Pathology associated with an aquareovirus in captive juvenile Atlantic halibut Hippoglossus hippoglossus and an experimental treatment strategy for aconcurrent bacterial infection

A large-scale mortality of larval and juvenile halibut Hippoglossus hippogl ossus occurred at a semi-commercial halibut farm in Atlantic Canada. Invest igation of the cause revealed aquareovirus particles in necrotic liver tiss ue of affected fish. Cytopathic effect on CHSE-214 cell lines occurred from all fish cultured for viruses, and the viral morphology of the particles i n culture was consistent with that observed in necrotic host tissue. The vi rus was placed in the family of Reoviridae, genus Aquareovirus based on mor phology and RT-PCR results. Multifocal hepatocellular necrosis was a consis tent finding in all fish as well as acute necrosis of proximal renal tubule s. Concurrent bacterial infections were present in some specimens. Fish exp erimentally treated with oxytetracycline or a combination of oxytetracyclin e and chloramine-T had a significantly lower mortality rate than untreated fish. Fish treated with chloramine-T alone had a significantly elevated mor tality rate compared to controls. Despite supportive medical therapy, morta lity levels in treated and untreated groups remained elevated, supporting t he hypothesis that the primary pathogen was of viral origin. This is the fi rst report of elevated mortalities in Atlantic halibut associated with an a quareovirus.