A One-Year Survey of Norovirus in UK Oysters Collected at the Point of Sale

Abstract

Contamination of bivalve shellfish, particularly oysters, with norovirus is recognised as a food safety risk and a potential contributor to the overall burden of gastroenteritis in the community. The United Kingdom (UK) has comprehensive national baseline data on the prevalence, levels, and seasonality of norovirus in oysters in production areas resulting from a previous two-year study (2009–2011). However, previously, data on final product as sold to the consumer have been lacking. As part of a wider project to establish the overall burden of foodborne norovirus in the UK, this study aimed to address this data gap. A one-year survey of oysters collected from the point-of-sale to the consumer was carried out from March 2015 to March 2016. A total of 630 samples, originating in five different European Union Member States, were collected from 21 regions across the UK using a randomised sampling plan, and tested for norovirus using a method compliant with ISO 15216-1, in addition to Escherichia coli as the statutory indicator of hygiene status. As in the previous production area study, norovirus RNA was detected in a high proportion of samples (68.7%), with a strong winter seasonality noted. Some statistically significant differences in prevalences and levels in oysters from different countries were noted, with samples originating in the Netherlands showing lower prevalences and levels than those from either the UK or Ireland. Overall, levels detected in positive samples were considerably lower than seen previously. Investigation of potential contributing factors to this pattern of results was carried out. Application of normalisation factors to the data from the two studies based on both the numbers of norovirus illness reports received by national surveillance systems, and the national average environmental temperatures during the two study periods resulted in a much closer agreement between the two data sets, with the notably different numbers of illness reports making the major contribution to the differences observed in norovirus levels in oysters. The large majority of samples (76.5%) contained no detectable E. coli; however, in a small number of samples (2.4%) levels above the statutory end product standard (230 MPN/100 g) were detected. This study both revealed the high prevalence of norovirus RNA in oysters directly available to the UK consumer, despite the high level of compliance with the existing E. coli-based health standards, while also highlighting the difficulty in comparing the results of surveys carried out in different time periods, due to variability in risk factors.Electronic supplementary materialThe online version of this article (10.1007/s12560-018-9338-4) contains supplementary material, which is available to authorized users.

Analysis of the European baseline survey of norovirus in oysters

“…These findings are consistent with expectations from the literature, indicating a real and ongoing potential for NoV contamination at the pre-harvest stage of oyster production, and for that to contribute to contamination of food placed on the market (FSA UK, 2012;Lowther et al, 2018). Quantitative contamination levels showed a mean of around 337 cpg in production area samples and around half that (168 cpg) in batches from dispatch centres.…”

“…Quantitative contamination levels showed a mean of around 337 cpg in production area samples and around half that (168 cpg) in batches from dispatch centres. These findings are consistent with expectations from the literature, indicating a real and ongoing potential for NoV contamination at the pre-harvest stage of oyster production, and for that to contribute to contamination of food placed on the market (FSA UK, 2012;Lowther et al, 2018). As with all bivalve molluscs, oysters filter water to extract food particles, and have a welldescribed potential for bioaccumulation of NoV present in the water resulting from contamination with human faeces (Lees, 2000).…”

Development of a method for direct extraction of viral RNA from bivalve molluscs

“…These values are in line with what reported in studies using the ISO 15216 method, where inhibition is usually in the range between null and 40% (Lowther et al . , ; Polo et al . ; Varela et al .…”

The use of capture-recapture methods to provide better estimates of the burden of norovirus outbreaks from seafood in England, 2004–2011

“…There are several reasons why reported summer NoV levels may be low and these may be separated into issues related to contamination and food consumption, and issues related to the reporting process. Considering the former, levels of norovirus in the UK produced oysters are shown to be higher during winter than summer [25, 26]. The effects of this could be exacerbated by specific consumption patterns, for example, consumption of shellfish on St. Valentine's Day [27].…”