Infectious hematopoietic necrosis virus (IHNV) occurs in North America as three major phylogenetic groups designated U, M, and L. In Coastal Washington State IHNV has historically consisted of U genogroup viruses found predominantly in sockeye salmon Oncorhynchus nerka. M genogroup IHNV, which has host-specific virulence for rainbow and steelhead trout O. mykiss, was detected only once in Coastal Washington prior to 2007, in an epidemic among juvenile steelhead trout in 1997. Beginning in 2007 and continuing through 2011, there were eight IHNV epidemics in juvenile steelhead trout, involving seven different fish culture facilities in four separate watersheds. During the same time period IHNV was also detected in asymptomatic adult steelhead trout from six coastal watersheds. Genetic typing of 283 recent virus isolates from Coastal Washington revealed the great majority were in the M genogroup of IHNV, and that there were two distinct waves of viral emergence between the years 2007–2011. IHNV type mG110M was dominant in Coastal steelhead trout during 2007–2009 and type mG139M was dominant between 2010–2011. Phylogenetic analysis of viral isolates indicated that all Coastal M genogroup viruses detected in 1997 and 2007–2011 were part of the MD subgroup and that several novel genetic variants related to the dominant types arose in the Coastal sites. Comparison of spatial and temporal incidence of Coastal MD viruses with that of the rest of the Pacific Northwest indicated that the likely source of the emergent viruses was Columbia River Basin steelhead trout.
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Emergence of MD type infectious hematopoietic necrosis virus in Washington State coastal steelhead trout