Viral Variability

Both genomic and antigenic analyses have shown that the noroviruses are highly variable. They are divided into five major genogroups within which are clustered a number of genotypes on the basis of sequence variations in polymerase and capsid genes. Most human infections

Fig. 1 Negative stain electron micrograph of noroviruses showing their ''feathery edge.''

Fig. 2 Negative stain electronmicrograph of a sapovirus demonstrating cup-shaped depressions and the classic ''Star of David'' appearance.

are due to genotypes I and II with genogroup II being responsible for the majority (73%) of outbreaks of disease.[6] Genotype IV viruses have also been detected in humans, but their role in disease is unclear. Genogroup II noroviruses have recently been detected in pigs,[7] but so far genogroup III viruses are restricted to cattle, and the recently described genogroup V norovirus was found in a mouse.[8] However, the genome is constantly mutating and within each genogroup and genotype variants can be found. There is even evidence for mutation of a genogroup II norovirus from a heart transplant patient with chronic diarrhea lasting over 2 years.[9] The mutations tend to accumulate in the P2 region of the capsid protein, which is a major target for the immune response. There are fewer studies of sapovirus variability. Studies based on part of the RNA polymerase (278-286 nucleotides) and capsid genes have divided sapoviruses into five genogroups. As with the noroviruses there are a number of genotypes within the genogroups and increasing evidence of great variability.[10]

REPLICATION, PATHOGENESIS, AND IMMUNITY

Neither human noroviruses nor sapoviruses have been cultured in cell lines or intestinal organ culture so little is known of their replication in vivo or in vitro.

Both viruses are excreted in feces and vomitus, and infection is acquired by ingestion or even inhalation of aerosolized vomit. The incubation period ranges from 10 to 51 hr with a mean of 24 hr in volunteers.[11] The site of infection in the gastrointestinal tract is unknown but from the short incubation period it is likely to include the jejunum.[12] The receptor(s) for noroviruses appear to be the ABH histo-blood group antigens and heparan sulfate proteoglycans.1-13-1 How human caliciviruses cause disease expression is not known. Although it was previously thought that virus shedding was short-lived, there is increasing evidence for virus excretion before onset of disease and for several weeks after clinical recovery.1-11,14,15-1 Immunity to human caliciviruses is poorly understood. In challenge experiments, six volunteers who developed gastroenteritis after ingesting Norwalk virus developed disease when challenged again with the same virus 27-42 months later, despite a brisk antibody response.[11] There does, however, appear to be a correlation between antisapovirus antibodies and immunity to infection.

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