Host specificity and ecology of infectious hematopoietic necrosis virus (IHNV) in Pacific salmonids

Some circumstances IHNV infection can cause acute disease with mortality ranging from 5-90% in host populations. Genetic typing of IHNV field isolates has shown that three major genetic groups of the virus occur in North America. These groups are designated the U, M, and L virus genogroups because they occur in the upper, middle, and lower portions of the geographic range of IHNV in western North America. Among field isolates there is some indication of host specificity: most IHNV isolated from sockeye salmon (Oncorhynchus nerka) is in the U genogroup, and most IHNV isolated from rainbow and steelhead trout (Oncorhynchus mykiss) is in the M genogroup. Experimental challenges confirm that U isolates are highly virulent for sockeye salmon, but not rainbow trout. In contrast, M isolates are virulent in rainbow trout but not in sockeye salmon. Studies comparing U and M virus infections show that virulence is associated with more rapid virus replication in the first few days after infection. In addition, high virulence isolates persist at higher viral loads in the host, while low virulence isolates do not persist. These host-specific aspects of the different IHNV genogroups are important for understanding the ecology of IHNV emergence events in the field. The recent emergence of U IHNV in Russian sockeye salmon of the Kamchatka Peninsula, and the emergence of M IHNV in steelhead trout on the Olympic Peninsula in the U.S.A, serve as examples of the relevance of IHNV host specificity.