Hygiene and health risks associated with the consumption of edible lamellibranch molluscs

Edible lamellibranch molluscs are frequently the cause of very serious outbreaks of food poisoning. Microbiological suitability for human consumption, according to EC Reg. n. 1441/2007, is only based on two faecal bacteriological contamination parameters (Escherichia coli and Salmonella) and does not consider other microorganisms which are naturally present within the marine environment and are potentially pathogenic such as those belonging to the genus Vibrio. In this study, a total of 540 samples of edible lamellibranch molluscs (mussels, clams and oysters) bred in Sardinia were analyzed for E. coli and Salmonella, according to EC Reg n. 1441/2005, and for the presence of Vibrios of major public health interest. Environmental parameters (seawater temperature and salinity) were also recorded. The 40% (n=216) of samples were positive for Vibrio while the levels of E. coli were above the legal European limit only in 6.4% of samples. A correlation between the presence of micro-organisms of faecal origin and potentially harmful Vibrios to human health was not observed. Vibrio alginolyticus was the most common in 68% of isolated species, followed by Vibrio parahaemolyticus in 22.6%, Vibrio cholerae in 6% and Vibrio vulnificus in 1.1%. Virulence gene (trh sequence) was present in 1.6% of the V. parahaemolyticus-positive samples. The V. cholerae, V. vulnificus and V. alginolyticus strains detected were non toxigenic. The absence of genes encoding major virulence factors in the studied strains does not exclude the possibility of exposure to pathogenic strains. This study indicates the potential health risk associated with the presence of pathogenic Vibrios in edible lamellibranch molluscs bred in Sardinia, where shellfish production is very important to the local economy.