An epizootic of infectious haematopoietic necrosis in 2-year-old kokanee, Oncorhynchus nerka (Walbaum) at Lake Cowichan, British Columbia

This report describes the first recorded outbreak of infectious haematopoeitic necrosis (IHN) in a feral population of 2-year-old kokankee (non-anadromous Oncorhynchus nerka ) in Lake Cowichan, British Columbia, since IHN virus was first recorded there in 1973. Dead and moribund specimens then were also 2 years old in contrast to the occurrence of the disease in sockeye salmon as very young fish. Viral assays of kidney tissue and gills proved positive for IHN virus. High virus titres occurred in both the kidneys and gills, making it difficult to ascertain whether the infection was acquired via the gills. Histopathological examination showed extensive kidney and pancreatic necrosis. The density of kokankee in Lake Cowichan is low so the transmission of IHN virus from fish to fish could only be explained on the basis of schooling behaviour which occurs more randomly among 2-year-old fish than in young fish.