Entry and spread of specialist and generalist infectious hematopoietic necrosis virus (IHNV) genotypes in Chinook salmon, steelhead and sockeye salmon

Infectious hematopoietic necrosis virus (IHNV) represents one of the most critical challenges for salmonids in the Pacific Northwest. There are three genogroups of IHNV, designated U, M, and L; the U is further delineated into two subgroups, UC and UP, and the M is further delineated into four subgroups (MA – MD). The UP, UC and MD subgroups co-occur in the Columbia River Basin where the host species sockeye salmon, Chinook salmon, and steelhead trout spawn and rear. Field prevalence data shows that UC viruses exhibit a generalist strategy in Chinook, and steelhead, while two other virus lineages, MD and UP, are more consistent with being specialists in steelhead or sockeye salmon, respectively. The L is found in Northern California and is considered a specialist of Chinook salmon. This study sought to understand early entry and replication of the specialist and generalist IHNV strains in three salmonid hosts. Chinook, steelhead and sockeye were exposed by immersion to their specialist viruses (L, MD and UP, respectively) and to the generalist UC virus. As controls, these hosts were also exposed to buffer (mock control) and IHNV variants that were not specialist for their species (non-specialists). Mortality was monitored throughout the experiments. Fish were sampled at early timepoints post-infection (2, 5, and 8 days post-exposure). Kidney and fin tissues were taken to represent external and internal tissues, respectively. Viral load was assessed by reverse transcriptase quantitative PCR (RT-qPCR) targeting the nucleocapsid (N) gene of IHNV. Additional fish from each experimental group were sampled for histopathology but only a subset were processed and analyzed.