key: cord-0046367-ak24kfvu
authors: Zaki, Sherif R.; Paddock, Christopher D.
title: Viral Diseases
date: 2020-06-22
journal: Pulmonary Pathology
DOI: 10.1016/b978-0-443-06741-9.50019-7
sha: 1ac115ac2c706cd0761fff9a0b76bda1c584c55a
doc_id: 46367
cord_uid: ak24kfvu

nan

Infl uenza viruses remain the most frequently identifi ed causes of viral infection in the lung. Nonetheless, the diversity of viral agents that cause pulmonary disease is extremely broad, and continues to expand (Table 13-1) . Several newly recognized viral pathogens have been identifi ed in the past two decades that are among the most feared and lethal of all emerging infections, including those caused by Hantaviruses, Nipah virus, and SARS coronavirus. Conversely, certain viral infections, particularly those that occur in vulnerable patient cohorts, have diminished during this same interval. For example, the U.S. incidence of varicella pneumonia has declined more than 65% since universal childhood vaccination for varicella was implemented in 1995, and advances in the clinical management of transplant recipients have reduced the incidence of cytomegalovirus pneumonia.

Adenoviruses are represented by a ubiquitous and diverse group of at least 51 serotypes found naturally in the upper respiratory tracts and gastrointestinal systems of humans, other mammals, and birds. More than 50% of the known adenovirus serotypes are associated with human diseases. The others are rarely encountered and may or may not cause recognizable disease.

It is estimated that approximately 5-10% of all pneumonias in infants and young children are caused by adenoviruses. Most pediatric cases of adenovirus pneumonia occur between 6 months and 5 years of age, and serotypes 3, 7, and 21 are the most common causes of pneumonia in this patient cohort. Serotypes 3 and 7 are particularly pathogenic adenoviruses that can cause disseminated and often fatal disease in previously healthy children. In adults, pneumonia is generally associated with serotypes 3, 4, and 7. Periodic epidemics of adenovirus pneumonia in young adults have been identifi ed, particularly among military recruits. In a manner similar to other pathogens, adenoviruses take advantage of impaired or destroyed immune systems to establish persistent and disseminated infections in immunocompromised hosts. Immunocompromised patients are also susceptible to a broader range of different adenovirus serotypes. Because some adenoviruses establish latency in lymphoid tissues and the kidneys of their host, it is believed that many, possibly most, cases of clinical disease caused by adenoviruses in immunocompromised patients are reactivated infections. 

The lungs of patients with adenovirus pneumonia are heavy and edematous, and the bronchi are fi lled with mucoid, fi brinous, or purulent exudates. The mucosae of the large airways are generally hemorrhagic and congested. Necrotic and infl ammatory foci in the pulmonary parenchyma are often represented by yellow palpable nodules.

The primary histopathologic fi ndings include necrotizing bronchitis and bronchiolitis with extensively denuded epithelium, particularly in medium-sized (1-2 mm in diameter) intrapulmonary bronchi ( Figure  13-1A) . Affected airways may be occluded by homogeneous eosinophilic material, mixed infl ammatory cells, detached epithelium, and cellular debris. The lamina propria of bronchi and bronchioles is typically congested and infi ltrated by predominantly mononuclear infl ammatory cell infi ltrates. Bronchial serous and mucous glands are also often involved and show necrosis and mixed infl ammatory infi ltrates. As the infection progresses, there is involvement of the more distal pulmonary parenchyma, forming foci of bronchocentric necrosis with hemorrhage, neutrophilic and mononuclear cell infi ltrates, and karyorrhexis. These fi ndings occur against a background of diffuse alveolar damage. Adenoviruses form intranuclear inclusions in respiratory epithelial cells of the trachea, bronchi, and bronchioles, in the acinar cells of bronchial glands, and in alveolar pneumocytes, and are generally most abundant at the viable edges of necrotic foci. On hematoxylin-eosin stain, early inclusions appear as small, dense, amphophilic structures surrounded by a cleared zone and peripherally marginated chromatin, similar to herpetic inclusions. As the cellular infection progresses, the inclusion becomes larger (as large as 14 microns in some cells) and more basophilic, and the margins of the nuclear membrane become blurred to form the characteristic "smudge cell" (Figure 13 -1B).

Various methods can be used to diagnose adenovirus infections, including antigen detection (fl uorescence antibody assays and enzyme immunoassays), cell culture, electron microscopy, molecular assays, and serologic testing for group-specifi c or type-specifi c antibodies. Immunohistochemical (IHC) staining methods can detect adenovirus-infected cells in formalinfi xed, paraffi n-embedded tissues using various commercially available adenovirus group-specifi c antibodies ( Figure 13-1C) . Electron microscopy of adenovirusinfected tissues reveals a paracrystalline array of virions represented by icosahedral capsids that measure 70 to 90 nm in diameter ( Figure 13-1D ). Most adenoviruses can be isolated in cell culture from bronchial washings, tracheal aspirates, or lung biopsy specimens during the early stage of the illness. Molecular assays, particularly gene amplifi cation using polymerase chain reaction (PCR) and in situ hybridization (ISH) methods, have been developed to detect adenovirus nucleic acid in respiratory secretions and in formalin-fi xed, paraffi n-embedded tissues.

The differential diagnosis includes those agents that cause necrotizing bronchiolitis, pneumonia, and intranuclear viral inclusions, particularly herpes simplex viruses, varicella-zoster virus, and cytomegalovirus. Histologic clues to distinguish these agents from adenovirus include the presence (herpes simplex virus [HSV] and varicella zoster virus [VZV] ) or absence (adenovirus) of multinucleated cells, cytoplasmic inclusions (CMV), or distinctive smudge cells (adenovirus); however, ancillary studies are generally required for confi rmation. 

In immunocompromised patients, the case fatality rate of adenoviral pneumonia approaches 60%, compared with an approximately 15% mortality in immu-nocompetent patients. There is no proven effective antiviral therapy for adenovirus infections. Most patients receive only supportive care for symptoms of the disease, which includes cessation of immunesuppressing drugs in those patients with iatrogenic immunosuppression.

Hantaviral diseases in humans are caused by a group of closely related, trisegmented, negative-sense RNA viruses of the genus Hantavirus, of the family Bunyaviridae. Two classes of hantavirus-associated illnesses have been described: Hemorrhagic Fever Renal Syndrome, (HFRS) for disease in which the kidneys are primarily involved, and Hantavirus Pulmonary Syndrome (HPS), for disease in which the lungs are primarily affected.

The initial symptoms of HFRS and HPS are similar and resemble those seen in early phases of many other viral diseases. Fever, myalgia, headache, vomiting, weakness, and cough are common symptoms in early phases of both HFRS and HPS. Renal involvement is seen in all cases of HFRS, and the clinical presentation ranges from a mild illness with minimal renal dysfunction to a more severe form with acute renal failure and shock. Only HFRS patients who die during the later phases of renal failure typically show signifi cant pulmonary edema. The clinical picture for HPS is quite different from that for HFRS. The initial prodrome is followed by rapidly progressive pulmonary edema, respiratory insuffi ciency, and shock. In fatal cases, the majority of deaths occur within 2 days of hospitalization. Hemorrhages and peripheral signs of vasomotor instability, such as fl ushing, conjunctival injection, and periorbital edema as seen in HFRS, are extremely rare.

Chest radiographs may be normal early in the course of HPS, but evidence of interstitial edema can be observed in the majority of cases within 48 hours of hospitalization.

Large quantities of protein-rich, gelatinous retroperitoneal edema fl uid are found in the hypotensive phase of severe HFRS, while all HPS patients have large bilateral pleural effusions and heavy, edematous lungs. In fatal Far Eastern HFRS, a distinctive triad of hemorrhagic necrosis of the junctional zone of the renal medulla, right atrium of the heart, and anterior pituitary can be seen. In patients with HPS, hemorrhages are exceedingly rare, and ischemic necrotic lesions, except those attributed to shock, are not seen.

Histologically, morphologic changes of the endothelium are uncommon but, when seen, consist of prominent and swollen endothelial cells. Vascular thrombi and endothelial cell necrosis are rare. In HFRS, the most severe and characteristic microscopic lesions involve the kidney; however, an interstitial pneumonitis can also be seen in some fatal cases. In contrast, the microscopic changes in HPS are principally seen in the lung and spleen. The lungs ( Figure 13 -2) show a mild to moderate interstitial pneumonitis characterized by variable degrees of edema and an interstitial mononuclear cell infi ltrate composed of a mixture of small and enlarged mononuclear cells with the appearance of immunoblasts. Focal hyaline membranes composed of condensed proteinaceous intraalveolar edema fl uid, fi brin, and variable numbers of infl ammatory cells are observed. Typically, neutrophils are scanty and the alveolar pneumocytes are intact with no evidence of cellular debris, nuclear fragmentation, or hyperplasia. In fatal cases, with a prolonged survival interval, tissues show features more characteristic of the exudative and proliferative stages of diffuse alveolar damage ( Figure 13 -2B). Other characteristic microscopic fi ndings in HPS cases include variable numbers of immunoblasts within the splenic red pulp and periarteriolar white pulp ( Figure 13 -2E), lymph nodal paracortical zones, hepatic portal triads, and peripheral blood.

Virus-specifi c diagnosis and confi rmation can be achieved through serology, PCR for hantavirus RNA, or IHC for hantaviral antigens. Serologic testing can detect hantavirus-specifi c immunoglobulin M or rising titers of immunoglobulin G in patient sera and is considered the method of choice for laboratory confi rmation of HPS. PCR detects viral RNA in blood and tissues and is extremely useful for diagnostic and epidemiologic purposes. Hantaviral RNA can also be detected in formalinfi xed, paraffi n-embedded archival tissues by RT-PCR. IHC testing of formalin-fi xed tissues is a sensitive method to confi rm hantaviral infections, and viral antigens are found primarily within capillary endothelia throughout various tissues in both HPS and HFRS ( Figure 13 -2C). In HPS, marked accumulations of hantaviral antigens are found in the pulmonary microvasculature and in splenic and lymph nodal follicular dendritic cells. Electron microscopic studies of HPS lung tissue demonstrate infection of endothelial cells and macrophages. The virus or virus-like particles observed are infrequent and extremely diffi cult to identify in autopsy tissues; in contrast, typical endothelial granulofi lamentous inclusions are seen more frequently (Figure 13-2D ).

HPS should be suspected in cases of acute respiratory distress syndrome (ARDS) without a known precipitating cause among previously healthy individuals. The level of suspicion should be particularly high when patients have a known exposure to rodents in areas where Peromyscus maniculatus or other reservoirs of hantavirus are found. Physicians need to differentiate HPS from other common acute respiratory diseases, such as pneumococcal pneumonia, infl uenza virus, and unexplained ARDS. Diseases that need to be distinguished pathologically from HPS include a relatively large number of different viral, rickettsial, and bacterial infections, as well as various noninfectious disease processes.

Recovery in HFRS is usually complete, with no apparent long-term sequelae. Mortality rates for HFRS range from 1-15%, with shock and uremia being the main contributing causes of death, although pulmonary edema has been implicated in some patients. In HPS, mortality rates may exceed 50%, depending on serotype involved. Management of patients with HPS or HFRS is often complex and phase-specifi c. Supportive therapy, such as dialysis and circulatory and respiratory support, is the basis of treatment. Controlled studies suggest that ribavirin, a nucleoside analog, is effective in the treatment of hantaviral infection if administered early. Ribavirin has not proven effective in therapy of HPS. China, suggesting that is the more likely species to be the natural reservoir from which the SARS coronavirus emerged.

The disease causes an infl uenza-like illness that typically presents with acute onset of fever, myalgia, malaise, and chills, with rhinorrhea and sore throat being less common features. A dry cough is common, but shortness of breath and tachypnea are prominent only later in the course of the disease. Watery diarrhea occurs in some patients, typically associated with clinical deterioration in the second week of illness. People of all ages can develop the illness and children tend to have a much milder clinical course than adults. Transmission is from person to person, and the estimated incubation period is 2 to 14 days.

The radiologic features of SARS include the peripheral appearance of lung opacities, lower lobe predominance, and a mixture of ground-glass opacities, interstitial thickening, and bronchiectasis. Pneumomediastinum without preceding positive-pressure ventilation or intubation can be seen later in the disease. Multifocal peripheral subpleural ground-glass opacifi cation or consolidation has been the most commonly observed CT feature at the time of diagnosis in patients with SARS.

In fatal cases of SARS, the lungs are usually heavy and edematous with varying degrees of red and gray hepatization. Multiple bilateral hemorrhagic infarcts are commonly seen in association with subpleural hemorrhages.

The main histopathologic pattern is diffuse alveolar damage (Figure 13 -3A). Increased mononuclear cell infi ltrates in the interstitium can be seen in some cases. Other fi ndings identifi ed in some patients include focal intra-alveolar hemorrhage, necrotic infl ammatory debris in small airways, and organizing pneumonia. In addition, multinucleated syncytial cells may be seen in the alveolar spaces of some pa-tients who die 14 days or more after onset of illness ( Figure 13 -3B).

ISH and IHC studies of tissues from SARS patients have identifi ed coronavirus in ciliated columnar epithelial cells in the trachea, bronchi, and bronchioles ( Figure 13 -3C) and in pneumocytes ( Figure 13 -3D,E) and occasional macrophages in some patients. Antigens are more readily identifi ed in patients who die within the fi rst 2 weeks of onset of illness. Electron microscopic examination can show coronavirus particles and nucleocapsid inclusions in cytoplasmic vesicles and along the cell membranes of pneumocytes, in phagosomes of macrophages, and associated with fibrin in alveolar spaces. Negative stains reveal particles averaging 80-100 nm in size with a characteristic crown-like fringe on the surface (Figure 13 -3F).

The histopathologic fi ndings seen in the lungs of patients who die from SARS are somewhat nonspecifi c and can also be seen in acute lung injury cases caused by infectious agents, trauma, drugs, or toxic chemicals. Multinucleated syncytial cells similar to those seen in some SARS patients can also be found in many viral infections, including measles, parainfl uenza viruses, respiratory syncytial virus, and Nipah virus infections. An unequivocal diagnosis can be made only by laboratory tests such as viral culture, direct fl uorescent antibody, serology, PCR, or IHC.

Patients with SARS can undergo complete recovery; however, the disease can progress to acute respiratory failure and death in about 5-10% of infected individuals. About 20-30% of all patients need observation in intensive care, and most of these require mechanical ventilation. The clinical management of patients with SARS includes respiratory support with intensive care support as needed. Ribavirin, lopinavir, and type I IFN show inhibition of SARS virus in tissue culture. However, their utility in SARS-infected patients is inconclusive, and they may actually be harmful. Similarly, studies of corticosteroid use are inconclusive and again these agents may possibly cause harm to the patient. Experimental and clinical trials are needed to evaluate the effi cacy of various treatments. 

Most CMV infections are inapparent, although cases of primary infection in otherwise healthy individuals can result in a self-limited mononucleosis syndrome resembling the illness caused by Epstein-Barr virus. Pulmonary involvement in CMV mononucleosis occurs in approximately 6% of these cases. Adults and children with advanced HIV disease and recipients of hematopoietic stem cell and lung transplants are particularly at risk for developing CMV pneumonia. Before the use of CMV screening and effective anti-viral prophylaxis regimens, 10-30% of all patients undergoing allogeneic bone marrow transplantation for leukemia, and 15-55% of solid organ transplants, developed CMV pneumonia with case fatality rates greater than 80% in some series. Neonates are also at risk. Symptomatology includes fever, cough, rales, and hypoxemia. Systemic dissemination and extrapulmonary involvement can occur in some patients.

Defi nition 8 

Pulmonary CMV disease typically appears as bilateral nodular or reticular opacities on chest radiographs. Pleural effusions are identifi ed in approximately 10-30% of patients. Because some patients may be co-infected with other pulmonary pathogens, radiologic fi ndings may be confusing. Some patients with documented infection have normal radiographs.

There are several general patterns of pulmonary CMV infection. The lungs are typically heavy and may appear diffusely consolidated, or show scattered nodular foci of hemorrhage and necrosis. Rarely, CMV infection of the lungs manifests as a single pulmonary nodule.

Multiple histopathologic patterns have been reported for CMV pneumonia. Extensive intra-alveolar hemorrhage with scattered cytomegalic cells and relatively scant infl ammatory cell infi ltrates may occur. In a similar manner, extensive involvement of the alveolar epithelium with minimal infl ammation or overt evidence of parenchymal injury has also been described. Other patterns include multifocal or miliary lesions with mixed infl ammatory cell infi ltrates, hemorrhage, necrosis, and cytomegalic cells, or a diffuse, predominantly mononuclear cell infi ltrate, interstitial pneumonitis with intra-alveolar edema and fi brin deposition, and diffusely distributed cytomegalic cells. The cytomegalic changes of CMV-infected cells are evident on standard hematoxylin-eosin staining and are virtually pathognomonic of active CMV infection. The cells are enlarged (25-40 microns) and contain amphophilic to deeply basophilic intranuclear and intracytoplasmic inclusions ( Figure 13 -4A,B). The single intranuclear inclusion is comprised of viral nucleoprotein and assembled capsids, and is a large (up to 20 microns), round-to-ovoid body with a smoothly contoured border that is generally surrounded by a clear halo that gives the inclusion a distinctive "owl's eye" appearance. Cytoplasmic inclusions are small (1-3 microns), granular bodies that appear after the intranuclear inclusion is well developed and are not uniformly present in all CMV-infected cells. These inclusions represent a mixture of virions and various cellular organelles, and increase in size and number as the infection progresses. Unlike the intranuclear inclusions, the cytoplasmic inclusions stain with periodic acid-Schiff stain and are deeply argyrophilic with methenamine silver stains.

CMV pneumonia is defi ned by the presence of signs or symptoms of pulmonary disease combined with the detection of CMV in bronchoalveolar lavage fl uid or lung tissue samples. Detection methods that support this defi nition include virus isolation, histopathologic observation of cytomegalic cells, ISH, or IHC stains ( Figure 13 -4C). Detection by PCR alone is considered too sensitive for the diagnosis of CMV pneumonia and is insuffi cient for this purpose. CMV is most often cultured in human diploid fi broblasts using a shell vial method to enhance infectivity and can usually yield diagnostic results within 48 hours.

Because the histopathologic features of CMV pneumonia are varied, the differential diagnosis depends on the predominant pattern of histologic pattern (hemorrhage, miliary infl ammatory lesions, or diffuse interstitial pneumonitis). The cytopathologic changes of CMV-infected cells are generally suffi cient to establish a diagnosis. CMV inclusions may on occasion, however, be confused with those of other herpesviruses, adenoviruses, or measles, but none of these pathogens collectively shows cytomegaly, a single large nuclear inclusion with a prominent halo, and multiple small cytoplasmic inclusions. Reactive pneumocytes can occasionally show enlarged nuclei, but the nuclei will be immunonegative with IHC for CMV.

Ganciclovir, foscarnet, and intravenous CMV immune globulin remain important lines of treatment for CMV pneumonia and have diminished mortality in immunosuppressed patients with this disease. Nonetheless, mortality attributable to CMV pneumonia is approximately 50%.

Human herpes simplex viruses (HSV) are large, enveloped, double-stranded DNA-viruses approximately 100-110 nm in diameter. Two serologic types are recognized, and each is most frequently associated with particular disease syndromes; however, either serotype may cause any of the clinical syndromes associated with either serotype. HSV-1 causes gingivostomatitis, pharyngitis, esophagitis, keratoconjunctivitis, and encephalitis, and is the serotype most commonly associated with adult HSV pneumonia. HSV-2 typically infects genital sites and is the serotype associated with approximately 80% of disseminated disease and pulmonary infections in newborn infants.

HSV, like all herpesviruses, has the ability to persist in an inactive state for varying periods of time and then recur spontaneously following undefi ned stimuli associated with physical or emotional stress, trauma to nerve roots or ganglia, fever, immunosuppression, or exposure to ultraviolet radiation. Tracheobronchitis and pneumonia are the primary respiratory tract manifestations of HSV infection. In adults, infection of the respiratory tract with HSV may be associated with disseminated herpetic infection, but is more commonly identifi ed as an isolated disease manifestation resulting from reactivation of latent herpetic infections in the oropharynx. Mucocutaneous herpetic infection generally precedes HSV pneumonia, and aspiration of virus-containing secretions into the lower respiratory tract is believed to be the most frequent cause of pulmonary infection with HSV; however, oral lesions may be absent in patients with herpetic laryngotracheitis and bronchopneumonia. Disease can also be associated with airway trauma caused by tracheal intubation or from hematogenous dissemination of HSV. Newborn infants, severely immunosuppressed or burned patients, and patients with severe trauma are at greatest risk of developing HSV pneumonia. Lower respiratory tract disease in neonates is most commonly associated with disseminated herpetic infections. Most cases of neonatal disease represent primary HSV infections and are acquired during parturition from HSV-infected mothers. The incidence of neonatal HSV infection is approximately 1 in 3,200 deliveries, and disseminated disease develops in approximately 25% of infected neonates. In disseminated infections, signs and symptoms appear a mean of 5 days after birth (range, 0 to 12 days), and approximately 40-50% of these patients develop pneumonia.

Chest radiographs of patients with HSV pneumonia show ill-defi ned nodular or reticular densities of various sizes scattered in both lung fi elds. During the early stages of disease, these nodules measure 2-5 mm and are best seen in the periphery of the lungs. As the disease progresses, these lesions coalesce and enlarge to form more extensive segmental and subsegmental infi ltrates. Computed tomography shows patchy ground-glass opacities with scattered areas of consolidation and nodular densities. Pleural effusions are common.

HSV tracheobronchitis appears as 5-15 mm ulcers covered by fi brinopurulent exudate on the mucous membranes. In HSV pneumonia acquired through the airways, the lungs are heavy and show nodular hemorrhagic foci that are generally distributed around bronchi and bronchioles. In hematogenously acquired HSV pneumonia, hemorrhagic foci usually have a random or miliary distribution. 8 Human CMV is a ␤-herpesvirus with the largest genome (230 kbp) of all the herpesviruses known to infect humans 8 Multiple histopathologic patterns have been reported, including extensive intra-alveolar hemorrhage, diffuse interstitial pneumonitis, and miliary infl ammatory foci with necrosis 8 Virally induced cytopathic changes include cytomegaly (25-40 microns) and amphophilic to deeply basophilic intranuclear and intracytoplasmic inclusions in various cell types including macrophages, pneumocytes, glandular epithelium, endothelium, and fi broblasts 8 The single intranuclear inclusion is a large (up to 20 microns), round to ovoid body with a smoothly contoured border that is generally surrounded by a clear halo 8 Cytoplasmic inclusions are small (1-3 microns), stain with periodic acid-Schiff stain, and are deeply argyrophilic with methenamine silver stains 

Herpetic tracheobronchitis is an ulcerative process characterized by large areas of denuded mucosal epithelium and fi brinopurulent exudate containing necrotic cells. Despite extensive tissue damage, cells with intranuclear inclusions may be sparse and are found most often at the margins of the ulcerated epithelium or occasionally in the mucous glands below the ulcerated mucosa. In the lung, herpetic lesions show extensive necrosis and karyorrhectic debris, and are associated with hemorrhage and a sparse-to-moderate neutrophilic infi ltrate (Figures 13-5A,B) . Intranuclear inclusions are best appreciated in cells at the leading edge of necrotic foci. Inclusions appear either as homogeneous, amphophilic, and glassy (e.g., Cowdry type B inclusions), or as eosinophilic with a halo separating the inclusion from the nuclear membrane (e.g., Cowdry type A inclusions). Other changes associated with HSV, including multinucleation and nuclear molding, ground-glass nuclear chromatin, and ballooning degeneration of the cytoplasm, are more frequently associated with squamous epithelium and less often encountered in the lung.

Virus isolation remains an important diagnostic method; however, because HSV can be isolated from oropharyngeal secretions and occasionally from the lower respiratory tract of patients who lack overt pulmonary disease, virologic cultures must be interpreted in the context of complementary clinical, radiographic, and histopathologic fi ndings as much as possible. PCR methods that amplify HSV DNA from clinical specimens, including tissue and blood, can be particularly useful for distinguishing between HSV-1 and HSV-2 infections. Commercially available antibodies exist for IHC detection of HSV in tissues (Figure 13 -5C). Electron microscopy can also be used to demonstrate encapsulated viral particles with a targetoid appearance arranged in a lattice-like pattern (Figure 13-5D ).

HSV, varicella-zoster virus (VZV), adenoviruses, measles virus, and CMV can cause necrotizing hemorrhagic pneumonias, and each produce intranuclear inclusions that may be diffi cult to differentiate. The viral inclusions of HSV are identical to those of VZV; separation can be accomplished by IHC, molecular methods, or culture. Distinction from adenoviruses can be accomplished if smudge cells are identifi ed (supporting the presence of adenovirus). HSV does not produce cyto-plasmic inclusions, which should be seen in CMV and in measles.

Prior to the discovery and use of antiviral therapies, 85% of neonates with disseminated HSV disease died from the infection. With early diagnosis and high-dose acyclovir therapy, however, mortality has been reduced to approximately 30%. Foscarnet has been used effectively in some acyclovir-resistant patients.

Variclla zoster virus (VZV), is a human alpha-herpesvirus closely related to HSV. Primary infection causes varicella (chickenpox), and reactivation of latent virus causes herpes zoster (shingles). VZV is ubiquitous in human populations around the world, and humans are the only known host. During the prevaccine era in the United States, approximately 4 million cases, 4,000 to 9,000 hospitalizations, and 50 to 140 deaths were reported annually. VZV-related deaths have declined sharply in the United States, however, since universal childhood vaccination was implemented in 1995.

Primary infection with VZV occurs by inoculation of respiratory mucosa with infectious aerosols or by direct contact with skin lesions of patients with varicella or herpes zoster. After a primary viremia in the reticuloendothelial system, and secondary viremia in circulating mononuclear cells, the virus is disseminated to the skin, where it initiates a pruritic vesicular rash (chickenpox), and is disseminated back to mucosal sites in the lungs. The attack rate for previously uninfected household contacts exposed to varicella is approximately 90%. VZV also establishes latent infection within satellite cells and neurons of the trigeminal and dorsal root ganglia and can reactivate under various conditions to cause herpes zoster, a painful unilateral vesicular eruption distributed in a dermatomal distribution. Although chickenpox is usually a relatively benign infection in children, adult patients are approximately 25 times more likely than children to develop pneumonia. Pneumonia occurs in approximately 10-15% of adults primarily infected with VZV; however, the incidence of pneumonia in bone marrow transplant recipients and acute leukemia patients may be as high as 30-45%. The greatest risk of severe disease and pneumonia occurs in those patients with chronic lung disease, immune-suppressing conditions, neonates, and pregnant women. The occurrence of pneumonia during herpes zoster is rare, and limited primarily to profoundly immunosuppressed patients, particularly bone marrow transplant recipients. VZV pneumonia develops 2 to 7 days following the onset of rash and is characterized by fever, cough, tachypnea, chest pain, and hemoptysis. Massive pulmonary hemorrhage and pulmonary infarcts are frequent terminal events. Hematopoietic cell transplant recipients may present with signs of visceral dissemination and pneumonia 1 to 4 days before the localized cutaneous eruption of herpes zoster appears, and lower respiratory tract disease has been described in the absence of skin lesions, particularly in neonates and bone marrow transplant recipients.

Defi nition 8 Human herpes simplex viruses (HSV) are large, enveloped, doublestranded DNA-viruses that exist in two serologic types 8 HSV-1 is the serotype most commonly associated with adult HSV pneumonia 8 HSV-2 is the serotype associated with approximately 80% of disseminated disease and pulmonary infections in newborn infants 

The lungs show multifocal, bilateral, poorly defi ned nodular densities that measure 5-10 mm in greatest dimension. These opacities may coalesce to form more extensive areas of consolidation. Hilar adenopathy may also occur, but pleural effusions are uncommon. Some patients who survive VZV pneumonia show persistent parenchymal nodules that may mineralize and persist as small (2-3 mm) calcifi cations, predominantly in the lower zones of the lungs.

The lungs of patients with fatal VZV pneumonia are 2 to 3 times heavier than normal, fi rm, and plum-colored. There are often multiple necrotic and hemorrhagic lesions on the visceral and parietal pleura that resemble the pox lesions of skin. The trachea and bronchi are generally edematous and erythematous with occasional vesicles on the mucosal surfaces, and there may be lobular consolidation of the lungs as well as randomly distributed hemorrhagic lesions.

The lungs show interstitial pneumonitis and diffuse miliary foci of necrosis and hemorrhage in the pulmonary parenchyma involving alveolar walls, blood vessels, and bronchioles ( Figure 13-6A,B) . Other fi ndings can include intra-alveolar collections of edema, fi brin, or hemorrhage, diffuse alveolar damage, and septal edema. Virally infected cells with intranuclear inclusions may be identifi ed in respiratory epithelial cells of the trachea and bronchi, pneumocytes, interstitial fi broblasts, or capillary endothelium. Eosinophilic intranuclear inclusions and multinucleated syncytial cells may be diffi cult to locate but are best identifi ed at the edges of necrotic foci. In cases of disseminated disease, similar necrotizing hemorrhagic lesions and occasional viral cytopathic changes in epithelial cells or fi broblasts may be observed in many other tissues and organs.

Because pulmonary symptoms most often occur several days following the onset of the characteristic rash of varicella, a pathologic diagnosis is seldom required for a real-time diagnosis of VZV pneumonia. Antigen detection kits using fl uorescein-conjugated VZV monoclonal antibodies can be helpful for rapid diagnosis of cutaneous VZV infection. Antibodies are also commercially available for IHC detection of VZV in tissue specimens (Figure 13-6C) ; however, relatively few laboratories are able to provide well-validated assays. Some commercial laboratories offer PCR amplifi cation to detect viral nucleic acid in clinical specimens. Isolation of the virus in cell culture remains the reference standard for the diagnosis of VZV. Infectious VZV is usually recoverable from the clear fl uid of cutaneous vesicles of varicella for approximately 3 days after the appearance of these lesions and for approximately 1 week from herpes zoster lesions. By using electron microscopy, VZV has an icosahedral nucleocapsid that is indistinguishable in appearance from other herpesviruses. The enveloped viral particle is pleomorphic to spherical in shape and 180-200 nm in diameter.

The histopathologic appearance of VZV pneumonia most closely resembles disease caused by HSV with respect to the general pattern of lung injury (e.g., multicentric, necrotizing, and hemorrhagic lesions) and to the appearance of the glassy intranuclear inclusions.

The U.S. incidence of varicella pneumonia declined markedly since universal childhood vaccination for varicella was implemented in 1995. Vaccine effi cacy at preventing severe disease is approximately 97%. Untreated adult varicella pneumonia is fatal in approximately 10% of cases, but mortality is as high as 25% to 40% in certain high-risk cohorts, including pregnant women, transplant recipients, and neonates. Intravenous acyclovir is recommended for use in all patients for whom the risk of disseminated disease is particularly high or unpredictable, including patients with leukemia, bone marrow transplant recipients, and severely immune suppressed persons.

Infl uenza viruses belong to the Orthomyxoviridae family, and include the two important infl uenza viruses, types A and B, which are associated with signifi cant human disease. All infl uenza viruses have a segmented, negative-sense RNA core surrounded by a lipid envelope. Infl uenza A viruses are further classifi ed into subtypes 

Infl uenza viruses are spread person-to-person primarily through the coughing and sneezing of infected persons. The typical incubation period is 1 to 4 days. Adults can be infectious from the day before symptoms begin through approximately 5 days after illness onset. Children can be infectious for 10 or more days, and young children can shed virus for several days before their illness onset. Severely immunocompromised persons can shed virus for weeks or months. Respiratory illness caused by infl uenza is diffi cult to distinguish from illnesses caused by other respiratory pathogens on the basis of symptoms alone. Uncomplicated infl uenza illness is characterized by the abrupt onset of constitutional and respiratory signs and symptoms including fever, myalgia, headache, malaise, nonproductive cough, sore throat, and rhinitis. Among children, otitis media, nausea, and vomiting are also commonly reported. Infl uenza typically resolves after 3 to 7 days in most patients, although cough and malaise can persist for more than 2 weeks. Complications include secondary bacterial pneumonias, febrile seizures, and, uncommonly, encephalopathy, transverse myelitis, Reye's syndrome, myositis, myocarditis, and pericarditis. The risks for complications, hospitalizations, and deaths from infl uenza are higher among persons aged 65 years or older, young children, and persons of any age with certain underlying health conditions, than among healthy older children and younger adults.

The main fi ndings include unilateral or bilateral patchy consolidation of the lungs, which may progress to confl uent lung disease. Pleural effusions are uncommon.

Lungs in infl uenza virus pneumonia, not associated with a bacterial infection, can have different degrees of hemorrhage and edema. Airways can be fi lled with varying amounts of exudate, and the mucosae of the trachea and large bronchi are hyperemic and swollen. Cross-sections of the lungs show a more or less granular appearance, in which the lower lobes are more affected than the upper lobes. The gross pathologic features in secondary infections depend largely on the specifi c microbial (usually bacterial) pathogen involved. The mucosae of the large airways can demonstrate hyperemia, hemorrhages, or purulent necrotic debris. In the lungs, the extent of the pathologic process in the lower lobes is generally greater than the upper and may include consolidation, abscesses, hemorrhages, and empyema. Secondary infl ammation in the regional lymph nodes may be present. Purulent mediastinitis and pericarditis may also be found in some cases.

The histopathologic features of non-fatal and fatal infl uenza include necrotizing bronchitis, diffuse alveolar damage, hemorrhage, edema, and thrombi. The pathology is more prominent in larger bronchi, and infl ammation may vary in intensity (Figure 13 -7A-C). Viral inclusions cannot be identifi ed by light microscopy ( Figure  13 -7F). Secondary bacterial infections with organisms such as Streptococcus pneumoniae, group A Streptococcus, Staphylococcus aureus, and Haemophilus infl uenzae may occur as a complication in about 50-75% of fatal cases and make it diffi cult to recognize the pathologic changes associated with the primary viral infection. The histopathologic features in other organs may include myocarditis, cerebral edema, rhabdomyolysis, and hemophagocytosis (Figures 13-8A,B) .

Recent studies suggest that, unlike human infl uenza viruses, avian virus H5N1 preferentially infects cells in the lower respiratory tract of humans, resulting in extensive damage of the lungs with minimal pathology in the upper respiratory tract (Figures 13-8C,D) . This may explain why the H5N1 avian infl uenza virus is so lethal to humans but so diffi cult to spread from person to person. These studies show that the avian virus preferentially binds to the ␣-2,3 galactose receptors, which are found only in and around the alveoli. This is in contrast to the human infl uenza viruses which preferentially bind to the ␣-2,6 receptors, which are found throughout the respiratory tract from the nose to the lungs.

Because of the absence of any characteristic viral inclusions and because the overall pathologic features of infl uenza may resemble other viral, rickettsial, and certain bacterial infections, an unequivocal diagnosis can be made only by laboratory tests such as viral culture, direct fl uorescent antibody and rapid antigen assays, serology, IHC, and ISH. IHC and ISH assays can demonstrate viral antigens and nucleic acids in epithelial cells of small airways (Figure 13-7D,E,G) . Antigens are more readily identifi ed in patients who die within 3 to 4 days of onset of illness.. 

Vaccination is an important strategy in the prevention of infl uenza virus infections. Infl uenza is seldom fatal in the immunocompetent host, and recovery is usually complete. Supportive management with bed rest, hydration, and antipyretics is the basis of treatment. Antiviral agents may be helpful early in the course of illness. NA, a major antigenic determinant of infl uenza viruses, catalyses the cleavage of glycosidic linkages to sialic acid and the release of progeny virions from infected cells. Accordingly, it has become an important target for drug inhibitors such as oseltamivir and zanamivir. The M2 surface component and channel of infl uenza A (not present in infl uenza B virus) regulates the internal pH of the virus and is blocked by the antiviral drug amantidine.

Measles (rubeola) is an infectious, acute febrile viral illness characterized by upper respiratory tract symptoms, fever, and a maculopapular rash. The causative agent, a member of the genus Morbillivirus, of the family Paramyxoviridae, is an enveloped virus that contains a negative sense, single-stranded RNA genome. Measles has a worldwide distribution. Although still a signifi cant problem in underdeveloped countries, measles infection became uncommon in the United States after the development and widespread use of an effective measles vaccine. However, a recrudescence of measles infection occurred in several large U.S. urban centers in recent years, associated with reduced use of the vaccine among children and young adults.

Measles virus is highly contagious and is spread by aerosols and droplets from respiratory secretions of acute cases. Children are usually infected by 6 years of age, resulting in lifelong immunity, and almost all adults are immune either from vaccination or exposure. Clinical infection in children younger than 9 months of age is generally uncommon because of passive protection afforded the infant by the transfer of maternal antibodies, although occasional infections have occurred in this age group. A person with acute measles is infective from just before the onset of symptoms to the end of fever.

After an incubation period of about 1 to 2 weeks, the prodromal phase of measles begins with fever, rhinorrhea, cough, and conjunctivitis. Koplik's spots, which are small, irregular red spots with a bluish-white speck in the center, appear on the buccal mucosa in 50-90% of cases shortly before rash onset. An erythematous maculopapular rash begins on the face 3 to 4 days after prodromal symptoms and usually spreads to the trunk and extremities. The symptoms gradually resolve, with the rash lasting for approximately 6 days, fading in the same order as it appeared.

Chest radiographs typically show fi ne reticular and ground-glass opacities as well as nodules and patchy consolidation. Bronchial thickening and peribronchial opacities may also be observed in some patients. Pleural effusions are rare.

In fatal cases, the lungs are heavy and show congestion, hemorrhage, and edema. The gross pathologic features in secondary infections depend largely on the specifi c microbial (usually bacterial) pathogen involved.

A focal or generalized interstitial pneumonitis, similar to that seen in many other viral infections, is seen in the lungs of measles patients. Histopathologic features include various degrees of peribronchial and interstitial mononuclear cell infi ltrates, squamous metaplasia of bronchial endothelium, proliferation of type II pneumocytes, and intra-alveolar edema with or without mononuclear cell exudates and hyaline membranes. Secondary changes created by bacterial or viral superinfection or organizational changes, may alter the original pathology. The hallmark of the disease is the formation of multinucleated epithelial giant cells. These cells, which are often numerous, are formed by fusion of bronchiolar or alveolar lining cells (Figure 13-9A) . These cells generally contain characteristic nuclear and cytoplasmic inclusions. The intranuclear inclusions are homogenous, eosinophilic, and surrounded by a slight indistinct halo (Figure 13-9B) . The cytoplasmic inclusions are deeply eosinophilic and may form large masses with a "melted tallow" appearance ( Figure 13-9B) . These giant cells may undergo degenerative changes with progressive loss of cytoplasm, increasing basophilia, and shrinkage of nuclei. The presence of measles virus in these giant cells may be demonstrated by immunofl uorescence, IHC, and ISH techniques (Figures 13-9C,D) . These giant cells can also be seen in extrapulmonary tissues (Figure 13-9E) .

Laboratory confi rmation is useful to avoid possible confusion with other rash-causing illnesses. Diagnostic laboratory procedures consist of either direct detection of the virus or viral antigens, usually by indirect immuno-fl uorescence or by serologic methods using hemagglutination inhibition, neutralization, or enzyme immunoassay. Specimens for serologic testing consist of acute and convalescent-phase serum pairs. The presence of specifi c IgM antibody can be used to diagnose recent infection. IHC and ISH can be performed on tissue specimens. Ultrastructurally, measles virions are pleomorphic, generally spherical, enveloped particles from 120-250 nm in diameter, with a lipid envelope surrounding a helical nucleocapsid composed of RNA and protein.

In typical cases, the diagnosis of measles can usually be made on the basis of clinical signs and symptoms. Other causes of a similar rash, but without other features of measles, include rubella, dengue virus, enteroviruses, and drug reactions, especially to ampicillin. The histologic diagnosis is facilitated by the identifi cation of the characteristic giant cells in a setting of interstitial pneumonitis. These giant cells are not seen in all cases of measles pneumonia, however, and their absence should not exclude the diagnosis. Furthermore, other viral pathogens, such as respiratory syncytial virus, parainfl uenza, metapneumovirus, VZV, and the recently discovered Henipa viruses, may also give rise to pneumonias with giant cells and should be considered in the differential diagnosis. IHC or ISH testing can demonstrate viral antigens or nucleic acids in the majority of cases, assisting in the histologic diagnosis.

Supportive therapy, such as bed rest, oral hydration, and antipyretics usually produces rapid and complete recovery. Immune globulins can be useful if treatment is given early in the infection. Vaccination can also be helpful in the treatment regimen if given within 3 days of exposure. In a smaller number of patients, complications can arise as a result of continued and progressive virus replication, bacterial or viral superinfections, or abnormal host immune response. The most common complications are secondary bacterial pneumonia and otitis media. In these settings, specifi c antibiotic therapy is administered. Other complications include febrile convulsions, encephalitis, liver function abnormalities, chronic diarrhea, and sinusitis. Several pulmonary and central nervous system syndromes that are often fatal have been described.

Death occurs in about 1 of every 1000 measles cases; however, the risk of death and other complications is substantially increased in infants, malnourished and immunocompromised individuals, persons with underlying illnesses, and non-immunized populations in underdeveloped countries.

Human parainfl uenza viruses (HPIVs) are second only to respiratory syncytial virus (RSV) as a cause of lower respiratory tract disease in young children. HPIVs are negative-sense, nonsegmented, single-stranded, enveloped RNA viruses that possess fusion and hemagglutinin-neuraminidase glycoprotein "spikes" on their surface. The four serotypes of HPIV belong in the family Paramyxoviridae, subfamily Paramyxovirinae, and genera Respirovirus (HPIV-1 and -3) and Rubulavirus (HPIV-2 and -4) .

HPIVs are spread from respiratory secretions through close contact with infected persons or contact with contaminated surfaces or objects. Infection can occur when infectious material contacts mucous membranes of the eyes, mouth, or nose, and possibly through the inhalation of droplets generated by a sneeze or cough.

HPIVs are ubiquitous and infect most people during childhood. Serologic surveys have shown that 90-100% of children aged 5 years and older have antibodies to HPIV-3, and about 75% have antibodies to HPIV-1 and -2. The different HPIV serotypes differ in clinical presentations, with HPIV-1 and HPIV-2 most frequently associated with outbreaks of croup and HPIV-3 more often associated with bronchiolitis and pneumonia. HPIV-4 is infrequently detected, possibly because it is less likely to cause severe disease. The incubation period is generally from 1 to 7 days. The HPIVs can also cause repeated infections throughout life, usually manifested by an upper respiratory tract illness (e.g., cold and sore throat). Serious lower respiratory tract disease (e.g., pneumonia, bronchitis, and bronchiolitis) can also occur with repeat infection, especially among the elderly, and among patients with compromised immunity.

The main fi ndings associated with HPIV pneumonia include diffuse interstitial opacities, bronchial wall thickening, and peribronchial consolidation. Infection is rarely associated with pleural effusions.

In fatal cases, the lungs are typically heavy and display congestion, hemorrhage, and edema.

In patients with severe HPIV infection, multinucleated giant cells derived from the respiratory epithelium may be seen in association with an interstitial pneumonitis, diffuse alveolar damage, bronchiolitis, and organizing changes (Figure 13-10A,B) . These giant cells, which may contain intracytoplasmic eosinophilic inclusions (Figure 13-10B) , have also been reported in extrapulmonary tissues such as kidney, bladder, and pancreas.

Diagnosis of infection with HPIVs can be made by virus isolation, direct detection of viral antigens by EIA or IFA in clinical specimens, detection of viral RNA by RT-PCR, demonstration of a rise in specifi c serum antibodies, or a combination of these approaches. HPIV infections of lung can be confi rmed by IHC testing of formalin fi xed tissues (Figure 13-10C) ; viral antigens can be detected in giant cells, pneumocytes, and respiratory epithelial cells. Ultrastructural studies demonstrate variably shaped virions of varying size (ranging from 150-300 nm), with a lipid envelope surrounding a helical nucleocapsid composed of RNA and protein.

Other viral causes of giant cell pneumonia, including measles and RSV, should be considered in the histopathologic differential diagnosis, and laboratory testing, including IHC, can be useful in determining the correct diagnosis. Gender, Race, and Age Distribution 8 Infection in children younger than 9 months of age is uncommon because of passive protection from immune mothers 8 Children (non-vaccinated) are usually infected by 6 years of age 8 Natural infection results in lifelong immunity and almost all adults are immune either due to exposure or vaccination 8 No recognized gender or racial predilection Clinical Features 8 Typical incubation period is 1 to 2 weeks 8 Brief prodrome characterized by fever, rhinorrhea, cough, and conjunctivitis 8 Koplik's spots can be seen in the buccal mucosa shortly before rash onset 8 An erythematous maculopapular rash begins on the face 3 to 4 days after prodromal symptoms and usually spreads to the trunk and extremities 8 The rash lasts for approximately 6 days, fading in the same order as it appeared 8 Fine reticular and ground-glass opacities in the lungs 8 Nodules and patchy consolidation throughout the lungs may be seen 8 Rarely associated with pleural effusions Prognosis and Therapy 8 Recovery is rapid and complete in most cases 8 Supportive therapy is administered, such as bed rest, oral hydration, and antipyretics 8 Immune globulins can be useful if treatment is given early in the infection 8 Vaccination can also be helpful if given within 3 days of exposure 8 Lungs are typically heavy, congested, hemorrhagic, and edematous 8 Gross fi ndings in cases with secondary infections depend largely on the specifi c microbial (usually bacterial) pathogen involved, and may include consolidation, abscess formation, hemorrhage, and empyema 

Most HPIV infections cause a mild, self-limited illness. The highest rates of serious HPIV illnesses occur among young children. HPIV infections are also being increasingly recognized as an important cause of severe morbidity and mortality in immunocompromised adults. The mortality of bone marrow transplant patients with HPIV-3 infection has been reported to be as high as 60%. Supportive management with bed rest, oral hydration, and antipyretics is the basis of treatment. Aerosolized ribavirin has shown some effi cacy in the treatment of severe cases of HPIV infection.

RSV is a negative-sense, nonsegmented, single-stranded, enveloped RNA virus. RSV is a member of the family Paramyxoviridae, and can be further distinguished genetically and antigenically into two subgroups, A and B.

The subgroup A strains are usually associated with more severe infections.

RSV is the most common cause of bronchiolitis and pneumonia among infants and children under 1 year of age. In temperate climates, RSV infections usually occur during annual community outbreaks, often lasting several months, during the late fall, winter, or early spring months. The timing and severity of outbreaks in a community vary from year to year. RSV spreads effi ciently during the annual outbreaks, infecting as many as 50% of children in their fi rst year of life. Most children will have serologic evidence of RSV infection by 2 years of age. Illness begins most frequently with fever, runny nose, cough, and sometimes wheezing. During their fi rst RSV infection, between 25% and 40% of infants and young children have signs or symptoms of bronchiolitis or pneumonia, and 0.5-2% require hospitalization. The majority of children hospitalized for RSV infection are under 6 months of age, or are children with cyanotic congenital heart disease, cystic fi brosis, bronchopulmonary dysplasia, or immunosuppression. RSV can also cause repeated infections throughout life, and severe lower respiratory tract disease may occur at any age, especially among the elderly or among those with compromised cardiac, pulmonary, or immune systems.

Children with RSV infection most commonly show multifocal air space consolidation and peribronchial thickening. In adults, disease is characterized by bilateral interstitial opacities and multifocal consolidations.

Large and small airways can contain necrotic debris and mucus, and may show ulceration. The lungs may be heavy and diffusely fi rm and may show areas of hyperexpansion or atelectasis.

The major histopathologic changes described in fatal RSV infections are necrotizing bronchiolitis and interstitial pneumonia. Bronchial lumens and airways are usually fi lled with necrotic debris and infl ammatory cells. Airways show mixed or predominantly mononuclear infi ltrates with hyperplastic epithelial changes (Figure 13-11A) . These fi ndings may be accompanied by diffuse alveolar damage ( Figure 13-11E) , and secondary bacterial superinfection. Giant cell pneumonia is seen in some cases ( Figure  13-11C) . The multinucleated giant cells represent epithelial cells in bronchi, bronchioles, and alveoli, and sometimes contain irregular, intracytoplasmic, eosinophilic inclusions surrounded by a clear halo (Figures 13-11C,D) .

Diagnosis of RSV infection can be made by virus isolation, direct detection of viral antigens in clinical specimens by EIA, IFA, or IHC ( Figure 13-11B, F) , detection of viral RNA by RT-PCR, or demonstration of a rise in RSV-specifi c serum antibodies. The virus is labile and attempts at culture isolation are often unsuccessful if there is delay or mishandling of the clinical specimen. Ultrastructural studies reveal virions of variable shape and size that range from 120 to 300 nm, with numerous 12-nm glycoprotein spikes.

Other viral causes of giant cell pneumonia should be considered in the histopathologic differential diagnosis, primarily parainfl uenza viruses and measles viruses. Herpes simplex and varicella zoster viruses less commonly produce multinucleated giant cells in the lung.

Mortality in otherwise healthy children hospitalized for RSV pneumonia is less than 1%. However, the disease is fatal in as many as 15-40% of patients with immune suppression or underlying disease. Mortality is greatest in infants with congenital heart disease and pulmonary hypertension, where it approaches 70%. At present, the only antiviral drug with in vitro effi cacy against RSV is ribavirin.

Human metapneumovirus (HMPV), fi rst identifi ed in 2001 from clinical specimens obtained from patients with acute respiratory illnesses, is a negative-sense, non-segmented, single-stranded, enveloped RNA virus. HMPV has been categorized in the family Paramyxoviridae, subfamily Pneumovirinae, genus Metapneumovirus. HMPV can be further distinguished genetically and antigenically into two subgroups, A and B.

HMPV infection is ubiquitous and occurs during infancy and early childhood, with annual epidemic peaks occurring late in the winter and spring months in temperate regions, often overlapping in part or in whole with the annual RSV epidemic. Seroprevalence studies reveal that 25% of all children aged 6 to 12 months have antibodies to HMPV; by age 5 years, 100% of patients have evidence of past infection. The incubation period is generally from 2 to 8 days. Although HMPV has been associated with a spectrum of respiratory disease, most infections cause a mild, self-limited illness. The patient may be asymptomatic, or symptoms may range from mild upper respiratory tract illness to severe bronchiolitis and pneumonia. During their fi rst HMPV infection, about 10-15% of infants and young children have signs or symptoms of bronchiolitis or pneumonia. About one-half of the cases of lower respiratory illness in children occur in the fi rst 6 months of life, suggesting that young age is a major risk factor for severe disease. Underlying pulmonary disease, especially asthma, may increase the risk of hospitalization for HMPV pneumonia. Like RSV and the HPIVs, studies suggest that HMPV may also contribute to respiratory disease in elderly adults and the immunocompromised.

Radiographic fi ndings include interstitial infi ltrates with focal consolidation commonly involving the lower lobes of the lung. 

In fatal cases, the lungs are typically heavy and display congestion, hemorrhage, and edema.

Histopathologic descriptions are few, and assessment of their validity is complicated by the uncertainty, in some cases, of the clinical signifi cance of detecting this ubiquitous virus. Nonetheless, BAL specimens collected from patients within a few days of a positive HMPV assay show degenerative changes and cytoplasmic inclusions within epithelial cells, multinucleated giant cells, and histiocytes. The intracytoplasmic inclusions are ill-defi ned, eosinophilic structures that measure 3-4 m. Necrotizing bronchiolitis may be found on lung biopsy. Lung tissue later in the disease shows chronic airway infl ammation, intra-alveolar foamy and hemosiderin-laden macrophages, acute and organizing lung injury, and organizing pneumonia (Figure 13 -12A-C). In such cases, typical multinucleated giant cells or viral inclusions cannot be identifi ed. ISH studies on a limited number of human cases suggest infection of alveolar and bronchial epithelial cells.

HMPV is diffi cult to identify with commonly used viral diagnostic procedures. The virus replicates slowly in primary and tertiary monkey kidney cell lines, and cytopathic effects can be diffi cult to discern. Antibodies to HMPV are not widely available; however, they can be used for identifi cation of the virus by IFA. Most HMPV studies have been conducted using RT-PCR assays or by demonstration of a rise in HMPV-specifi c serum antibodies. The enveloped virion is variable in shape and size and ranges from 150 to 300 nm.

Other viral causes of giant cell pneumonia and diffuse alveolar damage, including measles, RSV, HPIV, measles, VZV, and HSV, may be considered, as well as noninfectious causes of diffuse alveolar damage. Laboratory testing, including IHC and ISH, can be useful in making this differentiation possible.

Supportive management with bed rest, oral hydration, and antipyretics is the basis of treatment, and usually leads to complete recovery. There are no licensed therapies or prophylactic treatments for HMPV at this time.

Ribavirin and intravenous immunoglobulin, which have activity against RSV, were tested against HMPV in vitro and were found to have equivalent activity against HMPV and RSV.

Hendra and Nipah viruses belong to the recently designated genus Henipavirus within the family Paramyxoviridae, subfamily Paramyxovirinae, and are nonsegmented, negative-stranded RNA viruses. These zoonotic pathogens were fi rst identifi ed in Australia and Malaysia and have been associated with acute febrile encephalitis and respiratory tract disease.

Hendra was identifi ed in 1994 when patients who came in close contact with sick horses developed an infl uenza-like illness with fever, myalgia, headache, lethargy, sore throat, nausea and vomiting. Two patients died with pneumonitis and multiorgan failure. The closely related Nipah virus was identifi ed during an outbreak in Malaysia and Singapore during 1998-1999 that included more than 250 patients. The incubation period ranged from 2 days to 1 month, but in most cases lasted between 1 and 2 weeks. Patients presented with a severe acute encephalitic syndrome, but some also had significant pulmonary manifestations. In Bangladesh in 2001 and 2003, outbreaks of Nipah encephalitis occurred. Similar to the Malaysian outbreak, the most prominent symptoms were fever, headache, vomiting, and an altered level of consciousness. Respiratory illness was much more common in the Bangladesh cases, however, with 64% having cough and dyspnea. Epidemiologic and laboratory investigations identifi ed fruit bats of the Pteropus genus as asymptomatic carriers of Hendra and Nipah viruses and possible animal reservoirs. Food-borne transmission has also been reported in an individual who consumed fruit contaminated by Pteropus bats.

In patients with respiratory illness, chest radiographs reveal bilateral infi ltrates consistent with ARDS.

In fatal infections, the lungs are heavy, congested, edematous, and hemorrhagic.

Histopathologic fi ndings in fatal cases of Hendra and Nipah infections are similar, with varying degrees of central nervous system and respiratory tract involvement. Findings include a systemic vasculitis with extensive thrombosis, endothelial cell damage, necrosis, and syncytial giant cell formation in affected vessels (Figures 13-13A,B) . Multinucleated giant cells with intranuclear inclusions can occasionally be seen in lung, spleen, lymph nodes, and kidneys. In the lung, vasculitis and fi brinoid necrosis can be seen in the majority of cases (Figure 13-13A) . Multinucleated giant cells with intranuclear inclusions are usually noted in alveolar spaces adjacent to necrotic areas (Figure 13-13C ).

The diagnosis of Nipah virus infection, suspected by patient history and clinical manifestations, can be supported by characteristic histopathological fi ndings. The most unique histopathologic fi nding is the presence of syncytial and parenchymal multinucleated endothelial cells. However, this feature occurs in only about onefourth of the cases and cannot be used as a sensitive criterion for the diagnosis of Henipa virus infections; fur-thermore, similar cells can also be seen in measles virus, RSV, HPIV, herpesviruses, and other infections. In addition to these viral infections, other non-infectious causes of diffuse alveolar damage may also be considered in the differential diagnosis. Unequivocal diagnosis can be made only by laboratory tests such as IHC, cell culture isolation, PCR, or serology. IHC can reveal widespread presence of Nipah virus antigens in endothelial and smooth muscle cells of blood vessels as well as in various parenchymal cells (Figures 13-13D-F) . Ultrastructural studies can also demonstrate the pleomorphic viral particles which are composed of helical nucleocapsids enclosed within an envelope.

Only 3 persons are known to have been infected with Hendra virus, and 2 of them died. Death occurs in about 30-40% of patients infected with Nipah virus and is more frequent in patients with rapidly developing severe neurologic signs. Residual neurologic signs are common among survivors. Treatment is supportive, including mechanical ventilation for patients in a deep 8 8 Necrotizing bronchiolitis that evolves to chronic bronchiolitis has been described, as well as interstitial pneumonitis, acute or organizing diffuse alveolar damage, and increased intra-alveolar macrophages 8 Organizing DAD and chronic airway disease in patients who die later in the course of the illness 8 BAL specimens may show multinucleated giant cells with cytoplasmic inclusions Ultrastructural Features 8 The virion is variable in shape and size, ranging from 150-300 nm, and is morphologically indistinguishable from other members of the Paramyxoviridae family when viewed by negative-stain electron microscopy Pathologic Differential Diagnosis 8 Other viral causes of giant cell pneumonia, necrotizing bronchiolitis, and interstitial pneumonitis and diffuse alveolar damage coma who are unable to maintain airways. Ribavirin was used in humans during the Nipah outbreak in Malaysia, with equivocal results.

The combination of fever and hemorrhage can be caused by different viruses, rickettsiae, bacteria, protozoa, and fungi. However, the term "viral hemorrhagic fever" (VHF) is usually reserved for systemic infections characterized by fever and hemorrhage caused by a special group of viruses transmitted to humans by arthropods and rodents. VHFs are febrile illnesses characterized by abnormal vascular regulation and vascular damage and are caused by small, lipid-enveloped RNA viruses. This syndrome can be caused by RNA viruses belonging to four different families that differ in their genomic structure, replication strategy, and morphologic features (Arenaviridae, Bunyaviridae, Flaviviridae, and Filoviridae). Arenaviruses, bunyaviruses, and fi loviruses are negative-stranded, whereas fl aviviruses are positive-stranded RNA viruses. Hemorrhagic fever viruses are distributed worldwide, and the diseases they cause are traditionally named according to the location where they were fi rst described. The oldest and best known is yellow fever virus; others include Lassa fever, lymphocytic choriomeningitis, Ebola, and dengue viruses. The distributions of the individual VHFs are related to the distributions of their specifi c arthropod and rodent vectors.

Defi nition 8 Hendra and Nipah viruses are nonsegmented, negative-stranded RNA viruses, members of the family Paramyxoviridae and subfamily Paramyxovirinae Incidence and Location 8 Rare cases have occurred in Australia and Asia throughout the distribution of the animal reservoir, the fruit bat of the Pteropus genus 8 Infection has occurred primarily in patients who have contact with sick horses and pigs 8 Food-borne transmission has also been reported in an individual who consumed fruit contaminated by Pteropus bats 8 Death occurs in about 30-40% of Nipah cases associated with rapidly developing severe neurologic signs 8 2 of 3 of the known Hendra cases died with pneumonitis and multiorgan failure 

VHF is characterized clinically by its disproportionate effect on the vascular system. Typical manifestations are related to a loss of vascular regulation (vasodilatation and hypotension), vascular damage (leakage of protein into the urine, edema in soft tissues of the face and other loose connective tissues, and petechial hemorrhage in the skin and internal organs), and severe systemic derangement that presents as fever, myalgia, and asthenia proceeding to a state of prostration. Hemorrhage is common with most of these diseases and usually originates from mucosal surfaces. Patients with severe hemorrhagic fever generally develop shock, diffuse bleeding, and central nervous system dysfunction.

In patients with respiratory illness, chest radiographs may reveal bilateral interstitial and alveolar edema and hemorrhage.

In fatal VHF, the lungs show congestion, hemorrhage, and edema. Pleural effusion may be found with certain infections.

At autopsy, common fi ndings include widespread petechial hemorrhages and ecchymoses involving skin, mucous membranes, and internal organs. However, in many VHF patients manifestations of bleeding may be minimal or absent. Effusions, occasionally hemorrhagic, are also frequently seen. Widespread, focal, and sometimes massive necrosis is commonly observed in all organ systems and is often ischemic in nature. Necrosis is usually most prominent in the liver and lymphoid tissues. The most consistent microscopic feature is found in the liver and consists of multifocal hepatocellular necrosis with cytoplasmic eosinophilia, Councilman bodies, nuclear pyknosis, and cytolysis ( Figure 13-14D ). Infl ammatory cell infi ltrates and necrotic areas are usually mild and, when present, consist of neutrophils and mononuclear cells. Commonly observed histopathologic changes in the lung include various degrees of hemorrhage, intra-alveolar edema, interstitial pneumonitis, and diffuse alveolar damage (Figure 13-14A,E,H) .

The diagnosis of VHF should be suspected in patients with appropriate clinical manifestations returning from an endemic area, particularly if there is travel to rural areas during seasonal or epidemic disease activity. The diagnosis suspected by history and clinical manifestations can also be supported histopathologically. However, because of similar pathologic features seen in VHF and a variety of other viral, rickettsial, and bacterial infections, as well as noninfectious causes of hemorrhage, edema, and diffuse alveolar damage, unequivocal diagnosis can be made only by laboratory tests such as cell culture isolation, serology, PCR, and IHC ( Figure  13 -14B,F,G,I,J). Ultrastructural studies can also demonstrate the presence of virions. All viruses have a lipid envelope that is acquired by budding at either the cell surface or the internal membranes. The size and shape of these viruses vary from relatively small (35-50 nm), uniform, round particles, as seen with fl aviviruses, to more pleomorphic, rod-shaped particles (measuring occasionally up to 15,000 nm) in the case of fi loviruses (Figure 13-14C ).

Case mortality ranges from about 15% with infections such as Lassa fever up to 90% with fi lovirus infections such as Ebola. Treatment depends on the particular agent and may include the use of passive antibodies, antiviral drugs such as ribavirin, or supportive therapy. Supportive therapy should include the reasonable measures that would be employed in any very ill patient with a fragile vascular bed. Volume replacement may be particularly important in some patients, especially with dengue hemorrhagic fever. 

Pathology of Infectious Disease

Viral infections

Viral community-acquired pneumonia in nonimmunocompromised adults

Katzenstein and Askin's Surgical Pathology of Non-neoplastic Lung Diseases

American Registry of Pathology and the Armed Forces Institute of Pathology

Microbiology and Microbial Infections: Virology

Electron Microscopy in Viral Diagnosis

Becroft DM. Histopathology of fatal adenovirus infection of the respiratory tract in young children

Fatal pneumonia associated with adenovirus type 7 in three military trainees

Adenoviruses in the immunocompromised host

Fatal disseminated adenovirus infections in immunocompromised patients

Adenovirus pneumonia

Hantavirus pulmonary syndrome: a clinical description of 17 patients with a newly recognized disease. The Hantavirus Study Group

The pathology of thirty-nine fatal cases of epidemic hemorrhagic fever

Hantavirus pulmonary syndrome in the United States: a pathological description of a disease caused by a new agent

Hantavirus pulmonary syndrome. Pathogenesis of an emerging infectious disease

Retrospective diagnosis of hantavirus pulmonary syndrome, 1978-1993: implications for emerging infectious diseases

Analysis of deaths during the severe acute respiratory syndrome (SARS) epidemic in Singapore: challenges in determining a SARS diagnosis

The clinical pathology of severe acute respiratory syndrome (SARS): a report from China

Lung pathology of severe acute respiratory syndrome (SARS): a study of 8 autopsy cases from Singapore

Ultrastructural characterization of SARS coronavirus

A novel coronavirus associated with severe acute respiratory syndrome

Lung pathology of fatal severe acute respiratory syndrome

Immunohistochemical, in situ hybridization, and ultrastructural localization of SARS-associated coronavirus in lung of a fatal case of severe acute respiratory syndrome in Taiwan

Tissue and cellular tropism of the coronavirus associated with severe acute respiratory syndrome: an in-situ hybridization study of fatal cases

Cytomegalovirus pneumonia in bone marrow transplant recipients: miliary and diffuse patterns

Cytomegalovirus-induced alveolar hemorrhage in patients with AIDS: a new clinical entity?

Cytomegalovirus pneumonia in transplant recipients

Defi nitions of cytomegalovirus infection and disease in transplant recipients

Cytomegalovirus pneumonitis in patients with AIDS: fi ndings in an autopsy series

Herpes Simplex Viruses

Varicella zoster and herpes simplex virus pneumonias

Neonatal herpes simplex infection

Necrotizing tracheobronchitis and bronchopneumonia consistent with herpetic infection

Herpes simplex virus pneumonia: clinical, virologic, and pathologic features in 20 patients

Pathology of neonatal herpes simplex virus infection. Perspect

Varicella Zoster Virus

Varicella-zoster virus pneumonitis

Risk factors and outcome of varicella-zoster virus pneumonia in pregnant women

Varicella pneumonia

Varicella pneumonia in adults: report of seven cases and a review of literature

Severe forms of chickenpox in adults

Infl uenza Viruses

Histopathologic and immunohistochemical features of fatal infl uenza virus infections in children during the 2003-2004 season

Immunohistochemical and in situ hybridization studies of infl uenza A virus infection in human lungs

Bacteriology and histopathology of the respiratory tract and lungs in fatal Asian infl uenza

Changes in the respiratory mucosa resulting from infection with infl uenza virus B

Asian infl uenza A in Boston, 1957-1958: I. Observations in thirty-two infl uenzaassociated fatal cases

Infl uenza A virus infection complicated by fatal myocarditis

Avian fl u: infl uenza virus receptors in the human airway

Probable person-to-person transmission of avian infl uenza A (H5N1)

H5N1 virus attachment to lower respiratory tract

Pathologic features of lung biopsy specimens from infl uenza pneumonia cases

Measles

Measles giant cell pneumonia in an adult following long-term chemotherapy

Measles pneumonia and the nature of the inclusion-bearing giant cells: a light-and electron-microscope study

Fatal measles infection in children with leukemia

Virus pneumonia following measles: a virological and histological study of autopsy material

Coffi n CM. Disseminated measles infection after vaccination in a child with a congenital immunodefi ciency

Analysis of viral antigens in giant cells of measles pneumonia by immunoperoxidase method

Fatal measles (rubeola) pneumonia in adults

Parainfl uenza

Parainfl uenza virus pneumonitis in an adult

Human parainfl uenza virus giant cell pneumonia following cord blood transplant associated with pulmonary alveolar proteinosis

Parainfl uenza pneumonia in severe combined immunodefi ciency disease

Giant cell pneumonia associated with parainfl uenza virus type 3 infection

Pathology of parainfl uenza virus infection in patients with congenital immunodefi ciency syndromes

Giant cell pneumonia caused by parainfl uenza type 3 in a patient with acute myelomonocytic leukemia

Parainfl uenza virus respiratory infection after bone marrow transplantation

Respiratory Syncytial Virus

Giant cell pneumonia due to respiratory syncytial virus. Occurrence in severe combined immunodefi ciency syndrome

Respiratory syncytial virus infection in immunocompromised adults

Demonstration of respiratory syncytial virus in an autopsy series

Human Metapneumovirus

Human metapneumovirus in a haematopoietic stem cell transplant recipient with fatal lower respiratory tract disease

Experimental human metapneumovirus infection of cynomolgus macaques (Macaca fascicularis) results in virus replication in ciliated epithelial cells and pneumocytes with associated lesions throughout the respiratory tract

Detection of severe human metapneumovirus infection by real-time polymerase chain reaction and histopathological assessment

A newly discovered human pneumovirus isolated from young children with respiratory tract disease

Nipah virus: a recently emergent deadly paramyxovirus

Isolation of Hendra virus from pteropid bats: a natural reservoir of Hendra virus

Comparative pathology of the diseases caused by Hendra and Nipah viruses

A Morbillivirus that caused fatal disease in horses and humans

Fatal encephalitis due to novel paramyxovirus transmitted from horses

Outbreak of Nipah-virus infection among abattoir workers in Singapore

Nipah virus infection: pathology and pathogenesis of an emerging paramyxoviral zoonosis

Hemorrhagic Fever Viruses

Pathology of Thailand haemorrhagic fever: a study of 100 autopsy cases

Dengue haemorrhagic fever: a pathological study

Immunohistochemical and in situ localization of Crimean-Congo hemorrhagic fever (CCHF) virus in human tissues and implications for CCHF pathogenesis

Pathologic and virologic study of fatal Lassa fever in man

Viral hemorrhagic fevers

Defi nition 8