Infectious Hematopoietic Necrosis Virus Traffic in the Columbia River Basin
Gael Kurath, Kyle A. Garver, and Ryan M. Troyer
Abstract.—For several decades infectious hematopoietic necrosis virus (IHNV) has been a serious pathogen impacting salmon and trout in the North American West. In the Columbia River basin, IHNV caused severe epidemics in sockeye salmon Oncorhynchys nerka hatcheries during the 1950s, contributing to a great reduction of sockeye culture efforts. Since the early 1980s, IHNV has been endemic at fluctuating prevalence levels in both steelhead O. mykiss and Chinook salmon O. tshawytscha stocks in the basin, causing frequent epidemics in cultured steelhead fry. Infectious hematopoietic necrosis has also been endemic and epidemic in the Idaho rainbow trout (nonanadromous O. mykiss) industry since its emergence in the Hagerman Valley in the late 1970s. Infectious hematopoietic necrosis virus strain typing methods based on genetic analyses of gene sequences have recently been applied to more than 270 virus isolates from the Columbia River basin, including 150 isolates from the Hagerman Valley. Phylogenetic analyses revealed that there are two distinct major genogroups of IHNV, designated U and M, that overlap in the Columbia River basin. The U genogroup has a long history of prevalence throughout the Columbia River basin with the exception that it has not been found in the upper Snake River watershed, including the Hagerman Valley. Genogroup M is prevalent throughout the Hagerman Valley rainbow trout industry, and it also occurs in the lower Snake and lower Columbia River, but it has never been found in the upper Columbia River basin. The ability to distinguish different IHNV genotypes has provided numerous insights into the epidemiology of IHNV throughout the basin, suggesting frequent viral traffic between cultured fish stocks and also between wild and cultured fish. The patterns of M genogroup IHNV in the lower Snake and lower Columbia River basins suggest that virus translocation does not occur by simple downstream water flow, but more likely involves fish translocations that are part of salmon resource management in the region. The novel insights gained from this genetic typing underscore the critical need to manage salmonid stocks to prevent further spread and establishment of M genogroup IHNV throughout the basin.