Weekly variations in norovirus genogroup II genotypes in Japanese oysters.

Increased levels of norovirus contamination in oysters were reportedly associated with a gastroenteritis epidemic occurring upstream of an oyster farming area. In this study, we monitored the norovirus concentration in oysters weekly between November 2014 and March 2015 and investigated the statistical relationship between norovirus genogroup II (GII) concentrations in oyster and sewage samples and the number of gastroenteritis cases in the area using cross-correlation analysis. A peak correlation coefficient (R = 0.76) at a time lag of +1 week was observed between the number of gastroenteritis cases and norovirus GII concentrations in oysters, indicating that oyster contamination is correlated with the number of gastroenteritis cases with a 1-week delay. Moreover, weekly variations in norovirus GII genotypes in oysters were evaluated using pyrosequencing. Only GII.3 was detected in November and December 2014, whereas GII.17 and GII.4 were present from January to March 2015. GII.17 Kawasaki 2014 strains were detected more frequently than GII.4 Sydney 2012 strains in oyster samples, as previously observed in stool and sewage samples collected during the same study period in Miyagi, Japan. Our observations indicate that there is a time lag between the circulation of norovirus genotypes in the human population and the detection of those genotypes in oysters.