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

Bivalve molluscs are awell 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 bivalvemolluscs (185mussels, 66 clams, 23 oysters and 62 samples from other species)
collected in harvesting areas of class A and B of four Italian Regionswere 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 b 0.0001) and quantity (average contamination level of 3.1 × 102
vs. 1.9 × 103 copies/g; p b 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.