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Chapter 48 : Human Caliciviruses
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Molecular cloning and characterization of the norwalk virus (NV) genome allowed this virus to be classified as a member of the family Caliciviridae. Although structural studies confirm that NV contains cup-shaped depressions, these depressions are often clearer in other strains of animal caliciviruses and human caliciviruses (HuCVs). Consistent with the fact that chemical agents are the most important cause of food-borne and waterborne disease, NV is resistant to inactivation following treatment with chlorine concentrations usually present in drinking water, and NV is more resistant to inactivation by chlorine than poliovirus 1, human rotavirus (Wa), simian rotavirus (SA11), or f2 bacteriophage. The cloning and expression of the NV genome resulted in the development of new assays and reagents that permit large-scale epidemiological studies. Much of the understanding of the epidemiology of noroviruses (NoVs) has come from studying the cause of outbreaks of water-borne and food-borne gastroenteritis. The first immunologic assays developed to detect antigen and amenable to large-scale use were radioimmunoassays (RIAs) and, subsequently, enzyme immunoassays (EIAs). Additional antigen detection EIAs were developed using other NoV and sapovirus (SaV) recombinant viruslike particles (rVLPs) as antigens for production of hyperimmune antisera. A number of different primer pairs and different reverse transcriptase PCR (RT-PCR) conditions have been described for the detection of NoVs and sapoviruses (SaVs). Serologic assays are used to determine the presence of virus-specific immunoglobulin M (IgM) in individual sera or rising total antibody levels in paired sera.
Electron micrographs of caliciviruses. Negative-stain electron micrographs of an NoV (previously called SRSV) from the stool of a volunteer given NoV/NV/8fIIa (A); an SaV with the classical calicivirus morphological features, including distinct cup-like indentations in the surface of the particles, taken from the stool of a child and containing SaV/Sapporo (B); 38nm rNV particles produced and purified from insect cells infected with a baculovirus recombinant that expresses NV ORF2 (C); and 19-nm particles produced and purified from insect cells infected with a baculovirus recombinant that expresses NV ORF2 (D). Bar, 50 nm. (Panel D kindly provided by L. White.)
Genomic organization of NoVs and SaVs. Schematic of the genomic organization of two human genogroup I and II NoVs, the murine genogroup V NoV, and a human genogroup II SaV. The NoVs have three predicted ORFs: ORF1, which encodes a polyprotein that contains the nonstructural proteins NS1 to NS7; ORF2, which encodes the major capsid protein (VP1); and ORF3, which encodes a minor capsid protein (VP2). For SaVs, ORF1 is longer and contains VP1 (see text for details). Nucleotide numbers denoting ORFs are indicated for each of the viruses. Molecular weights of each of the viral proteins are also indicated. Among the NoVs, NS1 and NS2 form a single protein, and for the SaV strain indicated, NS6 and NS7 form a single protein. This information is compiled from GenBank sequences M87661 (NV), AF145896 (Camberwell), AY228235 (murine), and AY237420 (Mc10) and references 42 , 97 , 98 , 115 , and 118 .
Histologic alterations observed in a volunteer after challenge with NV. Jejunal tissues from biopsy samples of a volunteer prior to challenge (A) and after challenge (B) with NV are shown. The villi are broadened and flattened during NV gastroenteritis illness. Hematoxylin and eosin stain; magnification × 62. (Reproduced from reference 2 with permission.)
Clinical outcome of infection with NV in two volunteers. The clinical course of two volunteers who became ill after challenge with 8fIIa NV inoculum (at 0 h) is depicted. Both volunteers were considered to have severe disease. Volunteer 503 was a 29-year-old man, while volunteer 516 was a 23-year-old woman ( 31 ). NEG, negative; POS, positive; Ab, antibody; Abd, abdominal; –, negative; +, positive; ±, equivocal.
Prototype strains of Norovirus and Sapovirus by genogroups and genotypes a
HuCV structural and nonstructural proteins
Responses of 50 adult volunteers (19 to 39 years old) given NV a
Comparison of methods to detect HuCVs