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 2 years of age. Worldwide, more than 500 000 deaths in children younger than 5 years of age annually are attributed to rotavirus infections; whereas such deaths in the United States are rather infrequent, the annual morbidity rate has been nonetheless considerable in recent years (Figure 15–3). 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.
Rotavirus structure. (Reproduced with permission from Willey JM: Prescott, Harley, & Klein's Microbiology, 7th edition. McGraw-Hill, 2008.)
Estimated annual morbidity due to rotavirus infections in the United States. (Centers for Disease Control and Prevention.)
Most common cause of winter gastroenteritis in children less than 2 years of age
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 15–4A). 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, 15–4A). 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 15–4B–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 and VP7), 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.
Structure of Rotavirus. (A). Eleven segments of rotavirus are shown on a gel, each segment encoding corresponding structural (VP1–VP7) or nonstructural (NSP1–NSP6) proteins are shown, (B) structure of rotavirus showing outer layer capsid proteins, including VP4 (spikes) and VP7 (outer capsid layer) (C). (Courtesy of BVV Prasad)
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. 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 but are difficult to propagate because the replicative cycle is usually incomplete, and mature, infectious virions are often not produced. However, successful propagation of human strains in vitro has been achieved in some instances.
Wheel-shaped naked capsid spherical viruses
Double-stranded RNA genome replicates in the cytoplasm
Rotavirus replication is depicted in Figure 15–5. 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.
Schematic diagram of Rotavirus replication. Rotavirus outer capsid spike (VP4) binds to the receptor (sialic acid-containing glycoprotein) followed by a conformational change, removal of outer layer, and penetration of the virus in the target cells. Following partial uncoating, viral RNA-dependent RNA polymerase directs the transcription of viral mRNAs followed synthesis of viral proteins, by genome replication by using the negative-strand RNA of the double-stranded RNA genome. Rotavirus assembles by associating its core with a nonstructural protein (NSP4) and acquiring VP7 and a membrane budding from the ER. The virus eventually loses the membrane in the ER and is released upon cell lysis. (Courtesy of MK Estes)
Double-shelled (triple-layered) outer capsid
Group A rotaviruses predominantly infect humans
Five antigenic types (serotypes) based on capsid proteins VP4 and VP7 detected worldwide
ISVP is infectious, and not the whole virion
VP4 binds to sialic acid-glycoprotein on epithelial cells
RNA-dependent RNA polymerase directs the synthesis of mRNA and genomic RNA by using negative-strand RNA of the double-stranded RNA genome
Virus assembly takes place at the ER
Viruses release upon cell lysis after losing the membrane
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).
Animal rotaviruses produce diarrhea, but interspecies spread not demonstrated in nature
Reassortment of the 11 RNA segments readily occurs
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.
Live vaccines can incorporate genes from animal viruses
HUMAN ROTAVIRUS INFECTIONS
Worldwide, an estimated 1 million infants die each year as a result of Rotavirus diarrhea. In the United States, the total annual deaths now are thought to be less than 100, but these viruses are still major causes of severe illness and hospitalization in early life. Vomiting, abdominal cramps, and low-grade fever, followed by watery stools that usually do not contain mucus, blood, or pus, are all characteristics of the acute phase of illness and can also be seen with infections due to caliciviruses, astroviruses, and adenoviruses.
Outbreaks of rotavirus infection are common, particularly during the cooler months, among infants and children aged 1 to 24 months. 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.
Primarily affects infants and children in colder months
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.
Most of the older children and adults are immune
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.
Destroys villous cells of jejunum and duodenum
Absorptive surface is decreased
Enterotoxin-like effects are also present
Patients with rotavirus infection respond with production of type-specific humoral antibodies that appear to last for years, perhaps a lifetime. 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.
Type-specific humoral and secretory IgA antibodies are protective
IgA and mucin glycoproteins confer protective role of breastfeeding
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.
Severe dehydration can lead to death, particularly in very small or malnourished infants
Short incubation period, vomiting, and watery diarrhea can lead to dehydration
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 direct examination of the specimen by electron microscopy or, more conveniently, by immunologic detection of antigen with EIA methods or virion RNA by RT-PCR.
Electron microscope, EIA or RT-PCR detects virus
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.
No specific treatment
Vigorous fluids and electrolyte replacement
Rigorous hygienic measures to prevent spread
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 (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, Rotarix (developed by GlaxoSmithKline) is also licensed 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.
Live oral rotavirus vaccines are available and recommended for infants
Vaccine dose administration important