More than a third of raw oyster samples from production areas in Europe were positive for norovirus, according to a survey.

Data submitted to the European Food Safety Authority (EFSA) showed human fecal contamination is present in oyster production areas.

The survey found a significant difference in noroviruses (NoV) between production areas compared to dispatch batches. The prevalence at production areas was estimated to be 34.5 percent, while for batches from dispatch centers it was 10.8 percent.

“The presented findings of a more than one-in-three likelihood of EU oyster production being NoV contaminated, with such contamination in more than 1-in-10 batches dispatched for human consumption highlights the potential hazard of NoV when producing oysters for consumption as live bivalve mollusks,” according to a report, which can be read here.

Bivalve mollusks are a source of noroviral infection since they accumulate and concentrate NoV particles by filtration of water contaminated with feces. Probability of becoming infected increases with the dose but depends on characteristics of the organism, food matrix and the host factors.

Public health concern

Sampling took place in 12 member states between November 2016 and October 2018, in 172 production areas and 207 dispatch centers. Norway only took samples from production areas. A total of 2,180 valid samples came from production areas and 2,129 from dispatch centers.

Quantitative contamination levels showed a mean of around 337 copies per gram (cpg) in production area samples and around half that at 168 cpg in batches from dispatch centers.

Half of the positive batches (17.15 percent in production area samples and 5.59 percent of dispatch batches) had values over 200 cpg, while those over 500 cpg were 8.71 percent and 1.17 percent, respectively. Values below 100 cpg are unlikely to cause outbreaks but there is increased risk when levels exceed 500 cpg.

“This extent of contamination could be regarded as a public health concern, highlighting the particular risks associated with this food production system, supporting the need for active risk management strategies to mitigate NoV risk in this food chain in addition to those currently in place,” according to the study.

In the survey, the real-time PCR-based ISO (15216-1:2017) method to detect and quantify NoV was applied. This methodology can potentially amplify RNA from viable and non-viable viruses.

Seasonal and sampling area impact

Results for both genogroups were above the respective limit of quantification (LOQ) in less than 10 percent of the samples taken. For genogroup I the LOQ ranged between 40 and 298 copies and LOD between 13 and 264 copies. For genogroup II the LOQ was between 75 and 389 copies and the LOD between 20 and 196 copies.

Analyses showed a strong seasonal effect, with higher contamination in November to April, as well as lower contamination for Class A areas than other classes.

A total of 60 percent of samples from production sites were taken from Class B areas, 39 percent from Class A and less than 1 percent from Class C. Oysters from Class A areas do not require post-harvest treatment. Those from Class B must go through purification or relaying before being put on the market for direct human consumption as live animals. Conditioning was unexpectedly found to be associated with lower prevalence levels.

Findings suggest use of E. coli as a generalized indicator of fecal contamination in European shellfish hygiene regulations provides a useful indication on likelihood of contamination with NoV. However, Class A areas were not free from NoV, so other criteria should be considered when managing risk.

In the survey of dispatch centers, 61 percent of samples originated from Class A areas, 36 percent from Class B and 2.5 percent had an unknown origin of production area status.

Prevalence of NoV was generally lower in farmed oysters than wild ones and there was a trend of lower prevalence in Pacific cupped oysters, which are generally farmed, than European flat oysters, which are usually wild.

The current two bacteriological microbiological criteria applicable to bivalve mollusks placed on the market as live food products, might be complemented by a NoV criterion for operators of dispatch centers. Any microbiological criterion should take into account the point in the oyster production chain at which it would apply and seasonal variation in impact.