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The human intestinal rotaviruses were first found in 1973 by electron microscopic examination of duodenal biopsy specimens from infants with diarrhea (Figure 15–2). Since then, they have been found worldwide and are believed to account for 40% to 60% of cases of acute gastroenteritis occurring during the cooler months in infants and in children less than 5 years of age, with most serious disease in 3 to 35 months of age. Worldwide, more than 528 000 deaths in children younger than 5 years of age in 2000 were attributed to rotavirus infections mainly in Sub-Saharan Africa, South Asia, and Southeast Asia, which has dropped to 215 000 in 2013. Four countries, including India, Nigeria, Pakistan, and Democratic Republic of Congo accounted for 49% of rotavirus-related deaths under 5 years of age in 2013, with 22% alone in India. In the United States, more than 400 000 doctor visits, 200 000 emergency room visits, 50 000 to 70 000 hospitalizations, and 20 to 60 deaths were reported before the rotavirus vaccine was introduced in 2006. Now such deaths in the United States are rather infrequent; the annual morbidity rate has significantly dropped. Before the introduction of rotavirus vaccines in 2006, almost all children were infected in the United States before their fifth birthday. The routine use of rotavirus vaccine in infants has significantly reduced rotavirus infection in the United States. These viruses have been detected in intestinal contents and in tissues from the upper gastrointestinal tract.
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✺ Most common cause of winter gastroenteritis in children less than 5 years of age and serious diarrheal disease in 3 to 35 months of age
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The rotaviruses belong to the family Reoviridae. The genome of rotaviruses is unique in the sense that they have 11 segments of double-stranded RNA. The 11 segments of the genome encode six structural (VP1–VP4 and VP6–VP7) and six nonstructural (NSP1–NSP6) proteins (Figure 15–3A). There are three types of virus particles, including triple layered (previously called double shelled), double layered (previously called single shelled) and single layered (empty capsids, usually lacking genomes) (Figure 15–1A, 15–2). The complete virus particle of rotavirus is a wheel-shaped virus and the name is derived from the Latin rota (“wheel”) because of the outer capsid, which resembles a wheel attached by short spokes to the inner capsid and core (Figures 15–1, 15–2, 15–3A). Eleven segments of double-stranded RNA genome are packaged into an icosahedral capsid making the spherical particles of 65 to 75 nm in diameter in size (smaller forms have also been described) (Figure 15–3B–D). The virus particle has a virion-associated RNA-dependent RNA polymerase, and a double-shelled outer capsid; two segments encode proteins of the outer capsid (VP4 or P and VP7 or G), which are targets for neutralizing antibodies. The major outer capsid proteins are VP4 and VP7. VP4 performs several functions, including viral attachment protein, whereas VP7 is a type-specific antigen and facilitates viral attachment and entry.
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Wheel-shaped naked capsid spherical viruses
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✺ Eleven segments of double-stranded RNA genome replicates in the cytoplasm
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Rotaviruses are classified into seven groups, A to G, based on the internal capsid protein, VP6. Human infections are predominantly caused by group A and less commonly by group B or C. Based on VP4 and VP7 type-specific antigens on the outer capsid, G (VP7 is a glycoprotein) and P (VP4 is protease-sensitive) serotypes have been designated. Five serotypes (G1, G2, G3, G4, and G9), are of major epidemiologic importance because they represent more than 90% of all serotypes detected worldwide. G1 serotype represents more than 75% of the isolates. The outer capsid is proteolytically cleaved in the gastrointestinal tract to generate intermediate infectious subviral particle (ISVP), which activates the virus for infection. Rotaviruses can replicate in the cytoplasm of infected cell cultures in the laboratory and successful propagation of human strains in vitro has been achieved in cell lines.
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✺ Double-shelled (triple-layered) outer capsid
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Group A rotaviruses predominantly infect humans
Five antigenic types (serotypes) based on capsid proteins VP4 and VP7 detected worldwide
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Rotavirus replication is depicted in Figure 15–4. Rotavirus is transmitted by fecal–oral route, and the virus particle is partially digested in the gastrointestinal tract and activated by protease cleavage resulting in the loss of VP7 and cleavage of VP4 to generate ISVP. The VP4 binds to sialic acid containing glycoproteins on epithelial cells, and the ISVP penetrates the target cells. The generation of ISVP is necessary for rotavirus infection because the double-shelled virus particle, after entering the cells via receptor-mediated endocytosis, is unable to establish infection owing to a dead-end pathway. After entry of the ISVP, the core containing double-stranded RNA genomes and the RNA-dependent RNA polymerase is partially released into the cytoplasm. Rotaviruses use negative-sense RNA strategy for transcription and replication. RNA-dependent RNA polymerase directs the synthesis of early and late mRNAs followed by genome replication by using the negative-strand RNA of the double-stranded RNA genome. Early proteins are produced that are required for virus replication, whereas late proteins are mainly the structural proteins. Rotavirus assembles by associating its core with a nonstructural protein (NS28, a product of NSP4) and by acquiring VP7 and a membrane budding into the endoplasmic reticulum (ER). The virus eventually loses the membrane in the ER and is released upon cell lysis.
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Fecal–oral transmission
ISVP is infectious, and not the whole virion
VP4 binds to sialic acid-glycoprotein on epithelial cells
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✺ RNA-dependent RNA polymerase directs the synthesis of mRNA and genomic RNA by using negative-strand RNA of the double-stranded RNA genome
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Virus assembly takes place at the ER
Viruses release upon cell lysis after losing the membrane
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Why does rotavirus contains 11 segments of RNA?
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Rotaviruses of animal origin are also highly prevalent and produce acute gastrointestinal disease in a variety of species. Very young animals, such as calves, suckling mice, piglets, and foals, are particularly susceptible. The animal rotaviruses can often replicate in cell cultures, and infection across species has been accomplished experimentally; however, there is no evidence that such interspecies spread occurs in nature (eg, animal rotaviruses are not known to affect humans and vice versa).
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Animal rotaviruses produce diarrhea, but interspecies spread not demonstrated in nature
Reassortment of the 11 RNA segments readily occurs
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Think ➱ Apply 15-1. Rotavirus has 11 segments of RNA so it can follow the monocistronic rule of one RNA one protein.
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One unique feature of rotaviruses is the ease with which the 11 RNA segments can undergo reassortment. This has enabled the development of live vaccines that combine genes from readily cultivated animal rotaviruses with human rotavirus genes that encode serotype-specific capsid proteins.
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Live vaccines can incorporate genes from animal viruses
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HUMAN ROTAVIRUS INFECTIONS
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Outbreaks of rotavirus infection were common in pre-vaccine era, particularly during the cooler months, among infants and children of less than 5 years of age, but the incidence of clinical illness was highest among 3 to 35 months of age. Older children and adults can also be affected, but attack rates are usually much lower and the disease is milder. Outbreaks among elderly, institutionalized patients have also been recognized.
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✺ Primarily affects infants and children in colder months
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Although newborn infants can be readily infected with the virus, such infections often result in little or no clinical illness. This finding is illustrated by reported infection rates of 32% to 49% in some neonatal nurseries, but mild illness in only 8% to 28% of the infants. It is unclear whether this transient resistance to disease is a result of host maturation factors or transplacentally conferred immunity. Seroepidemiologic studies have been useful in demonstrating the ubiquity of these viruses and may help to explain the age-specific attack rates. By the age of 5 years, almost all individuals have humoral antibodies, suggesting a high rate of virus infection early in life.
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Most of the older children and adults are immune
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Rotaviruses appear to localize primarily in the duodenum and proximal jejunum, causing destruction of villous epithelial cells with blunting (shortening) of villi and variable, usually mild, infiltrates of mononuclear and a few polymorphonuclear inflammatory cells within the villi. The gastric and colonic mucosa is unaffected; however, for unknown reasons, gastric emptying time is markedly delayed. The primary pathophysiologic effects are a decrease in absorptive surface in the small intestine and decreased production of brush border enzymes, such as the disaccharidases. The net result is a transient malabsorptive state, with defective handling of fats and sugars. It may take as long as 3 to 8 weeks to restore the normal histologic and functional integrity of the damaged mucosa. Although the specific gene product associated with virulence is not yet known, some evidence suggests that one nonstructural protein, NSP4, may behave as an enterotoxin in a manner similar to that of the heat-labile enterotoxin (LT) of Escherichia coli and cholera toxin. This may further explain the excess fluid and electrolyte secretion in the acute phase of illness. Viral excretion usually lasts 2 to 12 days but can be greatly prolonged in malnourished or immunodeficient patients with persistent symptoms.
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✺ Destroys villous cells of jejunum and duodenum
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Absorptive surface is decreased
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✺ Enterotoxin-like effects are also present
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Why does rotavirus infection cause malabsorption in infected children?
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Rotavirus infection responds with production of type-specific humoral antibodies that probably does not last for a lifetime after the first infection. Recovery from the first infection provides 38% protection against infection, 77% protection against diarrhea, and 87% protection against severe diarrhea. Subsequent infections provide long-term immunity. In addition, type-specific secretory IgA antibodies are produced in the intestinal tract, and their presence seems to correlate best with immunity to reinfection. Breastfeeding also seems to play a protective role against rotavirus disease in young infants. Secretory IgA antibodies to rotaviruses appear in colostrum and continue to be secreted in breast milk for several months postpartum. Human breast milk mucin glycoproteins have also been shown to bind to rotaviruses, inhibiting their replication in vitro and in vivo.
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✺ Long-term immunity after subsequent infections
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Type-specific humoral and secretory IgA antibodies are protective
IgA and mucin glycoproteins confer protective role of breastfeeding
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After an incubation period of 1 to 3 days, there is usually an abrupt onset of vomiting, followed within hours by frequent, copious, watery, brown stools. In severe cases, the stools may become clear; the Japanese refer to the disease as hakuri, the “white stool diarrhea.” Fever, usually low grade, is often present. Vomiting may persist for 1 to 3 days, and diarrhea for 4 to 8 days. The major complications result from severe dehydration, occasionally associated with hypernatremia.
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✺ Severe dehydration can lead to death, particularly in very small or malnourished infants
✺ Short incubation period, vomiting, and watery diarrhea can lead to dehydration
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Diagnosis of acute rotavirus infection is usually by detection of virus particles, antigen or virion RNA in the stools during the acute phase of illness. This can be accomplished by immunologic detection of antigen with EIA methods or virion RNA by RT-PCR. Direct examination of the specimen by electron microscopy can also be done primarily in research setting.
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Antigen by EIA or viral RNA by RT-PCR in stool specimen to detects virus
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Think ➱ Apply 15-2. Rotavirus damages villous cells that reduce the absorptive space in the intestine causing transient malabsorption.
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TREATMENT AND PREVENTION
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There is no specific treatment for rotavirus infection. Vigorous replacement of fluids and electrolytes is required in severe cases and can be lifesaving. The rotaviruses are highly infectious and can spread quickly in family and institutional settings. Control consists of rigorous hygienic measures, including careful handwashing and adequate disposal of enteric excretions.
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✺ Vigorous fluids and electrolyte replacement
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Rigorous hygienic measures to prevent spread
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Previously developed live attenuated or reassortant rhesus-based rotavirus vaccine was developed and licensed in the United States in 1998, but withdrawn because of some side effects (intussusception). In 2006, a live, oral bovine/human reassortant vaccine that contains five reassortant rotaviruses (RV5) developed from human and bovine strains (RotaTeq developed by Merck) was licensed for routine use in the United States. It is a three-dose series at 2, 4, and 6 months of age. A second live oral vaccine, RV1 (Rotarix) that contains one live, attenuated human strain (developed by GlaxoSmithKline) was licensed in 2008 for a two-dose series, administered at 2 and 4 months. The minimum age for the first dose administration is 6 weeks and maximum age is 14 weeks and 6 days. The minimum interval between doses is 4 weeks and all doses should be completed by 8 months of age. To date, its efficacy after a three-dose series has been excellent, and no safety concerns have arisen. The efficacy of the vaccine in preventing infection is between 85% and 98%. However, rotavirus vaccine should not be given to infants aged 15 months and above due to lack of availability of safety data. While the vaccine is safe, mild problems such as temporary diarrhea or vomiting may occur. In addition, 1 in 20 000 to 1 in 100 000 infants may have intussusception (a bowel blockage) with rotavirus vaccination.
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✺ Live oral rotavirus vaccines are available and recommended for infants
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Vaccine dose administration important