Immunological and pathological responses of salmonids to infectious pancreatic necrosis virus (IPNV)

Abstract

The aquatic birnavirus IPNV is commonly found in association with apparently healthy, mature salmonids. Some birnaviruses cause lethal diseases in fry; however, many of those discovered in fish may not be pathogenic for the species from which they were isolated. More pathogenic virus may be produced from the acinar cells of the pancreas; less pathogenic virus by skin and gut cells. Skin infection could explain the lack of virus clearance after development of circulating antibody; the less pathogenic form of the virus may not induce protective antibodies. True vertical transmission to progeny fish would seem not to occur, but virus may adhere to egg cases and spread to fry which ingest them at the time of first feeding. When viruses are found with moribund mature fish, alternative causes of death must be considered. Atlantic salmon Salmo salar smolts, which have high levels of virus during pre-smolting, can develop pancreatic lesions (possibly mediated by the immune system) on transfer to salt water. IPNV infection may be linked to immunosuppression; possible controlling genes have been found.