The Food Safety Authority of Ireland asked the Panel on Biological Hazards to issue a scientific Opinion on: (i) The use of real-time reverse-transcription polymerase chain reaction (rRT-PCR) as a means of detection and quantification of Norovirus (NoV) in oysters; (ii) on Limits that do not pose an unacceptable risk to consumers for NoV genogroups GI and GII in oysters as determined by real-time PCR, and (iii) on Treatment regimes (post-harvest interventions) that can be relied upon to reduce NoV counts in oysters.).
On the use of PCR, the BIOHAZ Panel concluded that PCR-based detection methods exist for NoV in bivalve shellfish. Harmonization and standardization are currently ongoing for NoV in shellfish, and publication of methods is expected soon. To achieve a good sensitivity separate assays are required for NoV GI and GII detection. With the appropriate quality assurance measures, including accreditation and proficiency testing, the standardised CEN method is considered suitable for use for detection and quantification of NoV in oysters within a legislative context. The Panel also recommended that research needs to be conducted to establish the relationship between detection of NoV in oysters by PCR and human health consequences.
On the question of NoV limits, the Panel concluded that: NoV can be frequently detected by rRT-PCR in oysters during winter in European areas compliant with current legislative requirements (E. coli standards) for which data are available. NoV is highly infectious, and exposure of human volunteers to serial dilutions yielded a dose-dependent probability of becoming ill ranging from 0.1 (at a dose of 103 NoV genome copies) to 0.7 (at a dose of 108 virus genome copies). However, there is no threshold infectivity limit for NoV detected by rRT-PCR. From outbreak published data, it can be concluded that NoV concentrations detected in oysters linked to human cases varied greatly from less than hundred copies to more than ten thousand per gram of material analysed. 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 may vary depending on environmental conditions including time from the initial release from the host. Furthermore, the number of genome copies detected by quantitative PCR may not relate to infectious NoV particles, and as a consequence the method can only be used to provide an indirect measure of risk, and when considering what is an acceptable level of NoV in oysters, it is important to realise that the infectious risk associated with low level positive oysters as determined by rRT-PCR may be overestimated.
Despite observed differences between the ability of NoV GI and GII to cause human infection via different transmission routes, there is insufficient knowledge on the dose response for each genogroup to allow a distinction. Therefore it is appropriate to consider the total NoV load (GI+GII) when establishing microbiological criteria. Quantitative data on viral load from areas compliant with current EU legislative requirements (E. coli standards) during January-March 2010 in 3 member states, show that a viral limit of 100, 200, 500, 1000 or 10,000 NoV PCR genome 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. The lower the limit the greater the consumer protection achieved. However, it is not currently possible to quantify the public health impact of establishment of different limits.
Microbiological criteria for NoV in oysters are useful for validation and verification of HACCP-based processes and procedures, and can be used to communicate to food business operators and other stakeholders what is an acceptable or unacceptable viral load for oysters to be placed on the market. Microbiological criteria for NoV in oysters could 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 on the basis of the data presented in this Opinion, risk managers should consider establishing an acceptable limit for NoV in oysters to be harvested and placed on the market. Competent authorities should consider the use NoV testing of oysters (standardized CEN method) to verify compliance with the acceptable NoV limit established. Food business operators should consider incorporating NoV testing of oysters to verify their HACCP plans to demonstrate compliance with the acceptable level. The quantitative levels of NoV within production areas and batches should be investigated further, in order to optimise sampling strategies. Furthermore, sampling schemes to comply with NoV criteria should be risk based, e.g. considering seasonality, faecal pollution levels, community outbreaks, and variability from year to year. Finally, an EU-wide baseline survey on NoV contamination in oysters should be considered, in order to estimate consumer exposure and to allow quantification of the impact on human exposure related to establishment of microbiological criteria.
On the question of post-harvest interventions to reduce NoV counts in oysters, the Panel concluded that: Current treatment regimes for products placed live on the market (depuration and relaying) as commonly practised do not effectively reduce NoV in oysters. Depuration and relaying may be improved by optimising process parameters to enhance NoV reduction (e.g. depuration times, water temperature). However, limited data is currently available. Alternative treatments such as commercial heat treatment and high pressure may be effective for NoV inactivation, but give rise to organoleptic changes which may be unacceptable to consumers. 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 control regimes.
The Panel recommended that control measures for NoV in oysters should focus on avoiding contamination by either preventing human faecal contamination in mollusc production areas, or restricting commercial harvesting from faecally-contaminated areas. In addition, further studies are needed to establish and optimise the effectiveness of depuration and relaying for NoV reduction using the standardised CEN method.