Abstract

Chinook salmon Oncorhynchus tshawytscha returning to certain hatcheries in the Sacramento River basin of Northern California are known to be infected with infectious hematopoietic necrosis virus (IHNV) and epidemics due to the virus in juvenile salmon occur sporadically.^1,3-6 The virus is usually not detected in the first returning adults to these hatcheries, but as the run progresses, virus detection increases.⁶ Whether this increased virus prevalence is due to adult-to-adult transmission of the virus or a reactivation of persistent or latent virus in the later adults is unknown. That adult salmonids can become infected with virus from exogenous sources has been demonstrated with rainbow trout and sockeye salmon.^2,7 In the Sacramento River, four stocks of Chinook salmon return to spawn, with sufficient overlap in run timing providing the potential for transmission of IHNV between adults and from adults to juvenile salmon. The purpose of our study was to determine if naïve adult Chinook salmon could be infected following experimental exposures to IHNV.

Sexually mature female Chinook salmon were exposed to IHNV by additions of virus to the water. Virus was detected as early as 4 d post exposure and subsequently in all virus-exposed fish that died or that were examined at 14 d when the study was terminated. The greatest concentrations of virus, up to 10⁸ plaque forming units (pfu) ml^-1, were found in the ovarian fluid at 13 to 14 d post virus exposure but virus was also found in high concentrations in the gill, kidney/spleen and plasma. Virus was not recovered from unexposed control adult salmon that died or were sampled at the end of the study. Despite detecting concentrations of IHNV in excess of 10⁷ pfu g^-1 of tissue, there were no specific microscopic lesions found in IHNV-exposed compared to control unexposed salmon. In a second study, virus was detected in tissues of adult fish that were exposed to effluent water from IHNV-infected fish. These initial studies suggest that virus found in the spawning environment, either from adult salmon or other sources, may contribute to its rapid spread among adult Chinook salmon, thereby considerably increasing the prevalence of IHNV infection in wild and hatchery populations of adult Chinook salmon.

Acknowledgements

This research was conducted in collaboration with the U.S. Fish and Wildlife Service and the California Department of Fish and Game under a permit from the National Marine Fisheries Service and was supported by funds from the U.S. Bureau of Reclamation and the California Department of Water Resources (contract 4600001540).

References

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