Publication Abstract

Norovirus is the principal agent of bivalve molluscan shellfish-associated gastroenteric illness worldwide.Currently, non-cultivable human noroviruses (NoV) are detected in bivalve molluscan shellfish using molecular assays such as real-time reverse transcription-polymerase chain reaction (qRT-PCR). In addition to infectious viruses, this methodology may also detect non-infectious NoV including fragments of the NoV genome. This study addresses, in part, the implications of qRT-PCR results for the detection of NoV in shellfish in the absence of an infectivity assay. Firstly, to evaluate environmental persistence, the stability of a short fragment of the NoV genome, spanning the qRT-PCR target in the ORF 1/2 junction, was assessed in seawater under artificial environmental conditions simulating winter in the U.K. (1 mW/cm2 UV irradiation, 8°C) over a 4 week period. Detectable RNA levels decreased exponentially (T90 of approximately 141 hours), however sequences were still detectable for up to 2 weeks. Secondly, the ability of Pacific oysters (Crassostrea gigas) to bioaccumulate NoV particles (from human faeces) and RNA fragments was compared using qRT-PCR. While oysters exposed to NoV particles subsequently tested positive by qRT-PCR at levels several orders of magnitude in excess of the theoretical limit of detection, oysters exposed to similar quantities of NoV RNA tested either negative or positive at significantly lower levels. This study suggests therefore that whilst non-infectious fragments of norovirus RNA may persist in the environment under winter conditions, this type of material will not be efficiently bioaccumulated by Pacific oysters and should not therefore significantly contribute to positive qRT-PCR results.