Prevalence of norovirus in Dutch shellfish production area with low E.coli contamination

Bivalve molluscs are a well-known source of viral contamination as a result of contaminated production areas and consumer habits in eating raw and lightly cooked shellfish. Current legislation prescribes monitoring of E.coli,... [ view full abstract ]

Bivalve molluscs are a well-known source of viral contamination as a result of contaminated production areas and consumer habits in eating raw and lightly cooked shellfish. Current legislation prescribes monitoring of E.coli, however, criteria for norovirus are being considered. The success of monitoring is strongly dependent on virus prevalence, virus concentrations and on the sample size. In follow-up of the EFSA opinion (2012), the norovirus status was evaluated in two production areas in the South-western part of the Netherlands.

To calculated prevalence, 250 oysters for Area 1 and 150 oysters for Area 2 were harvested at one time. Since the norovirus concentration was expected to be low, oysters were analysed individually according to ISO/TS 15216-1. Area 1 is a stable class A area and results obtained for Norovirus are in line with its class A status. Neither norovirus GI nor GII could be detected. Samples were analysed in pools of 10 animals and individually. Area 2 is an area with E.coli levels above 230 once or twice a year. Norovirus results reveal that in almost all samples norovirus GI and GII could be detected in rather high concentrations, while E.coli levels were < 230 in the sampling period. Shellfish were not implicated in outbreaks. Analysing the data shows a homogenous distribution of GI and GII in the individual oysters over the different sample locations in area 2, indicating a similar source.