| Summary: | Infectious hematopoietic necrosis (IHN) is a serious viral disease of salmonid fish. This disease was first reported at fish hatcheries in Oregon and Washington in the 1950s. The causative virus now exists in many wild and farmed salmonid stocks in the Pacific Northwest region of North America. It has also spread to Europe and some Asian countries. Clinical infections are most common in young fish, particularly fry and juveniles. Infectious hematopoietic necrosis can have a major economic impact on farms that rear young rainbow trout or salmon; the cumulative mortality rates on these farms can reach 90-95%. Occasional epizootics have also been reported in wild salmon. Etiology Infectious hematopoietic necrosis is caused by infectious hematopoietic necrosis virus (IHNV), a member of the genus Novirhabdovirus and family Rhabdoviridae. Virus strains vary in their pathogenicity. IHNV isolates can be grouped into three genetic types, which are correlated mainly with geographic regions. The U genogroup includes isolates from Alaska, British Columbia, coastal Washington watersheds and the Columbia River basin, as well as a few isolates from Oregon, California and Japan. The L genogroup contains most of the viruses from California and the Oregon coast. The M genogroup contains isolates from Idaho, the Columbia River basin and Europe, as well as a virus from the Washington coast. The M genogroup has significantly higher genetic diversity than the L or U groups. Species Affected Infectious hematopoietic necrosis affects rainbow / steelhead trout (Oncorhynchus mykiss), cutthroat trout (Salmo clarki), brown trout (Salmo trutta), Atlantic salmon (Salmo salar), and Pacific salmon including chinook (O. tshawytscha), sockeye/
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