Caliciviruses are a group of enteric viruses that infect a broad variety of terrestrial and marine animals (1-3). Although human caliciviruses are currently classified in three genogroups, the human viruses are of two basic morphological and biological types: the classic human caliciviruses and the Norwalk-like viruses (NLVs). The latter are also referred to in the literature as small round structured viruses (SRSVs). Although examples of classic calicivirus strains causing gastroenteritis in adults have been reported (4), classic caliciviruses are normally associated with self-limiting gastroenteritis in infants and children. Overall, the classic caliciviruses only account for a small portion of infantile gastroenteritis cases (5). Consequently, classic caliciviruses are generally considered of less medical and foodborne significance than the NLVs.

The more common NLVs are associated with gastrointestinal illness in persons of all ages. The prototypical Norwalk virus was identified by Kapikian and coworkers from an outbreak of gastroenteritis among students and staff at an elementary school in Norwalk, Ohio, in 1968 (6). Since then, numerous NLV strains have been implicated in gastroenteritis outbreaks worldwide. Most of these strains have been named for the locations in which the outbreaks occurred, such as the Hawaii virus, Snow Mountain virus, Mexico virus, Desert Shield virus, etc. (7,8). While there are well over 100 different human enteric viruses capable of causing gastroenteritis, molecular epide-miological studies indicate that most of the recent nonbacterial gastroenteritis cases are caused by NLVs. In the United States, a recent study by the Centers for Disease Control and Prevention (CDC) implicated NLVs in 86 of 90 (96%) of the outbreaks of nonbacterial gastroenteritis reported to 33 state health departments between January 1996 and June 1997, with the consumption of food being the most commonly identified mode of transmission (9). Perhaps of even greater significance, Norwalk-like (NL) illness is common among the general population worldwide. The CDC estimates that approximately 23 million cases of NL illness occur annually in the United States, with 40% of these (9.2 million cases) being associated with the consumption of contaminated food (10). Overall, the CDC estimates that among foodborne illness mediated by known agents, NLVs account for 76% of all cases, 33% of the hospitalizations, and 7% of the deaths (10).

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