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Scientific Opinion on Norovirus (NoV) in oysters: methods, limits and control options

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NoV is highly infectious, and there is no threshold infectivity limit for NoV detected by PCR. The probability of becoming infected increases with the dose but depends also on the characteristics of the organism, the food matrix and the host factors. The relationship between the number of infectious virus particles and the number of virus genome copies detected by quantitative PCR is not a constant, and it is important to realise that the infectious risk associated with low level positive oysters as determined by real-time PCR may be overestimated.

Quantitative data on viral load from areas compliant with current EU legislative requirements (E. coli standards) during January-March 2010 in 3 selected member states, show that a viral limit of 100, 200, 500, 1000 or 10.000 NoV PCR copies would result in 33.6-88.9%, 24.4-83.3%, 10.0-72.2%, 7.7-44.4% or 0-11.1% of non-compliant batches, respectively. Compliance with any of the above NoV limits would reduce the number of contaminated oysters placed on the market and therefore the risk for consumers to become infected. It is currently not possible to quantify the public health impact of different limits.

Microbiological criteria for NoV in oysters are useful for validation and verification of HACCP-based processes and procedures, and can also be used by competent authorities as an additional control to improve risk management in production areas, during processing and retail. The Panel recommended that risk managers should consider establishing an acceptable limit for NoV in oysters to be harvested and placed on the market. NoV testing of oysters (standardized CEN method) should be used to verify compliance with the acceptable NoV limit established.

The most effective public health measure to control human NoV infection from oyster consumption is to produce oysters from areas which are not faecally contaminated, particularly given the ineffectiveness of current depuration and relaying procedures.