The rotavirus is particularly photogenic (Fig. 34-1). Smooth and round in outline, it is seen to have two concentric icosahedral shells. The 50 ran core of the virion, composed mainly of the structural protein VP2 (but also the tran-scriptase/replicase VP1 and the guanylyltransferase VP3) is surrounded by a 60 nm inner icosahedral capsid composed of the major group-specific protein VP6, which in turn is enclosed within a 70 nm outer icosahedral capsid composed mainly of the glycoprotein VP7, with dimers of the hemagglutinin/cell attachment protein VP4 projecting from the surface as 60 short spikes; 132 channels pass through both the outer and inner capsid to communicate with the core. The genome consists of II molecules of dsRNA, total size 18 kbp.
Rotaviruses are ubiquitous. Essentially every species of domestic animal
or bird that has been thoroughly searched has at least one indigenous rotavirus, causing diarrhea ("scours") in the newborn. Rotaviruses are classified into six serogroups, A-F, on the basis of differences between the major group-specific capsid antigen VP6. Most human rotaviruses fall into group A, but others, once called "atypical rotaviruses" or "pararotaviruses," belong to recently defined groups B and C.
Differentiation into serotypes within group A is based on neutralization tests. As both of the outer capsid proteins carry type-specific epitopes recognized by neutralizing antibodies, a binary system of classification of serotypes has been developed, akin to that used for influenza viruses. So far, 14 serotypes (G1-G14) have been defined on the basis of different VP7 antigens, whereas 8 serotypes (P1-P8) have been defined on the basis of different VP4 antigens. Human group A rotaviruses fall into nine G serotypes, 1, 2, 3, 4, and rarely 6, 8, 9, 10, or 12, and into four P serotypes, 1, 2, 3, and 4. Monoclonal antibodies can be used to make finer distinctions, for example, between subtypes within the G1 serotype, which is the major cause of severe rotavirus enteritis worldwide.
Large numbers of electropherotypes can be differentiated by poly-acrylamide gel electrophoresis (PAGE) of the viral RNA (Fig. 34-2). Because the patterns reflect differences in the migration of any of the 11 RNA segments, there is no direct relationship between electropherotypes and serotypes, but they are a valuable tool for tracking the epidemiology of individual strains.
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