Evidence for a carrier state of infectious hematopoietic necrosis virus inchinook salmon Oncorhynchus tshawytscha

In British Columbia, Canada, infectious hematopoietic necrosis virus (IHNV) is prevalent in wild sockeye salmon Oncorhynchus nerka and has caused dise ase in seawater net-pen reared Atlantic salmon Salmo salar. In this study, chinook salmon Oncorhynchus tshawytscha experimentally exposed to an isolat e of IHNV found in British Columbia became carriers of the virus. When Atla ntic salmon were cohabited with these virus-exposed chinook salmon, IHNV wa s isolated from the Atlantic salmon. Identification of chinook salmon popul ations that have been exposed to IHNV may be difficult, as virus isolation was successful only in fish that were concurrently infected with either Ren ibacterium salmoninarum or Piscirickettisia salmonis. Also, IHNV-specific a ntibodies were detected in only 2 of the 70 fish experimentally exposed to the virus. Two samples collected from chinook salmon exposed to IT INV whil e at a salt water net-pen site had a seroprevalence of 19 and 22 %; however , the inconsistencies between our laboratory and field data suggest that fu rther research is required before we can rely on serological analysis for i dentifying potential carrier populations. Because of the difficulty in dete rmining the exposure status of populations of chinook salmon, especially if there is no concurrent disease, it may be prudent not to cohabit Atlantic salmon with chinook salmon on a farm if there is any possibility that the l atter have been exposed to the virus.