. If a research facility is conditioning random-source dogs for long-term use, the presence of circulating adult heartworm antigen should disqualify an animal from the conditioning program.

Treatment. Treatment for eradication of heartworms (adults, juveniles, and microfilaria) is a long process that can pose a significant risk to the patient with regard to both drug side effects (Hoskins, 1989) and immunologic reactions to dead worms lodged in the pulmonary vasculature. For this reason, medical treatment of heartworm disease is not usually attempted in research dogs. In a rare instance when such treatment was in the best interest of a long-term canine experiment, thiacetarsamide (Caparsolate) and ivermectin (Ivomec) were used to eradicate adults and microfilariae, respectively (authors' personal experience). Alternative choices include melarsomine (Immiticide) as an adulticide and milbemycin (Interceptor), levamisole (Levasol), or fenthion (Spotton) as microfilaricidal agents. Dosing regimens for these agents are detailed in Dillon (2000) .

Research complications. The physiomodulative properties of heartworm infection have been studied. Such studies have looked at factors released by adult heartworms, as well as changes in the function of host tissues in response to the worm presence. Probably the most consistent finding is that endothelial cell-dependent relaxation of pulmonary arterial smooth muscle is depressed in heartworm-infected dogs as compared with control dogs, indicative of alterations in local endothelial cell behavior (Maksimowich et al., 1997; Matsukura et al., 1997; Mupanomunda et al., 1997) . The extension of this effect on peripheral arteries (in vivo and in vitro) has been supported in some studies (Kaiser et al., 1992) but refuted in others (Tithof et al., 1994) . It is thought that the endothelium is perturbed by a factor released from the adult Dirofilaria, possibly a cyclooxygenase product such as prostaglandin D2 (Kaiser et al., 1990 (Kaiser et al., , 1992 . These products have also been demonstrated to cause constriction in in vitro rat tracheal ring preparations (Collins et al., 1994) , suggesting that bronchoconstriction could be an aspect of the pathogenesis of the infestation. Platelet reactivity was also been found to be enhanced in dogs naturally infected with Dirofilaria, when compared with uninfected controis (Boudreaux and Dillon, 1991) . Based on these data, dogs that are positive for adult heartworm antigen should be considered inappropriate for use as research subjects and, if used, should be restricted to nonsurvival preparations that do not require physiological measurements.

Etiology. Several species of cestodes (tapeworms) parasitize the small intestine of dogs. The most common is Dipylidium caninum. Other species include Taenia pisiformis and, more rarely, Echinococcus granulosus, Multiceps spp., Mesocestoides spp., and Spirometra spp.

Clinical signs. Most cestode infestations are subclinical. Severe infestations with Dipylidium can be associated with diarrhea, weight loss, and poor growth.

Epizootiology and transmission. The cestode life cycle requires an intermediate host. For Dipylidium caninum, the intermediate hosts are fleas and lice. Thus this species of tapeworm can be readily transmitted by ingestion of arthropods that are canine parasites in and of themselves. Taenia pisiformis requires small ruminants, rabbits, or rodents for intermediate hosts, so spread is less likely, especially in a research setting. Echinococcus granulosus uses not only sheep as an intermediate host but also human beings, and thus the zoonotic potential of this cestode must be considered.

Pathologic findings. Adult cestodes in the small intestine are usually an incidental finding at necropsy.

Diagnosis and differential diagnosis. Definitive diagnosis is usually made by the identification of egg capsules or proglottids (tapeworm segments) on the surface of the feces or around the anus. Dipylidium egg packets are large (100 X 150 Bm) and contain 1-63 eggs per packet (Hall and Simpson, 2000) .

Prevention and control. The most significant means to limit cestode infestation is to control the population of fleas and/or lice infesting the colony. See the sections on these ectoparasites for effective means to treat infested dogs and kennels.

Treatment. Praziquantel at 5-12.5 mg/kg orally or subcutaneously is the standard treatment for cestodiasis, especially Taenia or Echinococcus species. Fenbendazole, mebendazole, or oxfendazole may also be effective against Dipylidium caninum (Hall and Simpson, 2000) . Clinical signs. Most lung fluke infestations are inapparent, but coughing can develop in cases that prompt a strong inflammatory response. Pneumothorax has been a sequela of cyst rupture, in which case dyspnea with reduced lung sounds would be the typical presentation.

Epizootiology and transmission. The lung fluke life cycle requires two intermediate hosts: a snail and then a crayfish. Dogs become infested after eating crayfish, which essentially limits this disease to random-source dogs. On ingestion, the immature flukes (metacercariae) migrate to the lungs and encyst in the pulmonary parenchyma. Eggs produced by adult flukes are passed into the bronchioles, coughed up, swallowed, and passed in the feces to complete the life cycle.

Pathologic findings. Grossly, the trematode cysts containing adult flukes can be seen in the lung parenchyma. Areas of eosinophilic inflammation surround the cysts, and eosinophilic granulomas can also be seen encircling released eggs. Pleural hemorrhages may also be caused by the migrating metacercariae (Lopez, 1995) .

Pathogenesis. Clinical illness is usually a result of a severe eosinophilic inflammatory response, pneumothorax caused by cyst rupture, or secondary bacterial pneumonia.

Diagnosis and differential diagnosis. Definitive diagnosis of Paragonimus infestation requires identification of the characteristic ovoid eggs (80-115 ~tm long) with a single operculum in either the feces or a transtracheal wash. Identification from fecal samples requires sedimentation techniques. Other causes of coughing in dogs (e.g., infectious tracheobronchitis, dirofilariasis, congestive heart failure) need to be considered. Radiographically, the appearance of (multi)focal densities within the air-filled lung field needs to be differentiated from pulmonary neoplasia (primary or metastatic) or systemic fungal pneumonias.

Prevention. Use of purpose-bred dogs virtually eliminates the chance of pulmonary trematodiasis in a research animal.

Treatment. Praziquantel (at 25 mg/kg q8 hr X 3 days) or fenbendazole (25-50 mg/kg q12 hr X 10-14 days) are recommended for treatment of canine Paragonimus infestation (Hawkins, 2000) . Effectiveness is monitored by fecal sedimentation tests for eggs and resolution of radiographic lesions (which may never resolve entirely). Early diagnosis of pulmonary trematodiasis should warrant discontinuation of a dog from a long-term study because of the possibility of more serious clinical sequelae, such as pneumothorax.

Research complications. Experimental studies involving the immune system, especially eosinophilic or local pulmonary responses, would be significantly affected by even minor infestations. Clinical illness would complicate almost any research project and makes dogs poor anesthetic risks. Radiographic lesions may confound diagnostic evaluation for pulmonary metastasis of tumors.

e. Mites i. Demodicosis Etiology. Canine demodicosis is caused by Demodex canis, a commensal mite that lives in the hair follicles. It is considered to be normal fauna of dog skin, but certain conditions (i.e., immunosuppression) cause development of clinical illness.

Clinical signs. Demodex canis infestation is typically asymptomatic. Clinical demodicosis presents with variable and nonspecific clinical signs, such as alopecia, erythema, pruritus, crusts, and hyperpigmentation. It can occur anywhere on the body but is often seen on the feet and the face and around the ears (DeManuelle, 2000a). Secondary bacterial pyoderma is a common complication.

Epizootiology and transmission. Demodex canis mites pass to nursing pups from the dam. They live their entire lives on one dog and are not considered contagious to other dogs or humans. Certain breeds are predisposed to the generalized form of Demodex dermatitis (see "Pathogenesis," below). Beagles are among the predisposed breeds, as are German shepherds, Doberman pinschers, Old English sheepdogs, collies, boxers, and shorthair brachycephalic breeds (Muller et al., 1983) .

Pathologic findings. Histologically, Demodex infections are characterized by perifolliculitis and folliculitis with mites and keratin debris visible in the hair follicles. Cases with generalized demodicosis (see "Pathogenesis," below) may have a minimal cellular response with no eosinophils, indicative of severe immunosuppression .

Pathogenesis. When clinical demodicosis develops, it is classified into "localized" or "generalized" (e.g., more than one foot affected, or five or more small areas, or one large body area). Localized demodicosis is typically seen in juvenile dogs (< 18 months) and usually resolves without treatment as natural immunological control develops. Generalized demodicosis can develop in juvenile or adult populations. Juvenile-onset generalized demodicosis occurs in dogs with a genetic predisposition, thought to be an inherited T-lymphocyte dysfunction. Adult-onset generalized demodicosis is usually indicative of an underlying endocrine (hyperadrenocorticism, diabetes mellitus, hypothyroidism) or neoplastic disorder or can develop as a result of immunosuppressive therapy (such as corticosteroid administration).

Diagnosis and differential diagnosis. Demodex is readily identified from deep skin scrapings of lesioned areas (Campbell, 2000; Noli, 2000) . Demodex canis has a characteristic "cigar shape," with short, stubby legs on a body 100-400 ~tm long. Differential diagnoses for local demodicosis include dermatophytosis, allergic contact dermatitis, and seborrheic dermatitis. The primary differential diagnosis for generalized demodicosis is primary bacterial pyoderma; remember, however, that bacterial pyoderma is a common secondary complication of the generalized form of this parasitism.

Prevention and control. Dogs with generalized demodicosis should not be maintained in a breeding colony.

Treatment: Ivermectin (Ivomec) at 200-600 ~tg/kg and oral milbemycin (Interceptor) at 1-2 mg/kg/day have been found to be effective treatments. These parasiticides are probably the most practical to use in a research setting, although they are not labeled for treatment of Demodex canis. Amitraz (Mitaban) dips (250 ppm every 14 days) can be used for more problematic cases. Treatment duration can be extensive and must be accompanied by repeated skin scrapings.

Research complications. Dogs with generalized demodicosis should not be used in research studies, because this disease is indicative of another underlying disorder (endocrine or immunological). Dogs that receive immunosuppressive agents or paradigms could develop generalized demodicosis as an unexpected consequence of the experimentation.

ii. Sarcoptic mange Etiology. Canine sarcoptic mange is caused by Sarcoptes scabiei var. canis.

Clinical signs. The most common clinical sign is an intense pruritus, usually beginning at sparsely furred areas such as the ear pinnae, elbows, and ventral thorax and abdomen. Lesions are characterized by alopecia and yellowish dry crusts with a macular papular eruption. These lesions may be exacerbated by excoriation due to the pruritic nature of the condition.

Epizootiology and transmission. Sarcoptes mites live their entire lives in the stratum corneum of the host animal; however, they can survive for 1-3 weeks away from the host, and it is this ability that enables them to spread from dog to dog. Sarcoptes scabiei var. canis can also infect cats and humans.

Pathologic findings. Histologic examination can be unrewarding because mites are rarely seen on tissue sections, and the associated dermatitis is nondiagnostic: perivascular and interstitial dermatitis with hyperkeratosis, with or without eosinophilic infiltration. Suggestive histopathologic lesions are epidermal "nibbles," small foci of edema, exocytosis, degeneration, and necrosis .

Pathogenesis. Lesions and illness are a result of the female mites burrowing through the epidermal layers to deposit eggs, and the larvae migrating back to the surface. The typical locations of mange lesions are a result of the mite's preference for relatively hairless areas.

Diagnosis and differential diagnosis. Sarcoptic mange can be difficult to diagnose because multiple skin scrapings can yield negative results with this parasitic disorder. Hopefully, adult mites, mite eggs, or mite feces can be observed on superficial skin scrapings. Even if scrapings are negative, however, a therapeutic trial should be initiated if the clinical signs and history suggest a Sarcoptes etiology. Demonstration of anti-mite IgE in either the serum or via an intradermal antigen test can be used as a diagnostic aid (Campbell, 2000 ). An important differential diagnosis is flea allergy dermatitis; in contrast, mange is nonseasonal and contagious.

Prevention and control. Use of purpose-bred dogs limits the possibility of having research animals with sarcoptic mange. For random-source dogs, an ectoparasite control program should be in place to limit possible infestations. Many institutions use ivermectin as a means to control both endoparasites and ectoparasites.

Treatment. Unless treatment would interfere with research objectives, all dogs with sarcoptic mange (no matter how minor the lesions) and their kennel mates should be treated because of the contagious nature of the disease and its zoonotic potential. In research colonies, the usual means of treatment is either ivermectin (Ivomec) at 200-400 ~tg/kg q14 days or milbemycin (Interceptor) at 3 oral doses of 2 mg/kg q7 days . Neither of these agents is approved for treatment of sarcoptic mange, but they are considered to be effective. Acaricidal dips (e.g., lime sulfur, organophosphates, amitraz) can also be used.

Research complications. The local skin inflammation and systemic immune response to sarcoptic mange probably make infected dogs poor subjects for dermatologic and immunologic studies.

f Lice and Ticks i. Lice Etiology. Dogs can be infested by one species of sucking louse (Linognathus setosus) and two species of biting lice (Trichodectes canis and Heterodoxus spiniger).

Clinical signs. Mild cases of pediculosis may be asymptomatic or may cause pruritic areas of dry skin. More severe infestations can cause significant pruritus and produce alopecia, papules, and crusts. These lesions lead to excoriation and secondary bacterial dermatitis. Severe Linognathus infestations could cause anemia, because this species feeds on blood.

Epizootiology and transmission. Louse infestations are uncommon in both pet animal practice and the research setting. They would most likely be seen in random-source dogs that were obtained from a pound or shelter. Transmission is usually by direct contact, for lice spend their entire lives on the host species. Lice are host-specific and not zoonotic.

Pathogenesis. The biting lice usually cause more local irritation than the sucking louse and therefore are more apt to induce clinical dermatologic signs. Trichodectes canis can serve as vector for the canine tapeworm Dipylidium caninum. The most severe complication of infestations by the sucking louse is the potential anemia.

Diagnosis and differential diagnosis. Pediculosis is diagnosed by direct observation of the lice or nits (eggs) on the dog's skin. Cellophane tape can be used to pick up surface debris from skin lesions, which may include nits or immobilized lice (Muller et al., 1983) . Differential diagnoses include dermal acariasis, flea allergy dermatitis, and seborrhea.

Prevention. Use of high-quality conditioned dogs for research should prevent pediculosis from ever being seen within a research facility. Random-source dogs should be shampooed or treated prophylactically with topical insecticide before being permitted to enter the research colony.

Treatment. Most commercially available insecticide shampoos and dips readily treat louse infestations. Treatment should be repeated in 10-14 days, because any nits that were not killed would have hatched by that time (Muller et al., 1983) .

There is probably minimal interference with research, unless severe Linognathus infestations cause anemia.

ii. Ticks Etiology. Ticks are obligate arachnid parasites that require vertebrate blood as their sole food source. Except for the brown dog tick (Rhipicephalus sanguineus), ticks have a wide host range and are not especially host-specific; so any number of tick genera and species can be found on dogs. Genera that more commonly infest dogs in the United States include species of Rhipicephalus, Dermacentor, and Ixodes. The primary significance of tick infestation is the tick's ability to be a vector for many other infectious diseases, including Rocky Mountain spotted fever (caused by Rickettsia rickettsii), Lyme disease (Borrelia burgdorferi), and the canine forms of ehrlichiosis (Ehrlichia canis and E. platys), babesiosis (Babesia canis), haemobartonellosis (Haemobartonella canis), and hepatozoonosis (Hepatozoon canis).

Clinical signs. As an entity unto itself, tick infestation causes minimal clinical signs. Most infestations are subclinical, although some dogs may lick and bite at the site, aggravating the local lesion. Some dogs can develop a hypersensitivity reaction after several tick bites; these dogs develop a more granulomatous response at the location of the bite (Merchant and Taboada, 1991) . Some species of ticks (primarily Dermacentor andersoni and D. variabilis) produce a salivary neurotoxin that can cause an ascending flaccid paralysis (Malik and Farrow, 1991) . The paralysis develops within 5-9 days of tick attachment and can result from a single tick. This paralysis is fatal once the respiratory musculature is affected.

Epizootiology and transmission. In dogs used for biomedical research, tick infestation may occasionally be seen in randomsource dogs, because these dogs are more likely to have been in tick habitats than purpose-bred dogs. Ticks commonly reside in wooded areas until they contact a suitable host for a blood meal. The brown dog tick may reside within kennels (attics, bedding, wall insulation) (Garris, 1991) .

Pathologic findings. Under most circumstances, tick infestation will be an incidental finding on necropsy (unless tick paralysis was the cause of death).

Pathogenesis. Tick-bite paralysis is caused by the presence of a salivary neurotoxin released by female ticks of certain genera (e.g., Dermacentor) while consuming a blood meal (Malik and Farrow, 1991) . Interestingly, dogs seem to be most affected by this condition, whereas cats appear to be resistant. The primary dysfunction appears to be at the neuromuscular junction, as stimulation of the motor nerves fails to elicit a response, but direct stimulation of the muscle tissue results in contractions. Tick bites can also transmit pathogen microorganisms to the dog, because ticks serve as vectors for several infectious diseases, including Lyme borreliosis, ehrlichiosis, babesiosis, and Rocky Mountain spotted fever.

Diagnosis and differential diagnosis. For uncomplicated tick bites and tick-bite paralysis, definitive diagnosis is made by identification of the offending arachnid (and improvement of paralysis after removal). Differential diagnoses for tick-bite paralysis include botulism, snakebite, polyradiculoneuritis, and idiopathic polyneuropathy (Malik and Farrow, 1991) .

Prevention. Purpose-bred dogs should be free from all ectoparasites, but ticks can occasionally be seen on randomsource animals. Research dogs should not be exercised in outdoor areas infested with ticks, and kennels must be cleaned properly and regularly so as to remain free of ticks and other parasites.

Treatment. Removal of the offending tick is the primary treatment for both local inflammation as well as tick-bite paralysis. Dogs with tick-bite paralysis usually show improvement within 24 hr, with complete recovery within 72 hr (Malik and Farrow, 1991) To remove an attached tick from a dog, forceps should be used to grasp the tick as close to the dog's skin as possible. The tick should not be grabbed by the body, as this may cause the parasite to either rupture or inject its body contents into the dog. The tick should be pulled away from the dog with steady pressure. Many of the diseases transmitted by ticks are zoonotic so precautions, such as wearing gloves, should be taken. Use of topical acaricide/insecticides on newly arrived random-source dogs should help to limit infestations.

probably have minimal impact on research variables. The significant concern for tick infestation is the possible development of tick-bite paralysis or of any one of a number of systemic diseases spread by ticks (see Sections III,A,l,e-g).

g. Other i. Flea infestation Etiology. Fleas are laterally flattened wingless insects that feed on animal blood. The most common flea to infest dogs is Ctenocephalides felis, the cat flea. Other fleas that can affect dogs are Ctenocephalides canis, Pulex irritans, and Echidnophaga gallinacea. The fleas are speciated by the shape of their head and by the presence or absence of ctenidae (spiny combs on or behind the head) (Campbell, 2000) .

Clinical signs. Flea infestations usually cause foci of alopecia and pruritus. Dogs that are hypersensitive to antigenic proteins in flea saliva develop the more severe "flea allergy dermatitis," which features papules and crusting. Acute moist dermatitis ("hot spots") can also be seen in these cases, and secondary pyoderma or seborrhea can develop. Lesions from flea allergy dermatitis generally appear in the dorsal lumbosacral region, as well as the flanks, thighs, and abdomen (Muller et al., 1983) .

The lesions are typically worse in the summer and autumn months and are progressively more severe as the dog ages.

Epizootiology and transmission. Fleas are readily transmitted between animals and even between host species. They move readily between the host and the environment, making transmission easy and control difficult. Because fleas require host blood for food, they can survive off of a host for only 1-2 months (Muller et al., 1983) .

Pathologic findings. Biopsy samples are usually nondiagnostic in cases of flea allergy dermatitis. Lesions are typically characterized by perivascular eosinophilic inflammation and may feature pustules and folliculitis if secondary pyoderma develops (Muller et al., 1983) .

Pathogenesis. Fleas are parasites that require animal blood for their meals. When they bite host animals, they inject some saliva into the host's skin. If the host develops an allergic response to the flea saliva, it will develop the more pruritic flea allergy dermatitis. Fleas can also transmit or serve as vectors for other pathogens (e.g., Dipylidium tapeworms).

flea allergy dermatitis are definitively diagnosed by observing the fleas on the host's skin. Given that this may be difficult because of the mobility of the flea and the majority of the time it spends off of the host, diagnosis is often based on clinical signs, history, and lesion distribution. Sometimes the presence of flea excrement ("flea dirt") on the dog's skin can support a presumptive diagnosis (DeManuelle, 2000b) . Circulating eosinophilia is seen in some dogs with flea allergy dermatitis. Differential diagnoses include mite and louse infestations, bacterial folliculitis, and allergic or atopic conditions that present with skin lesions in dogs (e.g., food, drug, or contact hypersensitivity).

Prevention. Most dogs obtained from high-quality purposebred facilities should be free from flea infestations. Dogs received from pounds, shelters, or licensed dealers would be more likely to be affected by fleas (or any ectoparasitism). Thorough knowledge of prevention, control, and treatment measures at these facilities should be obtained, and dogs from sources where proper prevention and/or therapy are not practiced should be evaluated and/or empirically treated upon arrival at the facility.

Control. Thorough cleaning of the dog's housing environment should remove the risk of perpetuating or transmitting flea infestation in the colony.

Treatment. Treatment for fleas needs to address treatment of both the dog and the environment. Many insecticide formulations such as shampoos, sprays, dips, powders, and oral systemics can be used for initial treatment of the individual dog. The active ingredients include pyrethrins, pyrethroids, carbamates, and organophosphates. Flea control in the kennel may need to include outdoor areas in warm climates. Typically combinations of adult insecticides and juvenile growth regulators are used for environmental treatment. Directed sprays are the most effective means of treating housing areas, because flea "bombs" or foggers do not penetrate adequately into tight areas where fleas might hide (DeManuelle, 2000b) .

In addition to insecticide therapy, dogs with flea allergy dermatitis may also require anti-inflammatory medication to relieve clinical signs. Oral prednisone or prednisolone at 0.5 mg/kg q12 hr for 5-7 days has been proposed as a starting therapy (Muller et al., 1983) . The use of hyposensitization with flea-bite antigens is controversial and not practical for the research setting.

Research complications. Mild flea infestation probably has minimal impact on most research protocols, and treatment measures may in fact be more detrimental to the experimental objective than the actual ectoparasitism. In a research setting, the residual effects of insecticides may preclude their use in experimental animals. Such treatments should be used judiciously to ensure that experimental results are not more seriously affected by the therapy rather than the infestation. Dogs with flea-allergy dermatitis are more severely affected by the flea infestation and should be treated apigropriately; however, systemic corticosteroids may also interfere with experimental objectives, especially in studies involving functions of the immune system. The ability of fleas to transmit other parasitic diseases must also be considered.

Etiology. Dermatophytoses ("ringworm") are fungal skin infections, which in dogs in the United States are usually caused by either Microsporum canis, M. gypseum, or Trichophyton mentagrophytes (Muller et al., 1983) .

Clinical signs. Uncomplicated superficial dermatophytoses are characterized by circumscribed circular areas of alopecia, usually with minimal to no inflammation. These skin lesions are usually seen around the face, neck, and forelimbs but can be found anywhere on the body. Secondary bacterial infections can develop; these lesions are called kerions and are selflimiting, for the fungus cannot survive in inflamed skin (Muller et al., 1983) .

Ep&ootiology and transmission. The fungi that cause skin infections are very contagious and readily transmissible between dogs and other species (including human beings), but they can also be obtained from the soil.

Pathologic findings. On close inspection of skin samples, broken hair shafts (and not complete hair loss) would be seen with uncomplicated dermatophytosis. Histologically, fungal elements can be seen within the stratum corneum or in and around the hair and hair follicles (Muller et al., 1983) . Stains that facilitate visualization of fungal elements include periodic acid-Schiff (PAS) or Gomori methenamine-silver. The pattern of inflammation in the affected foci is very variable and can feature folliculitis, perivascular dermatitis, hyperkeratosis, and/or vesicular dermatitis.

Pathogenesis. The dermatophytes typically infect the hair shaft itself, the hair follicle, and possibly the skin around the affected hair. The hair follicle is not destroyed (unless by secondary bacterial infection), but the hair itself becomes brittle and breaks. This causes short stubbly hair to be seen within the lesion. As the lesion progresses, the hairs in the center recover from the infection, thus leading to the classic "ringworm" appearance of the alopecic areas. It is postulated that the inflammatory process produces an environment that is unfavorable for dermatophyte survival, whereas the periphery of the lesion still enables continued fungal growth (Muller et al., 1983) .

Diagnosis and differential diagnosis. Diagnosis of dermal fungal infection is typically made by scraping the affected area to obtain hair and superficial epidermal cells. These scrapings are then digested with potassium hydroxide to facilitate observation of fungal elements. Fungal elements can also be seen on skin biopsy samples. For speciation of a fungus, skin scrapings can also be inoculated onto agars that promote fungal growth, such as Sabouraud's medium or dermatophyte test medium (DTM). Incubation should be at 30~ with 30% humidity for 14-30 days. Lesions caused by M. canis may fluoresce when inspected using a Wood's (253.7 nm ultraviolet) light. Unfortunately, some strains of M. canis do not fluoresce, and neither does M. gypseum or T. mentagrophytes. Differential diagnoses for dermatomycosis include seborrhea, localized demodecosis, folliculitis, histiocytoma, and acral lick dermatitis (Muller et al., 1983) .

Prevention. Purpose-bred dogs are typically free of infectious dermatophytes, but ringworm may be diagnosed on randomsource animals.

Control. In cases of dermatophytosis, isolation of the affected animal(s) is prudent, because the fungi are easily spread to other dogs, as well as to people. Treatment, if acceptable, should be started immediately.

Treatment. Topical antifungal therapy is most commonly used. Shampoos, rinses, and creams containing miconazole, ketoconazole, enilconazole, or chlorhexidine are commercially available to treat ringworm (Stannard et al., 2000) . Severe cases may require systemic therapy with griseofulvin, ketoconazole, itraconazole, or fluconazole. However, these systemic antifungal agents may have considerable side effects (such as vomiting and teratogenicity with griseofulvin). Many of the newer agents are also expensive and not labeled for use in dogs.

impact on most research applications for dogs. Unfortunately, the zoonotic implications of dermatophytoses force the issue of aggressive treatment, and many antifungal agents may not be compatible with biomedical research studies.

Systemic fungal infections disseminate to multiple organ systems from a single mode of entry (usually through the respiratory tract). Dogs are susceptible to several fungi that characteristically cause systemic mycosis, including Blastomyces dermatitidis, Histoplasma capsulatum, Coccidioides immitis, and Cryptococcus neoformans var. neoformans. These diseases are not typically seen in the research setting, because of the low overall incidence and noncontagious nature of these disorders and because of the use of purpose-bred animals. These conditions could, however, present in the rare random-source dog that was subclinical at its point of origin, especially if the animal becomes immunosuppressed (either naturally or by virtue of experimental manipulation). Typical clinical signs include weight loss, fever, lymphadenopathy, and cough and dyspnea (if the lungs are affected). The reader is advised to read veterinary medical text chapters (e.g., Taboada, 2000) for more complete information on these disorders and their possible treatments.

Although the incidence of hypothyroidism in the canine population is not high (Kemppainen and Clark, 1994) , deficiency in thyroid hormone can significantly affect basal metabolism and immune function. Because these factors are important in many biomedical research studies, it is imperative that laboratory animal veterinarians be able to recognize, diagnose, and treat this problem.

Etiology. The majority of cases of canine hypothyroidism are due to lymphocytic thyroiditis, an autoimmune disorder, or idiopathic atrophy of the thyroid gland. Both of these causes result in a gradual loss of functional thyroid tissue (Kemppainen and Clark, 1994) . Lymphocytic thyroiditis is the major cause of hypothyroidism in laboratory beagles and appears to be familial in that breed (Tucker, 1962; Beierwaltes and Nishiyama, 1968; Manning 1979) . Rarely, congenital defects or nonfunctional tumors may cause hypothyroidism (Peterson and Ferguson, 1989; Kemppainen and Clark, 1994) .

Clinical signs. Because it affects metabolism in general, hypothyroidism can produce a large number of clinical signs referable to many organ systems. An individual dog with hypothyroidism may have one or any combination of clinical signs. Hypothyroidism reduces the dog's metabolic rate, which then produces such signs as obesity, lethargy, cold intolerance, and constipation. Additionally, hypothyroidism can produce several dermatologic abnormalities, including alopecia, hyperpigmentation, seborrhea, and pyoderma (Peterson and Ferguson, 1989; Panciera, 1994) . Several clinicopathologic abnormalities have also been reported in a large percentage of hypothyroid dogs. These aberrations include increased serum cholesterol and triglycerides due to a decrease in lipolysis and decreased numbers of low-density lipopolysaccharide receptors (Peterson and Ferguson, 1989; Panciera, 1994) . Normocytic normochromic nonregenerative anemia and increased serum alkaline phosphatase and creatine kinase have also been reported in a significant number of hypothyroid dogs (Peterson and Ferguson, 1989; Panciera 1994) . Neurologic signs of hypothyroidism, which include lameness, foot dragging, and paresis, may be caused by several mechanisms such as segmental nerve demyelination or nerve entrapment secondary to myxedema (Peterson and Ferguson, 1989) . Mental impairment and dullness have also been reported in hypothyroid dogs, secondary to atherosclerosis and cerebral myxedema (Peterson and Ferguson, 1989) . Hypothyroidism has been implicated in other neurological abnormalities such as Horner's syndrome, facial nerve paralysis, megaesophagus, and laryngeal paralysis; however, these conditions do not always resolve with treatment (Bischel et al., 1988; Panciera, 1994) , and so the relationship between hypothyroidism and these problems has not been completely defined (Panciera, 1994) . Myopathies associated with hypothyroidism are caused by metabolic dysfunction and atrophy of type II muscle fibers and can present with signs similar to neurological disease (Peterson and Ferguson, 1989) . Hypothyroidism can also cause bradycardia as a result of decreased myocardial conductivity. Abnormalities that may be detected by ECG include a decrease in P and R wave amplitude (Peterson and Ferguson, 1989) and inverted T waves (Panciera, 1994) . These electrocardiographic abnormalities are caused by lowered activity of ATPases and calcium channel function. Several reports have suggested that hypothyroidism is associated with von Willebrand's disease and bleeding abnormalities. However, the relationship is probably one of shared breed predilection and not a true correlation. It has been demonstrated that dogs with hypothyroidism are not deficient in von Willebrand's factor when compared with other dogs. In addition, the replacement of thyroid hormone in dogs did not increase the levels of vWF:Ag in naturally occurring (Panciera and Johnson, 1994) or experimentally induced (Panciera and Johnson, 1996) hypothyroidism.

Epizootiology. The prevalence of hypothyroidism in the general canine population has been reported to be less than 1% (Panciera, 1994) . The disorder occurs most often in large-breed dogs but has been reported in several other breeds as well as mongrels. Doberman pinschers and golden retrievers appear to have a higher incidence of hypothyroidism when compared with other breeds (Panciera, 1994; Peterson and Ferguson, 1989; Scarlett, 1994) . There have been several reports about hypothyroidism in laboratory colonies of beagles (Manning, 1979; Tucker, 1962; Beierwaltes and Nishiyama, 1968) . In general, the problem is usually recognized in middle-aged animals, and some reports state that there is a higher incidence of hypothyroidism in spayed female dogs (Panciera, 1994; Peterson and Ferguson, 1989 ).

Diagnosis and differential diagnosis. Because of the large number of clinical manifestations in dogs, the recognition of hypothyroidism is not always straightforward. Likewise, the diagnosis of hypothyroidism can be difficult because of the lack of definitive diagnostic tests available for the dog. The tests currently available and in popular use will be discussed further. However, a complete understanding of the diagnosis of hypothyroidism requires a familiarity with thyroid hormone metabolism and function that is beyond the scope of this writing. For additional information, the reader is referred to one of several manuscripts available (Peterson and Ferguson, 1989; Ferguson, 1994) .

Currently, the ability to diagnose hypothyroidism relies heavily on the measurement of serum total T 4 (thyroxine) and free T 4 (Peterson and Ferguson, 1989; Ferguson, 1994) . T 4 serves primarily as a precursor for T 3 in the body and is heavily proteinbound. Free T4 represents the unbound fraction that is available to the tissues (Peterson and Ferguson, 1989) . Using the measurement of serum total T 4 and free T4, hypothyroidism can usually be ruled out if the values are within the normal range or higher. If both hormone concentrations are low, it is highly likely that the patient has hypothyroidism, and a therapeutic trial is in order (Peterson and Ferguson, 1989) . However, it must be noted that nonthyroidal illnesses and some drugs (e.g., glucocorticoids, anticonvulsants, phenylbutazone, salicylates) can falsely lower these values (Peterson and Ferguson, 1989; Ferguson, 1994) . Therefore, low values do not always indicate that hypothyroidism is present, and animals should not be treated solely on the basis of serum hormone levels if clinical signs are absent. If the clinical signs are equivocal or if only total T 4 or free T 4 is decreased, further diagnostic testing is warranted (Peterson and Ferguson, 1989) . Although T 3 is the most biologically active form of thyroid hormone in the body, the measurement of serum T 3 levels is an unreliable indicator of hypothy-roidism (Peterson and Ferguson, 1989; Ferguson, 1994) . Like T4, serum T 3 can be falsely lowered by many nonthyroidal illnesses and many drugs (see above). In addition, T 3 may be preferentially released, and conversion of T 4 to T 3 may be enhanced in the hypothyroid dog (Peterson and Ferguson, 1989; Ferguson, 1994) . T 3 was within normal limits in 15% of the hypothyroid dogs in one study (Panciera, 1994) . Autoantibodies can be responsible for false elevations in the concentrations of T 3 and T 4 found in these respective assays. It has been recommended that free T4, measured by equilibrium dialysis, be assayed in dogs that are suspected of hypothyroidism and have autoantibodies with normal or high T 3 and T 4. Autoantibodies have been found in less than 1% of the samples submitted to one laboratory (Kemppainen and Behrend, 2000) .

Other means of diagnosing hypothyroidism have been described. In humans, endogenous TSH (thyroid-stimulating hormone) levels provide reliable information on thyroid status, and an assay for endogenous TSH is now available in dogs. However, TSH levels can be normal in some dogs with hypothyroidism, and high TSH levels have been noted in normal dogs. Therefore, it is recommended that TSH levels be considered along with other information (clinical signs, T4) prior to diagnosis and treatment (Kemppainen and Behrend, 2000) . TSH stimulation testing using exogenous bovine TSH provides a good and reliable method for establishing a diagnosis. Unfortunately, the availability and expense of TSH limit the use of this diagnostic tool (Peterson and Ferguson, 1989; Ferguson, 1994) . Another drawback of TSH testing is that the test must be postponed for 4 weeks if thyroid supplementation has been given (Peterson and Ferguson, 1989) . When TSH is available for testing, there are several recommendations for dosage, routes of administration, and sampling times. One recommendation is 0.045 U of TSH per pound of body weight (up to a maximum of 5 U) to be administered IV. For this protocol, blood samples are taken prior to administration of TSH and 6 hours after. A normal response to the administration of TSH should create an increase of T 4 levels at least 2 ktg/dl above the baseline levels or an absolute level that exceeds 3 ~tg/dl (Peterson and Ferguson, 1989; Wheeler et al., 1985) .

Treatment. The treatment of choice for hypothyroidism in the dog is L-thyroxine (sodium levothyroxine). A recommended dosing regimen is 0.02 mg/kg once a day or 0.05 mg/m 2 (body surface area)/day for very small or very large dogs. If drugs that decrease thyroxine levels are being administered concurrently, it may be necessary to divide the thyroxine dose for twice daily administration. After the supplementation has begun, the thyroid hormone level should be rechecked in 6-8 weeks, and blood samples should be drawn 4-8 hours after the morning pill. A clinical response is usually seen in 6-8 weeks and would include weight loss, hair regrowth, and resolution of other signs (Panciera, 1994) . ECG abnormalities also return to normal (Peterson and Ferguson, 1989) . For dogs with neurologic signs, the prognosis is guarded, because the signs do not always resolve with supplementation (Panciera, 1994) .

Weight gain and eventual obesity are also frequent findings in dogs in the research environment. Because obesity can adversely affect several body systems as well as general metabolism, the laboratory animal veterinarian must be aware of the development of obesity and the potential effect that it can have on research.

Etiology. Obesity is defined as a body weight 20-25% over the ideal. In general, obesity occurs when the intake of calories exceeds the expenditure of energy. Excessive caloric intake resuits from overeating or eating an unbalanced diet. Overeating is a common cause of obesity in pet dogs and may be triggered by boredom, nervousness, or conditioning (MacEwen, 1992) . In addition, pet animals are often subjected to unbalanced diets supplemented with high-fat treats. In the laboratory animal setting, overeating is less likely than in a household, because access to food is more restricted and diets are usually a commercially prepared balanced ration. However, obesity can still be a problem if specific guidelines for energy requirements are not followed. In addition, the necessary caging of dogs in the research environment and thus the limitation to exercise reduces energy expenditure and predisposes dogs to weight gain. It is also important to realize that other factors may predispose dogs to obesity, even when guidelines for caloric intake and energy expenditure are followed (Butterwick and Hawthorne,. 1998 ). As in humans, genetics plays an important role in the development of obesity in dogs. It has been established that certain breeds are more predisposed toward obesity. In a study of dogs visiting veterinary clinics in the United Kingdom, Labrador retrievers were most likely to be obese. Other breeds affected included Cairn terriers, dachshunds, basset hounds, golden retrievers, and cocker spaniels. The beagle was also listed as a breed predisposed to obesity in the household environment (Edney and Smith, 1986) .

In addition to genetics, several metabolic or hormonal changes are associated with obesity. It has been well established that neutering promotes weight gain. In one study, spayed female dogs were twice as likely to be obese when compared with intact females (MacEwen, 1992) . The authors proposed that the absence of estrogen promotes an increase in food consumption. A similar trend toward obesity was found in castrated male dogs (Edney and Smith, 1986) . In addition, hypothyroidism and hyperadrenocorticism may present with obesity as one of the clinical signs (MacEwen, 1992) .

Epizootiology. Ewen, 1992) .

Obesity affects up to 40% of pet dogs (Mac-Diagnosis and differential diagnosis. The diagnosis of obesity is somewhat subjective and relies on an estimate of ideal body weight. The ideal body condition for dogs is considered to be achieved when the ribs are barely visible but easily palpated beneath the skin surface. When the ribs are not easily palpated and/or the dog's normal function is impaired by its weight, the animal is considered obese. There are few objective, quantifiable methods for establishing this diagnosis. Ultrasound has been evaluated for measurement of subcutaneous fat in dogs, and measurements taken from the lumbar area can be used to reliably predict total body fat (Wilkinson and McEwan, 1991) . After a diagnosis of obesity has been made, additional diagnostic tests should be performed to determine if there is an underlying cause for the problem. A complete physical exam should be performed to look for signs of concurrent disease and to establish if obesity has adversely affected the individual. Serum thyroid hormones should be evaluated (see Section III,B,l,a), and serum chemistry may reveal an increased alkaline phosphatase associated with hyperadrenocorticism.

Treatment. Restricting food intake readily treats obesity, and this is easily done in the research setting. It has been suggested that a good weight loss program involves restriction of intake to 60% of the calculated energy requirement to maintain ideal body weight. It has been shown that restriction of calories down to 50% produces no adverse health effects. However, T 3 levels will decrease in direct proportion with caloric intake. Ideally, weight loss will occur at a rate of 1-2% of body weight per week (Laflamme et al., 1997) . With more severe calorie restriction and more rapid weight loss, the individual is more likely to rebound and gain weight after restrictions are relaxed.

There has been agreat deal of attention in humans as to the correct diet to be fed to encourage weight loss. Likewise, the type of diet fed to dogs has been examined. As mentioned above, the restriction of calories is most important, and feeding less of an existing diet can do this. Alternatively, several diet dog foods are available, and there is some evidence that these diets are superior to simple volume restriction (MacEwen, 1992) . There has been much concern about the addition of fiber to the diet in both humans and animals as a method for reducing caloric intake while maintaining the volume fed. Studies in dogs have examined the addition of both soluble and insoluble fibers to calorierestricted diets. These studies have shown that the addition of fiber does not have an effect on satiety in dogs and therefore does not have a beneficial effect in weight loss protocols (Butterwick and Thorne, 1994; Butterwick and Thorne, 1997) .

It is important to control weight gain in research animals, because of the association of obesity and several metabolic changes. Although an association between obesity and reproductive, dermatologic, and neoplastic problems has been reported (MacEwen, 1992) , this relationship is not consistently apparent (Edney and Smith, 1986) . Obesity in dogs over 10 years of age appears to be related to an increase in cardiovascular problems (Edney and Smith, 1986) , and obesity has been linked to hypertension. Joint problems including osteoarthritis and hip dysplasia have also been related to obesity (MacEwen, 1992; Kealy et al., 1997) . In addition, diabetes mellitus has been linked to obesity, and obesity induces hyperinsulinism in several experimental models (MacEwen, 1992) .

In the laboratory setting, the majority of traumatic wounds will be small in size. In facilities with good husbandry practices and a diligent staff, traumatic wounds will generally be observed quickly and attended to promptly. Under these conditions, proper initial treatment will lead to uncomplicated wound healing. Complications such as infection and delayed healing arise when wounds are not noticed immediately or. when the basic principles of wound management are not followed.

To aid in the description of wounds and in decision making about wound therapy, several classification systems have been developed for traumatic injuries. At one time, decisions about wound therapy were largely based upon the length of time since wounding, or the concept of a "golden period." It is now recognized that several factors must be considered prior to initiating wound care, including (but not limited to) the type and size of the wound, the degree of wound contamination, and the capability of the host's defense systems (Swaim, 1980; Waldron and Trevor, 1993) . One of the most widely used classification systems is based upon wound contamination and categorizes wounds as either clean, clean-contaminated, contaminated, or dirty (see Table V ).

The vast majority of the wounds seen in the laboratory setting will fall into the clean and clean-contaminated categories. These wounds may be treated with the basic wound care described below and primary closure of the wound. Contaminated and dirty wounds, which are seen infrequently in the laboratory setting, require more aggressive therapy. Dirty wounds can occur as postsurgical infections or complications of initial wound therapy. When one is in doubt as to the classification of a wound, the worst category should be presumed in order to provide optimal therapy and reduce the chance for complications.

The initial treatment of a wound is the same regardless of the wound's classification. When first recognized, the wound should be covered' with a sterile dressing until definitive treatment is rendered. Bleeding should be controlled with direct Waldron and Trevor (1993) .

pressure; tourniquets are discouraged because of the complications that may arise with inappropriate placement (Swaim, 1980) . It is best to avoid using topical disinfectants in the wound until further wound treatment (culture, debridement, lavage) has been performed (Swaim, 1980) . When the treatment of a wound begins, anesthesia or analgesia may be necessary, and the choice of anesthetic regimen will depend on the size and location of the wound as well as the preference of the clinician. If the wound is contaminated or dirty, bacterial cultures, both aerobic and anaerobic, should be performed at this time. Then a water-soluble lubricant gel may be applied directly to the wound. A wide margin of hair should then be clipped from around the wound, using a #40 blade. After the clipping, a surgical scrub is performed around the edges of the wound. Povidone-iodine alternating with alcohol or chlorhexidine gluconate scrub alternating with water is most often recommended for surgical preparation of the skin surface (Osuna et al., 1990a,b) . Simple abrasions that involve only a partial thickness of the skin do not generally require further treatment. Full-thickness wounds require further attention, including irrigation with large quantities of a solution delivered under pressure. Two solutions, 0.05% chlorhexidine diacetate in water (Lozier et al., 1992) and 1% povidone-iodine in saline, are most often recommended for wound lavage (Waldron and Trevor, 1993) . The chlorhexidine solution may offer the advantage of greater bactericidal activity but does not significantly alter wound healing when compared with povidone-iodine (Sanchez et al., 1988) . Actually, the type of solution chosen may not be as important to wound care as the volume and pressure at which the solution is delivered. It has been suggested that 8 psi is required to obtain adequate tissue irrigation, and this may be achieved by using a 35 ml syringe and an 18-or 19gauge needle (Waldron and Trevor, 1993) .

For wounds that are contaminated or dirty, debridement is an important part of initial therapy. Debridement usually proceeds from superficial to deeper layers. Skin that is obviously necrotic should be removed. Although it is often recommended to remove skin back to the point at which it bleeds, this may not be feasible with large wounds on the limbs. In addition, other factors such as edema or hypovolemia may reduce bleeding in otherwise viable skin (Waldron and Trevor, 1993) . If one is unsure about tissue viability in areas that are devoid of extra skin, the tissue may be left (Swaim, 1980; Waldron and Trevor, 1993) , and nonviable areas will demarcate within 2-3 days (Waldron and Trevor, 1993) . Necrotic fat should be resected liberally, because it does not have a large blood supply and will provide an environment for infection. Often, resection of subcutaneous fat is necessary to remove debris and hair that could not be removed during wound irrigation. Damaged muscle should also be liberally resected (Swaim, 1980) . The wound should be irrigated several times during debridement and again after completion.

After initial wound treatment, the options concerning wound closure must be weighed. The principles of basic surgery are discussed in several good texts, and readers are encouraged to pursue additional information. Primary wound closure is defined as closure of the wound at the time of initial wound therapy and is the treatment of choice for clean and clean-contaminated wounds. Closure is performed in two or more layers, carefully apposing tissues and obliterating dead space. If dead space will remain in the wound, a drain should be place d. Subcutaneous closure should be performed with absorbable suture such as polydioxanone (PDS), polyglactin 910 (Vicryl), or polyglycolic acid (Dexon). It is best to use interrupted sutures and avoid leaving excess suture material in the wound. It may be necessary to choose tension-relieving suture patterns, such as horizontal mattress. Skin closure is generally performed with nylon (3-0 or 4-0).

In situations where gross contamination cannot be completely removed, closure of the wound should be delayed or avoided. After debridement and irrigation, the wound should be bandaged. Initially, the wound can be covered by gauze sponges soaked in saline or chlorhexidine to create a wet-to-dry bandage. When the sponges are later pulled from the wound, dried exudates will also be removed. When the wound appears clean, the layer in contact with the wound may be changed to a nonadherent dressing such as vaseline-impregnated gauze (Swaim, 1980) . The contact layer is covered by cotton padding, and the entire bandage is covered by a supportive and protective layer. The bandages should be changed once or twice daily, depending upon the amount of discharge coming from the wound. Wound closure within 3-5 days of wounding (prior to the formation of granulation tissue) is considered delayed primary closure. When the wound is closed after 5 days, this is considered secondary closure (Waldron and Trevor, 1993) . Secondintention healing involves allowing the wound to heal without surgical intervention. This type of healing is often used on limbs when there is an insufficient amount of skin to allow complete closure (Swaim, 1980) . It is important to note that second-intention healing will take longer than surgical repair of a wound, and in the case of large wounds it will be more expensive because of the cost of bandaging materials.

Several factors must be weighed concerning the use of antibiotics in traumatic wounds, including the classification and site of the wound, host defenses, and concurrent research use of the animal. When wounds are clean or clean-contaminated, antibiotics are seldom necessary unless the individual is at high risk for infection. When wounds have been severely contaminated or are dirty, antibiotics are indicated, and the type of antibiotic will ultimately depend on culture and sensitivity results. Until such results are available, the choice of antibiotic is based on the most likely organism to be encountered. In skin wounds, Staphylococcus spp. are generally of concern, whereas Pasteurella multocida should be considered in bite wounds. Cephalosporins, amoxicillin-clavulanate, and trimethoprim sulfas are often recommended for initial antibiotic therapy (Waldron and Trevor, 1993) .

Etiology. Pressure sores (decubital ulcers) can be a problem in long-term studies that require extended periods of recumbency. Decubital ulcers usually develop due to continuous pressure from a hard surface contacting a bony prominence such as the elbow, the tuber ischii, tarsus, or carpus. The compression of the soft tissues between the hard surfaces results in vascular occlusion, ischemia, and ultimately tissue death (Swaim and Angarano, 1990) . Several factors that increase pressure at the site and/or affect the integrity of the skin will predispose an individual to develop pressure sores. These factors include poor hygiene, self-trauma, low-protein diet, preexisting tissue damage, muscle wasting, inadequate bedding, and ill-fitting casts or bandages (Swaim and Angarano, 1990) .

Clinical signs. At first, the skin at the developing site will appear red and irritated. Over time, constant trauma can result in full-thickness skin wounds and can progress to necrosis of underlying structures such as bone. The severity of the sores may be graded from I to IV, according to the depth of the wound and the tissues involved, from superficial skin irritation to bone necrosis.

Epizootiology. The problem usually occurs in large-breed dogs, but any type of dog can be affected.

Prevention and control. Minimizing or eliminating those factors that can predispose to decubital ulcers is important to both the prevention and the control of this condition. If the dogs are going to experience long periods of recumbency, adequate bedding or padding must be provided. Skin hygiene is of the utmost importance when trying to prevent or treat pressure sores. The skin should be kept clean and dry at all times. If urine scalding is a problem, the affected area should be clipped, bathed, and dried thoroughly at least once or twice daily. Finally, an appropriate diet to maintain good flesh and adequate healing is also important (Swaim and Angarano, 1990) .

Treatment. The treatment of pressure sores must involve care of the wound and attention to the factors causing the wound. The extent of initial wound management will largely depend on the depth of the wound. For simple abrasions and small wounds involving the skin only, simple wound cleansing and openwound management provide adequate treatment. When wounds involve deeper tissues, including fat, fascia, or bone, more aggressive therapy must be performed. The affected area should be radiographed to assess bone involvement, and the wound should be cultured. All of the damaged tissue should be debrided, and wound management guidelines should be followed (see Section III,C,1). When a healthy granulation bed has formed over the entire wound, a delayed closure over a drain may be performed (Swaim and Angarano, 1990) . With extensive lesions, reconstruction with skin flaps may be necessary.

Bandaging should be performed on all full-thickness wounds; however, it is important to remember that ill-fitting or inadequately padded bandages or casts may worsen the problem. The area over the wound itself should not be heavily padded, because this will increase the pressure over the wound. The wounded area should be lightly covered and then a doughnut, created from rolled gauze or towel, should be fitted around the wound. This will displace the forces acting on the wound over a larger area and over healthier tissue. Then the doughnut is incorporated into the bandage. If a cast has been applied to the area for treatment or for research purposes, a hole can be cut over the wound to reduce pressure in that area and allow treatment of the wound (Swaim and Angarano, 1990 ). Bandages should be removed at least once or twice a day to allow wound care.

After wound care has been initiated the causative factors for the pressure sore must be addressed (see "Prevention and control," above). Recumbent animals should be moved frequently to prevent continuous compression on the wound. If the dog tends to favor a position that aggravates the problem, splinting the body part to reduce contact with hard surfaces may be necessary.

Etiology. Acral lick granuloma is a psychodermatosis, a skin lesion caused by self-trauma. In a few cases, self-trauma begins because of identifiable neurologic or orthopedic causes (Tarvin and Prata, 1988) . However, the majority of the cases begin because of repetitive licking by dogs that are confined and lack external stimuli (Swaim and Angarano, 1990) . It has been theorized that the self-trauma promotes the release of endogenous endorphins, which act as a reward for the abnormal behavior (Dodman et al., 1988) . The laboratory setting is an environment that could promote this abnormal behavior and lead to acral lick granuloma.

Epizootiology. The lesions associated with acral lick granuloma are seen most often in large-breed dogs, but any type of dog can be affected (Walton, 1986) .

Clinical signs. At first, lesions appear as irritated, hairless areas usually found on the distal extremities (Swaim and Angarano, 1990 ). The predilection for the limbs may be due to accessibility or possibly may be caused by a lower threshold for pruritus in these areas. As the lesions progress, the skin becomes ulcerated, and the wound has a hyperpigmented edge. The wounds may partially heal and then be aggravated again when licking resumes.

Diagnosis and differential diagnosis. Acral lick granulomas must be differentiated from several other conditions, including bacterial or fungal infection, foreign bodies, and pressure sores. In addition, mast-cell tumors and other forms of neoplasia can mimic the appearance of acral lick granuloma. Many of these problems can be ruled out by the history of the animal. When in doubt, a biopsy should be taken. An uncomplicated acral lick granuloma would feature hyperplasia, ulceration, and fibrosis without evidence of infection or neoplasia (Walton, 1986) .

Prevention and control. Behavior modification and relief of boredom are important aspects of preventing (and treating) acral lick granuloma. The environment of a dog with this problem can be enriched with exercise and the introduction of toys. In addition, the relief of boredom or anxiety can be attempted through the use of drugs such as phenobarbital, megestrol acetate, and progestins. These drugs may produce side effects, however (Swaim and Angarano, 1990) , and may interfere with experimental results.

Treatment. Several treatments have been reported for acral lick granuloma, and none of them have been proven to be successful in aH cases. One of the most important aspects of treatment is to break the cycle of self-trauma. Mechanical restraint with an Elizabethan collar is one of the easiest methods to accomplish this goal. Several direct treatments have been examined, including intralesional and topical steroids, perilesional cobra venom, acupuncture, radiation, and surgery (Swaim and Angarano, 1990; Walton, 1986) . Opioid antagonists have been used in an attempt to treat acral lick granuloma by blocking endogenous opioids. In one study, either naltrexone (1 mg/kg SQ) or nalmefene (1-4 mg/kg SQ) successfully reduced the excessive licking behavior in 7 of 11 dogs; however, lesions returned after the drug was discontinued (Dodman et al., 1988) . The use of a mixture of flunixin meglumine, steroid, and dimethyl sulfoxide (3 ml of Banamine [Schering] mixed with 8ml of Synotic [Diamond Laboratories]) applied topically twice daily has also been shown to be effective (Walton, 1986) . The prognosis for acral lick granuloma should be considered guarded, because the lesions often recur or new lesions develop when treatment is discontinued.

Etiology. Hygromas are fluid-filled sacs that develop as a result of repeated trauma over a bony prominence. The area over the olecranon is most frequently affected, but hygromas have been reported in association with the tuber calcis, greater trochanter, and stifle (Newton et al., 1974) .

Epizootiology. Elbow hygromas are most frequently reported in large and giant breeds of dogs around 6-18 months of age (Johnston, 1975; Bellah, 1993) . Elbow hygromas are seen infrequently in the laboratory animal setting because the commonly affected breeds are seldom used in research. However, the housing environment for research dogs predisposes them to hygromas, because these animals spend a large amount of time on hard surfaces such as cage bottoms or cement runs. For this reason, laboratory animal veterinary and husbandry staff should be familiar with this condition.

Clinical signs. A dog with an elbow hygroma presents with a unilateral or bilateral, painless, fluctuant swelling over the point of the elbow. The animals are not usually lame. Over a long period of time, elbow hygromas may become inflamed and ulcerated. If the hygroma is secondarily infected, the animal may exhibit pain and fever (Johnston, 1975; Bellah, 1993) .

Pathology. The fluid-filled cavity in the hygroma is lined by granulation and fibrous tissue. Hygromas lack an epithelial lining and therefore are not true cysts. The fluid within the cavity is yellow or red and is a serous transudate. This fluid is less viscous than joint fluid, and elbow hygromas do not communicate with the joint (Johnston, 1975) .

Treatment. The treatment of elbow hygromas should be conservative whenever possible, and surgical options should be reserved for complicated or refractory cases. Conservative management of the elbow hygroma is aimed at relieving pressure at the point of the elbow by providing a padded cage surface and/or bandaging the elbow in a manner similar to that used to treat pressure sores (see Section III,C,2). More aggressive therapy, including needle drainage and the injection of corticosteroid into the hygroma, has been described but is not recommended, because infection is a serious complication of this treatment (Johnston, 1975) . Likewise, simple surgical excision of elbow hygromas can be associated with complications such as wound dehiscence and ulceration (Johnston, 1975) . A technique that has been used successfully involves placement of multiple Penrose drains. The drains are kept in place for 2-3 weeks, and the limb remains bandaged for 4 weeks with this technique (Bellah, 1993) . Another technique has been described that involves the removal of a crescent-shaped piece of the skin and capsule. The remaining dead space is closed with mattress sutures over stents, and then the wound is closed in a routine fashion. The stents are removed in 5-7 days, and the wound is bandaged until suture removal in 10-14 days (Newton et al., 1974) . Regardless of the method used to treat an elbow hygroma, recurrence of the problem is likely unless the predisposing factors are identified and relieved.

Etiology. In the research environment, corneal ulcers are most often associated with either direct trauma, contact with irritating chemicals, or exposure to the drying effects of air during long periods of anesthesia. Chronic or recurrent corneal ulcers may also be associated with infection or hereditary causes in some breeds of dogs; however, these cases would be rare in the laboratory setting.

Clinical signs. The signs of corneal ulceration are blepharospasm, epiphora, and photophobia. The eye may appear irritated and inflamed. In minor cases, the cornea may not appear abnormal; however, in cases of deeper ulceration, the cornea may appear roughened or may have an obvious defect. In addition, the periocular tissues may be swollen and inflamed because of self-inflicted trauma from rubbing at the eye.

A tentative diagnosis of corneal ulcer or abrasion may be based on the clinical signs. A definitive diagnosis of corneal ulcers may be made by the green appearance of the cornea when stained with fluorescein dye. When a corneal ulcer has been diagnosed, the eye should be inspected for underlying causes such as foreign bodies or abnormal eyelids or cilia.

Treatment. The treatment of corneal ulcers will depend on the depth and size of the affected area. Deep ulcers may require debridement and primary repair. In such cases, a third eyelid or conjunctival flap may be applied to the eye until experienced help can be obtained. Superficial abrasions are generally treated with topical application of antibiotics. A triple antibiotic ointment that does not contain steroids given 3 times a day for 2-3 days usually provides adequate treatment. Ointments are preferred over drops, because use of the former requires less frequent. Simple corneal ulcers are restained with fluorescein after 3 days and should show complete healing at that time. If the ulcer is not healed, this may indicate that the ulcer has an undermined edge impeding proper healing. Topical anesthetic should be applied to the eye, and a cotton-tipped applicator can be rolled over the surface of the ulcer toward its edge. This will remove the unattached edge of the cornea and healing should progress normally after debridement. In all cases, an Elizabethan collar or other restraint may be necessary to prevent additional trauma to the eye.

Indwelling intravascular catheters, including intracaths and vascular access ports, often play a vital role in research protocols. The catheters are most often placed in a central vein or artery where they may be used for repeated blood sampling, administration of anesthetics and experimental compounds, or measurement of hemodynamic parameters. Although catheters vary in composition, number of ports, and port placement, the basic principles of their implantation and maintenance are similar. It is important that the laboratory animal veterinarian be familiar with these principles and the potential complications of catheter use. When appropriately maintained, indwelling catheters may remain functional for months without serious complication.

The actual incidence of complications associated with indwelling vascular catheters in dogs is unknown. This is due largely to the fact that many of the problems may be incidental findings or related to a particular research protocol. One study (Hysell and Abrams, 1967 ) examined the lesions found at necropsy in animals with chronic indwelling catheters (exact vascular locations not specified). The lesions found were categorized as traumatic cardiac lesions, visceral infarcts, and fatal hemorrhages. The traumatic cardiac lesions consisted primarily of masses of fibrin and inflammatory cells on the heart valves. The visceral infarcts were noted in the spleen, kidney (Fig. 5) , and brain and resulted from fibrin embolization from either the valvular lesions or the catheter tip. Fatal hemorrhages were most often found in animals with experimentally induced hypertension. These animals developed clinical signs of sepsis and later ruptured a major vessel associated with mycotic infection and aneurysm.

Etiology. The leading complication associated with the use of indwelling vascular catheters is infection, either systemic or local at the point of entry through the skin. Septicemia may develop from bacterial colonization of either the tract around the catheter or the catheter lumen.

Clinical signs. The signs and treatment of systemic infection are covered in Section III,D,3. Problems with the skin defect associated with the catheter port vary from mild skin irritation to obvious infection. The signs may include redness and swelling of the skin around the external port, discharge from the skin wound, or even abscess formation.

Prevention. Because indwelling catheters play an important role in many research protocols, it is highly desirable to prevent catheter complications that may result in loss of the device. The catheter should be made of nonthrombogenic material. In addition, it is recommended that catheters be as simple as possible. A catheter with extra ports or multiple lumens requires addi- tional management and supplies more routes for infection. The use of vascular access ports that lie entirely under the skin eliminates many problems with infection. It has also been found that a long extension of tubing connected to the port may actually reduce the potential for infection of the catheter (Ringler and Peter, 1984) . The initial placement of an indwelling catheter must be done under aseptic conditions by individuals who are familiar with the procedure. The placement of the catheter should be verified by radiography. Catheters that are used for delivery of drugs or blood sampling should be positioned in the vena cava and not in the right atrium, thereby minimizing trauma to the tricuspid valve. After catheter placement, the animals should be observed daily for signs of either local or systemic infection. The catheter entry site should be disinfected, coated with antibiotic ointment, and rebandaged every other day. Once a month, the catheter line may be disinfected with chlorine dioxide, as described below (see "Treatment"). Throughout the life of the catheter, injections into and withdrawals from the catheter should be done in a sterile manner, and the number of breaks in the line should be kept to a minimum.

Treatment. The treatment of catheter infections almost invariably involves removal of the catheter, as demonstrated in both dogs and monkeys (Ringler and Peter, 1984; DaRif and Rush, 1983) . Superficial wound irritation or infection may be treated locally with antibiotic ointment, sterile dressing changes and efforts to minimize catheter movement; however, more extensive problems require aggressive therapy. Systemic antibiotic therapy should be initiated for a 10-day period. The choice of drug will ultimately be based on previous experience and culture results. Aerobic and anaerobic cultures of blood and locally infected sites should be performed (Ringler and Peter, 1984) . Localized abscesses or sinus tracts may be managed by establishing drainage and flushing with chlorhexidine. Again, the catheter should be removed. If retention of a catheter is important, the catheter lumen may be disinfected by filling with chlorine dioxide solution. It has been shown that there are no adverse effects from the use of chlorine dioxide in catheters (Dennis et al., 1989) . The solution is removed after 15 min and replaced with heparinized saline. All of the extension lines and fluids used in the catheter should be discarded. The blood cultures should be repeated 3 days after the antibiotic therapy has ceased. If bacteria are still cultured, the catheter must be removed.

Intestinal access ports have been used to study the pharmacokinetics of drugs at various levels in the intestinal tract. These catheters are usually vascular access ports with several modifications to allow secure placement in bowel (Meunier et al., 1993) . When placed and managed correctly, these ports may remain in place for months without complications.

The most frequently reported complication associated with these catheters is infection around the port site (Meunier et al., 1993 , Kwei et al., 1995 . These infections lead to removal of the catheters despite treatment with local lavage and systemic antibiotics. There have also been reports of catheters dislodging from the intestinal tract and resulting in peritonitis. This complication has largely been eliminated with the improved security afforded by a synthetic cuff added to the end of the catheter (Meunier et al., 1993) . The chapter authors have also seen migration of the catheter end within the lumen of the intestine (caused by peristaltic motion to egest the catheter), extensive intra-abdominal adhesions, and intestinal torsion (Figs. 6a,b) as complications of intestinal access ports.

The procedures for placement and maintenance of the catheters are similar to those outlined previously for indwelling vascular catheters. It is important that the catheters be firmly secured to the intestine to prevent migration or dislodgment. An omental patch placed over the site of entry may help form a firm adhesion. In addition, it is important to place the proper length of catheter within the peritoneal cavity; excess catheter length can promote adhesion formation, whereas insufficient catheter length to account for visceral organ motion can result in detachment. The placement and patency of the catheters can be verified periodically by contrast radiography using iodinated contrast material or by fecal occult blood testing after a small amount of blood has been injected through the catheter (Meunier et al., 1993) .

Etiology. Sepsis is defined as the systemic response to infection. Most often, sepsis is a result of infection with gramnegative bacteria; however, sepsis may also be associated with gram-positive bacteria and fungi. In laboratory animals, sepsis is seen as a complication of surgical procedures or associated with chronic implants. Sepsis may also be seen as a complication of infectious diseases such as parvovirus.

Clinical signs. The signs of sepsis can vary, depending on the source of the infection and the stage of the disease. Early in the course of sepsis, dogs will present with signs of a hyperdynamic response, including an increased heart rate, increased respiratory rate, red mucous membranes, and a normal to increased capillary refill time. Systemic blood pressure and cardiac output will be increased or within the normal range. The animals will often be febrile. Later in the course of the syndrome, the animals will show the classic signs of septic shock, including decreased temperature, pale mucous membranes, and a prolonged capillary refill time. Cardiac output and blood pressure are decreased as shock progresses. Peripheral edema and mental confusion have also been reported (Hauptman and Chaudry, 1993) . Pathogenesis. The pathophysiology of sepsis is complex and is mediated by immune responses involving mediators such ~ as cytokines, eicosinoids, complement, superoxide radicals, and nitric oxide. The body responds to overwhelming infection with an attempt to optimize metabolic processes and maximize oxygen delivery to tissues. However, if inflammation is left unchecked, the system may be unable to compensate, and the result is cardiovascular collapse.

Diagnosis. In general, a presumptive diagnosis of sepsis is made based on the occurrence of several in a group of signs, including altered body temperature, increased respiratory and/or heart rate, increased or decreased white blood cell count, increased number of immature neutrophils, decreased platelet count, decreased blood pressure, hypoxemia, and altered cardiac output. However, extreme inflammation without infection (e.g., pancreatitis, trauma) may create similar signs. One study examined the diagnosis of sepsis in canine patients at a veterinary hospital based on easily obtainable physical and laboratory findings. That study found that septic individuals had higher temperatures, WBC counts, and percentage of bands than nonseptic individuals, whereas platelet counts were lower in the septic dogs. There were no differences in respiratory rate or glucose levels between the groups. Using these criteria, the results had a high sensitivity and a tendency to overdiagnose sepsis (Hauptman et al., 1997) . Ultimately, the presence of a septic focus simplifies diagnosis greatly; however, the focus may not be obvious. If the signs of sepsis are evident but the focus is not, several systems should be evaluated for infection, including urinary tract, reproductive tract, abdominal cavity, respiratory tract, teeth, and heart valves (Kirby, 1995) .

Treatment. The treatment of sepsis has three aims. The first aim is to support the cardiovascular system. All septic animals should be treated with fluids to replace deficits and to maximize cardiac output. Crystalloids are most frequently used to maintain vascular volume, primarily because of their low cost. Colloids offer the advantage of maintaining volume without fluid overload and may have other positive effects on the cardiovascular system. Acid-base and electrolyte imbalances should also be addressed.

After the animal has stabilized, the treatment of sepsis should be aimed at removing the septic focus. Obvious sources of infection should be drained or surgically removed. If an implant is associated with the source of infection, the implant should be removed. Antibiotic therapy should also be instituted. The choice of antibiotic will ultimately depend upon the results of culture; however, the initial choice of antibiotics is based on previous experience, source of infection, and Gram stains. The organisms associated with sepsis are often gram-negative bacteria of gastrointestinal origin or are previously encountered nosocomial infections. Ideally, the antibiotic chosen for initial therapy should be a broad-spectrum, bactericidal drug that can be administered intravenously. Second-or third-generation cephalosporins provide good coverage, as does combination therapy with enrofloxacin plus metronidazole or penicillin.

Finally, the treatment of sepsis is aimed at blocking the mediators of the systemic response. Several studies have examined the effects of steroids, nonsteroidal anti-inflammatory drugs, and antibodies directed against endotoxin, cytokines, or other mediators of the inflammatory response; however, none of these treatments have proven greatly effective in clinical trials. Consequently, there is no "magic bullet" for the treatment of sepsis at this time. Successful therapy remains dependent on aggressive supportive care coupled with identification and elimination of the inciting infection.

Etiology. In research animals, aspiration into the lungs may occur accidentally during the oral administration of various substances or by the misplacement of gastric tubes. Aspiration of gastric contents may also occur as a complication of anesthesia. In pet animals, aspiration is often seen as a result of metabolic and anatomical abnormalities; however, such occurrence would be rare in the research setting.

Clinical signs. The signs of aspiration lung injury may include cough, increased respiratory rate, pronounced respiratory effort, and fever. When respiration is severely affected, the oxygen saturation of blood will be decreased. The diagnosis of this problem is based on a history consistent with aspiration and the physical findings. Classically, radiographs of the thorax demonstrate a bronchoalveolar pattern in the cranioventral lung fields. However, these lesions may not appear for several hours after the incident of aspiration. In addition, the location of the lesions may be variable, depending on the orientation of the animal at the time of aspiration.

Pathogenesis. Aspiration of gastric contents or other compounds can create lung injury of variable severity, depending upon the pH, osmolality, and volume of the substance. The compounds aspirated can produce direct injury to lung tissue, but more importantly, the aspiration provokes an inflammatory response probably mediated by cytokines. The result is a rapid influx of neutrophils into the lung parenchyma and alveolar spaces. The inflammation leads to increased vascular permeability with leakage of fluid into the alveolar spaces and can eventually lead to alveolar collapse. If the condition is severe, it may result in adult respiratory distress syndrome and respiratory failure. It should be noted that infection is not present in the early stages of this condition but may complicate the problem after 24-48 hr.

Treatment. The treatment of aspiration lung injury is largely supportive and depends upon the severity of the inflammation and the clinical signs. In cases in which a small amount of a relatively innocuous substance (e.g., barium) has been aspirated, treatment may not be necessary. When severe inflammation is present, systemic fluid therapy should be instituted. Support of the cardiovascular system should be performed judiciously; fluid overload could lead to an increase in pulmonary edema. The use of colloids is controversial because of the increase in vascular permeability that occurs in the lungs. Oxygen therapy is also controversial, because it may increase lung injury if administered at high concentrations for long periods of time (Nader-Djahal et al., 1997) . Several studies have addressed the use of anti-inflammatory agents to reduce lung injury associated with aspiration; however, none are used clinically in human or veterinary medicine at this time.

In humans, antibiotics are reserved for use in cases with confirmed infection, in order to prevent the development of antibiotic-resistant pneumonia. It has been suggested that dogs should be treated with antibiotics immediately when the aspirated material is either not acidic or has potentially been contaminated by oral bacteria associated with severe dental disease. Amoxicillin-clavulanate has been recommended as a first line of defense, reserving enrofloxacin for resistant cases (Hawkins, 2000) . The presence of pneumonia should be verified by tracheal wash and cultures.

Etiology. In laboratory animals, accidental burns usually result from thermal injury (heating pads, water bottles) or harsh chemicals (strong alkalis, acids, disinfectants). The insult to the skin results in desiccation of the tissue and coagulation of proteins. In addition, the severely injured area is surrounded by a zone of vascular stasis, which promotes additional tissue damage. Even small burns can result in significant inflammation that could affect the outcome of some research investigations and cause considerable discomfort to the animal. The proper and immediate treatment of burn wounds can reduce the effects of the injury on both the individual and the research.

Clinical signs. The clinical signs vary with the type and degree of burn injury. Initially, the injury may not be noticed. The first signs may be oozing from the skin and matting of the overlying hair. Within a couple of days, progressive hair and skin loss may be observed (Johnston, 1993) . The wounds may vary in severity from very superficial (involving only the epidermis) to those in which the epidermis and dermis are completely destroyed. Superficial wounds appear as red, inflamed skin similar to sunburn in humans. The pain associated with these injuries usually subsides in 2-3 days, and the wound reepithelializes without complications in 3-5 days. Deeper burns develop a thick covering, or eschar, composed of the coagulated proteins and desiccated tissue fluid. The wound heals by granulation under the eschar, which eventually sloughs or is removed to allow further healing by contraction and reepithelialization. Within 2-3 days of injury, the burn wound will be colonized by grampositive bacteria that rapidly cover the entire wound. Several days later, gram-negative organisms can appear in the burn wound (Johnston, 1993) . At this point, signs of wound infection and sepsis may occur (see Section III,D,3).

Treatment. Appropriate and timely treatment of a burn wound will reduce the extent of the injury. Thermal injuries should be immediately cooled to reduce edema and pain (Demling and Lalonde, 1989) . Chemical burns should be thoroughly lavaged for 60 min after wounding. The damaged tissues may be unable to mount appropriate responses to changes in temperature; therefore, the lavage should be performed with warm water to prevent hypothermia. After the initial treatment, all burn wounds should be gently cleansed 2-3 times a day (Demling and Lalonde, 1989) . Burns involving the epidermis and part of the dermis can be extremely painful, and analgesia should be addressed throughout the treatment period.

Systemic antibiotics are unable to penetrate eschar and are not adequately distributed through the abnormal blood supply of burned tissues. Therefore, topical wound dressings are recommended in the early stages of treatment. A thin film of a water-soluble broad-spectrum antibiotic ointment should be applied to the wound surface after each cleaning. Silver sulfadiazine has a broad spectrum, penetrates eschar well, and is often the preparation of choice for burn wound therapy. Povidone-iodine ointment will also penetrate thin eschar and provides a broad spectrum. Mafenide has a good spectrum that covers gram-negative organisms well and is often used to treat infected wounds, although it is associated with pain upon application (Demling and Lalonde, 1989) . When signs of wound or systemic infection are present, systemic antibiotics should be employed, and their ultimate selection should be based on culture and sensitivity results.

After the topical antibiotic has been applied, a nonadherent dressing should be placed on the wound. Burn wounds covered in such a manner tend to epithelialize more rapidly and are less painful than uncovered wounds. When the eschar over a burn wound has formed and become fully defined, a small or moderately sized wound may be completely resected.

Prevention. Obviously, prevention of burn wounds is preferable to a long course of treatment. Care should be taken to prevent direct exposure to harsh chemicals. Tables, floors, and other surfaces should be rinsed thoroughly after chemical use, prior to allowing any animal contact. Electric heating pads should be avoided, and only heated water blankets or circulating warm-air devices should be used to provide warmth to the animals. In rare instances, heated water blankets have also caused burns; therefore these devices should be carefully monitored. As a precaution, a thin towel may be placed between the animal and the water blanket.

Etiology. Research and/or anesthetic protocols may require the intravenous injection of various solutions. When these substances have a pH or osmolarity significantly different from that of the surrounding tissues, the accidental perivascular extravasation of the solutions may result in tissue damage. Several drugs have been shown to cause problems when injected perivascularly, including pentobarbital, thiamylal, thiopental, thiacetarsemide, vincristine, vinblastine, and doxorubicin (Swaim and Angarano, 1990; Waldron and Trevor, 1993) .

Clinical signs. The immediate signs of perivascular injection are swelling at the injection site and withdrawal of the limb or other signs of discomfort. Later, the area may appear red, swollen, and painful as inflammation progresses. Often there will be eventual necrosis of the skin around the injection site. In cases of doxorubicin extravasation, signs may develop up to a week after the injection, and the affected area may progressively enlarge over a 1 to 4 month period. This is because the drug is released over time from the dying cells (Swaim and Angarano, 1990) .

Prevention. Because the degree of injury and extensive treatment associated with perivascular extravasation of a drug can be detrimental to research protocols and can cause severe discomfort to the dog, prevention of these injuries is preferred. Prior to the use of any substance, the investigator should be aware of its chemical composition and the potential for problems. If a potentially caustic compound is to be used in a fractious subject, sedation of the dog is warranted if this will not interfere with the research protocol. Whenever possible, insertion of an indwelling catheter is extremely important. Access to a central vessel such as the cranial or caudal vena cava is preferred over the use of peripheral vessels. When peripheral catheters are used, the injection should be followed by a vigorous amount of flushing with saline or other physiological solution and removal of the catheter. Additional injections are best given through newly placed catheters in previously unused vessels. The repeated use of an indwelling peripheral catheter should be approached cautiously and done only out of necessity. Prior to use, the catheter should be checked repeatedly for patency by withdrawal of blood and injection of saline. Any swelling at the catheter site or discomfort by the subject indicates that the catheter should not be used.

Treatment. The treatment of perivascular injections will depend on the amount and type of substance injected. In most cases, dilution of the drug with subcutaneous injections of saline is recommended. In addition, steroids may be infiltrated locally to reduce inflammation. Topical application of dimethyl sulfoxide (DMSO) may also be helpful in reducing the immediate inflammation and avoiding the development of chronic lesions (Swaim and Angarano, 1990 ). The addition of lidocaine to subcutaneous injections of saline has been used in cases of thiacetarsemide injection (Hoskins, 1989) , and local infiltration of hyaluronidase accompanied by warm compresses has been suggested for use in cases of vinblastine injection (Waldron and Trevor, 1993) . Despite these treatments, necrosis of skin may be observed and would require serial debridement of tissues with secondary wound closure or skin grafting. In cases of doxorubicin extravasation, early excision of affected tissues is advocated to prevent the progressive sloughing caused by sustained release of the drug from dying tissues (Swaim and Angarano, 1990) . In all cases, the condition can be painful, and analgesia should be addressed.

Etiology. Hepatic encephalopathy is the result of the derangements in metabolism associated with abnormal liver function. This condition may be seen in young dogs with congenital portosystemic shunting of blood flow. However, in the research setting, encephalopathy occurs more often in canine models of hepatic disease that lead to liver failure. A well-developed knowledge of the pathophysiology of liver disease is necessary for the initial treatment and long-term management of hepatic encephalopathy.

Pathogenesis. When the liver function is severely impaired because of either portosystemic shunting of blood flow or loss of metabolically active hepatic tissue, the result is an accumulation of ammonia, toxic amines, aromatic amino acids, and short-chain fatty acids (Hardy, 1989; Center, 1998) . These compounds have several toxic effects that result in a decrease in cerebral energy metabolism and a decrease in excitatory neurotransmitter synthesis. Concurrently, there is an increase in the concentration of false neurotransmitters and the inhibitory substance 7-aminobutyric acid (GABA).

Clinical signs. The signs of hepatic encephalopathy include lethargy, depression, muscle tremors, and convulsions.

Diagnosis and differential diagnosis. A presumptive diagnosis of hepatic encephalopathy may be based on the appearance of clinical signs following experimental manipulation of the liver. Additional diagnostic tests to verify the loss of liver function can be performed to confirm the diagnosis. Serum glucose and protein levels may be low if hepatic function is severely impaired. A low serum urea nitrogen level suggests that the normal hepatic metabolism of ammonia into urea has been impaired. Elevated levels of serum bile acids and blood ammonia also verify the loss of liver function (Hardy, 1989) . Measurement of serum hepatic leakage enzymes are nondiagnostic, because they can be low, high, or normal.

Treatment. Because of the severity of hepatic encephalopathy, treatment may be initiated based on a presumptive diagnosis. During initial treatment, supportive care with fluids and electrolytes should be instituted, based on the results of serum chemistry and blood gas analysis. The majority of animals with hepatic dysfunction will be hypokalemic, alkalotic, and hypernatremic; therefore, either 0.45% sodium chloride or 0.45% sodium chloride with 2.5% dextrose, supplemented with potassium chloride, is recommended (Hardy, 1989) . The type of drug to be used for seizure control is controversial. The short halflife of diazepam makes it an attractive choice compared with barbiturates, which have prolonged metabolism when hepatic function is impaired (Maddison, 1995) . However, endogenous benzodiazepines mediate some of the CNS signs seen with hepatic encephalopathy. Therefore, the use of diazepam has been discouraged in favor of phenobarbital (Johnson, 2000) . The drug selected for seizure control should be titrated carefully, given the altered liver metabolism.

Most importantly, the treatment of dogs with hepatic encephalopathy must be aimed at reducing the levels of toxic metabolites in the bloodstream. Because protein metabolism is a major source of ammonia, all oral food intake should cease until the signs of hepatic encephalopathy have abated. Because gastrointestinal bleeding may occur in individuals with liver failure and this is also a source of protein, the use of H2 blockers such as cimetidine or ranitidine is suggested (Swalec, 1993) . In addition, lactulose retention enemas should be performed (10-15 ml/lb of a 30% solution in water, retained for 20-30 min) (Hardy, 1989) . Lactulose is an indigestible semisynthetic sugar that is metabolized in the gut to lactic and other acids. The decrease in colonic pH reduces ammonia levels in the bloodstream by converting intestinal ammonia into less diffusible ammonium ions. Lactulose will also cause an osmotic diarrhea. Antibiotics such as neomycin (10 mg/lb, 3-4 times/ day) or metronidazole (9 mg/lb, 3 times/day) should also be used to reduce the intestinal load of urease-producing bacteria responsible for splitting urea into ammonia (Hardy, 1989) .

When the signs of hepatic encephalopathy have resolved, the dog may be fed a low-protein diet. Diets suitable for dogs with renal insufficiency are recommended initially. This type of diet is not suitable for long-term use, however, because it appears that individuals with some types of hepatic disease actually have increased protein requirements. These requirements may be met by slowly increasing protein in the diet as long as signs of hepatic encephalopathy do not recur. To maintain the appropriate balance of aromatic and branched-chain amino acids, the diet should be based on vegetable and dairy protein instead of meat or fish protein (Center, 1998) . In addition, the antibiotics suggested above should be continued to reduce the effects of increasing dietary protein levels.

The prevalence of cancer in the general canine population has increased over the years (Dorn, 1976) . This can be attributed to the longer life spans resulting from improvements in nutrition, disease control, and therapeutic medicine. Because of these changes, cancer has become a major cause of death in dogs (Bronson, 1982) .

In a lifetime cancer mortality study of intact beagles of both sexes, Albert et al. (1994) found death rates similar to the death rate of the at-large dog population (Bronson, 1982) . Approximately 22% of the male beagles died of cancer. The majority of the tumors were lymphomas (32%) and sarcomas (29%), including hemangiosarcomas of the skin and fibrosarcomas. Of the female beagles dying of cancer (26% of the population studied), three-quarters had either mammary cancer (40%), lymphomas (18%), or sarcomas (15%). Of the sarcomas in females, one-third were mast cell tumors. In addition to these tumors that cause mortality, the beagle is also at risk for thyroid neoplasia (Hayes and Fraumeni, 1975; Benjamin et al., 1996) .

Because of the popularity of the beagle as a laboratory animal, discussion of specific neoplasms will focus on the tumors for which this breed is at risk, as well as tumors that are common in the general canine population.

Fine-needle aspirates are generally the first diagnostic option for palpable masses, because they can easily be performed in awake, cooperative patients. This technique allows for rapid differentiation of benign and neoplastic processes. In cases where cytologic results from fine-needle aspirates are not definitive, more invasive techniques must be used.

Needle-punch or core biopsies can also be performed in awake patients but typically require local anesthesia. An instrument such as a Tru-Cut needle (Travenol Laboratories, Inc., Deerfield, Illinois) is used to obtain a 1 mm X 1 to 1.5 cm biopsy of a solid mass. A definitive diagnosis may be limited by the size of the sample acquired using this technique.

Incisional and excisional biopsies are utilized when less invasive techniques fail to yield diagnostic results. Excisional biopsies are the treatment of choice when surgery is necessary, because the entire mass is removed. Surgical margins should extend at least 1 cm around the tumor, and 3 cm if mast cell tumors are suspected (Morrison et al., 1993) . Incisional biopsies are performed when large soft-tissue tumors are encountered and/or when complete excision would be surgically difficult or life-threatening. When performing an incisional biopsy, always select tissue from the margin of the lesion and include normal tissue in the submission.

Etiology. Lymphomas are a diverse group of neoplasms that originate from lymphoreticular cells. Whereas retroviral etiologies have been demonstrated in a number of species (e.g., cat, mouse, chicken), conclusive evidence of a viral etiology has not been established in the dog. In humans, data implicate the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) as a cause of non-Hodgkin's lymphoma, but studies in dogs with similar conclusions have come under scrutiny (MacEwen and Young, 1991) .

Clinical signs. Multicentric and alimentary lymphomas account for most cases of canine lymphoma. In multicentric lymphoma, animals usually present with enlarged lymph nodes and nonspecific signs such as anorexia, weight loss, polyuria, polydypsia, and lethargy. When the liver and spleen are involved, generalized organomegaly may be felt on abdominal palpation. Alimentary lymphoma is associated with vomiting and diarrhea, in addition to previous clinical signs.

Less commonly, dogs develop mediastinal, cutaneous, and extranodal lymphomas. Dogs with mediastinal lymphoma often present with respiratory signs secondary to pleural effusion. Hypercalcemia is most frequently associated with this form of lymphoma and may result in weakness. Cutaneous lymphoma varies in presentation from solitary to generalized and may mimic any of a number of other skin disorders. The tumors may occur as nodules, plaques, ulcers, or dermatitis. Approximately half of the cases are pruritic. A number of extranodal forms of lymphoma have been reported, including tumors affecting the eyes, central nervous system, kidneys, or nasal cavity. Clinical presentation varies, depending on the site of involvement.

Epizootiology. The incidence of lymphoma is highest in dogs 5-11 years old, accounting for 80% of cases. Although the neoplasm generally affects dogs older than 1 year, cases in puppies as young as 4 months have been reported (Dorn et al., 1967) .

Pathologic findings. Enlarged neoplastic lymph nodes vary in diameter from 1 to 9 cm and are moderately firm. Some may have areas of central necrosis and are soft to partially liquefied. The demarcation between cortex and medulla is generally lost, and on cut section, the surface is homogenous. The spleen may have multiple small nodular masses or diffuse involvement with generalized enlargement. The enlarged liver may have disseminated pale foci or multiple large, pale nodules. In the gastrointestinal tract, both nodular and diffuse growths are observed. These masses may invade through the stomach and intestinal walls.

Histologically, the most common lymphomas are classified as intermediate to high grade and of large-cell (histiocytic) origin. The neoplastic lymphocytes typically obliterate the normal architecture of the lymph nodes and may involve the capsule and perinodal areas.

Pathogenesis. All lymphomas regardless of location should be considered malignant. A system for staging lymphoma has been established by the World Health Organization. The average survival time for dogs without treatment is 4-6 weeks. Survival of animals undergoing chemotherapy is dependent on the treatment regimen as well as the form and stage of lymphoma (MacEwen and Young, 1991) .

Hypercalcemia is a paraneoplastic syndrome frequently associated with lymphoma. The pathogenesis of this phenomenon is not fully understood but may be a result of a parathormone-like substance produced by the neoplastic lymphocytes.

Diagnosis and differential diagnosis. Differential diagnoses for multicentric lymphoma include systemic mycosis; salmonpoisoning and other rickettsial infections; lymph node hyperplasia from viral, bacterial, and/or immunologic causes; and dermatopathic lymphadenopathy. Alimentary lymphoma must be distinguished from other gastrointestinal tumors, foreign bodies, and lymphocytic-plasmacytic enteritis. In order to make a definitive diagnosis, whole lymph node biopsies and full-thickness intestinal sections are frequently needed.

Treatment. Therapy for lymphoma typically consists of one or a combination of several chemotherapeutic agents. The treatment regimen is based on the staging of the disease, the presence of paraneoplastic syndromes, and the overall condition of the patient. MacEwen and Young (1991) provide a thorough discussion of therapeutic options for the treatment of lymphomas in the dog.

Research complications. Given the grave prognosis for lymphoma with or without treatment, euthanasia should be considered for research animals with signifcant clinical illness.

Etiology. The fibrosarcoma group of tumors encompasses not only malignant tumors of fibroblasts but also a number of indistinguishable tumors, all of which are capable of collagen production (Pulley and Stannard, 1990) . Frequently classified in this group are undifferentiated leiomyosarcomas, liposarcomas, malignant melanomas, and malignant schwannomas.

Clinical signs. Although these neoplasms can arise throughout the body, they are most commonly found in the skin, subcutaneous tissues, and oral cavity. Fibrosarcomas are extremely variable in size and can grow to be quite large. In general, they are irregular and nodular, poorly demarcated, and nonencapsulated, and they frequently invade deeper tissues.

Epizootiology. Most fibrosarcomas develop in adult and aged animals but can affect dogs as young as 6 months or less. Pathogenesis. Fibrosarcomas exhibit rapid, invasive growth, recurring frequently after excision. Metastasis occurs in only one-fourth of cases, usually by the bloodstream to the lungs. Less frequently, spread to local lymph nodes is observed.

Diagnosis and differential diagnosis. Differential diagnoses for fibrosarcomas vary with the location of the tumor. Histopathologic exam should be used to distinguish these tumors from round cell tumors (mast cell tumors, histiocytomas, transmissible venereal tumors), papillomas, and other neoplasms.

Treatment. Treatment of any soft-tissue sarcoma would begin with wide surgical excision. If the tissue margins indicate incomplete resection, radiotherapy could be used. For any highgrade tumors, adjuvant chemotherapy would be recommended (see MacEwen and Withrow, 1991a , for a complete discussion).

Research complications. Because fibrosarcomas are locally invasive and often recur, dogs with these neoplasms should not be considered good subjects for long-term studies.

Etiology. Neoplasms of lipocytes and lipoblasts are welldifferentiated tumors referred to as lipomas.

Clinical signs. These growths can be found as single or multiple round, ovoid, or discoid masses in the subcutaneous tissues of the lateral and ventral thorax, abdomen, and upper limbs. Generally they are well circumscribed, encapsulated, and soft on palpation. Further, the skin is freely movable over the tumor.

Epizootiology. Lipomas occur principally in aged animals (average 8 years), and the incidence increases with age (Pulley and Stannard, 1990) . The tumors are most commonly seen in overweight female dogs, but no breed predisposition is observed.

Pathologic findings. Histologically, lipomas are indistinguishable from normal adipose tissue except when a fibrous capsule is present.

Pathogenesis: Lipomas are typically slow-growing and do not recur after complete surgical excision.

Diagnosis and differential diagnosis. Lipomas are not frequently confused with other tumors but can sometimes be difficult to distinguish from normal adipose tissue. Generally, the distinction can be made from the clinical history.

Treatment. Treatment for lipomas is not usually necessary unless the mass is causing problems with normal ambulation. In such cases, surgical excision is usually curative.

Research complications. Lipomas usually do not complicate research studies unless they are interfering with other systemic functions or ambulation.

Etiology. Histiocytomas are benign skin growths that arise from the monocyte-macrophage cells in the skin. Some debate exists as to whether this growth is actually a neoplasm or a focal inflammatory lesion (Pulley and Stannard, 1990) .

Clinical signs. The most frequent sites for histiocytomas are the head (especially the pinna) and the skin of the distal forelegs and feet. The masses are usually domelike or buttonlike (often referred to as "button tumors") and usually measure 1-2 cm in diameter.

Epizootiology. Histiocytomas are the most common tumors of young dogs, mostly occurring in dogs less than 2 years of age.

Pathologic findings. Histologically, these tumors contain round to ovoid cells with pale cytoplasm and large nuclei. The cells infiltrate the dermis and subcutis, displacing collagen fibers and skin adnexa. Despite being benign lesions, histiocytomas characteristically have a high mitotic index.

Pathogenesis. This tumor typically exhibits rapid growth (1-4 weeks) but does not spread. Most histiocytomas will spontaneously regress in less than 3 months.

Diagnosis and differential diagnosis. Histiocytomas must be distinguished from potentially metastatic mast cell tumors. This is accomplished by staining with toluidine blue, which would stain the cytoplasmic granules of mast cells red or purple.

Treatment. Although most histiocytomas will spontaneously resolve, conservative surgery or cryosurgery will provide an expeditious resolution.

Research complications. Histiocytomas should not interfere with most studies.

Etiology. Neoplastic proliferations of mast cells are the most commonly observed skin tumor of the dog (Bostock, 1986) . Mast cells are normally found in the connective tissue beneath serous surfaces and mucous membranes, and within the skin.

Clinical signs. Well-differentiated mast cell tumors are typically solitary, well-circumscribed, slow-growing, 1 to 10 cm nodules in the skin. Alopecia may be observed, but ulceration is not usual. Poorly differentiated tumors grow rapidly, may ulcerate, and may cause irritation, inflammation, and edema. Mast cell tumors can be found on any portion of the dog's skin but frequently affect the hindquarters, especially the thigh and in-guinal and scrotal areas. Mast cell tumors usually appear to be discrete masses, but they frequently extend deep into surrounding tissues.

Epizootiology. These tumors tend to affect middle-aged dogs but have been observed in dogs ranging from 4 months to 18 years (Pulley and Stannard, 1990 ).

Pathologic findings. Because of the substantial variation in histologic appearance of mast cell tumors, a classification and grading system described by Patnaik et al. (1986) has become widely accepted. In this system, grade I has the best prognosis, and grade III the worst prognosis. Grade I tumors are well differentiated, with round to ovoid uniform cells. The nuclei are regular, the cytoplasm is packed with large granules that stain deeply, and mitotic figures are rare to absent. Grade II (intermediately differentiated) mast cell tumors have indistinct cytoplasmic boundaries with higher nuclear-cytoplasmic ratios, fewer granules, and occasional mitotic figures. Grade III (anaplastic or undifferentiated) mast cell tumors have large, irregular nuclei with multiple prominent nucleoli. The cytoplasmic granules are few, but mitotic figures are much more frequent. In addition to skin lesions, mast cell tumors have been associated with gastric ulcers. These lesions are most likely secondary to tumor production of histamine. Histamine stimulates the H2 receptors of the gastric parietal cells, causing increased acid secretion. Gastric ulcers have been observed in large numbers (>75%) of dogs with mast cell tumors (Howard et al., 1969) .

The ulcers can be found in the fundus, pylorus, and/or proximal duodenum.

Although all mast cell tumors should be considered potentially malignant, the outcome in individual cases can be correlated with the histologic grading of the tumor. Grade III tumors are most likely to disseminate internally. This spread is usually to regional lymph nodes, spleen, and liver and less frequently to the kidneys, lungs, and heart. Diagnosis and differential diagnosis. Mast cell tumors can be distinguished histologically from other round cell tumors (such as histiocytomas and cutaneous lymphomas) by using toluidine blue, which metachromatically stains the cytoplasmic granules of the mast cells red or purple.

Treatment. Initial treatment for mast cell tumors is generally wide surgical excision (1 to 3 cm margins). Even with wide surgical margins, approximately 50% of mast cell tumors may recur. If the site is not amenable to wide surgical excision, debulking surgery and radiation therapy may be used. Other alternatives include amputation (if on a limb) or radiation therapy alone. As an adjunct to surgery, Grier et al. (1990 Grier et al. ( , 1995 found that deionized water injected into surgical margins reduced tumor recurrence by hypo-osmotically lysing any mast cells left behind. This technique has recently been refuted by Jaffe et al. (2000) . For systemic mastocytosis, and nonresectable or incompletely excised mast cell tumors, chemotherapy can be used. Treatment options would include oral prednisolone, intralesional triamcinalone, and the combination of cyclophosphamide, vincristine, and prednisolone (Graham and O'Keefe, 1994) .

Research complications. Because of the possibility of systemic histamine release and tumor recurrence, dogs with mast cell tumors are not good candidates for research studies. Grade I mast cell tumors may be excised, allowing dogs to continue on study; however, monitoring for local recurrence should be performed on a regular basis (monthly). Grade II tumors are variable; animals that undergo treatment should be monitored for recurrence monthly, and evaluation of the buffy coat should be performed every 3-6 months for detection of systemic mastocytosis. Because of the poor prognosis for grade III tumors, treatment is unwarranted in the research setting.

Etiology. Hemangiosarcomas are malignant tumors that originate from endothelial cells.

Clinical signs. These tumors may arise in the subcutis but are more commonly found in the spleen and the right atrium. Clinical signs are associated with the site of involvement. Vascular collapse is frequently observed secondary to rupture and hemorrhage from splenic masses. Heart failure can be observed secondary to tumor burden or hemopericardium. When found in the skin, hemangiosarcomas are poorly circumscribed, reddish black masses that range in size from 1 to 10 cm in diameter. The most common cutaneous sites are the ventral abdomen, the prepuce, and the scrotum.

Epizootiology: Hemangiosarcomas occur most frequently in 8-to 13-year-old dogs. The German shepherd dog is most commonly affected.

Pathologic findings. Grossly, splenic hemangiosarcomas resemble nodular hyperplasia or hematomas (Fig. 7) . The masses are spherical and reddish black and can range in size up to 15-20 cm in diameter. On cut section the masses may appear reddish gray or black and have cavernous areas of clotted blood. When the masses are found in the heart, the endocardium may be covered by a thrombus, giving the 2 to 5 cm tumors a reddish gray or yellow appearance. Histologically, hemangiosarcomas are composed of immature endothelial cells that form vascular channels or clefts. These spaces may be filled with blood or thrombi. The neoplastic cells are elongated with round to ovoid, hyperchromatic nuclei and frequent mitotic figures. Pathogenesis. Hemangiosarcomas can be found in one or many sites. In cases where multiple sites are involved, it may be impossible to identify the primary tumor. This neoplasia is highly malignant and spreads easily. Metastasis occurs most frequently to the lungs but can be found in any tissue.

Diagnosis and differential diagnosis. Splenic hemangiosarcoma may resemble nodular hyperplasia or some manifestations of lymphoma. When the heart is affected, other causes of heart failure must be ruled out. Echocardiography is a valuable tool for identifying the primary lesion. Histopathology should be used to differentiate dermal hemangiosarcoma from hemangiomas and other well-vascularized tumors.

Treatment. Surgery is generally the first choice of treatment for hemangiosarcoma. Dermal tumors are treated with radical resection, splenic tumors by total splenectomy, and heart tumors by debulking and pericardiectomy. Because of the high likelihood of metastasis, adjunct chemotherapy should always be considered.

Research complications. Dogs with dermal hemangiosarcoma may be cured after complete resection with margins, but monitoring should be done regularly for recurrence. The other forms of hemangiosarcoma have a much poorer long-term prognosis, and treatment is typically unwarranted in the research setting.

Etiology. Also known as infectious or venereal granuloma, Sticker tumor, transmissible sarcoma, and contagious venereal tumor, the transmissible venereal tumor is transmitted to the genitals by coitus (Nielsen and Kennedy, 1990) . The origin of this tumor is still unknown but has been described as a tumor of lymphocytes, histiocytes, and reticuloendothelial cells. Although this tumor has been reported in most parts of the world, it is most prevalent in temperate climates (MacEwen, 1991) .

Clinical signs. The tumors are usually cauliflower-like masses on the external genitalia, but they can also be pedunculated, nodular, papillary, or multilobulated. These friable masses vary in size up to 10 cm, and hemorrhage is frequently observed. In male dogs, the lesions are found on the caudal part of the penis from the crura to the bulbus glandis or on the glans penis (Fig. 8) . Less frequently, the tumor is found on the prepuce. Females typically have lesions in the posterior vagina at the junction of the vestibule and vagina. When located around the urethral orifice, the mass may protrude from the vulva. These tumors have also been reported in the oral cavity, skin, and eyes.

Epizootiology and transmission. Transmissible venereal tumors are most commonly observed in young, sexually active dogs. Transmission takes place during coitus when injury to the genitalia allows for transplantation of the tumor. Genital to oral to genital transmission has also been documented (Nielsen and Kennedy, 1990) . Extragenital lesions are believed to be a result of trauma prior to exposure to the tumor. Pathogenesis. Tumor growth is rapid after implantation but later slows. Metastasis is rare (<5% of cases) but may involve the superficial inguinal and external iliac lymph nodes as well as distant sites.

Diagnosis and differential diagnosis. Transmissible venereal tumors have been confused with lymphomas, histiocytomas, mast cell tumors, and amelanotic melanomas. Cytology may be of benefit in making a definitive diagnosis, so impression smears should be made prior to processing for histopathology.

Prevention. Thorough physical examinations prior to bringing new animals into a breeding program should prevent introduction of this tumor into a colony.

Control. Removing affected individuals from a breeding program should stop further spread through the colony.

Treatment. Surgery and radiation can be used for treatment, but chemotherapy is the most effective. Vincristine (0.5-0.7 mg/m 2) IV once weekly for 4 -6 treatments will induce remission and cure in greater than 90% of the cases (MacEwen, 1991).

Research complications. Experimental implantation of transmissible venereal tumors has been shown to elicit formation of tumor-specific IgG (Cohen, 1972) . This response may occur in natural infections and could possibly interfere with immunologic studies.

Etiology. Dogs are susceptible to a wide variety of mammary gland neoplasms, most of which are influenced by circulating reproductive steroidal hormones.

Clinical signs. Single nodules are found in approximately 75% of the cases of canine mammary tumors. The nodules can be found in the glandular tissue or associated with the nipple. Masses in the two most caudal glands (fourth and fifth) account for a majority of the tumors. Benign tumors tend to be small, well circumscribed, and firm, whereas malignant tumors are larger and invasive and coalesce with adjacent tissues.

Epizootiology. Mammary tumors are uncommon in dogs under 5 years of age with the incidence rising sharply after that. Median age at diagnosis is 10-11 years. Mammary tumors occur almost exclusively in female dogs, with most reports in male dogs being associated with endocrine abnormalities, such as estrogen-secreting Sertoli cell tumors.

Pathologic findings. Based on histologic classification of mammary gland tumors, approximately half of the reported tumors are benign (fibroadenomas, simple adenomas, and benign mesenchymal tumors), and half are malignant (solid carcinomas, tubular adenocarcinomas, papillary adenocarcinomas, anaplastic carcinomas, sarcomas, and carcinosarcomas) (Bostock, 1977) . Extensive discussions of classification, staging, and histopathologic correlations can be found in MacEwen and Withrow (199 lb) and Moulton (1990) .

Pathogenesis. Mammary tumors of the dog develop under the influence of hormones. Receptors for both estrogen and progesterone can be found in 60-70% of tumors. Futher, Schneider et al. (1969) showed that the risk of developing mammary tumors increased greatly after the first and second estrus cycles. Dogs spayed prior to the first estrus had a risk of 0.8%, whereas dogs spayed after the first and second estrus had risks of 8% and 26%, respectively.

Malignant mammary tumors typically spread through the lymphatic vessels. Metastasis from the first, second, and third mammary glands is to the ipsilateral axillary or anterior sternal lymph nodes. The fourth and fifth mammary glands drain to the superficial inguinal lymph nodes where metastasis can be found. Many mammary carcinomas will eventually metastasize to the lungs.

Diagnosis and differential diagnosis. Both benign and malignant mammary tumors must be distinguished from mammary hyperplasia and mastitis.

Prevention. Mammary tumors can effectively be prevented by spaying bitches prior to the first estrus. This is commonly done in the general pet population at 6 months of age. Recently, the topic of spaying sexually immature dogs (8-16 weeks of age) has received much attention for the control of the pet population. Kustritz (1999) reviewed the techniques for anesthesia and surgery, as well as possible pros and cons of spaying at this young age.

Treatment. Surgery is the treatment of choice for mammary tumors, because chemotherapy and radiation therapy have not been reported to be effective. The extent of the surgery is dependent on the area involved. Lumpectomy or nodulectomy should be elected in the case of small discrete masses, while mammectomy and regional or total mastectomies are reserved for more aggressive tumors. At the time of surgery, axillary lymph nodes are removed only if enlarged or positive on cytology for metastasis. Inguinal lymph nodes should be removed any time the fourth and fifth glands are excised (MacEwen and Withrow, 199 lb).

Research complications. Because 50% of mammary tumors are benign, treatment may be rewarding, allowing dogs to con-tinue on study. If removed early enough, malignant masses could yield the same results. All dogs should be monitored regularly for recurrence and new mammary tumors.

Etiology. Beagles are among the breeds with the highest prevalence of thyroid carcinomas. Benjamin et al. (1996) reported a correlation between lymphocytic thyroiditis, hypothyroidism, and thyroid neoplasia in the beagle.

Clinical signs. Thyroid carcinomas generally present as palpable cervical masses. Affected animals may experience dysphagia, dyspnea, and vocalization changes. Precaval syndrome resulting in facial edema is also observed in some cases.

Epizootiology and transmission. The mean age of dogs presented with thyroid carcinomas is 9 years, with equal distribution of cases between the sexes.

Pathologic findings. Grossly, thyroid carcinomas are multinodular masses, frequently with large areas of hemorrhage and necrosis. They tend to be poorly encapsulated and invade local structures such as the trachea, esophagus, larynx, nerves, and vessels. The masses are unilateral twice as often as bilateral (Capen, 1990) . Histologically, thyroid carcinomasare divided into follicular, papillary, and compact cellular (solid) types (see Capen, 1990 , for complete discussion).

Pathogenesis. Thyroid carcinomas tend to grow rapidly and invade local structures. Early metastasis is common and occurs to the lungs by invasion of branches of the thyroid vein.

Diagnosis and differential diagnosis. Nonpainful cervical swellings such as seen with thyroid tumors are also consistent with abscesses, granulomas, salivary mucoceles, and lymphomas. Often a preliminary diagnosis can be made by fineneedle aspirate.

Treatment. Surgery is the treatment of choice for thyroid carcinomas that have not metastasized. When the tumor is freely movable, surgery may be curative. Surgical excision may be difficult for tumors that adhere to local structures, requiring excision of the jugular vein, carotid artery, and associated nerves. When bilateral tumors are observed, preservation of the parathyroid glands may not be possible. In such cases, treatment for hypoparathyroidism will be necessary. Both chemotherapy and radiation therapy have been suggested for extensive bilateral tumors and/or after incomplete excision, but no controlled trials have been performed (Ogilvie, 1991) .

In the research setting, treatment of this tumor may not be rewarding. Only freely movable tumors can be practically treated without seriously affecting research efforts. Euthanasia is warranted in the more advanced cases when clinical illness is apparent.

Beagles are subject to many of the inherited and/or congenital disorders that affect dogs in general. In a reference table on the congenital defects of dogs (Hoskins, 2000b) , disorders for which beagles are specifically mentioned include brachyury (short tail), spina bifida, pulmonic stenosis, cleft palate-cleft lip complex, deafness, cataracts, glaucoma, microphthalmos, optic nerve hypoplasia, retinal dysplasia, tapetal hypoplasia, factor VII deficiency, pyruvate kinase deficiency, pancreatic hypoplasia, epilepsy, GM1 gangliosidosis, globoid cell leukodystrophy, XX sex reversal, and cutaneous asthenia (Ehlers-Danlos syndrome). In addition, there are defects that affect so many breeds that the author simply lists "many breeds" for the breeds affected by those disorders. Thus these defects could also affect beagles and include pectus excavatum, polydactyly, radial and ulnar dysplasia, hypoadrenocorticism, entropion, lens coloboma, factor VIII deficiency (von Willebrand's disease), renal agenesis or ectopia, and developmental defects of the reproductive and lower urinary tracts.

At a commercial breeder of purpose-bred beagles, the most common birth defects were umbilical hernia (1.82% of births) and open fontanelle (1.44% of births) (R. Scipioni and J. Ball, personal communication, 1999) . Other defects observed include cleft palate and cleft lip, cryptorchidism, monorchidism, limb deformity, inguinal hernia, diaphragmatic hernia, hydrocephaly, and fetal anasarca. Each of these other congenital defects occurred at less than 1.0% incidence.

Etiology. Cataract is an opacification of the lens or the lens capsule. It is the pathologic response of the lens to illness or injury, because the lens has no blood supply. Cataracts can be caused by metabolic, inflammatory, infectious, or toxic causes and can be congenital, juvenile, or degenerative. Nuclear sclerosis is an apparent opacification of the lens caused by the compression of older lens fibers in the center of the lens (nucleus) as a consequence of the production of new fibers. Because the nucleus increases in size as the animal ages, the sclerosis is more apparent in older animals and may be mistaken as a senile cataract. The ability to see the fundus during ophthalmoscopy persists with nuclear sclerosis but is obstructed by a true cataract.

Clinical signs. The first clinical sign is typically the ability to visualize the opaque lens through the pupil of the dog's eye. Dogs have an impressive ability to tolerate bilateral lens opacity (especially when development is gradual), and often visual impairment is detected late in the development of the condition (Helper, 1989) . Moderate vision loss may cause the dog to be hesitant in moving in new surroundings or unable to locate movable objects (such as a toy). Rapid cataract development can result in a sudden vision loss, such as can occur with diabetic cataracts.

Epizootiology and transmission. Certain dog breeds can be predisposed to the development of juvenile or senile cataracts or to metabolic disorders that result in cataract development, such as diabetes mellitus. Dogs in studies for diabetes mellitus should be observed regularly for cataract development. Toxicological studies may also induce formation of cataracts.

Pathogenesis. Lens fibers respond to all biological or chemical insults by necrosis and liquefaction (Render and Carlton, 1995) , because they have no blood supply with which to recruit an inflammatory and repair process. Disruption of these fibers by any means, therefore, leads to opacification. The exact processes by which the varieties of congenital and juvenile cataracts are produced have not been determined. In diabetic cataracts, the excess glucose is metabolized to sorbitol and fructose. As these alcohols and sugars accumulate in the lenticular cells, they produce an osmotic imbalance, which brings fluid into the cells, causing swelling and degeneration of lens fibers and resultant opacity (Capen, 1995) .

Diagnosis and differential diagnosis. The ability to visualize the retina and fundus during ophthalmoscopy differentiates true cataracts from nuclear sclerosis. Dogs with cataracts should be evaluated for possible causes, especially diabetes mellitus. Diabetes mellitus will typically affect middle-aged dogs and feature rapid cataract formation, whereas juvenile and senile cataracts are slow to develop and affect younger and older dogs, respectively. Progressive retinal atrophy can also cause secondary cataract formation; pupillary light response is maintained with primary cataracts (even if the lens is completely opaque), whereas this reflex is obtunded by retinopathy.

Prevention. Most forms of cataracts cannot be prevented, for their exact etiologic pathogenesis is unknown. Diabetic cataracts, however, can be prevented by proper regulation of blood glucose concentrations with insulin therapy and proper diet.

Treatment. Because dogs do not need to focus visual images as accurately as human beings, proper lens clarity and function are not necessary for an adequate quality of life. Many dogs adjust quite well to the visual impairment caused by persistent cataracts. Lens removal can be performed for dogs seriously affected by cataracts, but this would not be anticipated for dogs in the research setting. Information on surgical lens extraction procedures can be found in Helper (1989) or other veterinary ophthalmology textbooks.

Research complications. Research complications would be minimal with cataracts, unless the dogs were intended for use in ophthalmologic or visual acuity-based studies.

Etiology. Hip dysplasia is a degenerative disease of the coxofemoral joint. A specific etiology is unknown, but the development of hip dysplasia has a strong genetic component (Pedersen et al., 2000) , modified by age, weight, size, gender, conformation, rate of growth, muscle mass, and nutrition (Smith et al., 1995) .

Clinical signs. The initial clinical abnormality caused by hip dysplasia is laxity of the coxofemoral joint. This may present as a gait abnormality without any indication of lameness or stiffness. Eventually, affected dogs will have periods of lameness and, in protracted cases, will be rendered immobile by severe pain.

Epizootiology and transmission. Hip dysplasia has been seen in most dog breeds, but it typically affects larger breeds of dogs. In the research setting, it is primarily a condition of randomsource large-breed dogs used for surgical research.

Diagnosis and differential diagnosis. Hip dysplasia is classically diagnosed by radiography of the pelvis and hip joints. Radiographic abnormalities consistent with hip dysplasia include shallow acetabula with remodeling of the acetabular rim, flattening of the femoral head, subchondral bone sclerosis (caused by erosion of articular cartilage and exposure of underlying bone), and osteophyte production around the joint (Pedersen et al., 2000) . Hip dysplasia needs to be differentiated from other musculoskeletal or neurological conditions that can cause unusual gaits and/or lameness. This may be somewhat difficult, because clinical signs of hip dysplasia may develop before radiographic abnormalities. Radiographic calculation of the distraction index (DI) to measure joint laxity has proven to be a good means to predict future hip dysplasia before other radiographic changes are evident (Smith et al., 1995) .

Prevention. Because of the genetic component, dogs with hip dysplasia should not be used in breeding colonies. Dogs should be provided a good plane of nutrition but not be allowed to become overweight. Dogs that were limit-fed at 75% of the food amount eaten by ad libitum-fed dogs had lower body weights and decreased severity of radiographic lesions of hip dysplasia (Kealy et aL, 1997) .

Treatment. In the stages when clinical signs are episodic, cage rest and analgesics for several days can be used to treat the symptoms. More advanced cases may require continuous analgesia. Sectioning of the pectineus muscle or tendon may provide some pain relief but does not affect the progression of the disease (Pedersen et al., 2000) . Surgical treatments for hip dysplasia include femoral head ostectomy and total hip replacement. Neither surgical treatment is likely in a research setting.

Research complications. Long-term studies using large-breed dogs may be affected by the eventual development of hip dysplasia. In studies where hip dysplasia would be a serious complication or confounding variable (e.g., orthopedic research), dogs should be radiographed upon arrival to assess possibility of early coxofemoral joint degeneration and suitability for use in the study.

Etiology. Benign prostatic hyperplasia (BPH) is an agerelated condition in intact male dogs. The hyperplasia of prostatic glandular tissue is a response to the presence of both testosterone and estrogen.

Clinical signs. BPH is often subclinical. Straining to defecate (tenesmus) may be seen because the enlarged gland impinges on the rectum as it passes through the pelvic canal. Urethral discharge (yellow to red) and hematuria can also be presenting clinical signs for BPH.

Epizootiology and transmission. BPH typically affects older dogs (>4 years), although it has been seen as early as 2 years of age.

Pathologic findings. In its early stages, canine B PH is hyperplasia of the prostatic glandular tissue. This is in contrast to human BPH, which is primarily stromal in origin. Eventually, the hyperplasia tends to be cystic, with the cysts containing a clear to yellow fluid. The prostate becomes more vascular (resulting in hematuria or hemorrhagic urethral discharge), and BPH may be accompanied by mild chronic inflammation.

Pathogenesis. BPH occurs in older intact male dogs because increased production of estrogens (estrone and estradiol), combined with decreased secretion of androgens, sensitizes prostatic androgen receptors to dihydrotestosterone. The presence of estrogens may also increase the number of androgen receptors, and hyperplastic prostate glands also have an increased ability to metabolize testosterone to 5a-dihydrotestosterone (Kustritz and Klausner, 2000) .

Diagnosis and differential diagnosis. BPH is diagnosed in cases of nonpainful symmetrical swelling of the prostate gland in intact male dogs, with normal hematologic profiles and urinalysis characterized by hemorrhage, at most. Differential diagnoses include squamous metaplasia of the prostate, paraprostatic cysts, bacterial prostatitis, prostatic abscessation, and prostatic neoplasia (primarily adenocarcinoma). These differential diagnoses also increase in frequency with age and, except for squamous metaplasia, can also occur in castrated dogs. As such, these conditions do not necessarily abate or resolve when castration is used for treatment of prostatic enlargement.

Prevention. Castration is the primary means for prevention of benign prostatic hyperplasia.

Treatment. The first and foremost treatment for B PH is castration. In pure cases of B PH, castration results in involution of the prostate gland detectable by rectal palpation within 7-10 days. For most dogs in research studies this is a viable option to rapidly improve the animal's condition. The alternative to castration is hormonal therapy, primarily with estrogens. This may be applicable in cases in which the dog is a valuable breeding male (e.g., genetic diseases), and semen collection is necessary. If the research study concerns steroidal hormone functions, then neither the condition nor the treatment is compatible. Newer drugs marketed for human males have also shown promise in treating canine BPH. Finasteride (Proscar) is a 5a-reductase inhibitor that limits metabolism of testosterone to 5a-dihydrotestosterone. Treatment at daily doses of 1-5 mg/kg has been effective in causing prostatic atrophy without affecting testicular spermatogenesis (Kustritz and Klausner, 2000) . Dogs given 1.0 mg/kg were proven to still be fertile. There are also indications that lower doses may be effective in relieving B PH. Androgen receptor antagonists (flutamide and hydroxyflutamide) have also been studied in the dog and found to be effective for treatment of BPH while maintaining libido and fertility (Kustritz and Klausner, 2000) . Unfortunately, both the 5areductase inhibitors and the androgen receptor antagonists are not presently labeled for use in male dogs in the United States.

Research complications. BPH can cause complications to steroidal hormone studies, in that the condition may be indicative of abnormal steroidal hormone metabolism, and neither castration nor estrogen therapy is compatible with study continuation. It is presently unknown whether the use of the newer antihyperplastic agents systemically alters physiologic parameters outside of the prostate itself. The development of tenesmus as a clinical sign may also affect studies of colorectal or anal function.

Etiology. Juvenile polyarteritis syndrome (JPS) is a painful disorder seen in young beagles (occasionally reported in other breeds). The lesion consistent with the syndrome is systemic necrotizing vasculitis. The cause of the vasculitis has not been established, but it appears to have an autoimmune-mediated component and may have a hereditary predisposition.

Clinical signs. Clinical signs of JPS include fever, anorexia, lethargy, and reluctance to move the head and neck. The dogs tend to extend the neck ventrally. Most dogs seem to be in pain when touched, especially in the neck region. The syndrome typically has a course of remissions and relapses characterized by 3-7 days of illness and 2-4 weeks of remission (Scott-Moncrieff et al., 1992) . There may be a component of this condition that is subclinical, given that a vasculitis has been diagnosed postmortem in beagles that had no presenting signs.

Epizootiology and transmission. JPS typically affects young beagles (6-40 months), with no sex predilection.

Pathologic findings. On gross necropsy, foci of hemorrhage can be seen in the coronary grooves of the heart, cranial mediastinum, and cervical spinal cord meninges (Snyder et al., 1995) . Local lymph nodes may be enlarged and hemorrhagic. Histologically, necrotizing vasculitis and perivasculitis of small to medium-sized arteries are seen. These lesions are most noticeable in the three locations where gross lesions are observed, but they may be seen in other visceral locations. The perivasculitis often results in nodules of inflammatory cells that eccentrically surround the arteries (Fig. 9a) . The cellular composition of these nodules is predominantly neutrophils, but it can also consist of lymphocytes, plasma cells, or macrophages (Snyder et al., 1995) . Fibrinous thrombosis of the affected arteries is also seen (Fig. 9b) . A subclinical vasculitis has also been diagnosed in beagles postmortem; it is not known whether this subclinical condition is a different disorder or part of a JPS continuum. This subclinical vasculitis often affects the coronary arteries (with or without other sites).

Pathogenesis. The initiating factors for JPS are unknown. It was once presumed to be a reaction to test compounds by laboratory beagles, but this may have been coincident to the fact that the beagle is the breed most often affected with JPS. Immune mediation of JPS is strongly suspected, because the clinical signs have a cyclical nature and respond to treatment with corticosteroids, and the affected dogs have elevated a2-globulin fractions and abnormal immunologic responses. There may be hereditary predisposition, given that pedigree analysis of some affected dogs has indicated that the offspring of certain sires are more likely to be affected, and breeding of two affected dogs resuited in 1/7 affected pups (Scott-Moncrieff et al., 1992) .

Diagnosis and differential diagnosis. Differential diagnoses include encephalitis, meningitis, injury or degeneration of the cervical vertebrae or disks, and arthritis. In the research facility, the disorder may be readily confused with complications secondary to the experimental procedure, or with postsurgical pain. Beagles with JPS that were in an orthopedic research study were evaluated for postsurgical complications and skeletal abnormalities prior to the postmortem diagnosis of systemic vasculitis (authors' personal experience).

are known at this time.

No prevention and control measures brane (third eyelid). This is not considered a congenital anomaly, but there is breed disposition for this condition, including beagles. A specific etiology is not known.

Clinical signs. The glandular tissue of the nictitating membrane protrudes beyond the membrane's edge and appears as a reddish mass in the ventromedial aspect of the orbit (Fig. 10) . Excessive tearing to mucoid discharge can result, and severe cases can be associated with corneal erosion.

Treatment. Clinical signs can be abated by administration of corticosteroids. Prednisone administered orally at 1.1 mg/kg, q12 hr, was associated with rapid relief of clinical symptoms. Maintenance of treatment at an alternate-day regimen of 0.25-0.5 mg/kg was shown to relieve symptoms for several months. However, withdrawal of corticosteroid therapy led to the return of clinical illness within weeks.

Pathologic findings. Typically the glandular tissue is hyperplastic, possibly with inflammation. Rarely is the tissue neoplastic.

Pathogenesis. Prolapse of the gland may be a result of a congenital weakness of the connective tissue band between the gland and the cartilage of the third eyelid (Helper, 1989) .

Research complications. Because of the potentially severe clinical signs and the need for immunosuppressive treatment, JPS is often incompatible with use of the animal as a research subject. It is unknown whether subclinical necrotizing vasculitis causes sufficient aberrations to measurably alter immunologic responses.

Etiology. "Cherry eye" is a commonly used slang term for hyperplasia and/or prolapse of the gland of the nictitating mem-Prevention. Hyperplasia of the third eyelid cannot be prevented, but dogs that develop this condition unilaterally should have the other eye evaluated for potential glandular prolapse. Preventative surgical measures might be warranted.

Treatment. Corticosteroid treatment (topical or systemic) can be used to try to reduce the glandular swelling. However, surgical reduction or excision of the affected gland is typically required to resolve the condition. In the reduction procedure, the prolapsed gland is sutured to fibrous tissue deep to the fornix of the conjunctiva (Helper, 1989 ). If reduction is not possible (as with deformed nictitating cartilage) or is unsuccessful, removal of the gland can be performed. Such excision is fairly straightforward and can be done without removal of the nictitating membrane itself. The gland of the third eyelid is important in tear production; although the rest of the lacrimal glands should be sufficient for adequate tear production, keratoconjunctivitis sicca is a possible consequence after removal of the gland of the nictitating membrane.

Research complications. In most cases, research complications would be minimal, especially if treated adequately. Either the presence of the hyperplastic gland, or its removal, might compromise ophthalmologic studies.

Etiology. Interdigital cysts are chronic inflammatory lesions (not true cysts) that develop in the webbing between the toes (Fig. 11 ). The cause for most interdigital cysts is usually not identified unless a foreign body is present. Bacteria may be isolated from the site, but the lesions may also be sterile (hence the synonym "sterile pyogranuloma complex").

Clinical signs. Dogs with interdigital cysts are usually lame on the affected foot, with licking and chewing at the interdigital space. Exudation may be noticed at the site of the lesion. The lesion appears as a cutaneous ulcer, usually beneath matted hair, with possible development of sinus tracts and purulent exudate.

Epizootiology and transmission. Interdigital cysts are common in a variety of canine breeds, including German shepherds. Beagles have been affected in the research setting. Interdigital cysts usually occur in the third and fourth interdigital spaces (Bellah, 1993) .

Pathologic findings. Histopathologically, interdigital cysts are sites of chronic inflammation, typically described as pyogranulomatous.

Pathogenesis. Initial development of the cysts is unknown, except for those cases in which a foreign body can be identified.

Diagnosis and differential diagnosis. Bacterial culture swabs and radiographs should be taken of the cysts to rule out bacterial infection, and radiopaque foreign bodies or bony lesions, respectively. A biopsy should be taken if neoplasia is suspected.

Treatment. If a foreign body is associated with the lesion, then removal is the first order of treatment. If biopsy of the site provides a diagnosis of sterile pyogranuloma complex, then systemic corticosteroid therapy (e.g., prednisolone at 1 mg/kg ql2h) can be initiated and then tapered once the lesion heals. Interdigital cysts that are refractory to medical therapy require Fig. 11 . Interdigital cyst between the third and fourth digits of the forelimb of a research beagle. surgical removal. Excision includes removal of the lesion and the interdigital web, and a two-layer closure of the adjacent skin and soft tissues is recommended (Bellah, 1993) . The foot should be put in a padded bandage and a tape hobble placed around the toes to reduce tension when the foot is weightbearing. The prognosis for idiopathic interdigital cysts is guarded, because the cysts tend to recur (Bellah, 1993) .

Research complications from the cysts are minimal, unless the dogs need to be weight-bearing for biomechanic or orthopedic studies. Treatment with systemic steroids could be contraindicated with some experimental designs.

Post-therapy antibody titers in dogs with ehrlichiosis: follow-up study on 68 patients treated primarily with tetracycline and/or doxycycline

Dog thyroiditis: occurrence and similarity to Hashimoto's struma

Surgical management of specific skin disorders

Bordetella and mycoplasma infections in dogs and cats

Associations between lymphocytic thyroiditis, hypothyroidism, and thyroid neoplasia in beagles

Diagnostic exercise: peracute death in a research dog

Saunders Manual of Small Animal Practice

Neurologic manifestations associated with hypothyroidism in four dogs

Neoplasms of the skin and subcutaneous tissues in dogs and cats

Neoplasia of the skin and mammary glands of dogs and cats

Platelet function, antithrombin-III activity, and fibrinogen concentration in heartworm-infected and heartworm-negative dogs treated with thiacetarsamide

Pancreatic adenocarcinoma in two grey collie dogs with cyclic hematopoiesis

Ehrlichia platys infection in dogs

The rickettsioses

Monoclonal gammopathy associated with naturally occurring canine ehrlichiosis

Variation in age at death of dogs of different sexes and breeds

Leptospira interrogans serovar grippotyphosa infection in dogs

Efficacy and dose titration study of mibolerone for treatment of pseudopregnancy in the bitch

Tropical canine pancytopenia: clinical, hematologic, and serologic response of dogs to Ehrlichia canis infection, tetracycline therapy, and challenge inoculation

Comparison of Campylobacter carriage rates in diarrheic and healthy pet animals. Zentralbl

Advances in dietary management of obesity in dogs and cats

Effect of level and source of dietary fiber on food intake in the dog

Effect of amount and type of dietary fiber on food intake in energy-restricted dogs

External parasites: identification and control

Tumors of the endocrine glands

Thomson's Special Veterinary Pathology

Infectious Diseases of the Dog and Cat

Nutritional support for dogs and cats with hepatobiliary disease

Specific amplification of Ehrlichia platys DNA from blood specimens by two step PCR

Detection of humoral antibody to the transmissible venereal tumor of the dog

Dirofilaria immitis: heartworm products contract rat trachea in vitro

Dogs: Laboratory Animal Management

Management of septicemia in rhesus monkeys with chronic indwelling catheters

Client information series: canine demodicosis

Client information series: fleas and flea allergy dermatitis

Management of the burn wound

Chlorine dioxide sterilization of implanted right atrial catheters in rabbits

Current concepts in the management of Helicobacter associated gastritis

Dirofilariasis in dogs and cats

Use of narcotic antagonists to modify stereotypic self-licking, self-chewing, and scratching behavior in dogs

Epidemiology of canine and feline tumors

Epizootiologic characteristics of canine and feline leukemia and lymphoma

Study of obesity in dogs visiting veterinary practices in the United Kingdom

Miller's Anatomy of the Dog

Update on diagnosis of canine hypothyroidism

Helicobacter-associated gastric disease in ferrets, dogs, and cats

The role of Helicobacter species in newly recognized gastrointestinal tract disease of animals

Serologic diagnosis of infectious cyclic thrombocytopenia in dogs using an indirect fluorescent antibody test

Hemorrhagic streptococcal pneumonia in newly procured research dogs

Control of ticks

Platelet aggregation studies in dogs with acute Ehrlichia platys infection

Health benefits of animal research: the dog as a research subject

Soft tissue sarcomas and mast cell tumors

Textbook of Veterinary Internal Medicine

Infectious Diseases of the Dog and Cat

Canine Lyme borreliosis

Mast cell tumor destruction by deionized water

Mast cell tumour destruction in dogs by hypotonic solution

Transmission of Ehrlichia canis to dogs by ticks (Rhipicephalus sanguineus)

Textbook of Veterinary Internal Medicine

Diseases of the liver and their treatment

Cyclic thrombocytopenia induced by a rickettsia-like agent in dogs

Shock

Evaluation of the sensitivity and specificity of diagnostic criteria for sepsis in dogs

Textbook of Veterinary Internal Medicine

Canine thyroid neoplasms: epidemiologic features

Magrane's Canine Ophthalmology

Helicobacter-like organisms: histopathological examination of gastric biopsies from dogs and cats

Thiacetarsamide and its adverse effects

Infectious Diseases of the Dog and Cat

Pediatrics: puppies and kittens

Canine viral diseases

Textbook of Veterinary Internal Medicine

Antibodies to Ehrlichia canis, Ehrlichia platys, and spotted fever group rickettsia in Louisiana dogs

Mastocytoma and gastroduodenal ulceration

Complications in the use of indwelling vascular catheters in laboratory animals

Deionised water as an adjunct to surgery for the treatment of canine cutaneous mast cell tumours

Helicobacter infection

Textbook of Veterinary Internal Medicine

Textbook of Veterinary Internal Medicine

Hygroma of the elbow in dogs

Thermal injuries

Dirofilaria immitis: Do filarial cyclooxygenase products depress endothelium-dependent relaxation in the in vitro rat aorta?

Depression of endotheliumdependent relaxation by filarial parasite products

Three cases of canine leptospirosis in Quebec

CVT update: interpretation of endocrine diagnostic test results for adrenal and thyroid disease

Etiopathogenesis of canine hypothyroidism

Five-year longitudinal study on limited food consumption and development of osteoarthritis in coxofemoral joints of dogs

Role of Bordetella bronchiseptica in infectious tracheobronchitis in dogs

Kirk's Current Veterinary Therapy 12: Small Animal Practice

The Fire of Life

Kirk's Current Veterinary Therapy 13: Small Animal Practice

Coinfection with multiple tick-borne pathogens in a Walker hound kennel in North Carolina

Tarsal joint contracture in dogs with golden retriever muscular dystrophy

Clinical and hematological findings in canine ehrlichiosis

Early spay-neuter in the dog and cat

Textbook of Veterinary Internal Medicine

Chronic catheterization of the intestines and portal vein for absorption experimentation in beagle dogs

Evaluation of weight loss protocols for dogs

The brown dog tick Rhipicephalus sanguineus and the dog as experimental hosts of Ehrlicha canis

The Clinical Chemistry of Laboratory Animals

Double-blinded crossover study with marine-oil supplementation containing high-dose eicosapentaenoic acid for the treatment of canine pruritic skin disease

Thomson's Special Veterinary Pathology

Effects of four preparations of .05% chlorhexidine diacetate on wound healing in dogs

Transmissible venereal tumors

Kirk's Current Veterinary Therapy 11: Small Animal Practice

Soft tissue sarcomas

Tumors of the mammary gland

Canine lymphoma and lymphoid leukemias

Kirk's Current Veterinary Therapy 12: Small Animal Practice

Effect of heartworm infection on in vitro contractile responses of canine pulmonary artery and vein

Tick paralysis in North America and Australia

Saunders Manual of Small Animal Practice

Thyroid gland and arterial lesions of beagles with familial hypothyroidism and hypedipoproteinemia

Bacterial gastroenteritis in dogs and cats: more common than you think

Urea protects Helicobacter (Campylobacter) pylori from the bactericidal effect of acid

Characterization of a new isolate of Ehrlichia platys using electron microscopy and polymerase chain reaction

Decreased pulmonary arterial endothelium-dependent relaxation in heartworm-infected dogs with pulmonary hypertension

Vaccination against canine bordetellosis: protection from contact challenge

Dermatologic aspects of tick bites and tick-transmitted diseases

A chronic access port model for direct delivery of drugs into the intestine of conscious dogs

Clinical trial of DVM Derm Caps in the treatment of allergic diseases in dogs: a nonblinded study

Diagnosis of neoplasia

Tumors of the mammary gland

Dirofilaria immitis: Heartworm infection alters pulmonary artery endothelial cell behavior

Survey of conjunctival flora in dogs with clinical signs of external eye disease

Hyperoxia exacerbates microvascular injury following acid aspiration

Nutrient Requirements of Dogs

Surgical closure of elbow hygroma in the dog

Tumors of the genital system

Practical laboratory methods for the diagnosis of dermatologic diseases

Walker's Mammals of the World

Tumors of the endocrine system

Beta hemolytic streptococcus isolated from the canine vagina

Comparison of three skin preparation techniques in the dog

Comparison of three skin preparation techniques in the dog. Part 2: Clinical trial in 100 dogs

Clinical Behavioral Medicine for Small Animals

Hypothyroidism in dogs: 66 cases (1987-1992)

Plasma von Willebrand factor antigen concentration in dogs with hypothyroidism

Plasma von Willebrand factor antigen concentration and bleeding time in dogs with experimental hypothyroidism

Canine cutaneous mast cell tumor: morphologic grading and survival time in 83 dogs

Joint diseases of dogs and cats

Target imbalance: disparity of Borrelia burgdorferi genetic material in synovial fluid from Lyme arthritis patients

Textbook of Veterinary Internal Medicine

Tumors of the skin and soft tissue

Thomson's Special Veterinary Pathology

Canine leptospirosis: a retrospective study of 17 cases

Dogs and cats as laboratory animals

Effects of chlorhexidine diacetate and povidoneiodine on wound healing in dogs

Epidemiology of thyroid diseases of dogs and cats

Factors influencing canine mammary cancer development and postsurgical survival

Muller and Kirk's Small Animal Dermatology

Systemic necrotizing vasculitis in nine young beagles

Thomson's Special Veterinary Pathology

Textbook of Veterinary Internal Medicine

Canine infectious tracheobronchitis (kennel cough complex)

Saunders Manual of Small Animal Practice

Serum protein alterations in canine erhlichiosis

Bacterial factors and immune pathogenesis in Helicobacter pylori

Evaluation of Rhipicephalus sanguineus as a potential biologic vector of Ehrlichia platys

Role of the eastern chipmunk (Tamias striatus) in the epizootiology of Lyme borreliosis in northwestern Illinois

Evaluation of risk factors for degenerative joint disease associated with hip dysplasia in dogs

Pathologic features of naturally occurring juvenile polyarteritis in beagle dogs

Textbook of Veterinary Internal Medicine

Clinical manifestations, pathogenesis, and effect of antibiotic treatment on Lyme borreliosis in dogs

Streptococcus zooepidemicus as the cause of septicemia in racing greyhounds

Trauma to the skin and subcutaneous tissues of dogs and cats

Chronic problem wounds of dog limbs

Portosystemic shunts

Textbook of Veterinary Internal Medicine

Lumbosacral stenosis in dogs

Experimental respiratory disease in dogs due to Bordetella bronchiseptica

Dirofilaria immitis: Depression of endothelium-dependent relaxation of canine femoral artery seen in vivo does not persist in vitro

Thyroiditis in a group of laboratory dogs: a study of 167 beagles

of Agriculture, Animal and Plant Health Inspection Service

Thomson's Special Veterinary Pathology

A retrospective study of 27 cases of naturally occurring canine ehrlichiosis

Role of canine parainfluenza virus and Bortedella bronchiseptica in kennel cough

Management of superficial skin wounds

Serum concentrations of thyroxine and 3,5,3'-triiodothyronine before and after intravenous or intramuscular thyrotropin administration in dogs

Use of ultrasound in the measurement of subcutaneous fat and prediction of total body fat in dogs

Ehrlichia platys in a Michigan dog

Ehrlichial diseases of dogs

Canine ehrlichiosis. Miss

Albert, R. E., Benjamin, S. A., and Shukla, R. (1994). Life span and cancer mortality in the beagle dog and human.