Family Caliciviridae

Caliciviruses are small, unenveloped positive-strand RNA viruses. The name calicivirus is derived from the Latin word “calyx” meaning cup or goblet describing the cup-shaped depressions on the capsid surface. Replication is cytoplasmic and RNA synthesis is primed with a virally encoded protein, VPg. Genome-length RNA serves as the mRNA for synthesis of a precursor polyprotein. A subgenomic mRNA, also linked to VPg, encodes one or two structural proteins. (There is some diversity in protein expression strategies.) In contrast to picornaviruses, caliciviruses do not have an IRES. Instead, cap-independent translation requires interactions of VPg with translation initiation factors. Caliciviruses have been isolated from humans, cattle, pigs, cats, chickens, reptiles, dolphins, and amphibians. As a group they have been difficult to study because many, including human noroviruses, are difficult to grow in cultured cells. Feline calicivirus and murine noroviruses are often used as surrogates for human noroviruses. Human noroviruses can cause large outbreaks of acute gastroenteritis and norovirus pathogenesis can only be studied by infecting adult human volunteers with bacterial-free fecal material. Noroviruses are easily transmitted (fecal–oral) and are extremely stable in the environment.