Negative correlation between the prevalence of norovirus and high bacterial loads of escherichia coli in oysters crassostrea gigas

Abstract

Seasonal variation in the prevalence of norovirus in oysters Crassostrea gigas was investigated and compared to levels of Escherichia coli, a fecal indicator in oysters. Oysters were collected from Iwon-myeon, Taean-gun, Korea, a primary production area for European Union export of oysters between 2013 and 2014. We observed seasonality in the prevalence of norovirus in oysters, with a higher prevalence and viral load detected during winter months. Oysters taken from production areas that complied with the European Union standard for raw consumption (< 230 MPN/100 g of E. coli) had 22.1% of the samples test positive for norovirus (15/68 samples). However, norovirus was not detected in any of the samples (0%, 0/4 samples) that were collected from production sites that exceeded the standard fecal contamination level for raw consumption (> 230 MPN/100 g of E. coli). These results indicated that there is a negative correlation between the prevalence of norovirus and high levels of E. coli in oysters. Therefore, our results suggest that current food safety guidelines using only a bacterial fecal contamination indicator, E. coli, may not adequately assess shellfish production areas for viral and bacterial contamination.