Genetic Analysis and Pathogenicity Against Masu Salmon <i>Oncorhynchus masou</i> Seedlings of Infectious Hematopoietic Necrosis Virus Strains Detected in Masu Salmon Broodstock in Hokkaido, Japan in 2021 and 2022

Infectious hematopoietic necrosis virus (IHNV) was detected in female masu salmon Oncorhynchus masou broodstock in Hokkaido, Japan, in the fall of 2021 and 2022. In 2021, an outbreak of the disease occurred in yearling masu salmon at another nearby hatchery. We investigated the origin of the IHNV strains detected in 2021 and 2022 by genetic analysis and assessed the pathogenicity of the 2021 strain in fish ranging from fingerlings to juveniles weighing up to 50 g. The results showed that the strains detected in both years shared nearly identical nucleotide sequences and belonged to the JN lineage, which is prevalent in Honshu. Furthermore, the 2021 strain was found to be highly pathogenic even to juveniles over 10 g in weight. These findings revealed that a highly pathogenic strain, similar to those found in Honshu, has spread into rivers in Hokkaido. Until this virus is eradicated from Hokkaido, it is essential to implement strict measures such as treating wastewater during egg collection, disinfecting eggs and rearing equipment, and continuing surveillance for IHNV in rivers.