Isavirus

Infectious salmon anemia
Virus classification
Group:	Group V ((−)ssRNA)
Family:	Orthomyxoviridae
Genus:	Isavirus
Species:	Infectious salmon anemia virus

Infectious salmon anemia or anaemia (ISA) is a viral disease of Atlantic Salmon (Salmo salar) that affects fish farms in Canada, Norway, Scotland and Chile, causing severe losses to infected farms. The disease is listed as a non-exotic disease of the EU and is therefore watched closely by the European Community Reference Laboratory for Fish Diseases.

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.

There are several distinct strains of the virus. The most common are a European strain and a North American strain.

As the name implies, it causes severe anemia of infected fish. The red blood cells of fish have DNA, and can become infected with viruses. The fish develop pale gills, and may swim close to the water surface, gulping for air. However, the disease can also develop without the fish showing any external signs of illness, the fish maintain a normal appetite, and then they suddenly die. The disease can progress slowly throughout an infected farm and, in the worst cases, death rates may approach 100%. Post-mortem examination of the fish has shown a wide range of causes of death. The liver and spleen may be swollen, congested or partially already dead. The circulatory system may stop working, and the blood may be contaminated with dead blood cells. Red blood cells still present often burst easily and the numbers of immature and damaged blood cells are increased.

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