Heart and skeletal muscle inflammation (HSMI) is a transmissible disease found in farmed Atlantic salmon (Salmo sailor). It is caused by a novel piscine retrovirus (double-stranded RNA viruses that infect a wide range of vertebrates). The disease destroys heart and muscle tissue and kills 20% of infected fish, but mortality may be higher in affected pens. Environmental stress during outbreaks also increases mortality.

It was first detected in Norway in 1999 and has also been identified in the United Kingdom and British Columbia.

Transmission: HSMI is transmitted by tissue extracts of diseased fish or by co-habitation. The disease has the potential to spread to wild salmon that migrate past salmon farms.

Symptoms and pathology: Atlantic salmon are commonly affected 5 to 9 months after being transferred to marine pens, but outbreaks have been recorded as early as two weeks post transfer. Affected fish are anorexic and display erratic and abnormal swimming behaviour.

Autopsies of diseased fish will include a pale heart, yellow liver, ascites, swollen spleen and petechiae in the perivisceral fat.

Diagnosis: HSMI is diagnosed on the basis of histopathology. The major changes occur in the myocardium and red skeletal muscle.

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