Some aspects of pathogenesis of infectious hematopoietic necrosis (IHN)

The histopathogenesis of infectious haematopoietic necrosis (IHN) virus infection was studied by exposing juvenile sockeye salmon (Oncorhynchus nerka) to the IHN virus. Fish samples were taken every 24 h for histological examination and for determination of virus concentration. A close correlation was found between histopathological changes and virus concentration. The most significant changes occurred 4 days after exposure. The haematopocitic tissue of the kidney was the most extensively involved but minor degenerative changes were seen in the liver, pancreas, and in the granular cells of the digestive tract. On the 4th day, maximum tissue concentration of virus was reached and the mortality increased. By the 5th day, 90% of the samples showed extensive pathological changes in the kidney, together with variable changes in spleen, liver, pancreas, and gut. Similarities in the histopathogenesis of IHN, Oregon sockeye disease (OSD), Sacramento River chinook disease (SRCD) and viral haemorrhagic septicaemia (VHS), are discussed.