Despite major advances in basic and applied research and the availability of several vaccines, viral diseases still account for a large proportion of the human infectious disease burden. Many viruses cause self-limiting and relatively mild infections, but several, including human immunodeficiency virus and influenza virus, are responsible for millions of deaths every year throughout the world. Several factors contribute to the enormous impact that viruses have on human health. For example, there are very few therapeutic options available for the treatment of viral infections, and many of those that are available possess a limited spectrum of activity or are designed for the treatment of diseases caused by specific viruses (e.g., oseltamivir is intended for the treatment of influenza only). In addition, the rapid evolution of viruses has led to the emergence of drug-resistant strains against which no currently available therapeutics are effective. Coupled with these and other issues are the appearance of never before seen viruses and the emergence of known but previously underappreciated viruses. Since the beginning of the twenty-first century, numerous “new” viruses, including the coronaviruses responsible for severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS), the 2009 pandemic influenza A virus, and Lujo hemorrhagic fever virus, have made their debut and have proved to be formidable threats to human health. Recently, the appearance of Ebola virus (Zaire ebolavirus) in West Africa, a region that has not previously seen an outbreak of this virus, was marked by an epidemic that afflicted nearly 30,000 individuals and killed more than 11,000 of those who were infected. Most recently, the far-reaching and rapid spread of Zika virus, a mosquito-borne virus that was discovered in the 1940s in Uganda, in the Western Hemisphere has invoked considerable public and scientific attention and has given rise to perhaps the largest concerted effort by scientists to rapidly develop a vaccine to halt the transmission of a virus. Each of these points underscores the importance of further research into improved surveillance, diagnosis, treatment, and prevention of viral diseases.
Virus Inclusion body Viral factories Syncytium Virion Cytopathic effect Councilman body
Despite major advances in basic and applied research and the availability of several vaccines, viral diseases still account for a large proportion of the human infectious disease burden. Many viruses cause self-limiting and relatively mild infections, but several, including human immunodeficiency virus and influenza virus, are responsible for millions of deaths every year throughout the world. Several factors contribute to the enormous impact that viruses have on human health. For example, there are very few therapeutic options available for the treatment of viral infections, and many of those that are available possess a limited spectrum of activity or are designed for the treatment of diseases caused by specific viruses (e.g., oseltamivir is intended for the treatment of influenza only). In addition, the rapid evolution of viruses has led to the emergence of drug-resistant strains against which no currently available therapeutics are effective. Coupled with these and other issues are the appearance of never before seen viruses and the emergence of known but previously underappreciated viruses. Since the beginning of the twenty-first century, numerous “new” viruses, including the coronaviruses responsible for severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS), the 2009 pandemic influenza A virus, and Lujo hemorrhagic fever virus, have made their debut and have proved to be formidable threats to human health. Recently, the appearance of Ebola (Zaire ebolavirus) virus in West Africa, a region that has not previously seen an outbreak of this virus, was marked by an epidemic that afflicted nearly 30,000 individuals and killed more than 11,000 of those who were infected. Most recently, the far-reaching and rapid spread of Zika virus, a mosquito-borne virus that was discovered in the 1940s in Uganda, in the Western Hemisphere has invoked considerable public and scientific attention and has given rise to perhaps the largest concerted effort by scientists to rapidly develop a vaccine to halt the transmission of a virus. Each of these points underscores the importance of further research into improved surveillance, diagnosis, treatment, and prevention of viral diseases.
The diagnosis of viral infections has traditionally relied upon the observation of clinical signs and symptoms alone or in conjunction with analysis of clinical specimens. Historical laboratory methods, including direct analysis of clinical specimens by transmission electron microscopy, immunofluorescence, and inoculation of cultured cells for recovery of infectious viruses have largely been supplanted by rapid immunoserologic and molecular methods that provide definitive answers in as little as a few hours or less. Despite their waning presence in clinical laboratories, these historical methods remain important for the detection and characterization of rare or new viruses and for basic virology research, among other things. The microscopic examination of fixed and paraffin-embedded tissue and fixed cell concentrates also plays an important role in the diagnosis of many viral infections. The recognition of virus infection–associated tissue architectural and cytologic changes (cytopathic effects) and the judicious use of immunohistochemical methods afford anatomic pathologists the ability to provide clinically meaningful information to healthcare providers regarding the type and extent of a viral infection. Chief among the requirements for optimal visualization of virus-induced histopathology and cytopathology is the procurement of high-quality clinical specimens, such as tissue biopsy specimens and body fluids. Sections of tissue destined for histopathologic examination should immediately be placed into an appropriate fixative (e.g., 10% buffered formalin) and allowed to thoroughly fix prior to processing, staining, and examination. Specimens for other viral diagnostic studies, including viral cultures, may not be possible using formalin-fixed specimens; therefore separate sections of tissue should be submitted in transport media appropriate for the desired test(s). The same general considerations should be given when bodily fluids or exfoliated cell preparations are harvested for cytopathologic analysis.
In this chapter, representative photomicrographs of histopathologic and cytopathologic features of several viral infections are presented with brief descriptions of the viruses that cause them. Viral infections with readily detectable and distinguishing features that are likely to be encountered by anatomic pathologists in routine private and academic settings are presented along with a few examples of uncommonly encountered viral diseases.
3.1 Viral Classification and Genomics
Viruses are most commonly grouped by the type of nucleic acid, either DNA or RNA, that comprises their genomes and by the means by which their genomes are transcribed into messenger RNA. So far, seven distinct viral genome types are known and, to date, all characterized viruses possess one of these seven types. DNA genomes can exist either as double-stranded (dsDNA), gapped-double-stranded (g-dsDNA), or as single-stranded molecules (ssDNA). RNA genomes can also be double-stranded (dsRNA) or single-stranded (ssRNA). ssRNA genomes are further divided into three distinct types, including negative-sense ssRNA (−ssRNA), positive-sense ssRNA (+ssRNA), and positive-sense ssRNA that is reverse transcribed into a DNA intermediate (+ssRNA-RT). This generalized system for categorization of viruses is the backbone of the most widely used grouping system for viruses, the Baltimore classification system.
Viruses are also formally categorized according to conventional Linnaean taxonomic classification into orders, families, subfamilies, genera, and species. As of this writing and according to the latest virus taxonomy release of the International Committee on Taxonomy of Viruses, there are seven accepted orders, 122 families (of which 84 are not assigned to an order), 35 subfamilies, and many genera and species. Most of these viruses are not human pathogens, however. Examples of human viruses categorized according to genome type and formal taxonomic naming are listed in Table 3.1. For the most up-to-date information regarding viral systematics, please refer to the viral taxonomy listing of the International Committee on Taxonomy of Viruses (https://talk.ictvonline.org/taxonomy/).
Human viruses according to genome type, ranked according to formal taxonomic hierarchy
Herpes simplex virus-1
Human alphaherpesvirus 1
Human mastadenovirus A–G
Hepatitis B virus
Hepatitis B virus
Human parvovirus B19
Primate erythroparvovirus 1
Torque teno virus
Torque teno virus 1
Colorado tick fever virus
Colorado tick fever virus
Influenza A virus
Influenza A virus
Hepatitis C virus
Human immunodeficiency virus-1
Human immunodeficiency virus 1
Human T-lymphotropic virus-1
Primate T-lymphotropic virus 1
3.2 Human Adenoviruses
In immunocompetent hosts, human adenoviruses (HAdVs) are common causes of usually self-limited respiratory tract and extrapulmonary infections, including coryza, conjunctivitis, and gastroenteritis; however, severe respiratory tract infections have been reported in otherwise healthy adults. Infections of immunosuppressed patients can lead to severe and sometimes fatal infections such as pneumonia, neurologic diseases, and diseases of the gastrointestinal and urinary tracts. Currently, there are 57 HAdV types spanning seven species (Human mastadenovirus A-G) that belong to the genus Mastadenovirus. Like all members of the family Adenoviridae, HAdV possess monopartite, linear dsDNA genomes that are covalently linked to a terminal protein (TP) at the 5′-ends of both DNA strands. HAdV genomes measure approximately 36 kilobase pairs (kbp) in length and encode roughly 40 proteins. Once susceptible and permissive host cells are infected, viral replication and particle assembly occurs within the host cell nucleus (Figs. 3.1 and 3.2).
3.3 Human Herpesviruses
The family Herpesviridae is currently organized into three subfamilies, including Alphaherpesvirinae, Betaherpesvirinae, and Gammaherpesvirinae, and a single unassigned genus. Scattered among these subfamilies are nine human herpesviruses (see Table 3.2). Although it is not considered a human herpesvirus, Macacine alphaherpesvirus 1 (herpes virus B or “B” virus) is a zoonotic agent that is associated with severe and often fatal central nervous system infections in human hosts that acquire the virus through bites from infected macacine nonhuman primates (e.g., rhesus macaques). Virions are enveloped and contain an amorphous tegument within the space between the viral membrane and the nucleocapsid. Herpesvirus genomes contain linear, monopartite dsDNA that ranges in length from approximately 120–140 kbp. As with many other DNA viruses, herpesvirus replication occurs within the nucleus, and infection with many of these viruses results in obvious cytopathic effects. Many herpesviruses, including HSV and CMV, can be detected by immunohistochemistry (Figs. 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 3.10, 3.11, and 3.12).
Human herpesviruses according to common name, formal taxonomic hierarchy, and common disease associations
Chickenpox (varicella), shingles (zoster), CNS diseases, disseminated infections, etc.
Human gammaherpesvirus 4
Infectious mononucleosis (IM), Burkitt lymphoma, Hodgkin lymphoma, etc.
Human betaherpesvirus 5
IM/glandular fever-like syndrome, various infections of the immunosuppressed, etc.
Human betaherpesvirus 6A
Roseola, IM-like syndrome, encephalitis, mesial temporal lobe epilepsy, various infections of the immunosuppressed, etc.
Human betaherpesvirus 6B
Roseola, IM-like syndrome, encephalitis, mesial temporal lobe epilepsy, various infections of the immunosuppressed, etc.
Human betaherpesvirus 7
Roseola, febrile seizures, pityriasis rosea (possibly), various infections of the immunosuppressed, etc.
Kaposi sarcoma-associated herpesvirus (HHV-8)
Human gammaherpesvirus 8
Febrile illness with rash, Kaposi sarcoma
3.4 Human Papillomaviruses
The family Papillomaviridae is comprised of 49 genera and numerous species of nonenveloped, dsDNA viruses containing genomes of approximately 8 kbp in length that produce particles that measure approximately 60 nm in size. Interestingly, papillomaviral genomes are complexed with cellular histones, a feature also seen in polyomaviruses. Human papillomaviruses (HPV) are the causative agents of warts, genital cancers, anal dysplasia, focal epithelial hyperplasia, oropharyngeal cancer, and a host of other dermatologic and mucous membrane-associated diseases. Of the 170 types of HPV currently recognized, only a small subset is associated with head, neck, anal, and genital cancers (e.g., HPV 16, 18, 31, 45). Replication of HPV is restricted to stratified epithelial basal cells; therefore, efforts to detect HPV should focus largely on molecular and histopathologic methods using exfoliative cytologic preparations and tissue biopsies (Figs. 3.13, 3.14 and 3.15).
3.5 Human Parvoviruses
Parvoviruses are nonenveloped viruses that possess short (4–6 kbp) linear ssDNA genomes. Parvoviral particles are among the smallest known, with average sizes ranging from approximately 18–26 nm. To date, only two parvoviruses, human parvovirus B19 and human bocavirus, are known to cause human disease. Human parvovirus B19 (Primate erythroparvovirus 1) is the causative agent of erythema infectiosum (fifth disease), a common childhood illness, and human bocavirus ( Primate bocaparvovirus1 and 2) has been suggested to cause gastroenteritis and lower respiratory tract infections. Parvoviruses, like many other DNA viruses, replicate within the cell nucleus and cause morphologic alterations, including nuclear swelling, chromatin margination, and inclusion production (Figs. 3.16 and 3.17).
3.6 Human Polyomaviruses
To date, there have been at least 13 species of human polyomavirus, including BK polyomavirus ( Human polyomavirus1), JC polyomavirus (Human polyomavirus 2), and Merkel cell polyomavirus (Human polyomavirus 5) shown to infect humans. Most species are not pathogenic, but those that are cause very mild diseases in immunocompetent hosts, most commonly young children. Severe polyomavirus-associated infections are most common in immunosuppressed hosts, including kidney and hematopoietic stem cell transplant recipients, and the spectrum of diseases in these individuals includes progressive multifocal leukoencephalopathy, nephropathy, hemorrhagic cystitis, and Merkel-cell carcinoma. Polyomavirus particles are roughly 50 nm in size, are nonenveloped, and contain a dsDNA genome that measures approximately 5 kbp in length. Like with papillomaviruses, the genomes of polyomaviruses are complexed with histones derived from host cells. Replication of polyomaviral genomes and assembly of new particles occurs within the cell nucleus (Figs. 3.18, 3.19 and 3.20).
Several viruses that are classified in the family Paramyxoviridae are human pathogens and cause diseases that range in severity from relatively mild respiratory tract infections to severe, life-threatening respiratory and central nervous system infections. Viruses belonging to this family that cause human disease are listed in Table 3.3. Like other members of the order Mononegavirales, paramyxoviruses have nonsegmented–ssRNA genomes that measure approximately 15 kbp in length and form complexes with viral structural proteins to produce helical nucleocapsids. Nascent particles bud from the cytoplasmic membrane, during which they are enveloped; resulting particles are roughly spherical or filamentous. Paramyxoviruses are transmitted between humans via aerosolized infectious respiratory secretions, through fomites, and some, the henipaviruses, are transmissible from infected animals such as bats, pigs, and horses (Figs. 3.21 and 3.22).
Examples of human-pathogenic paramyxoviruses listed according to common name, formal taxonomic hierarchy, and disease associations
Respiratory syncytial virus
Mild-to-severe bronchiolitis and pneumonia, coryza
Some poxviruses (e.g., molluscum contagiosum virus) are common human pathogens; however, natural variola virus transmission has been eradicated from the planet, and the last human cases of smallpox were witnessed in the later part of the 1970s. Poxviruses possess some of the largest genomes of any virus known to infect humans. On average, the linear dsDNA genomes of these viruses measure approximately 170–250 kbp in length and encode numerous proteins. Unlike many other DNA viruses, replication of poxvirus genomes and particle assembly occurs within the cytoplasmic compartment of host cells, a feature that is made possible by viral proteins encoded within poxviral genomes. Consequently, light microscopic examination of stained tissue sections often permits visualization of intracytoplasmic inclusions of poxviruses such as Guarnieri bodies and molluscum bodies seen in variola virus- and molluscum contagiosum virus-infected cells, respectively (Figs. 3.23, 3.24, 3.25 and 3.26).
3.9 Hemorrhagic Fever Viruses and Miscellaneous Human-Pathogenic Viruses
The following figures represent a broad diversity of viruses that can cause hemorrhagic fever or other severe infectious diseases that are rarely encountered in most developed nations. Included among this group are the filoviruses (e.g., Ebola virus), flaviviruses (e.g., yellow fever virus), hantaviruses (e.g., Sin Nombre virus), and rabies virus (Figs. 3.27, 3.28, 3.29 and 3.30).
The author wishes to thank Dr. Sherif Zaki (Infectious Diseases Pathology Branch, U.S. Centers for Disease Control and Prevention), Dr. Bobbi Pritt (Division of Clinical Microbiology, Mayo Clinic), and Dr. Dana Scott (National Institutes of Health Rocky Mountain Laboratories) for provision of slides of H&E-stained tissue sections of Ebola and yellow fever virus-infected tissues, variola virus-infected tissue, and hantavirus-infected nonhuman primate tissue, respectively. In addition, Dr. Relich wishes to thank Drs. Bryan Schmitt, Theodore Kieffer, Matthew Kuhar, Simon Warren, Mercia Gondim, and Dibson Gondim (Department of Pathology and Laboratory Medicine, Indiana University School of Medicine, Indianapolis, IN) for their professional consultations and interpretations of many of the images shown.
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