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Noroviruses have emerged as the leading worldwide cause of acute non-bacterial gastroenteritis in humans. The presence of noroviruses in diarrheic stool samples from calves on Michigan and Wisconsin dairy farms was investigated by RT-PCR. Norovirus-positive samples were found on all eight farms studied in Michigan and on 2 out of 14 farms in Wisconsin. Phylogenetic analyses of partial polymerase and capsid sequences, derived for a subset of these bovine noroviruses, showed that these strains formed a group which is genetically distinct from the human noroviruses, but more closely related to genogroup I than to genogroup II human noroviruses. Examination of 2 full and 10 additional partial capsid (ORF2) sequences of these bovine strains revealed the presence of two genetic subgroups or clusters of bovine noroviruses circulating on Michigan and Wisconsin farms. One subgroup is “Jena-like”, the other “Newbury agent-2-like”.