Investigation of the relationship between the presence of a Gram-positive bacterial infection and summer mortality of the Pacific oyster, Crassostrea gigas Thunberg

The occurrence of Pacific oyster nocardiosis (PON) was found to correspond with peak periods of summer mortality as described for Pacific oysters, Crassostrea gigas, reared in certain bays in Washington State and British Columbia. Areas in which 2-year or older Pacific oyster historically experience summer mortality and bacterial infection are embayments where water temperatures and nutrient levels become elevated during the summer months. Pacific oysters reared in three sites in both Washington State and British Columbia, Canada, contained the bacterial pathogen. An examination of the seasonality of the bacterial disease in Oakland Bay, Washington State, showed a peak prevalence of infection during the month of September. Although the ratio of male:female oysters was generally 1:1 at most locations sampled, between late August and November bacterial foci were more often found in males and reproductively immature oysters than in females. However, approximately equal numbers of male and female oysters were infected in early summer. Diseased oysters contained an infiltration of hemocytes surrounding tufts of Gram-positive bacteria of the genus Nocardia. Injection of the bacterium isolated from diseased oysters into healthy oysters resulted in infections with the same characteristic histopathological changes as observed in individuals with PON. The seasonal coincidence and involvement of older oysters containing bacterial foci suggests that PON is a causal factor in summer mortality in certain areas.