Comparative pathogenesis of three strains of infectious Hematopoietic necrosis virus in rainbow trout Oncorhynchus mykiss

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Abstract
Three strains of infectious hematopoietic necrosis virus (IHNV) were compared for their pathogenicity to alevin rainbow trout Oncorhynchus mykiss by waterborne exposure of the trout to approximately equal concentrations of each virus strain. The Idaho strain of IHNV (Type 2) was the most virulent and induced a 62% mortality over a 10 d period at water temperature of 10.degree.C. In contrast, strains of IHNV from Oregon (Type 1) and California (Type 3) caused only 4 and 6% mortality, respectively, among rainbow trout infected under the same conditions. Histopathological signs of infection (hematopoietic necrosis) were first detected 3 to 4 following exposure to virus. In trout exposed to the Idaho strain of IHNV, lesions were severe and progressive. The Oregon and California strains of the virus induced similar changes in the kidney, spleen, and pancreas but these were less severe, and by 8 to 10 d following exposure had resolved in most of the fish. These results showed the Type 2 strain to be the most pathogenic for rainbow trout.

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