Jawetz Chapter 37.pdf

37 C H A P T E R Reoviruses, Rotaviruses...

37 C H A P T E R Reoviruses, Rotaviruses, and Caliciviruses Reoviruses are medium-sized viruses with a double-stranded, Classification segmented RNA genome. The family includes human rota- The family Reoviridae is divided into 15 genera. Four viruses, the most important cause of infantile gastroenteri- of the genera are able to infect humans: Orthoreovirus, tis around the world (Figure 37-1). Acute gastroenteritis is a Rotavirus, Coltivirus, and Orbivirus. The genera are divided very common disease with significant public health impact. into two subfamilies: Spinareovirinae contains viruses In developing countries, it is estimated to cause as many as with large spikes at the 12 vertices on the particle (eg, 1.3 million deaths of preschool children annually, of which Orthoreovirus), whereas members of the Sedoreovirinae rotavirus is responsible for up to about 500,000 deaths. In the appear more smooth, lacking the large surface projections United States, acute gastroenteritis is second only to acute (eg, Rotavirus). respiratory infections as a cause of disease in families. There are at eight species or groups of rotaviruses (A–H), Caliciviruses are small viruses with a single-stranded of which three species (A, B, C) infect humans. Strains of RNA genome. The family contains noroviruses, the major human and animal origin may fall in the same serotype. cause of nonbacterial epidemic gastroenteritis worldwide. Other rotavirus groups and serotypes are found only in ani- Astroviruses also cause gastroenteritis. mals. Three different serotypes of reovirus are recognized, along with about 100 different orbivirus serotypes and two coltivirus serotypes. REOVIRUSES AND ROTAVIRUSES Important properties of reoviruses are summarized in Table 37-1. Reovirus Replication Viral particles attach to specific receptors on the cell surface (Figure 37-3). The cell attachment protein for reoviruses is Structure and Composition the viral hemagglutinin (σ1 protein), a minor component of The virions measure 60–80 nm in diameter and possess two the outer capsid. concentric capsid shells, each of which is icosahedral. (Rota- After attachment and penetration, uncoating of virus viruses have a triple-layered structure.) There is no enve- particles occurs in lysosomes in the cell cytoplasm. Only lope. Single-shelled virus particles that lack the outer capsid the outer shell of the virus is removed, and a core-associ- are 50–60 nm in diameter. The inner core of the particles is ated RNA transcriptase is activated. This transcriptase 33–40 nm in diameter (Figure 37-2). The double-shelled par- transcribes mRNA molecules from the minus strand of ticle is the complete infectious form of the virus. each genome double-stranded RNA segment contained in The reovirus genome consists of double-stranded RNA in the intact core. There are short terminal sequences at both 10–12 discrete segments with a total genome size of 16–27 kbp, ends of the RNA segments that are conserved among all iso- depending on the genus. Whereas rotaviruses contain 11 lates of a given subgroup. These conserved sequences may genome segments, orthoreoviruses and orbiviruses each pos- be recognition signals for the viral transcriptase. The func- sess 10 segments and coltiviruses have 12 segments. The indi- tional mRNA molecules correspond in size to the genome vidual RNA segments vary in size from 680 bp (rotavirus) segments. Most RNA segments encode a single protein, to 3900 bp (orthoreovirus). The virion core contains several although a few (depending on the virus) encode two pro- enzymes needed for transcription and capping of viral RNAs. teins. Reovirus cores contain all enzymes necessary for Rotaviruses are stable to heat at 50°C, to a 3.0–9.0 range transcribing, capping, and extruding the mRNAs from the of pH, and to lipid solvents, such as ether and chloroform, core, leaving the double-stranded RNA genome segments but they are inactivated by 95% ethanol, phenol, and chlo- inside. rine. Limited treatment with proteolytic enzymes increases After being extruded from the core, the mRNAs are infectivity. translated into primary gene products. Some of the full-length 547 Riedel_CH37_p547-p556.indd 547 04/04/19 5:06 PM  548   SECTION IV  Virology Unknown Unknown Rotavirus Rotavirus Parasites Other bacteria Toxigenic Escherichia coli Bacteria Adenovirus Adenovirus Astrovirus Astrovirus Calicivirus Calicivirus A Developed countries B Developing countries FIGURE 37-1 An estimate of the role of etiologic agents in severe diarrheal illnesses requiring hospitalization of infants and young children. A: In developed countries. B: In developing countries. (Reproduced from Kapikian AZ: Viral gastroenteritis. JAMA 1993;269:627.) TABLE 37-1 Important Properties of Reoviruses transcripts are encapsidated to form immature virus particles. Virion: Icosahedral, 60–80 nm in diameter, double capsid shell A viral replicase is responsible for synthesizing negative-sense strands to form the double-stranded genome segments. This Composition: RNA (15%), protein (85%) replication to form progeny double-stranded RNA occurs Genome: Double-stranded RNA, linear, segmented in partially completed core structures. The mechanisms that (10–12 segments); total genome size 16–27 kbp ensure assembly of the correct complement of genome seg- Proteins: Nine structural proteins; core contains several enzymes ments into a developing viral core are unknown. However, genome reassortment occurs readily in cells coinfected with Envelope: None (transient pseudoenvelope is present during rotavirus particle morphogenesis) different viruses of the same subgroup, giving rise to virus particles containing RNA segments from the different paren- Replication: Cytoplasm; virions not completely uncoated tal strains. Viral polypeptides probably self-assemble to form Outstanding characteristics: the inner and outer capsid shells. Genetic reassortment occurs readily Reoviruses produce inclusion bodies in the cytoplasm in which virus particles are found. These viral factories are Rotaviruses are the major cause of infantile diarrhea closely associated with tubular structures (microtubules and Reoviruses are good models for molecular studies of viral pathogenesis intermediate filaments). Rotavirus morphogenesis involves budding of single-shelled particles into the rough endoplas- mic reticulum. The “pseudoenvelopes” so acquired are then removed and the outer capsids are added (Figure 37-3). This unusual pathway is used because the major outer capsid pro- tein of rotaviruses is glycosylated. Cell lysis results in release of progeny virions. ROTAVIRUSES Rotaviruses are a major cause of diarrheal illness in human infants and young animals, including calves and piglets. Infections in adult humans and animals are also common. Among the rotaviruses are the agents of human infantile diarrhea, Nebraska calf diarrhea, epizootic diarrhea of infant mice, and SA11 virus of monkeys. Rotaviruses resemble reoviruses in terms of morphology and strategy of replication. FIGURE 37-2 Electron micrograph of a negatively stained Classification and Antigenic Properties preparation of human rotavirus. D, double-shelled particles; E, empty capsids; i, fragment of inner shell; io, fragments of a combination of Rotaviruses have been classified into eight species (A–H) inner and outer shell; S, single-shelled particles. Inset: Single-shelled based on antigenic epitopes and sequence of the internal particles obtained by treatment of the viral preparation with sodium structural protein VP6. Group A rotaviruses are the most dodecyl sulfate. Bars, 50 nm. (Courtesy of J Esparza and F Gil.) frequent human pathogens. Outer capsid proteins VP4 and Riedel_CH37_p547-p556.indd 548 04/04/19 5:06 PM  CHAPTER 37 Reoviruses, Rotaviruses, and Caliciviruses   549 VP6 VP4 spike conformational change VP7 with receptor interactions VP2 Penetration VP4 Nonclassical vesicular transport Transcription VP7 Loss of envelope and particle VP4 maturation NSP4 Budding and transient enveloped particles ER Double-layered NSP2,5,6 particle VP1,2,3,6 Replication intermediate and double-stranded RNA synthesis Cell lysis Nucleus Viroplasm FIGURE 37-3 Overview of the rotavirus replication cycle. ER, endoplasmic reticulum. (Courtesy of MK Estes.) VP7 carry epitopes important in neutralizing activity, with exhibit subclinical infection, perhaps reflecting the pres- VP7 glycoprotein being the predominant antigen. These ence of maternal antibody; overt disease is more common in type-specific antigens differentiate among rotaviruses and weanling animals. are demonstrable by neutralization tests. Five predomi- nant serotype strains of rotavirus species A (G1–G4, G9) are responsible for the majority of human disease. Serotype Propagation in Cell Culture distributions differ geographically. Multiple serotypes have Rotaviruses are fastidious agents to culture. Most group A been identified among human and animal rotaviruses. Some human rotaviruses can be cultivated if pretreated with the animal and human rotaviruses share serotype specificity. For proteolytic enzyme trypsin and if low levels of trypsin are example, monkey virus SA11 is antigenically very similar to included in the tissue culture medium. This cleaves an outer human serotype 3. The gene-coding assignments responsible capsid protein and facilitates uncoating. Very few non-group for the structural and antigenic specificities of rotavirus pro- A rotavirus strains have been cultivated. teins are shown in Figure 37-4. Molecular epidemiologic studies have analyzed isolates Pathogenesis based on differences in the migration of the 11 genome seg- ments after electrophoresis of the RNA in polyacrylamide Rotaviruses infect cells in the villi of the small intestine gels (Figure 37-5). These differences in electropherotypes can (gastric and colonic mucosa are spared). They multiply in the be used to differentiate species A viruses from other groups, cytoplasm of enterocytes and damage their transport mecha- but they cannot be used to determine serotypes. nisms. One of the rotavirus-encoded proteins, NSP4, is a viral enterotoxin and induces secretion by triggering a calcium- dependent signal transduction pathway. Damaged cells may Animal Susceptibility slough into the lumen of the intestine and release large quanti- Rotaviruses have a wide host range. Most isolates have been ties of virus, which appear in the stool (up to 1012 particles per recovered from newborn animals with diarrhea. Cross- gram of feces). Viral excretion usually lasts from 2 to 12 days species infections can occur in experimental inoculations, in otherwise healthy patients but may be prolonged in those but it is not clear if they occur in nature. Newborns often with poor nutrition and immunocompromised patients. Riedel_CH37_p547-p556.indd 549 04/04/19 5:06 PM  550   SECTION IV  Virology RNA VP7 VP6 VP4 RNA segment Protein VP1/3 1 VP1 2 VP2 3 VP3 4 VP4 5 NSP1 6 VP6 VP2 B C D 7 NSP2 8 NSP3 9 VP7 10 NSP4 11 NSP5,6 mRNA A E F FIGURE 37-4 Rotavirus structure. A: Gel diagram showing the 11 segments of the genome. The structural (VP) and nonstructural (NSP) proteins encoded by these segments are indicated. B: Surface representation of the rotavirus structure from cryoelectron microscopic analysis. The two outer layer proteins are VP4, which forms the spikes, and VP7, which forms the capsid layer. C: Cut-away view showing the triple-layered organization of the virion, with the intermediate VP6 layer and the innermost VP2 layer indicated. The enzymes required for endogenous transcription (VP1) and capping (VP3) are attached as heterodimeric complexes to the inner surface of the VP2 layer. D: Proposed organization of the double-stranded RNA genome inside the VP2 layer along with transcription enzyme complexes (VP1/3) depicted as balls. E: Exit of transcripts from the channels at the fivefold vertices of actively transcribing double-layered particles. F: Close-up view of one of the exit channels. (Courtesy of BVV Prasad.) Diarrhea caused by rotaviruses may also be due to impaired sodium and glucose absorption as damaged cells on villi are replaced by nonabsorbing immature crypt cells. It may take from 3 to 8 weeks for normal function to be restored. Clinical Findings and Laboratory Diagnosis Rotaviruses cause the major portion of diarrheal illness in infants and children worldwide but not in adults (Table 37-2). There is an incubation period of 1–3 days. Typical symptoms include watery diarrhea, fever, abdominal pain, and vomit- ing, leading to dehydration. In infants and children, severe loss of electrolytes and fluids may be fatal unless treated. Patients with milder cases have symptoms for 3–8 days and then recover completely. However, viral excretion in the stool may persist up to 50 days after onset of diarrhea. Asymptomatic infections with sero- conversion occur. In children with immunodeficiencies, rotavirus can cause severe and prolonged disease. Adult contacts may be infected, as evidenced by sero- conversion, but they rarely exhibit symptoms, and virus is infrequently detected in their stools. A common source of FIGURE 37-5 Electrophoretic profiles of rotavirus RNA segments. infection is contact with pediatric cases. However, epidemics Viral RNAs were electrophoresed in 10% polyacrylamide gels and of severe disease have occurred in adults, especially in closed visualized by silver stain. Different rotavirus groups and RNA patterns populations, as in a geriatric ward. Group B rotaviruses have are illustrated: a group A monkey virus (SA11; lane A), a group A human been implicated in large outbreaks of severe gastroenteritis in rotavirus (lane B), a group B human adult diarrhea virus (lane C), and a group A rabbit virus that exhibits a “short” RNA pattern (lane D). adults in China and southeastern Asia (Table 37-2). Rotaviruses contain 11 genome RNA segments, but sometimes two or Laboratory diagnosis rests on demonstration of virus three segments migrate closely together and are difficult to separate. in stool collected early in the illness and on a rise in anti- (Photograph provided by T Tanaka and MK Estes.) body titer. Virus in stool is demonstrated by antigen enzyme Riedel_CH37_p547-p556.indd 550 04/04/19 5:06 PM  CHAPTER 37 Reoviruses, Rotaviruses, and Caliciviruses   551 TABLE 37-2 Viruses Associated with Acute Gastroenteritis in Humansa Important as a Cause of Virus Size (nm) Epidemiology Hospitalization Rotaviruses Group A 60–80 Single most important cause (viral or bacterial) of endemic severe Yes diarrheal illness in infants and young children worldwide (in cooler months in temperate climates) Group B 60–80 Outbreaks of diarrheal illness in adults and children in China and SE Asia No Group C 60–80 Sporadic cases and occasional outbreaks of diarrheal illness in children No Enteric adenovirus 70–90 Second most important viral agent of endemic diarrheal illness of infants Yes and young children worldwide Caliciviruses Noroviruses 27–40 Important cause of outbreaks of vomiting and diarrheal illness in No older children and adults in families, communities, and institutions; frequently associated with ingestion of food Sapoviruses 27–40 Sporadic cases and occasional outbreaks of diarrheal illness in infants, No young children, and elderly adults Astroviruses 28–30 Sporadic cases and occasional outbreaks of diarrheal illness in infants, No young children, and elderly adults Source: Kapikian AZ: Viral gastroenteritis. JAMA 1993;269:627. immunoassays (EIAs) or PCR. Genotyping of rotavirus Treatment and Control nucleic acid from stool specimens by the polymerase chain Treatment of gastroenteritis is supportive to correct the loss of reaction (PCR) is the most sensitive detection method. water and electrolytes that may lead to dehydration, acidosis, shock, and death. Management consists of replacement of flu- Epidemiology and Immunity ids and restoration of electrolyte balance either intravenously or orally as feasible. The infrequent mortality from infantile Rotaviruses are the single most important worldwide cause diarrhea in developed countries is the result of the routine use of gastroenteritis in young children. Estimates range from 3 of effective replacement therapy. to 5 billion for annual diarrheal episodes in children younger In view of the fecal–oral route of transmission, wastewa- than 5 years of age in Africa, Asia, and Latin America, result- ter treatment and sanitation are significant control measures. ing in as many as 1 million deaths. Developed countries have An oral live attenuated rotavirus vaccine was licensed a high morbidity rate but a low mortality rate. Typically, up to in the United States in 1998 for vaccination of infants. It was 50% of cases of acute gastroenteritis of hospitalized children withdrawn a year later because of reports of intussusception throughout the world are caused by rotaviruses. (bowel blockages) as an uncommon but serious side effect Rotavirus infections usually predominate during the winter associated with the vaccine. In 2006, an oral live attenuated season. Symptomatic infections are most common in children pentavalent human–bovine reassortant rotavirus vaccine was between ages 6 months and 2 years, and transmission appears to licensed in the United States, followed by licensing of an oral be by the fecal–oral route. Nosocomial infections are frequent. live attenuated monovalent human rotavirus vaccine in 2008. Rotaviruses are ubiquitous. By age 3 years, 90% of Both vaccines are safe and effective, and neither is associated children have serum antibodies to one or more types. This with intussusception. Similar to other live attenuated vac- high prevalence of rotavirus antibodies is maintained in cines, immunization of immunocompromised individuals or adults, suggesting subclinical reinfections by the virus. their family members should be avoided as vaccine strains can Rotavirus reinfections are common; it has been shown that cause disease in these patients. A safe and effective vaccine young children can suffer up to five reinfections by 2 years remains the best hope for reducing the worldwide burden of of age. Asymptomatic infections are more common with rotavirus disease. successive reinfections. Local immune factors, such as secre- tory immunoglobulin A (IgA) or interferon, may be impor- tant in protection against rotavirus infection. Asymptomatic infections are common in infants before age 6 months, the REOVIRUSES time during which protective maternal antibody acquired passively by newborns should be present. Such neonatal The viruses of this genus, which have been studied most infection does not prevent reinfection, but it does protect thoroughly by molecular biologists, are not known to cause against the development of severe disease during reinfection. human disease. Riedel_CH37_p547-p556.indd 551 04/04/19 5:06 PM  552   SECTION IV  Virology Classification and Antigenic Properties TABLE 37-3 Important Properties of Caliciviruses Reoviruses are ubiquitous, with a very wide range of mam- Virion: Icosahedral, 27–40 nm in diameter, cup-like depressions on malian, avian, and reptilian hosts. Three distinct but related capsid surface types of reovirus have been recovered from many species and Genome: Single-stranded RNA, linear, positive-sense, are demonstrable by neutralization and hemagglutination- nonsegmented; 7.4–8.3 kb in size; contains genome-linked inhibition tests. Reoviruses contain a hemagglutinin for protein (VPg) human group O or bovine erythrocytes. Proteins: Polypeptides cleaved from a precursor polyprotein; capsid is composed of a single protein Epidemiology Envelope: None Reoviruses cause many inapparent infections because most Replication: Cytoplasm people have serum antibodies by early adulthood. Antibodies Outstanding characteristics: are also present in other species. All three types have been Noroviruses are major cause of nonbacterial epidemic gastroenteritis recovered from healthy children, from young children during Human viruses are noncultivable outbreaks of minor febrile illness, from children with enteri- tis or mild respiratory disease, and from chimpanzees with epidemic rhinitis. Human volunteer studies have failed to demonstrate a CALICIVIRUSES clear cause-and-effect relationship of reoviruses to human In addition to rotaviruses and noncultivable adenoviruses, illness. In inoculated volunteers, reovirus is recovered far members of the family Caliciviridae are important agents more readily from feces than from the nose or throat. of viral gastroenteritis in humans. The most significant members are the noroviruses, the prototype strain being Pathogenesis Norwalk virus. Properties of caliciviruses are summarized in Table 37-3. Reoviruses have become important model systems for the study of the pathogenesis of viral infection at the molecular level. Defined recombinants from two reoviruses with differ- Classification and Antigenic Properties ing pathogenic phenotypes are used to infect mice. Segrega- Caliciviruses are similar to picornaviruses but are slightly tion analysis is then used to associate particular features of larger (27–40 nm) and contain a single major structural pro- pathogenesis with specific viral genes and gene products. The tein (Figure 37-6). They exhibit a distinctive morphology in pathogenic properties of reoviruses are primarily determined the electron microscope (Figure 37-7). The family Caliciviri- by the protein species found on the outer capsid of the virion. dae is divided into five genera: Norovirus, which includes the Norwalk viruses; Sapovirus, which includes the Sapporo-like viruses; Nebovirus, which includes bovine enteric viruses; Lagovirus, the rabbit hemorrhagic disease virus; and Vesivirus, ORBIVIRUSES AND COLTIVIRUSES which includes vesicular exanthem virus of swine, feline cali- civirus, and marine viruses found in pinnipeds, whales, and Orbiviruses are a genus within the reovirus family. They fish. The first two genera contain human viruses that cannot commonly infect insects, and many are transmitted by be cultured; the latter two genera contain animal strains that insects to vertebrates. About 100 serotypes are known. None of these viruses cause serious clinical disease in humans, but they may cause mild fevers. Serious animal pathogens include bluetongue virus of sheep and African horse sickness virus. Antibodies to orbiviruses are found in many vertebrates, including humans. The genome consists of 10 segments of double-stranded RNA, with a total genome size of 18 kbp. The replicative cycle is similar to that of reoviruses. Orbiviruses are sensitive to low pH, in contrast with the general stability of other reoviruses. Coltiviruses form another species within the Reoviridae. The virus particle is 80 nm in diameter with a genome con- sisting of 12 segments of double-stranded RNA, totaling about 29 kbp. Colorado tick fever virus, transmitted by ticks, FIGURE 37-6 X-ray structure of the Norwalk virus capsid (left). is able to infect humans (see Chapter 38). It is found in the The capsid subunit structure is illustrated (right panel). S, P1, and P2 southwestern United States and can cause fever, rash, and domains are shaded in blue, red, and yellow, respectively. (Courtesy of systemic symptoms in infected patients. BVV Prasad.) Riedel_CH37_p547-p556.indd 552 04/04/19 5:06 PM  CHAPTER 37 Reoviruses, Rotaviruses, and Caliciviruses   553 A B C D E F G H FIGURE 37-7 Electron micrographs of virus particles found in stools of patients with gastroenteritis. These viruses were visualized after negative staining. Specific viruses and the original magnifications of the micrographs are as follows. A: Rotavirus (185,000×). B: Enteric adenovirus (234,000×). C: Coronavirus (249,000×). D: Torovirus (coronavirus) (249,000×). E: Calicivirus (250,000×). F: Astrovirus (196,000×). G: Norwalk virus (calicivirus) (249,000×). H: Parvovirus (249,000×). The electron micrographs in panels C–H were originally provided by T Flewett; panel E was originally obtained from CR Madeley. Bars, 100 nm. (Reproduced with permission from Graham DY, Estes MK: Viral infections of the intestine. In Gitnick G, et al [editors]. Principles and Practice of Gastroenterology and Hepatology. Elsevier 1988;566.) can be grown in vitro. Rabbit hemorrhagic disease virus was GI, GII, and GIV. Since 2001, genotype GII.4 viruses have introduced in 1995 in Australia as a biologic control agent to caused most viral gastroenteritis outbreaks worldwide. Noro- reduce that country’s population of wild rabbits. viruses appear to undergo antigenic drift over time, probably Human calicivirus serotypes are not defined. Multiple in response to population immunity. genotypes of noroviruses have been detected. Three geno- Cellular receptors for noroviruses are histocompatibil- groups are associated with human gastroenteritis, designated ity blood group antigens that are expressed on the mucosal Riedel_CH37_p547-p556.indd 553 04/04/19 5:06 PM  554   SECTION IV  Virology epithelia of the digestive tract. A person’s secretor status is Epidemiology and Immunity controlled by the fucosyltransferase 2 gene; secretor-negative Human caliciviruses have worldwide distribution. Norovi- individuals are resistant to infection with common strains of ruses are the most common cause of nonbacterial gastroen- Norwalk virus. teritis in the United States, causing an estimated 21 million cases annually. Clinical Findings and Laboratory Diagnosis The viruses are most often associated with epidemic outbreaks of waterborne, foodborne, and shellfish-associated Noroviruses (Norwalk virus) are the most important cause of gastroenteritis. All age groups can be affected. Outbreaks epidemic viral gastroenteritis in adults (Table 37-2). Epidemic occur throughout the year, with a seasonal peak during cooler nonbacterial gastroenteritis is characterized by (1) absence of months. Fecal–oral spread is the primary means of transmis- bacterial pathogens, (2) gastroenteritis with rapid onset and sion. Most outbreaks involve foodborne or person-to-person recovery and relatively mild systemic signs, and (3) an epi- transmission via fomites or aerosolization of contaminated demiologic pattern of a highly communicable disease that body fluids (vomitus, fecal material). Outbreaks in closed spreads rapidly with no particular predilection in terms of settings, such as cruise ships and nursing homes, are typical. age or geography. Various descriptive terms have been used Characteristics of norovirus include a low infectious in reports of different outbreaks (eg, epidemic viral gastroen- dose (as few as 10 virus particles), relative stability in the teritis, viral diarrhea, and winter vomiting disease) depend- environment, and multiple modes of transmission. It sur- ing on the predominant clinical feature. vives 10 ppm chlorine and heating to 60°C; it can maintain Norwalk viral gastroenteritis has an incubation period viability in steamed oysters. of 24–48 hours. The onset is rapid, and the clinical course No in vitro neutralization assay is available to study is brief, lasting 12–60 hours; symptoms include diarrhea, immunity. Volunteer challenge studies have shown that about nausea, vomiting, low-grade fever, abdominal cramps, head- 50% of adults are susceptible to illness. Norwalk virus anti- ache, and malaise. The illness can be incapacitating during body is acquired later in life than rotavirus antibody, which the symptomatic phase, but hospitalization is rarely required. develops early in childhood. In developing countries, most Norovirus infections are more likely to induce vomiting children have developed norovirus antibodies by 4 years of than those with Sapporo-like viruses. Dehydration is the age. most common complication in young and elderly individu- als. Viral shedding may persist for as long as 1 month. No sequelae have been reported. Treatment and Control Volunteer experiments have shown that the appear- Treatment is symptomatic. The low infectious dose permits ance of Norwalk virus coincides with clinical illness. Anti- efficient transmission of the virus. Effective handwashing is body develops during the illness and is usually protective on probably the most important method to prevent norovirus a short-term basis against reinfection with the same agent. infection and transmission. Because of the infectious nature Long-term immunity does not correspond well to the pres- of the stools, care should be taken in their disposal. Contain- ence of serum antibodies. Some volunteers can be reinfected ment and disinfection of soiled areas and bedding can help with the same virus after about 2 years. decrease viral spread. Careful processing of food and educa- Reverse transcriptase PCR is the most widely used tion of food handlers are important because many foodborne technique for detection of human caliciviruses in clini- outbreaks occur. Purification of drinking water and swim- cal specimens (feces, vomitus) and environmental samples ming pool water should decrease norovirus outbreaks. There (contaminated food, water). Because of the genetic diversity is no vaccine. among circulating strains, the choice of PCR primer pairs is very important. There can be up to 100 billion viral genome copies per gram of feces at peak shedding (2–5 days after infection). ASTROVIRUSES Electron microscopy can be used to detect virus particles Astroviruses are about 28–30 nm in diameter and exhibit a in stool samples. However, norovirus particles are usually distinctive starlike morphology in the electron microscope present in low concentration (unless the sample was collected (Figure 37-7F). They contain single-stranded, positive-sense at peak viral shedding) and are difficult to recognize; they can RNA, 6.4–7.4 kb in size. The family Astroviridae contains be identified by IEM. ELISA immunoassays based on recom- two genera; all human viruses are classified in the Mamas- binant virus-like particles can detect antibody responses, trovirus genus. At least eight serotypes of human viruses are with a fourfold or greater rise in IgG antibody titer in acute recognized by IEM and neutralization. and convalescent-phase sera indicative of a recent infection. Astroviruses cause diarrheal illness and may be shed However, the necessary reagents are not widely available, and in extraordinarily large quantities in feces. The viruses are the antigens are not able to detect responses to all antigenic transmitted by the fecal–oral route through contaminated types of noroviruses. food or water, person-to-person contact, or contaminated Riedel_CH37_p547-p556.indd 554 04/04/19 5:06 PM  CHAPTER 37 Reoviruses, Rotaviruses, and Caliciviruses   555 surfaces. They are recognized as pathogens for infants and (A) Mosquitoes or ticks, present in high numbers in the area children, elderly institutionalized patients, and immunocom- (B) Contaminated food served at the party promised persons (Table 37-2). They may be shed for pro- (C) A nearby stream used for fishing longed periods by immunocompromised hosts. (D) A visiting parent who was developing pneumonia (E) The swimming pool Animal astroviruses are found in a variety of mammals and birds and have recently been identified in several species 4. This viral gastroenteritis agent has a segmented, double- stranded RNA genome and a double-shelled capsid. It is a mem- of bats. Clinical testing for astroviruses is not commonly per- ber of which virus family? formed, but detection can be achieved with electron micros- (A) Adenoviridae copy, antigen, or RT-PCR methods. (B) Astroviridae (C) Caliciviridae (D) Reoviridae CHAPTER SUMMARY (E) Coronaviridae Reoviruses and rotaviruses are nonenveloped and contain 5. Rotavirus and Norwalk virus are distinctly different viruses. However, they share which one of the following characteristics? a double-stranded, segmented RNA genome. Reoviruses are not known to cause human disease but (A) Fecal–oral mode of transmission (B) They mainly cause disease in infants and young children. are important model systems for molecular pathogenesis (C) They induce generally mild disease in young children. studies. (D) Infection patterns show no seasonal variation. Rotaviruses are the most important cause of diarrheal ill- (E) A double-stranded RNA genome ness in infants and young children worldwide. 6. Because rotavirus infections can be serious, a vaccine would be Genetic reassortment occurs readily with rotaviruses. beneficial. Which of the following is most correct regarding a Caliciviruses are small nonenveloped viruses with a single- rotavirus vaccine? stranded, nonsegmented RNA genome. (A) A killed human rotavirus group A vaccine is licensed for Noroviruses, a genus of caliciviruses, are the major cause of use in the United States. nonbacterial epidemic gastroenteritis in the world. (B) Live attenuated vaccines are licensed for use in the Rotaviruses and noroviruses are transmitted primarily by United States. fecal–oral spread; noroviruses are associated with food- (C) Vaccine development is complicated by rapid antigenic borne and waterborne outbreaks. variation by the virus. Oral live attenuated rotavirus vaccines are available that (D) Available antiviral drugs make a vaccine unnecessary. are safe and effective; there is no norovirus vaccine. (E) Vaccine development is complicated because the virus can- not be grown in cell culture. 7. Rotaviruses and astroviruses share a number of characteristics. Which of the following is not shared? REVIEW QUESTIONS (A) Multiple serotypes exist 1. A 36-year-old man enjoyed a meal of raw oysters. Twenty-four (B) Can cause gastroenteritis in infants and children hours later, he became ill, with a sudden onset of vomiting, (C) Can cause gastroenteritis in elderly institutionalized diarrhea, and headache. The most likely cause of his gastroen- patients teritis is (D) Live vaccine available (A) Astrovirus (E) Fecal–oral route of transmission (B) Hepatitis A virus 8. A 20-year-old man was on a 3-week tour of Italy with other (C) Norwalk virus college students. One day he abruptly became ill with nausea (D) Rotavirus, group A and vomiting followed 5 hours later by abdominal cramps and (E) Echovirus watery diarrhea. No fever was noted. Which of these viruses is 2. This virus is the most important cause of gastroenteritis in the most likely cause of the man’s illness? infants and young children. It causes infections that are often (A) Calicivirus severe and may be life threatening, especially in infants. (B) Rotavirus (A) Echovirus (C) Reovirus (B) Norwalk virus (D) Adenovirus (C) Rotavirus, group A (E) Astrovirus (D) Orbivirus 9. Which statement about rotaviral gastroenteritis is false? (E) Parvovirus (A) The name of the causative agent was suggested by its 3. An outbreak of epidemic gastroenteritis occurred at a wooded appearance. summer camp 24 hours after a party for visiting families. Some (B) Most of the estimated 600,000 deaths occurring worldwide of the visiting parents also became ill. Samples taken 2 weeks from this disease are from dehydration. later from the well that was the source of drinking water at the (C) Most cases of the disease occur in infants and children. camp were negative for fecal coliforms. The most likely source (D) The causative agent infects mainly the stomach. of the outbreak was (E) The disease is transmitted by the fecal–oral route. Riedel_CH37_p547-p556.indd 555 04/04/19 5:06 PM  556   SECTION IV  Virology 10. Norwalk virus illness might be prevented by any of the follow- REFERENCES ing except Bresee JS, Nelson EA, Glass RI (guest editors): Rotavirus in Asia: (A) Avoidance of raw fruits Epidemiology, burden of disease, and current status of vac- (B) Live, reassortant vaccine cines. J Infect Dis 2005;192 (Suppl 1). [Entire issue.] (C) Careful handwashing Dennehy PH: Rotavirus vaccines: An overview. Clin Microbiol Rev (D) Avoidance of local drinking water 2008;21:198. (E) Avoidance of raw oysters Estes MK, Kapikian AZ: Rotaviruses. In Knipe DM, Howley PM 11. Which of the following statements about noroviruses is false? (editors-in-chief) Fields Virology, 5th ed. Lippincott Williams (A) They cause almost half the cases of viral gastroenteritis in & Wilkins, 2007. the United States. Green KY: Caliciviridae: The noroviruses. In Knipe DM, Howley (B) They can be responsible for epidemics of gastroenteritis. PM (editors-in-chief). Fields Virology, 5th ed. Lippincott (C) They generally produce an illness lasting 1–2 weeks. Williams & Wilkins, 2007. (D) Similar viruses are widespread among marine animals. McDonald SM, Patton JT: Assortment and packaging of the seg- (E) They typically cause disease in children and adults rather mented rotavirus genome. Trends Microbiol 2011;19:136. than infants. Monroe SS, Ando T, Glass RI (guest editors): International Work- 12. Each of the following statements regarding rotaviruses is cor- shop on Human Caliciviruses. J Infect Dis 2000;181(Suppl 12). rect except [Entire issue.] (A) The rotavirus vaccine contains recombinant RNA poly- Prevention of rotavirus gastroenteritis among infants and merase as the immunogen. children. Recommendations of the Advisory Committee on (B) Rotaviruses are a leading cause of diarrhea in young Immunization Practices (ACIP). MMWR Morb Mortal Wkly children. Rep 2009;58(RR-2). (C) Rotaviruses are transmitted primarily by the fecal–oral Rotavirus infection in Africa: Epidemiology, burden of disease, route. and strain diversity. J Infect Dis 2010;202(Suppl 1). [Entire (D) Rotaviruses belong to the reovirus family, which has a dou- issue.] ble-stranded segmented RNA genome. Rotavirus vaccines: An update. World Health Org Wkly Epidemiol Rec 2009;84:533. Updated norovirus outbreak management and disease prevention Answers guidelines. MMWR Morb Mortal Wkly Rep 2011;60:1. 1. C 5. A 9. D WHO position paper: Rotavirus vaccines. World Health Org Wkly 2. C 6. B 10. B Epidemiol Rec 2007;82:285. 3. B 7. D 11. C 4. D 8. A 12. A Riedel_CH37_p547-p556.indd 556 04/04/19 5:06 PM