Qualitative and quantitative assessment of viral contamination in bivalve molluscs harvested in Italy

• Quantitative data on NoV and HAV in shellfish harvested in Italy were provided.
• NoV contamination levels in class A and B areas were significantly different.
• NoV contamination levels in different mollusc species were evaluated.

Bivalve molluscs are a well documented source of viral infection. Further data on shellfish viral contamination are needed to implement European Regulations with sanitary measures more effective against viral pathogens. To this aim, 336 samples of bivalve molluscs (185 mussels, 66 clams, 23 oysters and 62 samples from other species) collected in harvesting areas of class A and B of four Italian Regions were analyzed for qualitative and quantitative determination of hepatitis A virus (HAV) and Norovirus (NoV) GI and GII, using real time RT-PCR.The results showed a wide diffusion of viral contamination in the shellfish production areas considered. HAV prevalence was low (0.9%) with contamination levels that varied from 5 to 7 × 102 copies/g. On the contrary, NoV showed a high prevalence (51.5%), with a large variability according to the group considered (e.g. 47.8% for Crassostrea in Veneto, 79.7% for Mytilus in Campania, 84.6% for Tapes in Sardinia). NoV contamination affected class A and class B production areas to a different extent, with a statistically significant difference in both contamination prevalence (22.1% vs. 66.3%; p < 0.0001) and quantity (average contamination level of 3.1 × 102 vs. 1.9 × 103 copies/g; p < 0.05). The different species analyzed from class B harvesting areas (Mytilus, Tapes/Ruditapes and Crassostrea) showed a NoV prevalence respectively of 70.3%, 66.0% and 47.8% but comparable NoV contamination levels (between 8.4 × 102 and 4.9 × 103 copies/g). Other two bivalve species considered in the study (Donax spp. and Solen spp.) showed a relevant NoV presence (40.0% and 34.4% of samples). Finally, samples analyzed before and after commercial purification treatment showed a decrease of contamination prevalence after the treatment, but inconsistent results were recorded on NoV levels.The data obtained, together with other quantitative information to estimate consumer exposure, in association with studies on dose–response and on the effectiveness of post-harvest treatments, will provide a useful tool for the definition of microbiological criteria related to the different shellfish species.