Occurrence and molecular characterization of enteric viruses in bivalve shellfish marketed in Vietnam

Viral contamination of bivalve molluscs is an extensively document food safety issue. Limited data are available, however, on the prevalence and quantitative levels of such viruses in countries, as Vietnam, with an important shellfish production, both for internal consumption and for export. The aim of this study was to evaluate both the occurrence and the quantitative levels of different viruses, including Norovirus (NoV genogroups I, II and IV), Hepatitis A (HAV), Hepatitis E (HEV), Astrovirus (AstV), and Aichi virus (AiV), in bivalve shellfish commercialized in Vietnam. A total of 121 samples (63 Pacific oysters and 58 white hard clams) were collected over nine months in two fish markets and two supermarkets in Hanoi, Vietnam. Norovirus was the most frequently detected virus (81.8% of samples) with viral loads ranging from below the quantification limit (LOQ) to 2.8 × 10⁴ genome copies (g.c.)/g of shellfish digestive tissue and an average contamination of 3.8 × 10³ g.c./g. HAV, HEV and AstV were detected in 1.7%, 11.6% and 12.4% of samples respectively (average concentrations: 1.3 × 10² g.c./g for HAV, 1.2 × 10² for HEV and 1.5 × 10³ for AstV). AiV were also frequently detected (11.6% of samples), while NoV GIV was never observed. Contamination by at least one virus and by more than one virus was present in 83.5% and 52.9% of samples, respectively. Molecular characterization of NoV revealed circulation, over the observed period, of a wide variety of strains (four and nine genotypes for NoV GI and NoV GII respectively: GI.2, GI.4, GI.5, GI.6, GII.3, GII.4, GII.6, GII.7, GII.13, GII.14, GII.17, GII.18, GII.21). To our knowledge, this study is the first to provide quantitative information on the occurrence of enteric viruses in bivalve shellfish marketed in Vietnam, which will help outlining monitoring plans and setting appropriate control measures.