Occurrence, seasonality and infectivity of Vibrio strains in natural populations of mussels Mytilus galloprovincialis.

Widespread and large-scale mortalities of bivalve molluscs significantly affect their production. A number of pathogens have been identified as the primary causes of death in oysters or clams, especially bacteria of the genus Vibrio. We evaluated the occurrence, seasonality and infectivity of Vibrio strains associated with natural mussel (Mytilus galloprovincialis) populations. In particular, different isolates of V. splendidus and V. aestuarianus were analysed because they were associated with major oyster mortalities in areas where mussels are cultured without presenting mortalities. The presence of both Vibrio spp. was analysed bimonthly in mussels, water, sediment, plankton and other associated fauna from 2 sites in Galicia (NW Spain), the region with the highest mussel production in Europe. Environmental factors were also considered. The pathogenicity of different Vibrio isolates was analysed by performing experimental infections in mussels with strains isolated from the field. Results showed that Vibrio populations were mainly influenced by changes in water temperature and salinity. V. splendidus was dominant during the warm months and V. aestuarianus was predominant throughout the cold season. The sediment was the most important natural reservoir for bacteria. Experimental infections showed the extreme resistance of mussels to bacterial pathogens. Isolates of V. splendidus and V. aestuarianus were only moderately pathogenic for mussels in intramuscular infections and bath infections, and mortalities only occurred when animals were infected with a high bacterial concentration in adverse environmental conditions (hypoxia and 25°C). Although the pathogenicity of the Vibrio strains isolated from the wild was low for mussels, their potential risk for other bivalves cannot be ignored.