The aetiological agent of ISA is the infectious salmon anemia virus (ISAV). ISAV, a RNA virus, is the only species in the genus "Isavirus" which is in the family Orthomyxoviridae and therefore related to the influenza viruses.
The genome encodes at least 10 proteins.[2]
There are several distinct strains of the virus. The most common are a European strain and a North American strain.[3]