key: cord-0937507-0qy1nngs
authors: Raj, G. Dhinakar; Jones, R. C.
title: Infectious bronchitis virus: immunopathogenesis of infection in the chicken
date: 2007-11-12
journal: Avian Pathol
DOI: 10.1080/03079459708419246
sha: bdb43c308b9a0f04d6d108e1b4ae71ad03c87d3f
doc_id: 937507
cord_uid: 0qy1nngs

The immunopathogenesis of infectious bronchitis virus (IBV) infection in the chicken is reviewed. While infectious bronchitis (IB) is considered primarily a disease of the respiratory system, different IBV strains may show variable tissue tropisms and also affect the oviduct and the kidneys, with serious consequences. Some strains replicate in the intestine but apparently without pathological changes. Pectoral myopathy has been associated with an important recent variant. Several factors can influence the course of infection with IBV, including the age, breed and nutrition of the chicken, the environment and intercurrent infection with other infectious agents. Immunogenic components of the virus include the S (spike) proteins and the N nucleoprotein. The humoral, local and cellular responses of the chicken to IBV are reviewed, together with genetic resistance of the chicken. In long-term persistence of IBV, the caecal tonsil or kidney have been proposed as the sites of persistence. Antigenic variation among IBV strains is related to relatively small differences in amino acid sequences in the S1 spike protein. However, antigenic studies alone do not adequately define immunological relationships between strains and cross-immunisation studies have been used to classify IBV isolates into ‘protectotypes’. It has been speculated that changes in the S1 protein may be related to differences in tissue tropisms shown by different strains. Perhaps in the future, new strains of IBV may arise which affect organs or systems not normally associated with IB.

The study of pathogenesis was formerly regarded as a messy, unsatisfactory area for virus research-a sad state of affairs that was lamented by early enthusiasts (Mimms, 1964) . That view has changed now and there is an urgent need to know how viruses cause diseases. The study of mechanisms of pathogenicity was recently placed at the top of the research objectives proposed for priority treatment by an ad hoc group on vaccinology (Bourne, 1996) . However, pathogenesis cannot be studied without also considering immunology and mechanisms of protection. This review attempts to concisely present the current state of knowl-edge on the immunopathogenesis of infection in chickens with infectious bronchitis virus (IBV).

In 1931 a 'new respiratory disease of baby chicks' was identified in the USA by Schalk & Hawn and named infectious bronchitis (IB). Following infection of chickens with IBV, falls in egg production and quality were reported for the first time by Van Roekel et al. (1951) and Broadfoot & Smith (1954) . In 1962, Cumming found that IBV was the cause of nephrosis-nephritis syndrome seen in Australia. In more recent years, IBV has also been isolated from cloacal swabs and gut tissues, but while infection has sometimes been associated with diarrhoea, no pathological changes have been described in the alimentary tract. Swollen head syndrome (SHS) in chickens has generally been associated with infection with the avian pneumovirus (APV), turkey rhinotracheitis virus (TRTV) (Picault et al, 1987; Lu et al, 1994) . However, in the first description of SHS, Morley & Thomson (1984) isolated a coronavirus, while in other reports, Massachusetts strains of IBV have also been isolated (Shirai et al., 1993; Droual & Woolcock, 1994) . Recently, a variant strain of IBV, 793/B was isolated from broiler breeder flocks where the affected birds had bilateral myopathy affecting both superficial and deep pectoral muscles (Gough et ah, 1992) .

Thus, it is clear that strains of IBV have wide and variable tissue tropisms, and the clinical manifestations of the disease can be diverse. Although the name IB does not encompass the varied clinical manifestations observed with IBV infections, for the present this name has still been retained to avoid confusion.

Replication of IBV in the respiratory tissues causes characteristic, but not pathognomonic signs such as gasping, coughing, tracheal rales and nasal discharge. Occasionally, puffy, inflamed eyes and swollen sinuses may be seen (Parsons et al, 1992; Capua et al, 1994) . In uncomplicated cases these signs last for only 5 to 7 days and disappear within 10 to 14 days. The affected chickens also appear depressed, and feed consumption and weight gain are significantly reduced from 3 days after infection (Otsuki et al, 1990) . In uncomplicated cases, mortalities are generally low and have been attributed to asphyxiation due to blocking of the lower trachea or bronchi by plugs of mucus.

The upper respiratory tract is the main site of IBV replication, following which a viraemia occurs and the virus gets widely disseminated to other tissues (Mc-Martin, 1993) . The virus is primarily epitheliotropic and enters the epithelial cells by viropexis (Patterson & Bingham, 1976) . Studies using immunofluorescence (IF) Qones & Jordan, 1972; Yagyu & Ohta, 1990) , immunoperoxidase (IP) (Nakamura et al, 1991; Owen et al, 1991) and electron microscopy (Purcell & Clarke, 1972; Nakamura et al, 1991) have shown virus replication in ciliated epithelial and mucus-secreting cells. During the clinical phase of the disease, maximum virus titres are recorded in the trachea between 5 and 10 days post-infection (p.i.) (Ambali & Jones, 1990; Otsuki et al, 1990 ), but occasionally virus may be present for up to 28 days p.i. Cook, 1968) .

Virus also replicates in the epithelial cells of lungs (Janse et al, 1994 ) and airsacs (Nauwynck & Pensaert, 1988) . High virus titres are seen in these tissues between 4 to 11 days p.i. (Nauwynck & Pensaert, 1988; Otsuki et al, 1990) .

Infected chickens have mucosal thickening with serous or catarrhal exudate in the nasal passages, sinuses and trachea. The incidence of nasal exudate in experimentally-infected chickens has been used to assess the severity of disease in different inbred lines (Parsons et al, 1992) and in bursectomised birds (Cook et al, 1991a) . Small areas of pneumonia may be observed in the lungs. The air sacs may appear cloudy or contain a yellow caseous exudate (King & Cavanagh, 1991) .

The progression of lesions in the trachea has been divided into three stages, degenerative, hyperplastic and recovery (Nakamura et al, 1991; Purcell & Mc-Ferran, 1972) . Deciliation and desquamation of epithelial and mucous-secreting cells occur in the first 1 or 2 days with a mild infiltration of heterophils and lymphocytes in the lamina propria. Heterophils are often seen infiltrating between ciliated epithelial cells and occasionally in the lumen of the trachea. During the hyperplastic stage, newly-formed epithelial cells are observed which usually have no cilia. By 4 to 6 days the reparative processes begin with complete recovery by 10 to 20 days (Chen et al, 1996) . In affected airsacs, epithelial cell desquamation, oedema and some fibrinous exudate may be seen (King & Cavanagh, 1991) .

Difficulty in quantifying severity of respiratory infection caused by IBV has always been a problem in studying its pathogenicity . The virus causes stasis of tracheal cilia both in vivo and in vitro, and this parameter has been used to assess severity of respiratory infection. Otsuki et al. (1990) found that following infection with the M41 strain, the duration and severity of tracheal ciliary damage was longer in a susceptible line of chicken (151) than in a resistant line (C). The authors proposed that this method could be used to compare the pathogenicity of different strains or susceptibility of different breeds to IBV. Cubillos et al. (1991) found that in unvaccinated chickens challenged with four IBV isolates from Chile (one belonging to the Massachusetts serotype and three variants), the tracheal damage in terms of ciliary activity was variable, suggesting differences in virulence of IBV strains for the trachea.

While tracheal organ cultures (TOC) have been used extensively for cultivation and assay of IBV, distinct differences in virulence among IBV strains have not been observed in this in vitro system. Cook et al. (1976) , while standardizing TOC for the isolation and assay of IBV, compared three strains of IBV on the basis of their effect on tracheal cilia, but found no marked differences. Dhinakar Raj & Jones (1996c) also reported little difference among seven IBV strains using measurement of ciliary activity as a criterion for damage to the tracheal epithelium. Of nine strains compared in TOC by Cubillos et al (1991) , the hyperplasia caused by two viruses was the only histological difference. IBV infections may not occur as a single entity in the field. With the presence of several respiratory diseases in chickens caused by bacteria and viruses, the role of these agents in increasing the severity of IBV or otherwise is an important contributory factor in influencing the pathogenesis of the disease (see later).

In layers, IB can cause a severe decline in egg production, and later, a deterioration in shell and internal quality (McDougall, 1968; Sevoian & Levine, 1957) . Such effects may be accompanied by mild (Muneer et al, 1986 (Muneer et al, , 1988 or no respiratory signs (Cook, 1984; . Some strains produce only a loss in shell colour . The severity of production decline varies with the period of lay, the virulence of the virus involved and other non-specific factors. The response of individual hens varies greatly (McMartin, 1968) . Production may start to increase after 2 to 3 weeks, but reaches only sub-optimal levels. When laying is resumed, some eggs have soft-shells, while others are mis-shapen or rough-shelled.

When laying hens were infected with IBV strain M41, viral antigen was demonstrated in the epithelium of the oviducts between 6 and 9 days p.i. (Jones & Jordan, 1971) . Areas of glandular hypoplasia caused by IBV leads to reduction in the synthesis of albumen proteins especially ovomucin, lysozyme and other major proteins which constitute the structural matrix of the thick albumen (Butler et al, 1972) . Hence, there is a decrease in the proportion of both thick and inner thin albumen, and an increase in the amount of outer thin albumen causing 'watery-whites'. Presence of blood or meat spots in the egg albumen has also been reported (McDougall, 1968; Muneer et al, 1987) .

Inspissated yolk material may be seen in the abdominal cavity of infected layers. Microscopic changes in the oviduct include reduction in the height of the epithelial cells, reduction in number or complete absence of cilia, dilation of glands, lymphocytic foci and cellular infiltration in the lamina propria and inter tubular stroma (Sevoian & Levine, 1957) .

The mechanisms whereby IBV infection causes egg production of some birds to cease and for varying periods of time have not been elucidated.

IBV infection of female chicks of less than 2 weeks of age can cause permanent damage to the developing reproductive tract, resulting in 'false layers' that do not lay normally at sexual maturity (Broadfoot et al, 1956; Jones & Jordan, 1970) . Following infection of day-old chicks with IBV strain M41, virus was isolated between days 5 and 11 p.i. (Jones & Jordan, 1972) and was also found to replicate in the epithelium of the oviducts (Crinion & Hofstad, 1972a,b; Jones & Jordan, 1972) .

Gross changes in the oviduct caused by early infection may vary from the presence of a continuous patent but underdeveloped structure to a blind sac projecting forward from the cloaca (Jones & Jordan, 1970 . The middle third of the oviduct is the most severely affected with areas of localised hypoplasia seen between normal patent oviducts. Caudal to the hypoplastic regions, macroscopic cysts filled with a clear serous fluid may be seen (Crinion et al., 1971a) .

The histopathological changes in the oviduct include decreased height and loss of cilia from epithelial cells, dilation of the tubular glands, infiltration of heterophils, lymphocytes and plasma cells, and oedema and fibroplasia of the lamina propria (Crinion et al, 1971a,b; Crinion & Hofstad, 1972a,b) .

The effect of IBV on the male reproductive tract has not been reported.

Variations in the ability of IBV strains to cause decreases in egg production and quality were reported by Guittet et al. (1988) ; D274 was the least virulent while M41 and a variant strain PL84084 had the same degree of pathogenicity. found that some variant strains of IBV caused only small decreases in egg production, but had a marked effect on egg colour. In contrast, a more recent variant (Parsons et al., 1992) caused a substantial decline in egg production with little loss of egg colour in the field, although no experimental work has been done with this virus. Differences in virulence of IBV strains for the immature chicken oviduct were reported by Crinion & Hofstad (1972a) ; Massachusetts and T strains were virulent, while Connecticut and Iowa 609 were not. Embryo passage of IBV strain M41 reduced its virulence for the oviduct (Crinion & Hofstad, 1972b) . In vitro, oviduct organ cultures (OOC) were highly susceptible to the H52 strain of IBV regardless of the age of the chickens and no differences in susceptibility were seen between magnum and uterus regions . Pradhan et al. (1984) showed that IBV strain M41 causes stasis of cilia in OOC prepared from precociously-induced oviducts in young chicks by oestrogen treatment. This work has been extrapolated to compare the virulence of seven strains of IBV in vitro using ciliostasis and a calmodulin assay to quantify the damage to oviduct epithelium (Dhinakar Raj & Jones, 1996c ). An IBV isolate belonging to serotype D207 was the most virulent while an enterotropic variant strain G was the least.

Although even those strains of IBV considered primarily to affect the respiratory tract such as M41 can occasionally cause kidney damage (Jones, 1974) , nephropathogenicity has been associated only with certain strains. Greater virulence of the virus for the kidney was first reported in Australia (Cumming, 1962) .

Since then, nephropathogenic IBV (NIBV) has been reported from USA and certain parts of Europe (Butcher et al., 1990; Kinde et al., 1991; Lambrechts et al, 1993; Picault et al, 1991; Zanella, 1988) .

NIBV strains initially cause respiratory symptoms followed by signs due to kidney damage which include increased water consumption and wet droppings (Winterfield & Hitchner, 1962; Cumming, 1963) . Mortalities occur and follow a consistent pattern (Cumming & Chubb, 1988) . The first deaths usually occur around 6 days p.i., increase rapidly to a peak around 10 days with the last deaths seen around 16 days p.i. However, the mortality rates depend on several intrinsic and extrinsic factors (see below).

Virus replication in the kidneys has been shown in the proximal convoluted tubules (Chong and Apostolov, 1982) , distal convoluted and collecting tubules (Owen et al, 1991) and collecting ducts (Chen et al, 1996) . Structural alterations in the tubular epithelial cells (Condron & Marshall, 1986) caused impaired fluid and electrolyte transport leading to acute renal failure. An increase in urinary water losses in chickens infected with NIBV was found to be associated with lower urine osmolality and higher rates of urinary excretion of sodium, potassium and calcium (Afanador & Roberts, 1994; Condron & Marshall, 1985; Heath, 1970) . Negative sodium balance was a direct effect of increased output of sodium in the urine, while negative potassium balance was due to decreased intake.

The kidneys of chickens infected with NIBV are swollen and pale, with tubules and ureters distended with urates (Cumming, 1963) . The relative kidney weight and kidney asymmetry are increased. The haematocrit values of infected birds was decreased and plasma uric acid levels were increased (Afanador & Roberts, 1994) . Despite lack of gross lesions microscopic changes of nephritis may still be present (Winterfield & Albassam, 1984) . Chen et al., (1996) proposed that IBV-induced renal lesions can be considered to be a ductotubular interstitial nephritis. The virus causes granular degeneration, vacuolation and desquamation of the tubular epithelium with massive infiltration of heterophils in the interstitium in acute stages of the disease. The changes in the chronic phase were classified as being active or inactive types of interstitial lymphocytic nephritis (Albassam et al., 1986) . The chronic inactive form of nephritis was indicative of IBV replication in the kidneys and subsequent clearance; while the chronic active type suggested a persistent viral replication and damage to epithelial cells of the kidney tubules. Virus was isolated from 50% of the birds with chronic nephritis (Chong & Apostolov, 1982) . The histopathological changes in the kidneys following NIBV infections have been described by several workers (Albassam et al., 1986; Chen et al., 1996; Pohl, 1974; Purcell et al, 1976; Siller & Cumming, 1974) .

The type of kidney lesions produced by different NIBV strains were similar but their severity varied (Albassam et al., 1986; Chandra, 1987) . The Australian strain 'T' induced the most rapid and severe lesions following both intra-venous (i.v.) (Chandra, 1987) or intra-ocular (Albassam et al, 1986) inoculations of susceptible chickens. The effect of IBV on the trachea is apparently independent of the effect on kidney (Ratanasethakul & Cumming, 1983) . A strain of NIBV, 'S' virus was less severe on the kidneys than 'T', but more severe on the tracheal mucosa. Using an i.v. inoculation model to titrate kidney infectivity, Lambrechts et al. (1991) found no significant differences in the infectivity among Belgian field NIBV isolates, but the infectivity of egg-passaged virus was highly reduced.

Several strains of IBV have been isolated from cloacal swabs, faeces and caecal tonsils Alexander et al, 1978; Cook, 1984; Lucio & Fabricant, 1990) . In vitro explants of several gut tissues have been shown to support the growth of IBV (Bhattacharjee, 1994; Bhattacharjee & Jones, 1997; Darbyshire et al, 1976) . In studies where virus isolation was attempted from several tissues, maximum virus isolations were obtained from the oesophagus of chickens infected with ECV2, an enteric isolate of IBV (Lucio & Fabricant, 1990) and 793/B-like virus (Dhinakar Raj & Jones, 1996a) . Oesophageal swabs have also been used to identify IBV using the polymerase chain reaction (D. Cavanagh, personal communication). However, it is not clear whether the virus actually multiplies in the oesophagus or whether virus is swallowed after being expelled from the trachea. IBV has also been isolated from proventriculus, duodenum and jejunum (Ambali & Jones, 1990; Lucio & Fabricant, 1990) . Darbyshire et al, (1976) have shown that proventriculus was inferior only to respiratory tissues and oviduct in supporting virus multiplication in vitro. However, the site of virus multiplication in these tissues has not been confirmed, but presumably occurs in the epithelial cells.

In contrast, in the tissues of the lower gut, IBV replication has been described (without photographs) in cells resembling histiocytes and lymphoid cells in the caecal tonsils (Owen et al, 1991) and demonstrated by IF in apical epithelial cells of the villi in ileum and rectum (Ambali & Jones, 1990; Dhinakar Raj & Jones, 1996a) . Although IBV has a wide tropism for gut tissues no gross or histological changes have been reported.

A variant strain of IBV, strain G, was classified as being enterotropic by virtue of its prolonged persistence in the gut compared to the trachea (El Houadfi et al, 1986) . Recently, it was seen that a variant IBV strain 793/B was more enterotropic than pneumotropic and was even associated with diarrhoea in broilers (Dhinakar Raj & Jones, 1996a) . It seems that studies on the replication of IBV in the gut have been neglected, probably because no IBV strain has been demonstrated to be enteropathogenic. MacDonald et al. (1983) reported that following crop inoculation of chicks with H52 and H120 vaccines, virus was rarely isolated from the gut or any other visceral tissues and no antibody response occurred. However, the kidneys were found to be resistant to i.v. challenge. More intensive studies on oral infection with enterotropic IBV strains or dual infections with the virus and other enteric pathogens, such as salmonella, coccidia or rotavirus, might yield more information on the effects of IBV replication in the gut.

The ability of IBV strains to survive in the presence of low pH, digestive enzymes and bile salts may be relevant to enteric replication. Otsuki et al (1979a) and Cowen & Hitchner (1975) showed that some strains suffer a greater loss of titre than others when kept at pH 3.0 for 3 or 4 h, but there was no indication that any of the more resistant strains were isolated from the gut. Ambali & Jones (1991a) compared strain M41 with an enterotropic variant (G). Both viruses had a similar sensitivity to trypsin, but the variant showed a 50-fold greater resistance to sodium tauroglycocholate, which might partly explain its ability to replicate in the gut.

The important variant strain of IBV, 793/B, was recently isolated from a broiler breeder flock where the affected birds had bilateral myopathy affecting both deep and superficial pectoral muscles (Gough et al, 1992) . There was marked swelling and pallor of deep pectoral muscles together with the presence of occasional fascial haemorrhages and a layer of gelatinous oedema over its surface. Experimental infection of 6 week old broilers with an isolate of IBV belonging to this group resulted in mild oedematous separation of muscle fibres but with no corresponding increase in serum creatine kinase concentrations (Dhinakar Raj & Jones, 1996a ). It has been clearly shown that the virus was not involved directly in the pathogenesis of the muscle lesions but formation and deposition of immune complexes, such as those found in the kidneys (Dhinakar Raj & Jones, 1996a) , in the capillary walls of the muscle could be a possible reason for the development of this strange lesion.

IBV has been isolated from the Harderian gland (Dhinakar Raj & Jones, 1996a; Gelb et al, 1991b; Toro et al., 1996a) and IBV-positive cells have been shown in the stroma of the gland by IF staining (Toro et al, 1996a) . IBV vaccination by eyedrop resulted in a massive infiltration of lymphocytes, increase in plasma cell numbers (Davelaar & Kouwenhoven, 1976; Survashe et al, 1979; Montgomery et al, 1994; Toro et al, 1996a) and desquamation of tubular epithelium in the Harderian gland with restoration from 14 days after vaccination (Toro et al, 1996a) . Increased numbers of plasma cells (Survashe et al, 1979) and lymphoid tissue development (Montgomery et al, 1994) were also seen in the lachrymal gland following IBV vaccination.

IBV has also been isolated from the bursa of Fabricius (El Houadfi et al, 1986; Ambali & Jones, 1990) , and gross and histopathological lesions have been shown following experimental H52 and HI20 infections (MacDonald & McMartin, 1976) . Although IBV has been isolated from a variety of other tissues such as liver Ambali & Jones, 1990 ) and spleen (Otsuki et al, 1990) it has not been documented to be involved with any functional damage. IBV has been isolated from semen and eggs of infected chickens (Cook, 1971 ), but vertical transmission appears to be of little importance.

Several intrinsic (age and breed of chickens) and extrinsic (nutrition, environment and intercurrent infections) factors influence the pathogenesis of IBV infections.

All ages are susceptible, but the clinical disease is more severe in young chicks (Animas et al., 1994b) although recovery of the virus from the trachea was similar compared to 6-week old infected chickens (Animas et al., 1994a) . However, the duration of virus excretion in the faeces was longer in 2-week old infected chicks compared to 4-and 6-week-old chickens. As age increases chicks become more resistant to IBV-induced mortality (Smith et al., 1985) , nephropathic effects (Albassam et al., 1986) and oviduct lesions (Crinion & Hofstad, 1972a) . Mac-Donald et al. (1980) described fewer pathological changes in kidneys of chickens infected at 3 weeks of age than those infected at day-old or 10 weeks of age, although the virus was inoculated i.v., which is an unnatural route of infection. Purchase et al. (1966) demonstrated variation in mortality following IBV inoculation of embryos from different inbred lines. Bumstead et al. (1989) compared mortalities in several inbred lines of chickens following inoculation with a pool of strains of IBV and/or Escherichia coli and found marked differences among them. On this basis, the lines of chickens were classified as being resistant or sensitive to IBV. However, these inbred lines did not show marked differences in susceptibility to the variant strain of IBV, 793/B (Parsons et al, 1992) .

Genetic differences in susceptibility to nephritis are also marked, with light breeds being more susceptible than heavy breeds (Cumming & Chubb, 1988) . The mortalities in 4-week-old cockerels of 10 Australian commercial egg-laying strains varied from 11 to 59%. NIBV has been shown to cause higher mortalities in broilers than layers (Ignjatovic, 1988; Zanella, 1988) although the virus multiplies to the same extent in both types of birds (Lambrechts et al., 1993) . Male chicks are twice as susceptible as females to nephritis (Cumming, 1969) .

High protein diets increase mortality from IBV-induced nephrosis; chickens fed meat meal or poultry by-product meal-based diets experience higher mortality than those fed soybean based diets (Cumming, 1969; Cumming & Chubb, 1988) .

Low temperatures have a dramatic effect on mortality due to NIBV. Reduction in temperature from 20°C to 16°C increased mortality from 8 to 50% (Cumming, 1969) with greater severity of histopathological lesions in kidneys (Ratanasethakul & Cumming, 1983) . Exposure to cold stress has been used to increase the severity of challenge for assessing protection afforded by IBV vaccines (Klieve & Cumming, 1990) . Cold stress also increased the severity of IBV-induced tracheal lesions (Ratanasethakul & Cumming, 1983) and promoted more extensive air sacculitis after combined infections with IBV and Mycoplasma synoviae (MS) (Yoder et al, 1977) .

Mycoplasmas Adler et al. (1962) found that intranasal inoculation of either the Massachusetts strain of IBV or M. gallisepticum (MG) given alone produced little by way of clinical signs, but dual infections resulted in coryza, tracheitis and airsacculitis. The incidence of airsacculitis was highest in chickens infected with IBV 1 week after MG inoculation. In adult chickens combined infections with IBV and MG has been shown to cause a more severe effect on egg production and quality than inoculation with either agent alone (Blake, 1962) .

Olson et al, (1964) exposed chickens to IBV and MS but found that dual infections were not synergistic. In contrast, later work by Kleven et al, (1972) found that airsacculitis resulted when chickens were infected with IB and Newcastle disease (ND) vaccines after MS infection. Hopkins & Yoder (1982) , in similar experiments with MS, gave different strains of IBV 2 to 5 days before the mycoplasma. They showed that different strains of IBV had differing abilities to induce airsacculitis. Field isolates of IBV had the most severe effects. Vaccines of high passage produced very mild lesions while those of low passage caused intermediate lesions. Gross (1958) , in one of the first studies, infected 5-week-old chickens with IBV and gave pathogenic E. coli by aerosol at intervals afterwards. The most severe pericarditis was seen when the bacteria were given 11 to 22 days after the virus. Smith et al. (1985) , using a combined inoculation of a pool of strains of IBV and E. coli, developed an experimental model for IBV which closely resembled natural outbreaks of the disease. The damage to tracheal epithelium caused by IBV facilitates E. coli invasion and multiplication leading to death or lesions in surviving chickens. Using this model, several reports have described differences in virulence of IBV strains, differences in susceptibility between genetic lines of chickens and detailed interactions between the agents (Avellanda et al., 1994; Bumstead et al, 1989; Cook et al, , 1991b Cubillos et al, 1991; Smith et al, 1985) . Raggi et al. (1967b) showed that when IBV and Haemophilus paragallinarum were given together intra-nasally, the incubation period was shorter, there was higher mortality and lesions were more severe.

There is no reported evidence of dual infections with IBV and Pasteurella multocida, the causative agent of fowl cholera. However, Bisgaard (1977) , in a study of chickens in the field, found that where the bacterium P. haemolytica was present with IBV, there was no apparent exacerbation of the viral infection.

IBV is known to interfere with the replication of NDV in eggs and Raggi & Lee (1964) have shown that interference also occurs in chickens, but this is only one way. They found that when IBV was given to chickens with NDB1 vaccine, there was subsequently no production of haemagglutination-inhibition (HI) antibodies to NDV, but NDV did not interfere with IBV infection. Raggi et al. (1967a) found no synergism between IBV and infectious laryngotracheitis virus (ILTV), but Pattison et al. (1971) reported that aerosol vaccination with IBV increased the mortalities due to ILTV. APV, the cause of TRT and mild respiratory disease in chickens may also occur in respiratory outbreaks involving IBV, but the significance of these two agents being present together is yet to be reported. Fabricant & Levine (1962) examined triple infections involving IBV, MG and pathogenic E. coli. They reported that with regard to disease lesions the descending order of severity was IBV, MG and E. coli; MG and E. coli; IBV and E. coli; and E. coli alone. A similar study by Nakamura et al. (1994) was performed with live IB and ND vaccines given to young chicks with MG and/or E. coli or both. Again, the most severe combination was when vaccine, E. coli and MG were given together.

Thus, it is clear that several respiratory pathogens interact synergistically with IBV or IB vaccines and enhance the severity and duration of the disease. In many instances, the interval between infections is important, as are the challenge doses. However, with few exceptions (e.g. Nakamura et al., 1992) , it is not known whether the synergisms are due to immunosuppression or simply that the epithelial damage caused by one agent permits greater penetration by others.

The two classical examples of immunosuppressive viruses of chickens are infectious bursal disease (II3D) which principally affects the B-cells and chicken anaemia virus (CAV) which affects the T-cells.

There is abundant evidence that IBDV infection impairs the humoral response to IBV. Giambrone et al. (1977) infected chicks with IBDV at day-old and IBV at 14 days. Compared to single IBV infection with both viruses, antibody titres were lower and airsac lesions greater. Rosenberger & Gelb (1978) also showed that IBDV at day-old affected the response to IB vaccines so that there was lowered resistance to challenge. Furthermore, Winterfield et al. (1978) showed that IBDV increased susceptibility to IBV, reduced IBV antibody levels and explained unsatisfactory immunity when replacement and broilers were vaccinated at an early age.

There are no reports of dual infection of chickens with IBV and CAV. Goodwin et al. (1992) reported that there were no significant differences in IBV-specific antibody titres between chicken flocks positive or negative for CAV antibodies. However, when chicks infected with IBV strain M41 were treated with the T-cell immunosuppressor drug cyclosporin A (CSA), the clinical signs in these birds were more severe and virus titres in tissues, especially kidneys, were greater than in infected but untreated birds, although the persistence of the virus was not increased (Dhinakar Raj & Jones, 1997a). IBV-induced mortality was also increased. It seems possible that infection with CAV at a critical time might induce some of these effects on IB.

Although IBV has been isolated from lymphoid tissues such as bursa of Fabricius and Harderian gland, the evidence for a direct immunosuppression is limited. It is not clear whether IBV multiplies in the lymphocytes but cultured macrophages were found to be resistant (von Bulow & Klasen, 1983) . A virulent IBV has been shown to induce transitory reduction in proliferative responses of whole blood lymphocytes to T-cell mitogens, PHA (Wakenell et ah, 1995) and concanavalin A (Dhinakar Raj & Jones, 1997a). IBV vaccines were found to depress the Harderian gland responses to killed Brucella abortus (Montgomery et al., 1991 (Montgomery et al., , 1994 . However, no correlation between histological responses and HI titres to IBV were found with depression of responses to Br. abortus.

IBV is a positive-stranded RNA virus and is the prototype of the Family Coronaviradae. It has three structural proteins. The spike 'S' glycoprotein is located at the surface of the virion, and consists of two subunits, SI and S2, with molecular weights of 92 and 84 K, respectively. The membrane 'M' glycoprotein is partially exposed at the surface of the virion with molecular weights ranging from 27 to 36 K, and the nucleocapsid 'N' protein is internally located with a molecular weight of 52 K (Wadey & Westaway, 1981; Cavanagh, 1983a,b,c) .

The SI glycoprotein of IBV induces virus neutralizing (VN) and HI antibodies (Cavanagh et al, , 1986 Kant et al, 1992; Koch et al, 1990; Mockett et al, 1984; Niesters et al, 1987) and has been considered as the most likely inducer of protection (Cavanagh et al, 1986; Ignjatovic & Galli, 1994) , but S2 and N proteins may also be important since they carried epitopes for induction of cross-reactive antibodies (Ignjatovic & Galli, 1995) . The time of appearance of SI, S2 and N ELJSA-antibodies have been shown to be similar, being detected 2 weeks after live IBV vaccination (Ignjatovic & Galli, 1995) . This coincides with the appearance of VN antibodies Mockett & Darbyshire, 1981; Darbyshire and Peters, 1984) . Epitopes on N and S2 proteins that gave rise to cross-reactive antibodies showed the same degree of conservation while SI epitopes were shown to be marginally less conserved (Ignjatovic & Galli, 1995) .

A T-cell epitope has been identified in the IBV 'N' protein (Boots et al, 1991) and has been shown to induce anti-viral responses (Boots et al, 1992) . Cellular immune responses elicited by a live IBV vaccine have also been found to be cross-reactive and the responses varied in magnitude with the serotype of IBV used for in vitro stimulation (Dhinakar Raj & Jones, 1997b) .

Immunity can be considered to be either innate or acquired. Innate immunity comprises a collection of factors which resist invasion by external agents, such as physical barriers provided by skin and mucous membranes, soluble factors like lysozyme, complement and acute phase proteins, and cells such as granulocytes, macrophages and natural killer (NK) cells. The main features of innate immunity are lack of specificity and immunological memory.

Heterophils (neutrophils) constitute the 'first line of defence' against infectious agents and are the first cells to be recruited to the site of infection, following initiation of an inflammatory response. In IBV-infected chickens, heterophils are the most numerous early inflammatory cells in respiratory lavage fluids (Fulton et al, 1993) . Using a heteropaenic chicken model (Kogut et al, 1993) the importance of these cells in limiting IBV replication was studied (Dhinakar Raj et al., 1997c) . It was found that they had no effect on virus multiplication and, in fact, contributed to the damage in the tracheal epithelium.

The role of macrophages in IBV infections is unknown, while no alterations in NK cell activity has been found following IBV infection (Wakenell et al, 1995) . Serum levels of an acute phase protein, °° i acid glycoprotein, have been found to peak on day 6 following IBV infection (Nakamura et al, 1996) .

Acquired immunity results in the activation of antigen-specific effector mechanisms including B-cells (humoral), T-cells (cellular) and macrophages, and the production of memory cells.

to IBV infections, measurable by enzyme-linked immunosorbent assay (ELISA), haemagglutiation inhibition (HI) or VN tests (De Wit et al, 1992; Monreal et al, 1985; Wilcox et al, 1983) . However, there is a lack of correlation between titres of circulating antibodies and resistance to infection (Raggi & Lee, 1965; Winterfield & Fadly, 1972; .

Immunoglobulin G (IgG), the major circulating Ig, is the antibody detected by HI and an ELISA developed to measure it is more sensitive (Mockett & Darbyshire, 1981) . Anti-IBV IgG can be detected as soon as four days pi, reaches a peak at about 21 days but can remain in high titre in the serum for many weeks (Mockett and Darbyshire, 1981) . This is the antibody measured in conventional serological tests to monitor IBV infections or vaccine uptake.

Immunoglobulin M (IgM), present only transitorily after infection, reaches peak concentrations about 8 days after IBV infection and levels then decline (Mockett and Cook, 1986) . Although an IgM-specific ELISA has been shown to be useful in the diagnosis of recent infections (Martins et al., 1991) , these antibodies needed to be separated either by sucrose density gradient centrifugation (Gillette, 1974) or column chromatography (Mockett & Cook, 1986) before performing the ELISA. The availability of an antibody-capture ELISA for IBVspecific IgM assays would facilitate IB diagnosis where demonstration of the virus is time-consuming.

The importance of B-cells in IBV infections has been studied by depletion experiments using the hormone testosterone propionate (Chubb, 1974) , the chemical cyclophosphamide (Chandra, 1988; Chubb, 1974) and surgical bursectomy (Cook et al, 1991a) . Cyclophosphamide-treated chickens showed increased clinical signs and more severe histopathological lesions in the kidney (Chandra, 1988) attributable to the prolonged persistence of virus. IBV infection of a surgically bursectomised resistant chicken line (line C) resulted in increased severity and duration of clinical infection but not mortality (Cook et al., 1991a) . However, humoral antibodies seemed to protect the tracheal epithelium following secondary challenge. Presence of high titres of humoral antibodies correlate well with the absence of virus recovery from kidneys and genital tract MacDonald et al, 1981; Yachida et al, 1985) and protection against drop in egg production (Box et al., 1988) . IBV-specific antibodies probably prevent the spread of virus by viraemia from the trachea to other susceptible organs such as the kidneys and oviduct.

Vaccination studies with IBV have always focussed on humoral immune responses in relation to protection. Nevertheless, the lack of correlation between antibodies and resistance, discrepancies between in vitro strain differentiation by VN tests and in vivo cross-protection results (Darbyshire, 1985) and re-excretion of virus in the presence of high titres of circulating antibodies (Jones & Ambali, 1987) all suggest that while humoral antibodies play a role in recovery from IBV infection, other immunological mechanisms are involved.

Maternally derived antibodies (MDA) can provide protection against IBV, but they are short-lived (Darbyshire & Peters, 1985; Cook et al, 1991b) . Presence of MDA has no adverse effect on the efficacy of live IBV vaccines administered at one-day of age (Davelaar & Kouwenhoven, 1977 , Cook et al, 1981 . Maternally-derived IgG has been demonstrated in tracheal washes (Mockett et al, 1987) .

Local immunity in the respiratory tract is of fundamental importance in protection against IBV (Gomez & Raggi, 1974; Gillette, 1981; Hawkes et al, 1983) . This has been exemplified by the use of an in vitro challenge model using TOC from immunized chickens, for cross-protection studies . IBVspecific IgA and IgG have been demonstrated in tracheal washes of infected chickens (Hawkes et al, 1983; Dhinakar Raj & Jones, 1996b ) and antibody-secreting cells were shown in tracheal sections (Nakamura et al, 1991) . Local immunity at the oviduct level has been shown by demonstration of virus-specific IgG and IgA in the oviduct washes of infected hens (Dhinakar Raj & Jones, 1996b) . It was found that in addition to local production, antibodies also transuded from the serum later in the course of infection, but their value in protection of the oviduct has not been determined. In young chicks, local antibodies in the oviduct were found to be less protective compared to those in the trachea using in vitro challenge of OOC prepared from vaccinated chickens (Dhinakar Raj & Jones, 1996d) .

Although IBV has been shown to multiply in the gut (Lucio & Fabricant, 1990; Ambali & Jones, 1990) , Lutticken et al. (1988) could not detect any antibodies in gut washings following vaccination of day-old chicks with HI20 vaccine and revaccination at 4 weeks of age with live H52 vaccine, inactivated M41 in an oil emulsion or inactivated M41 adjuvanted with avridine. In contrast, Dhinakar Raj & Jones (1996b) demonstrated local antibody production in the duodenum and caecal tonsils of 16-week-old hens infected with an enterotropic strain of IBV (strain G). It is not clear whether the induction of these antibodies is strain-related and their role in limiting the replication of IBV in the gut needs to be investigated.

The Harderian gland of the chicken contains a large age-dependent population of plasma cells and is the source of immunoglobulins in the lachrymal fluid (Baba et al, 1988) . It plays an important role in the development of vaccinal immunity since vaccines are generally given by spray or eye-drop. Davelaar & Kouwenhoven (1981) reported that the protection against IBV of day-old ocular-vaccinated chickens was localized mainly in the oculo-nasal mucosa and removal of the Harderian gland caused a decreased level of protection (Davelaar & Kouwenhoven, 1980) . IBV-specific IgA has been demonstrated in the lachrymal fluid Davelaar et al, 1982; and its synthesis in the Harderian gland has been shown (Davelaar et al, 1982) . IgG in tears was mainly serum-derived (Davelaar et al, 1982; Toro et al., 1993) . IgA levels in tears appeared to be better correlated with resistance to IBV re-infection (Toro & Fernandez, 1994) than levels of serum antibody (Yachida et al, 1985) and their measurement was recommended for antibody profiling of chicken flocks. Cook et al. (1992) found more IBV-specific IgA in the lachrymal fluids of chicken lines resistant to IBV while antibody titres in tracheal washes were similar. Variation in IBV-specific IgG levels in serum and IgA levels in lachrymal fluids has also been demonstrated in different chicken lines after ocular vaccination with IBV (Toro et al, 1996b) . It was found that light layer (white leghorn) chickens had a significantly higher and more homogenous serum IgG response between days 5 and 9 p.i., and lachrymal IgA response between days 5 and 14 p.i. than broiler or brown-egg (heavy) layer chicks.

Reports concerning a role for cell-mediated immunity in protection against IBV are limited. Antigen-specific proliferation of T-lymphocytes in IBV-infected or vaccinated chickens has been demonstrated (Timms et al, 1980; Timms & Bracewell, 1981 . In some chickens, a positive correlation between lymphoproliferative responses and resistance to challenge has been shown (Timms & Bracewell, 1981) .

Mouse monoclonal antibodies (Mab) that distinguish between chicken Tlymphocytes have been described (Chan et al., 1988; Iillehoj et al, 1988) . The CD4 and CD8 antigens are found on two main populations of T-cells, T-helper (T n ) and T-cytotoxic / suppresser (Tc/ S ) cells, respectively. Janse et al. (1994) contended that local immunity to IBV in the trachea is mediated by T-cells. CD4 and CD8 cells were shown in sections of trachea and lung of chickens infected with IBV (Janse et al, 1994; Dhinakar Raj & Jones, 1996a) . However, it is not clear which of these cells are more important in virus clearance, since Janse et al. (1994) found an increase in CD4 cells, while Dhinakar Raj & Jones (1996a) found higher proportions of CD8 cells. The differences may be related to the strains of IBV used.

When chickens were treated with CSA to suppress the T-cells, virus titres in the kidneys were 1 to 3 logio median ciliostatic doses (CD 50 ) higher than in intact birds (Dhinakar Raj & Jones, 1997a) . Thus, T-cells may also play an important role in limiting virus replication in the kidneys. Chubb et al (1987) demonstrated the presence of cytotoxic lymphocytes (CTL) in the spleen and peripheral blood following IBV infection using adherent cells as target cells and neutral red as indicator of lysis. However, Wakenell et al. (1995) could not demonstrate CTL using kidney cells as targets and the conven-tional chromium-release assay. Delayed-type hypersensitivity (DTH) responses were induced in response to live IBV and to affinity-purified SI, S2, N and M proteins (Igjnatovic & Galli, 1995) .

Cytokines secreted in response to a general stimulus such as concanavalin A (Con A) or by specific antigen are important mediators of cellular immunity. T-cell growth factor (TCGF) or IL-2 and IFN-y are among the most important soluble factors produced by lymphocytes. Only IFN-y has been studied in relation to IBV but results are conflicting. Otsuki et al (1988) detected variable levels of IFN-y in chickens with various strains of IBV whereas other workers (Lomniczi, 1974; Holmes & Darbyshire, 1978) could not detect IFN-y in serum or organ cultures of chickens infected with IBV. Furthermore, whether IBV is susceptible to anti-viral effects of IFN-y is controversial (Holmes & Darbyshire, 1978; Otsuki et al, 1979b) . Otsuki et al. (1991) found no differences in IFN-y levels of IBV-resistant and IBV-sensitive lines of chickens.

The identification of resistant and sensitive inbred lines of chickens to IBV (Bumstead et al, 1989) has provided an excellent model to study immune mechanisms in relation to resistance of chickens to IBV. Otsuki et al, (1990) found that although a resistant line (C) and a sensitive line (151) were equally susceptible to infection initially, recovery was more rapid in the resistant line. Ultrastructural and histochemical studies showed that though the type of damage to the tracheal epithelium following IBV infection was similar in both lines of chickens, lesions were more severe and longer lasting in the sensitive line (Nakamura et al, 1991) . The severity and duration of clinical infection in bursectomised line C chickens were similar to those seen in the sensitive line (Cook et al, 1991a) . However, no increase in mortality was observed, in contrast to high mortality recorded in line 151 chickens. Comparisons of secretory antibody responses between the two lines of chickens revealed increased local antibody secretion in the saliva and lachrymal fluid of the resistant line .

Suppression of T-cells by CSA in a line of chickens resistant to IBV (brown leghorn; BLH), induced them to behave like a sensitive line, in terms of mortality (Dhinakar Raj & Jones, 1997a) . The mortality of intact BLH chickens infected with a pool of ten IBV strains was 0% compared to 43% in CSA-treated BLH chickens. The virus titres in kidneys were much higher than in intact birds. The classes of T-cells involved in these effects need to be studied in further detail by analysing the kinetic changes in T-cell subsets following IBV infections using flow cytometry or by specific depletion in vivo by Mabs against T-cell subsets.

Although IB is generally considered as an acute respiratory disease, prolonged virus excretion has been reported (Cook, 1968; Alexander et al, 1978; Chong & Apostolov, 1982) . When day-old chicks were infected with an enterotropic IBV, strain G, faecal excretion could not be detected beyond day 35 p.i., but when birds reached sexual maturity re-excretion occurred (Jones & Ambali, 1987) . Virus re-excretion could not be induced earlier by hormone injections (Ambali & Jones, 1991b) , but it could after T-cell suppression by CSA (Bhattacharjee et al, 1995) . Reactivation of virus was confirmed by appearance of IBV-specific IgM in the serum (Bhattacharjee et al, 1995) .

Mainly because of the prolonged or intermittent recovery of IBV from these tissues, 2 candidate sites have been proposed for virus persistence in the chicken, the kidneys (Chong & Apostolov, 1982) and/or caecal tonsils (Cook, 1968; Alexander et al, 1978) . However, recent work has suggested that the kidneys are the more likely site. When strain M41 was used to infect day-old chickens and T-cell immunosuppression by CSA treatment was given from 5 weeks p.i., virus was recovered first from the kidneys and then from the trachea and lungs but never from the caecal tonsils (Dhinakar Raj & Jones, 1997a) . The target cells of the virus in the kidneys are in the tubular epithelium (Chong & Apostolov, 1982; Ambali & Jones, 1990 ) which provides an ideal site for virus persistence because of its immunologically privileged nature (Mimms, 1988) .

Occurrence of persistence was also found to be related to the age at infection (Dhinakar Raj & Jones, 1997a) . When chicks were infected at 2 weeks of age and treated with CSA from 5 weeks p.i. no virus re-excretion was observed, but when chicks were infected at day-old, virus re-excretion was successfully induced with CSA. Notably, virus re-excretion was not accompanied by clinical symptoms.

The persistence of IBV may have practical implications in the epizootiology of the disease and perhaps a role in evolution of variant strains of the virus. Furthermore, re-excretion of persistent virus may serve as an unnoticed source of infection to susceptible chickens.

IBV does not constitute a single homogenous antigenic type. The prototype virus is Massachusetts M41. Since the first identification of a different serotype (named Connecticut) by Jungherr et al, (1956) several new antigenic types have been reported in various countries (see Captua et al, 1994; Cook, , 1984 Gelb et al, 1991a; Parsons et al, 1992; Picault et al, 1986) . The major virus neutralizing antibody site of IBV, which defines serotype, resides in the SI subunit of the spike protein (Mockett et al, 1984; Cavanagh et al, 1986) . Differences in only a few amino acids in the SI protein can result in different VN serotypes (Cavanagh et al, 1992a) which accounts for the plethora of IBV strains which exists today.

Hence, different antigenic types identified by VN tests do not imply that the isolates have substantially different SI proteins or overall antigenic properties or greatly different evolutionary lineages.

As in other RNA viruses, antigenic variation is probably facilitated by the high error rate during the transcription of RNA template and the absence of a proof-reading mechanism. It has been shown that point mutations may lead to the generation of IBV variants in the field Qia et al, 1995) . However, both circumstantial (Cavanagh & Davis, 1988; Cavanagh et al, 1992b; Jia et al, 1995; Kusters et al, 1989 Kusters et al, , 1990 Wang et al, 1994) and experimental (Kottier et al, 1995) evidence suggest that the main mechanism of generation of variant strains of IBV is by recombination. This could be promoted by the use of more than one strain of IBV for vaccination or by a mixture of vaccine and challenge viruses.

Isolates of IBV shown to be distinct by the VN test can still induce partial or complete cross-immunity (Arvidson et al, 1990; Darbyshire, 1980 Darbyshire, , 1985 Hitchner et al, 1964; Raggi & Lee, 1965; Winterfield & Fadly, 1972; Winterfield et al, 1976) . For example, vaccination with H120 caused a 30,000-fold reduction in titres of the challenge virus, Australian 'T' strain (Darbyshire, 1985) , while on the basis of VN tests in vitro, no evidence of a serological relationship between these two viruses could be demonstrated . Hence, antigenic studies alone do not adequately define immunological relationships between strains. Thus, it was suggested that cross-immunization studies could be used to classify IBV isolates into protectotypes (Lohr, 1988) as this would reduce the large number of serotypes to a smaller number of protectotypes and provide more practical information to the field.

Cross-immunisation tests have been performed in experimental chickens (Darbyshire, 1980 (Darbyshire, , 1985 Lambrechts et al, 1993) or in in vitro conditions using TOC from immunized birds Dhinakar Raj & Jones, 1996d) to determine protective immunological relationships between IBV strains. Arvidson et al. (1990) have described a model to study immunogenic relationships between IBV strains based on the vaccinating dose required to prevent the multiplication of a standard challenge dose of a homologous strain in the lungs of chickens. These cross-immunization tests would help to determine whether an already existing vaccine could offer protection to a new variant. This information is critical before embarking on a long and cumbersome process of new vaccine production.

Since tissue affinities are a function of the viral peplomer-mediated attachment of virus to cells, changes in the spike protein might lead to altered tropism of the virus. Six of ten differences in amino acid sequences of Gray and JMK strains of IBV were found between residues 99 and 127; hence it was postulated that this region may play a role in differences in tissue tropism exhibited by these viruses (Kwon & Jackwood, 1995) . However, no correlation was found by Sapats et al. (1996) between SI amino acid sequences and nephropathogenicity of nine Australian strains of IBV. Nevertheless, it is not unreasonable to expect new variant strains of IBV to emerge with unusual tissue tropisms and such an example of this is strain 793/B. CONCLUSIONS IBV has been effectively controlled by the extensive use of vaccines but it still remains a major economic problem some 65 years after it was first reported. The constant emergence of variant strains has challenged vaccination strategies. With the advent of molecular techniques much of the recent work on IBV has concentrated on improved diagnostic and strain differentiation methods. However, the appearance and spread of the economically-important variant strain 793/B has given us a reminder of the need for a more detailed understanding of the immunopathogenesis of the disease and to be prepared for emergence of variants with unusual tissue tropisms and disease manifestations. tionstraktes angesehen wird, können verschiedene IBV-Stämme unterschiedliche Gewebstropismen aufweisen und, mit ernsten Folgen, auch den Eileiter und die Nieren befallen. Manche Stämme vermehren sich im Darm, anscheinend aber ohne pathologische Veränderungen. Pektorale Myopathie ist mit einer wichtigen neuen Variante in Verbindung gebracht worden. Mehrere Faktoren können den Verlauf der Infektion mit IBV beeinflussen und umfassen das Alter, die Rasse und die Fütterung des Huhnes, die Umgebung und die interkurrente Infektion mit anderen Infektionserregern. Die immunogenen Bestandteile des Virus sind die S (spike)-Proteine und das N-Nukleoprotein. Die humoralen, lokalen und zellulären Reaktionen des Huhnes gegen IBV sowie die genetische Resistenz des Huhnes werden besprochen. Bei der langfristigen IBV-Persistenz sind die Blinddarmtonsillen oder die Nieren als Orte der Persistenz vermutet worden. Die Antigenvariation bei IBV-Stämmen hängt mit relativ kleinen Unterschieden der Aminosäuresequenzen im Spike-Protein S1 zusammen. Antigenuntersuchungen alleine reichen jedoch nicht aus, um immunologische Beziehungen zwischen Virusstämmen ausreichend zu definieren, und Kreuzimmunisierungsversuche sind benutzt worden, um IBV-Isolate in 'Protektotypen' zu klassifizieren. Es sind Vermutungen darüber angestellt worden, daß Veränderungen im Spike-Protein S1 eine Beziehung zu Unterschieden der Gewebstropismen, die verschiedene Stämme aufweisen, haben könnten. Vielleicht könnten in der Zukunft neue IBV-Stämme auftauchen, die Organe oder Systeme befallen, welche normalerweise keinen Zusammenhang mit der IB haben.

Virus de la bronquitis infecciosa: Inmunopatogenia de la infeccion en la gallina Se revisa la inmunopatogenia de la infeccion por el virus de la bronquitis infecciosa en la gallina. Mientras que la bronquitis infecciosa (IB) es considerada primariamente una enfermedad del aparato respiratorio, distintas cepas de IBV pueden mostrar tropismos diversos y afectar tambien al oviducto y el riñón con consecuencias serias. Algunas cepas se replican en el intestino pero aparentemente sin producir lesiones. Una reciente variante de IBV ha sido relacionada con la miopatia pectoral. Diversos factores pueden influir el euros de la infección por IBV incluyendo la edad, estirpe y estado nutritivo del ave, el medio ambiente e infecciones intercurrentes con otros agentes infecciosos. Los componentes inmunogénicos del virus incluyen las proteinas S y la nucleoproteína N. Se revisan las respuestas inmunes humorales y celulares de las gallinas frente a IBV asi como la resistencia genética de la gallina. En situaciones de persistencia de IBV se considera que las tonsilas cecales o el riñón son los lugares probables de acantonamiento. Las variaciones antigénicas entre las distintas cepas de IBV están en relatión con diferencias aminoacidicas pequeñas en la proteína S1. No obstante, los estudios antigenicos por si solos no son capaces de definir adecuadamente las relaciones entre las cepas, y se han utilizado estudios de inmunización cruzada para clasificar los aislamientos de IBV en "protectotipos". Se especula que los cambios en la proteína S1 puedan estar relacionados con las diferencias en el tropismo tisular que presentan ciertas cepas. Probablemente en el futuro se aislarán nuevas cepas de IBV que afecten a organos o sistemas no asociados normalmente con IB.

The effect of infectious bronchitis virus on chickens infected with Mycoplasma gallisepticum

Effect of nephropathogenic infectious bronchitis virus on renal function in male broiler chickens

Comparison of the nephropathogenicity of four strains of infectious bronchitis virus

Isolation of avian infectious bronchitis virus from experimentally infected chickens

A long-term study of the pathogenesis of infection of fowls with three strains of avian infectious bronchitis virus

Early pathogenesis in chicks of infection with an enterotropic strain of infectious bronchitis virus

Effects of trypsin and sodium tauroglycocholate on an enterotropic variant infectious bronchitis virus

The effects of oestrogen and progesterone on re-excretion of infectious bronchitis virus strain G in specific-pathogen-free chickens

Experimental infection with avian infectious bronchitis virus (Kagoshima-34 strain) in chicks of different ages

Comparison of the susceptibility of chicks of different ages to infection with nephrosis-nephritis causing strain of infectious bronchitis virus

A model for determining immunogenic relationships between avian infectious bronchitis virus

In vivo evaluation of the pathogenicity of field isolates of infectious bronchitis virus

Harderian gland dependency of immunoglobulin A production in the lachrymal fluid of chickens

Enterotropism and persistence of avian infectious bronchitis virus in chickens

Susceptibility of organ cultures from chicken tissues for strains of infectious bronchitis isolated from the intestine

Re-excretion of infectious bronchitis virus in chickens induced by cyclosporin

Incidence of Pasteurella haemolytica in the respiratory tract of apparently healthy chickens and chickens with infectious bronchitis

Effects of experimental chronic respiratory disease and infectious bronchitis on pullets

Localisation of a T-cell epitope within the nucleocapsid protein of the avian corona virus

Induction of antiviral immune responses by immunisation with recombinant-DNA encoded avian coronavirus nucleocapsid protein

Research priorities for the future

Infectious bronchitis in laying hens: the relationship between haemagglutination-inhibition antibody levels and resistance to experimental challenge

Jr (1954) Effects of infectious bronchitis in laying hens on egg production, percent unsettable eggs and hatchability

Jr (1956) Effects of infectious bronchitis in baby chicks

Genetic differences in susceptibility to a mixture of avian infectious bronchitis virus and Escherichia coli

Pathogenesis of H13 nephropathogenic infectious bronchitis virus

Effect of infectious bronchitis virus on the structure and composition of egg albumen

A novel infectious bronchitis strain infecting broiler chickens in Italy

Coronavirus IBV glycopolypeptides: size of their polypeptide moieties and nature of their oligosaccharides

Coronavirus IBV: further evidence that the surface projections are associated with two glycopolypeptides

Coronavirus IBV: structural characterisation of the spike protein

Evolution of avian coronavirus IBV: sequence of the matrix glycoprotein gene and intergenic region of several serotypes

Induction of humoral neutralising and haemagglutination-inhibiting antibody by the spike protein of infectious bronchitis virus

Coronavirus IBV: virus retaining spike glycopolypeptide S2 but not S1 is unable to induce virus-neutralising of hamagglutination-inhibiting antibody or induce chicken tracheal protection

Location of amino acid differences in the S1 spike glycoprotein subunitof closely related serotypes of infectious bronchitis virus

Infectious bronchitis virus: evidence for recombination within the Massachusetts serotype

Identification of the avian homologue of mammalian CD4 and CD8 antigens

Comparative nephropathogenicity of different strains of infectious bronchitis virus in chickens

Comparative nephropathogenicity of infectious bronchitis virus in bursectomised and non-bursectomised chickens

Histopathology and immunochemistry of renal lesions due to infectious bronchitis virus in chickens

The pathogenesis of nephritis in chickens induced by infectious bronchitis virus

The effect of suppression of circulating antibody on resistance to the Australian avian infectious bronchitis virus. Research in Veterinary Science

The detection of cytotoxic lymphocyte activity in chickens infected with infectious bronchitis virus or fowlpox virus

The induction and control of delayed type hypersensitivity reactions induced in chickens by infectious bronchitis virus

Pathogenesis of infectious bronchitis nephritis. 2. Studies of water and electrolyte balance in colostomised chickens

Pathogenesis of infectious bronchitis nephritis 1. Morphometric analysis of kidney proximal tubular epithelium in chickens

Duration of experimental infectious bronchitis in chickens

Recovery of infectious bronchitis virus from eggs and chicks produced by experimentally inoculated hens

Isolation of a new serotype of infectious bronchitis-like virus from chickens in England

The classification of new serotypes of infectious bronchitis virus isolated from poultry flocks in Britain between

Newly isolated serotypes of infectious bronchitis virus: their role in disease

The use of chicken tracheal organ cultures for the isolation and assay of avian infectious bronchitis virus

Infectious bronchitis immunity: its study in chickens experimentally infected with mixtures of infectious bronchitis virus and Escherichia coli

Effect of in-ovo bursectomy on the course of an infectious bronchitis virus infection in line C white leghorn chickens

Use of an infectious bronchitis virus and Escherichia coli model infection to assess the ability to vaccinate successfully against infectious bronchitis in the presence of maternally-derived immunity

The secretory antibody response in inbred lines of chickens to avian infectious bronchitis virus infection

pH stability studies with avian infectious bronchitis virus (Coronavirus) strains

Pathogenicity of four serotypes of avian infectious bronchitis virus for the oviduct of young chickens of various ages

Pathogenicity of two embryo-passage levels of avian infectious bronchitis virus for the oviduct of young chickens of various ages

Pathogenesis of oviduct lesions in immature chickens following exposure to infectious bronchitis virus at one day-old

Abnormalities in laying chickens following exposure to infectious bronchitis virus at one day-old

Characterisation of strains of infectious bronchitis virus isolated in Chile

The etiology of uraemia of chickens

Infectious avian nephrosis (uraemia) in Australia

The control of avian infectious bronchitis/nephrosis in Australia

The pathogenesis of nephritis evoked by Australian IB viruses

Assessment of cross-immunity in chickens to strains of avian infectious bronchitis virus using tracheal organ cultures

A clearance test to assess protection in chickens vaccinated against infectious bronchitis virus

Sequential development of humoral immunity and assessment of protection in chickens following vaccination and challenge with avian infectious bronchitis virus. Research in Veterinary Science

Humoral antibody response and assessment of protection following primary vaccination of chicks with maternally-derived antibody against avian infectious bronchitis virus. Research in Veterinary Science

Organ culture studies on the efficiency of infection of chicken tissues with avian infectious bronchitis virus

Growth comparisons of avian infectious bronchitis virus strains in organ cultures of chicken tissues

Taxonomic studies on strains of avian infectious bronchitis virus using neutralisation tests in tracheal organ cultures

Changes in the Harderian gland of the chicken following conjunctival and intranasal infection with infectious bronchitis virus in one and 20 day-old chickens

Influence of maternal antibodies on vaccination of chicks of different ages against infectious bronchitis virus

Effect of removal of the Harderian gland in one day-old chicks on immunity following infectious bronchitis vaccination

Study on the local effect of eye-drop vaccination against infectious bronchitis in one day-old chicks with maternal antibodies

A study on the synthesis and secretion of immunoglobulins by the Harderian gland of fowl after eye-drop vaccination against infectious bronchitis at 1 day-old

Comparison of the enzyme linked immunosorbent assay, the haemagglutination-inhibition test and agar gel precipitation test for the detection of antibody against infectious bronchitis and Newcastle disease in commercial broilers

Immunopathogenesis of infection in SPF chicks and commercial broilers of a variant infectious bronchitis virus of economic importance

Local antibody production in the oviduct and gut of hens infected with an enterotropic strain of infectious bronchitis virus

An in-vitro comparison of the virulence of seven strains of infectious bronchitis virus using tracheal and oviduct organ cultures

Protectotypic differentiation of avian infectious bronchitis viruses using an in-vitro challenge model

Effect of T-cell suppression by cyclosporin on primary and persistent infections of infectious bronchitis virus in chickens

Cross-reactive cellular immune responses in chickens vaccinated with live infectious bronchitis vaccine

Effect of heterophil depletion by 5-fluorouracil on infectious bronchitis virus infection in chickens

Swollen head syndrome associated with Escherichia coli and infectious bronchitis virus in the Central Valley of California

The isolation and characterisation of six avian infectious bronchitis virus isolated in Morocco

Experimental production of complicated chronic respiratory disease (air-sac disease)

Cellular response of the respiratory tract of chickens to infection with Massachusetts 41 and Australian T infectious bronchitis viruses

Variant serotypes of infectious bronchitis virus isolated from commercial layer and broiler chickens

Attenuation of avian infectious bronchitis virus by cold adaptation

Effects of infectious bursal disease on the responsiveness of the chickens to Mycoplasma synoviae, Newcastle disease and infectious bronchitis

Avian infectious bronchitis: demonstration of serum IgG and IgM neutralising antibody by sucrose density gradient centrifugation and mercaptoethanol reduction

Local antibody responses to avian infectious bronchitis: virus neutralising antibody in tracheobronchial secretions

Local immunity to avian infectious bronchitis virus in tracheal organ cultures

Relationship of common avian pathogen antibody-titres in so-called chicken anaemia agent (CAA)-antibodypositive chicks to titers in CAA-antibody-negative chicks

Comparison of serological tests for the measurement of primary immune response to avian infectious bronchitis virus vaccines

A new strain of infectious bronchitis virus infecting domestic fowl in Great Britain

The role of Escherichia coli in the cause of chronic respiratory disease and certain other poultry diseases

Strategy of protection of the layer against infectious bronchitis virus

Presence of viral antigens and antibody in the trachea of chickens infected with avian infectious bronchitis virus

Chemical pathology of nephrosis induced by infectious bronchitis virus

Infectious bronchitis virus strain differentiation attempts by cross-immunity studies in tracheal organ cultures derived from immunised chickens

Evaluation of the immune response to infectious bronchitis virus

Induction of chicken interferon by avian infectious bronchitis virus. Research in Veterinary Science

Influence of infectious bronchitis strains and vaccines on the incidence of Mycoplasma synoviae airsacculitis

Epidemiology of infectious bronchitis in Australia

The S1 glycoprotein but not the N or M proteins of avian infectious bronchitis virus induces protection in vaccinated chickens

Immune responses to structural proteins of avian infectious bronchitis virus

Leukocyte subpopulations in kidney and trachea of chickens infected with infectious bronchitis virus

A novel variant of avian infectious bronchitis virus resulting from recombination from three different strains

Nephrosis in laying chickens caused by Massachusetts-type of infectious bronchitis virus

Re-excretion of an enterotropic infectious bronchitis virus by hens at point of lay after experimental infection at day-old

The exposure of day-old chicks to infectious bronchitis arid subsequent development of the oviduct

The site of replication of infectious bronchitis virus in the oviducts of experimentally infected hens

Persistence of the virus in the tissues and development of the oviduct in the fowl following infection at day-old with infectious bronchitis virus. Research in Veterinary Science

Immunological difference in strains of infectious bronchitis virus. 60th meeting of the U.S. Livestock Sanitary Association

Location of antigenic sites defined by neutralising monoclonal antibodies on the S1 avian infectious bronchitis virus glycopolypeptide

Viral pathogenesis of a nephrotropic infectious bronchitis virus isolated from commercial pullets

Infectious Bronchitis

Airsacculitis in broilers from Mycoplasma synoviae: effects on air sac lesions of vaccinating with infectious bronchitis or Newcastle disease vaccines

Respiratory disease and immunity to challenge produced by Australian strains of infectious bronchitis virus

Antigenic domains on the peplomer proteins of avian infectious bronchitis virus: correlation with biological functions

The effect of 5-fluorouracil treatment of chicks: a cell depletion model for the study of avian polymorphonuclear leukocytes and natural host defences

Experimental evidence of recombination in coronavirus infectious bronchitis virus

Phylogeny of antigenic variants of avian coronavirus, infectious bronchitis virus

Sequence evidence for RNA recombination in field isolates of avian corona virus, infectious bronchitis virus

Molecular cloning and sequence comparison of the S1 glycoprotein of the Gray and JMK strains of avian infectious bronchitis virus

Comparative study of the infectivity of Belgian NIBV isolates for the kidney after intravenous inoculation in chickens

Challenge experiments to evaluate cross-protection induced at the trachea and kidney level by vaccine strains and Belgian nephropathogenic isolates of avian infectious bronchitis virus

Functional and biochemical characterisation of chicken T-lymphocyte antigens

Differentiation of IBV strains

Immunological relationship between the New Zealand A and the Australian T strains of infectious bronchitis virus as measured by cross-immunisation tests in tracheal organ cultures from immunised birds

Interferon induction by different strains of infectious bronchitis virus

Swollen head syndrome in Taiwan-isolation of an avian pneumovirus and serological survey

Tissue tropism of three cloacal isolates and Massachusetts strain of infectious bronchitis virus

Aspects of local immune response to IBV

Observations on the effects of the H52 and H120 vaccine strains of infectious bronchitis virus in the domestic fowl

An inverse age resistance of chicken kidneys to infectious bronchitis virus

Active and passive immunisation against nephritis induced by avian infectious bronchitis virus

Immunity following inoculation of H120 and H52 vaccine strains of infectious bronchitis virus into the crop of the domestic fowl

IgM responses in chicken serum to live and inactivated infectious bronchitis virus vaccines

Infectious bronchitis in laying fowls: its effect upon egg production and subsequent egg quality

The pathogenicity of an infectious bronchitis virus for laying hens, with observations on pathogenesis

Infectious bronchitis

Aspects of pathogenesis of virus diseases

Immunobiology and pathogenesis of persistent virus infections

The detection of specific IgM to infectious bronchitis virus in chicken serum using an ELISA

Comparative studies with an enzyme linked immunosorbent assay for antibodies to avian infectious bronchitis virus

Monoclonal antibodies to the S1 spike and membrane proteins of avian infectious bronchitis corona virus strain Massachusetts M41

Maternally-derived antibody to infectious bronchitis virus: its detection in chick trachea and serum and its role in protection

Comparison of enzyme linked immunosorbent assay (ELISA), haemagglutination-inhibition test and agar-gel precipitation test for detection of antibodies to avian infectious bronchitis virus

Effects of an Arkansas strain of infectious bronchitis vaccine on the head-associated lymphoid tissue (HALT)

Effects of modified live infectious bronchitis virus vaccines on the head-associated lymphoid tissue

Swollen head syndrome in broiler chickens

Effects of infectious bronchitis (Arkansas strain) on laying chickens

Effects of avian infectious bronchitis virus (Arkansas strain) on vaccinated laying chickens

Efficacy of infectious bronchitis virus vaccines against heterologous challenge

Comparative study of respiratory lesions in two chicken lines of different susceptibility , infected with infectious bronchitis virus: histology, ultrastructure and immunochemistry

Escherichia coli multiplication and lesions in the respiratory tract of chickens inoculated with infectious bronchitis virus and/or Escherichia coli

Effect of mixed live vaccine (ND and IB) and Mycoplasma gallisepticum on the chicken respiratory tract and on Escherichia coli infection

Comparison of the effects of infectious bronchitis and infectious laryngotracheitis on the chicken respiratory tract

Studies on the pathogenesis of infections with a nephropathogenic variant of infectious bronchitis virus in chickens

Epitopes on the peplomer protein of infectious bronchitis virus strain M41 as defined by monoclonal antibodies

The effect of intranasal exposure to Mycoplasma synoviae and infectious bronchitis virus on the development of lesions and agglutinins

Studies on avian infectious bronchitis virus (IBV)

Resistance of IBV to chemical and physical treatments

Studies on avian infectious bronchitis virus (IBV). III. Interferon induction by and sensitivity to interferon of IBV

Interferon induction by several strains of avian infectious bronchitis virus, a corona virus, in chickens

Comparison of the susceptibility to avian infectious bronchitis virus infection of two inbred lines of white leghorn chickens

Interferon induction by avian infectious bronchitis virus

Detection of viral antigen following exposure of one-day old chickens to the Holland-52 strain of infectious bronchitis virus

Characterisation of an infectious bronchitis virus isolated from vaccinated broiler breeder flocks

Electron microscope observations on the entry of avian infectious bronchitis virus into susceptible cells

The effect of live infectious bronchitis vaccine on the course of infectious laryngotracheitis

The susceptibility of chicken kidney and oviduct organ cultures to a vaccine strain of infectious bronchitis virus

) Isolation, characterisation and preliminary cross-protection studies with a new pathogenic avian infectious bronchitis virus (strain PL-84084)

Isolation of a turkey rhinotracheitis-like virus from chickens with swollen head syndrome

Infectious bronchitis in France: epidemiological situation and prophylactic practices

The histopathogenesis of the nephrosis-nephritis syndrome

Comparative sensitivities of oviduct and tracheal organ cultures and chicken embryo kidney cultures to infectious bronchitis virus

The replication of infectious bronchitis virus in fowl trachea. Archiv fur die Geasmte Virusforschung

The histopathology of infectious bronchitis in the domestic fowl

The histopathology of infectious bronchitis in fowls infected with a nephrotropic T strain of virus

Genetic differences among chicken embryos in response to inoculation with an isolate of infectious bronchitis virus

Lack of correlation between infectivity, serologic response and challenge results in immunisation with an avian infectious bronchitis vaccine

Co-existence of infectious bronchitis and infectious laryngotracheitis

Synergism between avian infectious bronchitis and Haemophilus gallinarum

Effect of environmental temperature on the mortality in vaccinated chickens after challenge with Australian infectious bronchitis virus

Response to several avian respiratory viruses as affected by infectious bursal disease virus

Sequence analysis of the S1 glycoprotein of infectious bronchitis viruses-identification of a novel genotypic group in Australia

An apparently new respiratory disease of baby chicks

Effects of infectious bronchitis on the reproductive tract, egg production and egg quality of laying chickens

Swollen head syndrome is not associated with turkey rhinotracheitis virus

The histopathology of an interstitial nephritis in the fowl produced experimentally with infectious bronchitis virus

The experimental infection of chickens with mixtures of infectious bronchitis virus and Escherichia coli

The response of the Harderian gland of the fowl to antigen given by the ocular route. I. Histologic changes

Cell mediated and humoral immune response of chickens to live infectious bronchitis vaccines

Cell mediated and humoral immune response of chickens to inactivated oil-emulsion infectious bronchitis vaccines

Cell mediated and humoral immune response in chickens infected with avian infectious bronchitis

Avian infectious bronchitis-specific lachrymal IgA level and resistance to challenge

Transfer of IgG from serum to lachrymal fluid in chickens

Lachrymal antibody response of specific-pathogen-free chickens and chickens with maternal immunity after infectious bronchitis virus vaccination

Avian infectious bronchitis-viral persistence in the Harderian gland and histological changes after eye-drop vaccination

Local and systemic antibody response of different chicken lines after ocular vaccination against infectious bronchitis

Infectious bronchitis

Effects of avian viruses on cultured chicken bone marrow-derived macrophages

Structural proteins and glycoproteins of infectious bronchitis virus particles labelled during growth in chick embryo cells

Embryo vaccination of chickens with infectious bronchitis virus-histologic and ultrastructural lesion response and immunological response to vaccination

Evolutionary implications of genetic variations in the S1 gene of infectious bronchitis virus

Comparison of a microneutralisation test with ELISA and precipitin tests for detection of antibodies to infectious bronchitis virus in chickens

Nephropathogenicity of infectious bronchitis virus

Some characteristics of isolates of infectious bronchitis virus from commercial vaccines

Etiology of an infectious nephritis-nephrosis syndrome of chickens

Immunity to infectious bronchitis virus from spray vaccination with derivatives of Holland strain

Vaccination against infectious bronchitis virus and immunosuppressive effects of infectious bursal disease

Relationship between several criteria of challenge-immunity and humoral immunity in chickens vaccinated with avian infectious bronchitis vaccines

Detection of infectious bronchitis virus antigen from experimentally infected chickens by indirect immunofluorescent assay with monoclonal antibodies

Influence of environment on airsacculitis: effects of relative humidity and air temperature on broilers infected with Mycoplasma synoviae and infectious bronchitis

Avian infectious bronchitis: properties and application of attenuated vaccine prepared with nephropathogenic strain AZ-23/74

RÉSUMÉ Virus de la bronchite infectieuse: immunopathogènie de l'infection chez le poulet

Bien que la bronchite infectieuse (IB) soit d'abord considérée comme une maladie du système respiratoire, différentes souches d'IBV peuvent montrer des tropismes tissulaires variables et affectent aussi l'oviducte et les reins avec des conséquences sérieuses. Quelques souches se multiplient dans l'intestin mais apparemment sans troubles pathologiques

En ce qui concerne la persistance à long terme de l'IBV, les amygdales caecales ou les reins ont été proposés pour être des sites de persistance. La variation antigénique parmi lès souches d'IBV est reliée à des différences relativement faibles au niveau de la séquence en acides aminés de la protéine de la spicule S1. Néanmoins, les études antigéniques seules ne définissent pas de façon adéquat les relations immunologiques existant entre les souches, et des essais d'immunisation croisées ont été utilisés pour classer les isolats d'IBV en 'protectotypes'. Il a été spéculé que les changements au niveau de la protéine S1 peuvent être reliés à des différences de tropismes tissulaires mises en évidence pour certaines souches