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

Abstract

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.

Environmental Transmission of Human Noroviruses in Shellfish Waters

“…Such contaminated shellfish may require extended relaying before they can be safely introduced into the market (32,119). Surveillance of NoV in commercial oyster production areas in the United Kingdom, Ireland, and France detected total levels of NoV (GI plus GII) ranging from Ͻ100 genome copies/g to 10 4 genome copies/g (33). Levels of NoV GI in oysters collected from commercial beds in Georgia in the United States ranged from 10 3 to 10 8 copies/g (88).…”

“…Many studies have detected NoV, sometimes at elevated levels, in oysters harvested from areas compliant with health status classification A (Ͻ230 most probable number [MPN] of E. coli/100 g) and classification B (Ͻ4,600 MPN of E. coli/100 g in 90% of samples) under the European system (32,33,110). Such contaminated shellfish may require extended relaying before they can be safely introduced into the market (32,119).…”

“…However, evidence from laboratory studies has demonstrated that NoV may persist for much longer periods than FIB in actively filtering shellfish (31). The implication is that legislative standards based on FIB may not provide an accurate estimate of the risk of contamination by the pathogen and therefore may not adequately protect consumers (32,33). Agencies responsible for protecting and promoting food safety worldwide have considered options to reduce the risk of NoV infection due to shellfish.…”

Relevance of F-Specific RNA Bacteriophages in Assessing Human Norovirus Risk in Shellfish and Environmental Waters

“…However, molecular methods are not without significant drawbacks. These techniques may underestimate the viral load because of the low sample mass used for detection (17), low extraction yields, the presence of reverse transcription-PCR (RT-PCR) inhibitors (4), and the possible presence of other nontargeted viruses. Consequently, laboratories sometimes fail to detect the presence of HuNoV genomes in oysters involved in global HuNoV gastroenteritis outbreaks (17).…”

“…These techniques may underestimate the viral load because of the low sample mass used for detection (17), low extraction yields, the presence of reverse transcription-PCR (RT-PCR) inhibitors (4), and the possible presence of other nontargeted viruses. Consequently, laboratories sometimes fail to detect the presence of HuNoV genomes in oysters involved in global HuNoV gastroenteritis outbreaks (17). On the other hand, a very high overestimation of the infectious risk may also occur (17)(18)(19) because of the persistence of viral genomes being greater than viral infectivity (20)(21)(22)(23), especially after disinfection treatment.…”