A mixed infection in sevenband grouper Epinephelus septemfasciatus affected with viral nervous necrosis (VNN)

Viral nervous necrosis (VNN) of groupers by fish nodavirus is one of the most serious diseases in aquaculture around the world. In the present study, we introduced a mixed infection of fish nodavirus with another filterable pathogen(s) in VNN-affected sevenband grouper of Nagasaki Prefecture, Japan. In challenge tests with the brain-homogenate of VNN-affected grouper, relative percent survival (RPS) of fish immunized with inactivated brain-homogenate was 95%, but that of fish with inactivated RGNNV, which was isolated from the VNN-affected grouper, was only 28%, although the same degree of RGNNV antigens were included in both of the immunized materials. Although 50% lethal dose (LD 50) of RGNNV was 10 3.5 TCID 50/100 µl/fish, an inoculum of the VNN-affected grouper brain-homogenate with LD 50 against sevenband grouper contained only 10 1.5 TCID 50/100 µl. Infectivity titer of RGNNV was not influenced by Triton X-100 (TrX) treatment, but cumulative mortalities of fish inoculated with non- and TrX-treated brain-homogenate of VNN-affected fish were 70% and 40%, respectively, meaning that virulence of the brain-homogenate distinctly decreased by TrX-treatment. Therefore, it was suggested that a TrX-sensitive virus(es) participating fish mortality existed as a mixed infection with RGNNV in VNN-affected sevenband grouper of Nagasaki Prefecture, Japan.