Infection source of herpesvirus disease in coho salmon culture and its control

Abstract

Epizootiological surveys on salmonid herpesvirus-2 (SaHV-2) infection in coho salmon (Oncorhynchus kisutch) culture were undertaken at freshwater hatcheries and seawater farms in order to clarify the infection source. At seawater farms, the outbreaks were observed only in the fish populations derived from particular hatcheries, and in some cases, the disease was observed during the seawater acclimatization period. Outbreaks during mariculture period tended to occur repeatedly year after year when the seedlings were introduced from hatcheries where coho salmon was reared together with other salmonids. On the other hand, in fish derived from hatcheries where only coho salmon was reared, the outbreak occurred only when the hatchery replenished the juveniles from virally contaminated hatcheries. In the surveillance of virus carrier fish, SaHV-2 highly virulent to coho salmon was isolated from apparently normal rainbow trout collected at a hatchery, where coho salmon seedlings supplied every year had been found to be contaminated. These results suggest that the SaHV-2 infection to coho salmon occurred at freshwater hatcheries and that the transportation of infected juveniles spread the virus among the hatcheries. It was also suggested that the infection source might be asymptomatically infected other salmonid species which were reared together with coho salmon.