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To investigate the prevalence, seasonality and genetic diversity of genogroup IV noroviruses (GIV NoVs) in wastewater in Japan.Untreated and treated wastewater samples were collected monthly for a year from a wastewater treatment plant in Japan. The concentrated wastewater samples were examined for the presence of GIV NoV genomes with seminested RT-PCR assay targeting partial capsid gene. Among 12 untreated and 12 treated wastewater samples tested, GIV NoV genomes were detected in three (25%) untreated and two (17%) treated wastewater samples with a high positive ratio in winter season. Genetic analysis revealed that the GIV NoVs in the wastewater samples were genetically diverse and were classified into three different genetic clusters.Frequent detection of GIV NoVs in winter season, which is a common epidemic period of human NoVs in Japan, indicates that GIV NoVs exhibit temporal trends similar to GI and GII NoVs. Based on the partial capsid gene sequences, we identified several unique GIV NoV strains belonging to the novel genetic cluster, demonstrating that GIV NoVs are more genetically diverse than previously appreciated.Our findings provide novel evidence of considerable genetic diversity among the GIV NoV strains.