A natural epizootic of infectious haematopoietic necrosis in imported Atlantic salmon, Salmo salar L., reared in the enzootic region

In June 1984, fry of Atlantic salmon, Salmo salar L., (0-3 g mean weight), being reared in a hatehery operated by the University of Washington, began to die and infectious haematopoietic necrosis (IHN) was diagnosed. A total of 40 000 fry were being held in 11 tanks. Mortality due to IHN was noticeable in three tanks, losses being heavy in one and moderate in two. Gross examination of the fish failed to reveal an obvious cause of death. Signs included: evidence of haemorrhage ventrally, anterior to the anus; pale gills and liver; an empty gut, except for the presence of a white caseous material; and abdominal distension. The spleen and kidney were normal. When examinations of the fish for bacteria and parasites were negative, virus isolation was attempted.