Abstract
During the winter of 1988, a mass mortality occurred in the population of maricultured coho salmon, Oncorhynchus kisutch, in Shizugawa Bay, Miyagi Prefecture of Japan. Since the first outbreak, the disease was reported every winter at more than 10 coho salmon farms in the same area. Diseased fish were 100-1, 000 g in body weight and showed excoriations of fin, erosion and ulcers on the body surface and pale spots in the liver. Cumulative mortalities ranged from 10 to 31% in the epizootic periods. Necrosis of hepatocytes was the most evident histopathological change of the diseased fish. Numerous viral capsids having icosahedral shape were observed in the necrotic hepatocytes. A virus was isolated in CHSE-214 cells from spontaneously diseased fish which had the signs mentioned above. The isolant was identified as salmonid herpesvirus type 2 by morphological observation and serological tests. In infection trials, more than 90% of the coho salmon juveniles which received intraperitoneal injection of the isolant died. The virus was reisolated from all the dead fish showing signs similar to spontaneously affected fish. From these results, the virus was considered to be the causative agent of the disease.
