Reduction of norovirus concentrations in oysters during application of risk management procedures

Norovirus contamination in oysters represents a significant public health risk. Current control measures applied to oyster production fail to fully protect consumers from this risk and the extent to which current risk... [ view full abstract ]

Norovirus contamination in oysters represents a significant public health risk. Current control measures applied to oyster production fail to fully protect consumers from this risk and the extent to which current risk management procedures can reduce NoV concentrations is not well defined. In this study, we used a standardised real-time RT-PCR (ISO/TS 15216-1) method to determine the reduction of NoV concentrations in oysters during the application of risk management procedures during commercial production of oysters. Risk management procedures included relay of contaminated oysters in a clean site and depuration for extended periods (up to 7 days) at elevated temperatures.  Extensive monitoring at the commercial production site was conducted between January 2015 and until February 2017. Combined treatments showed reductions in NoV concentrations. Ninety-two percent of samples from treated oyster contained NoV concentrations below 100 genome copies per gram (LoQ for the test) compared to just 29% prior to treatment.  In addition, we undertook laboratory scale depuration experiment under controlled conditions to investigate the impact of seawater temperature on NoV reduction and optimise depuration conditions. This study reveals that the risk management procedures, such as relaying of oysters as well as post-harvest depurations can reduce the concentration of norovirus in market ready oysters to low levels but in general does not fully eliminate NoV. The public health significance of the residual concentrations of NoV observed in oysters following treatment in this study remains unclear.