Introduction
Taxonomic Structure of the Family
Virion Properties
Morphology
Virions are nonenveloped with icosahedral symmetry. They are 27-40 nm in diameter by negative stain electron microscopy and 35-40 nm by electron cryomicroscopy. The capsid is composed of 90 dimers of the major structural protein arranged on a T = 3 icosahedral lattice. A characteristic feature of the capsid architecture is the 32 cup-shaped depressions at each of the icosahedral 5-fold and 3-fold axes. In some negative stain virus preparations, the cup-shaped depressions appear distinct and well-defined, while in others, these depressions are less prominent (Fig. 1).
Physicochemical and Physical Properties
Virion Mr is approximately 15 
106. Virion buoyant density is 1.33-1.41 g/cm3 in CsCl and 1.29 g/cm3 in glycerol-potassium tartrate gradients. Virion S20w is 170-187S. Physicochemical properties have not been fully established for all members of the family. Rabbit hemorrhagic disease virus (RHDV) in the genus Lagovirus has been reported as stable over a wide range of pH values (4.5 to 10.5). The genus “Norwalk-like viruses” has been shown in studies with one of its members (Norwalk virus, NV) to be acid, ether, and relatively heat stable. For those strains examined in the genus Vesivirus, inactivation occurs at pH values between 3 and 5, thermal inactivation is accelerated in high concentrations of Mg++ ions, and virions are insensitive to treatment with ether, chloroform, or mild detergents.
Nucleic Acid
The genome consists of a linear, positive sense, ssRNA molecule of 7.4-8.3 kb. A protein (VPg, Mr 10-15 103) is covalently attached to the 5
-end of the genomic RNA and the 3-end is polyadenylated. Subgenomic RNA (2.2-2.4 kb) is synthesized intracellularly and is VPg-linked in RHDV and Feline calicivirus (FCV). The gene order for RHDV as determined by in vitro translation studies is 5-p16-p23-p37(helicase)-p30-VPg-protease-polymerase-VP60(capsid)-VP10-3.
Proteins
Virions are constructed predominantly from one major species of capsid protein (CP) (Mr 58-60 103). A second minor structural protein (Mr 10 103) has been found in association with RHDV virions, but this protein has not yet been confirmed for other caliciviruses. Nonstructural proteins have homology with those of the family Picornaviridae of replicative enzymes and include 2C helicase, 3C cysteine protease, and 3D RNA-dependent RNA polymerase domains. The calicivirus VPg (Mr 10-15 103) is covalently linked to the viral RNA and maps to the region of the calicivirus genome analogous to the 3B region of the picornaviruses, but has no apparent amino acid homology with those of picornaviruses. Mapping studies are in progress to establish precursor and product relationships of the calicivirus nonstructural proteins.
Lipids
None reported.
Carbohydrates
None reported.
Genome Organization and Replication
The positive strand genomic RNA is organized into either two or three major ORFs. The nonstructural proteins are encoded in the 5-end of the genome and the structural proteins in the 3-end. Replication occurs in the cytoplasm and two major positive-sense RNA species are found in infected cells. The genome-sized positive-sense RNA serves as the template for translation of a large polyprotein that undergoes cleavage by a virus-encoded protease to form the mature nonstructural proteins. A subgenomic-sized positive strand RNA co-terminal with the 3-end of the genome is the template for translation of the major viral CP as well as the 3-terminal ORF product. A dsRNA corresponding in size to the full-length genomic RNA has been identified in FCV and San Miguel sea lion virus (SMSV)-infected cells, indicating that replication occurs via a negative strand intermediate (Fig. 2).
Antigenic Properties
Cross-challenge studies in the natural host and experiments with monoclonal antibodies indicate that RHDV and European brown hare syndrome virus (EBHSV) are antigenically distinct. Serotypes have been defined by cross-challenge studies, immune electron microscopy or solid phase immune electron microscopy for noncultivatable strains in the genera “Norwalk-like viruses” and “Sapporo-like viruses”. Numerous serotypes have been established by neutralization for Vesicular exanthema of swine virus (VESV) and SMSV strains. One serotype has been described for FCV strains, but considerable antigenic variation within this serotype has been reported. Recombinant virus-like particles (rVLPs) have been generated by expression of the major calicivirus structural CP in baculovirus and plant expression systems. These VLPs are highly immunogenic and similar in antigenicity to native virions.
Biological Properties
Caliciviruses infect a broad range of animals that includes hares, rabbits, pigs, cats, pinnipeds, cattle, reptiles, skunks, cetaceans, chimpanzees, and humans. Although individual calicivirus species generally exhibit a natural host restriction, the VESV species of the genus Vesivirus is an exception, showing a broad host range. For example, VESV-like viruses have been isolated from several marine animal species (including fish), birds, reptiles, and land mammals. The geographic distribution of each calicivirus species usually reflects the host distribution.
Transmission is via contaminated food, water, fomites, and on occasion via aerosolization of fecal material, vomitus or respiratory secretions. In general, no vectors appear to be involved in transmission; however, mechanical arthropod vector transmission of RHDV has been described.
Caliciviruses are associated with a number of disease syndromes. RHDV is associated with a generalized viremic infection in which there is massive liver necrosis that triggers a disseminated intravascular coagulation and rapid death in rabbits greater than three months of age. A nonvirulent RHDV strain has been described. EBHSV is similar to RHDV but appears to be less virulent. Human caliciviruses in the genera “Norwalk-like viruses” and “Sapporo-like viruses” induce a generally self-limited gastroenteritis with symptoms that may include nausea, diarrhea, vomiting, abdominal cramping, fever, and malaise. VESV produces clinical signs in swine that are sometimes indistinguishable from foot-and-mouth disease, including vesicles in the mouth, tongue, lips, snout and feet between the digits. In addition, the virus may cause encephalitis, myocarditis, fever, diarrhea, abortion and failure of infected animals to thrive. SMSV is similar to VESV, although there have been limited studies of natural infection in the marine host. Primate calicivirus causes mucosal vesiculation and persistent infection. FCV is associated in cats with conjunctivitis, rhinitis, pneumonia, mucosal vesiculation, diarrhea, and paresis and can produce a persistent infection with virus latent in the tonsils.
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