Reoviruses are medium-sized viruses with a double-stranded, segmented RNA genome. The family includes human rotaviruses, the most important cause of infantile gastroenteritis around the world (Figure 37-1). Acute gastroenteritis is a very common disease with significant public health impact. In developing countries, it is estimated to cause as many as 1.3 million deaths of preschool children annually, of which rotavirus is responsible for up to about 500,000 deaths. In the United States, acute gastroenteritis is second only to acute respiratory infections as a cause of disease in families.
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.)
Caliciviruses are small viruses with a single-stranded RNA genome. The family contains noroviruses, the major cause of nonbacterial epidemic gastroenteritis worldwide. Astroviruses also cause gastroenteritis.
REOVIRUSES AND ROTAVIRUSES
Important properties of reoviruses are summarized in Table 37-1.
TABLE 37-1Important Properties of Reoviruses |Favorite Table|Download (.pdf) TABLE 37-1 Important Properties of Reoviruses
|Virion: Icosahedral, 60–80 nm in diameter, double capsid shell |
|Composition: RNA (15%), protein (85%) |
|Genome: Double-stranded RNA, linear, segmented (10–12 segments); total genome size 16–27 kbp |
|Proteins: Nine structural proteins; core contains several enzymes |
|Envelope: None (transient pseudoenvelope is present during rotavirus particle morphogenesis) |
|Replication: Cytoplasm; virions not completely uncoated |
|Outstanding characteristics: |
Genetic reassortment occurs readily
Rotaviruses are the major cause of infantile diarrhea
Reoviruses are good models for molecular studies of viral pathogenesis
Structure and Composition
The virions measure 60–80 nm in diameter and possess two concentric capsid shells, each of which is icosahedral. (Rotaviruses have a triple-layered structure.) There is no envelope. Single-shelled virus particles that lack the outer capsid are 50–60 nm in diameter. The inner core of the particles is 33–40 nm in diameter (Figure 37-2). The double-shelled particle is the complete infectious form of the virus.
Electron micrograph of a negatively stained preparation of human rotavirus. D, double-shelled particles; E, empty capsids; i, fragment of inner shell; io, fragments of a combination of inner and outer shell; S, single-shelled particles. Inset: Single-shelled particles obtained by treatment of the viral preparation with sodium dodecyl sulfate. Bars, 50 nm. (Courtesy of J Esparza and F Gil.)
The reovirus genome consists of double-stranded RNA in 10–12 discrete segments with a total genome size of 16–27 kbp, depending on the genus. Whereas rotaviruses contain 11 genome segments, orthoreoviruses and orbiviruses each possess 10 segments and coltiviruses have 12 segments. The individual RNA segments vary in size ...