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Chapter 16 : Foodborne Noroviruses
Category: Applied and Industrial Microbiology; Food Microbiology
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The noroviruses (NVs) cause approximately one illness for every 30 persons per year on a worldwide basis and are now recognized as the number one cause of foodborne illness in the United States, causing about 9 million foodborne infections per year. Human calciviruses fall into two major genera, Norovirus and Sapovirus. Sapoviruses also cause gastroenteritis, principally in infants, and are less commonly associated with foodborne transmission than noroviruses (NVs). NVs are nonenveloped, icosahedral viruses ranging from 27 to 35 nm in diameter. The icosahedral virion has T=3 symmetry and is composed of 90 dimers of the single capsid protein VP1. The first 225 amino acids form the shell domain, which is critical for icosahedron formation. One or two copies of VP2 are believed to be within the capsid. The high isoelectric point of VP2 suggests the possibility of an interaction between the positively charged virus protein and the viral RNA during virus assembly. STAT-1 mice are deficient for interferon production and interferon-mediated responses. On the surface, this information suggests that the innate immune response may be critical for successful defense against NV. For a protective immune response in normal mice inoculated with MNV-1, it has recently been shown that both adaptive CD4 and CD8 T-cell responses are critical for a protective immune response. The high degree of variability of different NV strains suggests that there is significant evolutionary immune selection on NVs. Currently, outbreaks from virus strains from the GII.4 cluster account for approximately 80% of outbreaks worldwide.
Electron micrograph of Norwalk virus, the prototypical G1.1 human norovirus. Reprinted from reference 95 with permission.