Occurrence of Vibrio parahaemolyticus in oysters before and after depuration
Résumé
The Thau lagoon (French Mediterranean coast), is of economic importance due to bivalve mollusk farming (15,000 t of mussels and oysters a year). The sanitary classification, based on E. coli concentrations in shellfish, of this production area is B. Shellfish must thus be stored in a depuration system for at least 24 hours before sale. However, these sanitary controls do not apply for vibrios of human health importance which are present in this lagoon. The aim of this study was to evaluate the presence of Vibrio parahaemolyticus in oysters before and after depuration, using MPN-PCR, and to understand the depuration practices of shellfish farmers. Oysters were collected from a production area every 15 days (May-October 2017-2018). Water temperature, salinity and rainfall values were recorded simultaneously. V. parahaemolyticus was first detected when water temperature was above 20°C and showed a seasonal dynamic in oysters collected from the production area. The highest concentrations of total V. parahaemolyticus were recorded during intense autumn rainfall events (1500 MPN/g). For depurated oysters, V. parahaemolyticus was first detected in June 2018 (230 MPN/g). Between July and September (T°C in tanks > 23°C), V. parahaemolyticus concentrations (up to 1800 MPN/g) were significantly higher than those in oysters collected in the lagoon at the same period (up to 390 MPN/g). V. parahaemolyticus trh2 + was detected in depurated oysters from storage tanks in August 2017 (2.5 MPN/g). These concentrations were significantly correlated with water temperature. An investigation of the stop-sales processes and practices of the professionals in the use and maintenance of the depuration tanks was conducted with a representative sample of 81 shellfish farmers. This survey showed that depuration practices should be improved in order to limit V. parahaemolyticus presence in oysters and thus, infections and health issues in humans.