The veterinary authority sent an Immediate Notification dated 20 June to the World Organisation for Animal Health (OIE).
The report describes the outbreak, which started on 11 June 2014 at Osland in the country of Sogn og Fjordane, in 300,000 farmed rainbow trout in fresh water. Morbidity and mortality are put at 2 on a scale from zero to 5.
Scientists at the Norwegian Veterinary Institute have reported a presumably new emerging disease in rainbow trout in Western Norway, according to the OIE report.
Occurrence of diseased fish is primarily in fresh water, where four different hatcheries have been affected. After sea transfer, mortality and clinical signs of the disease are reportedly reduced.
In a recent pilot trial, the Norwegian Veterinary Institute has established an association between a newly detected virus, so-called 'virus Y' and disease in rainbow trout. The infectivity trial indicates that the virus Y is contagious for rainbow trout by horizontal transmission.
Further investigations will be carried out.
According to the web site of the Norwegian Veterinary Institute, sick fish show signs of circulatory failure and may have pale viscera due to anaemia. Ascites may also be observed. Histopathological findings are inflammation in heart- and red muscle tissue and cellular necrosis in the liver.