Change of pathogenicity in Olive flounder Paralichthys olivaceus by co-infection of Vibrio harveyi, Edwardsiella tarda and marine birnavirus

Recently, mass mortality of adult flounder showing ascites occurred at many aquaculture farms in Korea. While we investigated the cause of the mortality, marine birnavirus (MABV-F) and bacteria (Vibrio harveyi and Edwardsiella tarda) were isolated from diseased fish. By the intraperitoneal (IP) injection with MABV-F, the virus strain did not show pathogenicity to the flounder. When the fish co-infected with MABV-F (IP) and V. harveyi or E. tarda (immersion), the cumulative mortality did not show any significant difference with control group. In contrast, the case of co-infection with MABV-F and bacteria by IP, cumulative mortality reached more than 90%. Most of the moribund and dead fish except for the mock-infected fish displayed ascites, and the virus and bacteria were re-isolated from all the dead and moribund fish except for the control fish. The results indicate that the cause of mass mortality of the adult fish may be due to bacterial infection. Higher mortalities may occur due to co-infection of MABV-F and bacteria, although MABV-F alone did not cause direct mortality.