Two-Year Systematic Study To Assess Norovirus Contamination in Oysters from Commercial Harvesting Areas in the United Kingdom

Abstract

The contamination of bivalve shellfish with norovirus from human fecal sources is recognized as an important human health risk. Standardized quantitative methods for the detection of norovirus in molluscan shellfish are now available, and viral standards are being considered in the European Union and internationally. This 2-year systematic study aimed to investigate the impact of the application of these methods to the monitoring of norovirus contamination in oyster production areas in the United Kingdom. Twenty-four monthly samples of oysters from 39 United Kingdom production areas, chosen to represent a range of potential contamination risk, were tested for norovirus genogroups I and II by using a quantitative real-time reverse transcription (RT)-PCR method. Norovirus was detected in 76.2% (643/844) of samples, with all sites returning at least one positive result. Both prevalences (presence or absence) and norovirus levels varied markedly between sites. However, overall, a marked winter seasonality of contamination by both prevalence and quantity was observed. Correlations were found between norovirus contamination and potential risk indicators, including harvesting area classifications, Escherichia coli scores, and environmental temperatures. A predictive risk score for norovirus contamination was developed by using a combination of these factors. In summary, this study, the largest of its type undertaken to date, provides a systematic analysis of norovirus contamination in commercial oyster production areas in the United Kingdom. The data should assist risk managers to develop control strategies to reduce the risk of human illness resulting from norovirus contamination of bivalve molluscs.T he contamination of bivalve shellfish with norovirus from human fecal sources is recognized as a major human health risk (21). Risk assessment and management currently rely on the use of Escherichia coli as an indicator of fecal (sewage) contamination in shellfish (3). However, this approach has been repeatedly demonstrated to inadequately indicate the risk from human enteric viruses (2,8,16). Over the last decade, considerable progress has been made toward the development of sensitive quantitative detection methods for norovirus in molluscan shellfish, and there are numerous reports describing various approaches to virus testing (19,24,25). All available methods are based on the detection of virus genomes by using molecular techniques (PCR), as the successful cultivation of norovirus by conventional virological techniques has been reported only rarely (33, 34) and is not currently feasible for application to foodstuffs. At present, no internationally accepted reference method exists; however, the European Committee on Normalization (CEN) has an active working group addressing the development of an ISO standard method for the detection of norovirus and hepatitis A virus (HAV) in foodstuffs, including bivalve shellfish (22). The method is scheduled for publication in 2012, and publication will be followed by an internatio...

Environmental Transmission of Human Noroviruses in Shellfish Waters

“…Several studies have documented higher prevalences and levels of both NoV GI and NoV GII in commercially harvested shellfish during the winter (102,103,104,105,106,107), although some studies have reported no substantial summer-winter seasonality with respect to the prevalence of these genogroups in shellfish (108) and shellfish-growing waters (109). This discrepancy could be due to high fluxes of sewage contamination throughout the year in the latter study sites.…”

“…In the United Kingdom, average levels of NoV in Pacific oysters (Crassostrea gigas) during winter months (October to March) may be as much as 17 times higher than those during the remainder of the year (110). This seasonal pattern has been observed to mirror variations in water temperature (104) and in salinity and rainfall (103). Richards et al showed that long-term (120-day) frozen conditions do not decrease NoV RNA titers and that repeated freezing and thawing do not inactivate NoV in the environment (111).…”

Application of Long-Range and Binding Reverse Transcription-Quantitative PCR To Indicate the Viral Integrities of Noroviruses

“…The observed prevalence ranged from 3.9 to 76.2% in shellfish (14,28,29), 6.7 to 55.5% in fresh produce (30), and 15.1 to 45% in drinking water sources and surface water (31,32). These positive NoV signals may induce recalls or rejections of food batches associated with NoV contamination, causing economic loss in international markets.…”

Norovirus

“…Fruits, vegetables, and shellfish pose a significant risk for disease transmission because they are consumed raw and may be subject to norovirus contamination from water sources. Norovirus has been recovered from a variety of food products ranging from oysters to romaine lettuce, raspberries, and other produce (119)(120)(121)(122)(123)(124)(125). Additionally, the use of contaminated water for reconstituting food products has been implicated in widespread outbreaks (e.g., custard in Bristol, United Kingdom) (126).…”