Strategies to reduce norovirus (NoV) contamination from oysters under depuration conditions.

Depuration of oysters can effectively reduce levels of E. coli, however, may not be effective in safeguarding against viral contamination (EFSA, 2012). These trials assess the removal of Norovirus Genogroups I and II (NoV GI and GII) and F + RNA bacteriophage genogroup II (FRNAP-II) from oysters under depuration using molecular and viability assay methods. Our results show consistently better removal of NoV GII compared with Nov GI. We found approximately 46% removal of NoV GII at 18 °C after 2 days and 60% after 5 days compared with a maximum of 16% NoV GI removal. Twice the rate of NoV GII removal was achieved at 18 °C compared with 8 °C after 5 days. Results suggest better NoV removal when depuration water salinity is close to that prevailing in the harvesting area. Trials investigating algal feeding, light/dark and disturbance from pump vibration did not show any significant effect. We found that FRNAP-II was more readily removed than NoV. No significant difference was found between the rate of removal (as measured by RT-qPCR) and inactivation (as measured by bioassay) of FRNAP-II. This indicates that reduction in FRNAP-II may be primarily due to physical removal (or destruction) rather than in situ inactivation of the virus.

Assessment of the Applicability of Capsid-Integrity Assays for Detecting Infectious Norovirus Inactivated by Heat or UV Irradiation.

Human noroviruses are the leading cause of viral gastroenteritis. In the absence of a practical culture technique for routine analysis of infectious noroviruses, several methods have been developed to discriminate between infectious and non-infectious viruses by removing non-viable viruses prior to analysis by RT-qPCR. In this study, two such methods (RNase and porcine gastric mucin) which were designed to remove viruses with compromised capsids (and therefore assumed to be non-viable), were assessed for their ability to quantify viable F-specific RNA bacteriophage (FRNAP) and human norovirus following inactivation by UV-C or heat. It was found that while both methods could remove a proportion of non-viable viruses, a large proportion of non-viable virus remained to be detected by RT-qPCR, leading to overestimations of the viable population. A model was then developed to determine the proportion of RT-qPCR detectable RNA from non-viable viruses that must be removed by such methods to reduce overestimation to acceptable levels. In most cases, nearly all non-viable virus must be removed to reduce the log overestimation of viability to within levels that might be considered acceptable (e.g. below 0.5 log10). This model could be applied when developing alternative pre-treatment methods to determine how well they should perform to be comparable to established infectivity assays.