Evaluation of Norovirus Reduction in Environmentally Contaminated Pacific Oysters During Laboratory Controlled and Commercial Depuration

Abstract

Norovirus contamination of oysters is the lead cause of non-bacterial gastroenteritis and a significant food safety concern for the oyster industry. Here, norovirus reduction from Pacific oysters (Crassostrea gigas), contaminated in the marine environment, was studied in laboratory depuration trials and in two commercial settings. Norovirus concentrations were measured in oyster digestive tissue before, during and post-depuration using the ISO 15216-1 quantitative real-time RT-PCR method. Results of the laboratory-based studies demonstrate that statistically significant reductions of up to 74% of the initial norovirus GII concentration was achieved after 3 days at 17–21 °C and after 4 days at 11–15 °C, compared to 44% reduction at 7–9 °C. In many trials norovirus GII concentrations were reduced to levels below 100 genome copies per gram (gcg−1; limit of quantitation; LOQ). Virus reduction was also assessed in commercial depuration systems, routinely used by two Irish oyster producers. Up to 68% reduction was recorded for norovirus GI and up to 90% for norovirus GII reducing the geometric mean virus concentration close to or below the LOQ. In both commercial settings there was a significant difference between the levels of reduction of norovirus GI compared to GII (p < 0.05). Additionally, the ability to reduce the norovirus concentration in oysters to < LOQ differed when contaminated with concentrations below and above 1000 gcg−1. These results indicate that depuration, carried out at elevated (> 11 °C) water temperatures for at least 3 days, can reduce the concentration of norovirus in oysters and therefore consumer exposure providing a practical risk management tool for the shellfish industry.

Norovirus Persistence in Oysters to Prolonged Commercial Purification

“…Most of the studies regarding the purification rate of noroviruses from oysters have been done with artificially contaminated oysters [ 11 , 13 , 19 , 25 , 26 ]. As far as we know, only two research studies have evaluated the effectiveness of depuration process in the reduction of NoV in naturally contaminated oysters (species Crassostrea gigas ) subjected to purification for 5 [ 18 ] and 7 days [ 27 ]. Therefore, in our investigation, we chose to indagate NoV reduction in naturally contaminated oysters subjected to purification processes of different lengths (1, 4, and 9 days postharvest) in a commercially authorized plant.…”

Impact of COVID-19 Nonpharmaceutical Interventions on the Extent of Norovirus Contamination in Oyster Production Areas in Ireland during Winter 2020 to 2021

“…As a result, mitigation measures used by some oyster farmers in Ireland to reduce norovirus contamination in final product oysters were not required, unlike during previous winter periods. Mitigation measures used by the oyster farmers include relocation of product during the winter months to a site less impacted by sewage pollution to prevent oyster contamination, enhanced depuration, or a combination of both (11,32,33). Despite the high concentrations detected in prepandemic winters, there were no outbreaks associated with the oysters from these production areas.…”

Assessment of the Impact on Human Health of the Presence of Norovirus in Bivalve Molluscs: What Data Do We Miss?

“…However, despite the achievement of bacterial end-product standards, depuration may not be effective in safeguarding against viral contamination [13,[152][153][154]. In the European Union, depuration requirements vary according to the classification of harvesting areas but minimum time and water temperature are not stipulated for commercial depuration [155], which lasts generally 24-48 h [156]. Viral depuration is usually considered to be be 'two phase', where elimination in the first few days is more rapid than in subsequent days [157][158][159][160].…”