The concentrations of norovirus (NV) genes in water samples from rivers A, B and C flowing into oyster farms in Miyagi Prefecture, Japan were determined from October 2002 to March 2004. Norovirus genes in 35 specimens collected over 17 months were measured using genogroup 1 (G1) and genogroup 2 (G2) specific real-time quantitative reverse transcription PCR techniques. The respective positive rates of NV genes in rivers A, B and C were 51% (18 of 35), 97% (34 of 35) and 26% (9 of 35). The respective concentrations of the NV G1 gene ranged from 85 - 5.3 × 103 copies, 80 - 9.6 × 104 copies and 85 - 1.7 × 103 copies in 100 ml of water samples from rivers A, B and C. The respective concentrations of the NV G2 gene ranged from 93 - 2.0 × 103 copies, 65 - 1.5 × 105 copies and 72 - 1.2 × 103 copies in 100 ml of the same water sample. These values were high during winter, and low but detectable during summer.
