Seasonal distribution and genetic diversity of genogroups I, II, and IV noroviruses in the Tamagawa River, Japan.

We investigated the prevalence, seasonal dependence, and genetic diversity of noroviruses (NoVs) in the Tamagawa River, which runs through a densely populated region in Tokyo, Japan, from April 2003 to March 2004. A total of 60 water samples were collected from five sites from upstream to downstream along the river, and 500 mL of which was concentrated using the "cation-coated filter method". Of the 60 samples tested, genogroup I (GI) and GII NoVs were detected from 28 (47%) and 18 (30%) samples, respectively. GIV NoV was successfully detected from 2 (3%) samples with a newly developed seminested RT-PCR assay specific for GIV. The occurrence of NoVs in the river was significantly higher in winter/spring than in summer/autumn and also in mid- to downstream rather than upstream. A total of 176 different NoV strains were identified from river water samples based on the phylogenetic analysis of partial capsid gene sequences. GI, GII, and GIV strains were clustered into 7 (GI/1, 2, 4, 5, 8, 9, and 11), 8 (GII/2, 3, 4, 5, 6, 11, 12, and 16), and 1 (GIV/1) genotypes, respectively. The results suggest that genetically diverse NoV strains are circulating between human populations and aquatic environments.