Analysis of the European baseline survey of norovirus in oysters

Abstract

The European Commission requested scientific technical assistance for the analysis of a European Union coordinated monitoring programme on the prevalence of norovirus in raw oysters. A total of 2,180 valid samples were taken from production areas and 2,129 from dispatch centres, taken over two consecutive years, ensuring the precision and the confidence desired in the estimation. The prevalence at production areas was estimated to be 34.5% (CI: 30.1-39.1%), while for batches from dispatch centres it was 10.8% (CI: 8.2-14.4%). The analyses show a strong seasonal effect, with higher contamination in the period November to April, as well as lower contamination for Class A areas than other classes. These associations were observed in both production areas and batches from dispatch centres. The results for both genogroups were above the respective limit of quantification (LOQ) in less than 10% of the samples taken. The simple substitution of not-detected and positive samples below the LOQ, by half of the limit of detection and half of the LOQ, respectively, produced estimates of the proportion of samples above or equal to 300 copies per gram (cpg) comparable to the statistical model. The current bacteriological microbiological criteria applicable to live bivalve molluscs might be complemented by a norovirus criterion. The analyses of the substitution approach show that selection of a potential limit within a microbiological criterion close to or lower than the LOQ (for example, less than 300 cpg, given the current test used in this survey) would be difficult to apply. This survey only assessed thresholds from the perspective of the analytical capability and not that of human health risk.

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

“…Data published in a recent EFSA baseline survey highlights the high prevalence of norovirus contamination in oysters from classified production areas during the winter months (EFSA 2019 ). Peak norovirus concentration and prevalence was observed in the months of January and February when almost 65% of samples tested were positive for norovirus with a mean concentration of 661 gcg −1 (EFSA 2019 ). This important data demonstrates the need for effective mitigation measures to reduce norovirus concentrations before reaching the consumer.…”

“…Maximum reductions of the norovirus concentrations in such artificially contaminated shellfish were previously reported in the region of 1–1.2 log 10 unit (equivalent to 90–94% reduction) and based on that evidence the process was deemed ineffective (McLeod et al 2017 ). However, the initial viral loads in such conditioned animals would be considered to be much higher than those seen in oysters from classified production areas across Europe as confirmed by the EFSA baseline survey conducted between November 2016 and October 2018 (EFSA 2019 ). Much less is therefore known about the efficacy of norovirus reduction of environmentally contaminated oysters from classified production areas under controlled laboratory conditions or in commercial operations.…”

“…In order to comply with EU regulations shellfish harvested from class B areas, accounting for approximately 60% of overall oyster production across the EU (EFSA 2019 ) must be depurated prior to going to market. Despite these regulatory obligations virtually eliminating bacterial illness associated with bivalve shellfish, there are many reports of such product causing outbreaks of viral illness (Morse et al 1986 ; Chalmers and McMillan 1995 ; Lee et al 1999 ; Kingsley et al 2002 ; Baker et al 2010 ; LeBlanc et al 2016 ).…”

“…The introduction of quantitative real-time PCR for the detection and quantification of norovirus in bivalve shellfish, particularly the method described in ISO 15216-1:2017 (Anonymous 2017 ), has been an important step in the area of risk management of norovirus. This tool has allowed better understanding of the relative differences in norovirus concentrations when applying risk management procedures (Rupnik et al 2018 ), as well as understanding the concentrations of norovirus found in classified production areas across the EU (EFSA 2019 ).…”

Passive Samplers, a Powerful Tool to Detect Viruses and Bacteria in Marine Coastal Areas

“…For samples presenting a ΔCt > 1, the Ct value used for quantification was the value obtained for the tenfold-diluted sample and then corrected using the slope of the standard curve. For a few samples (<7%), Ct values were between 39 and 40, thus under the limit of quantification; in those cases, quantification required substitution with LQ/2 value (LQ = 1.3 Log 10 ) ( EFSA, 2019 ).…”