Infectious salmon anaemia

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

Infectious salmon anemia (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. ISA has been a World Organisation for Animal Health notifiable disease since 1990. In the EU, it is classified as a non-exotic disease, and is monitored by the European Community Reference Laboratory for Fish Diseases.

ISA is caused by 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.

ISA virus causes severe anemia in infected fish. Unlike the mature red blood cells of mammals, the mature red blood cells of fish contain DNA, and can become infected by viruses. The fish develop pale gills, and may swim close to the water surface, gulping for air. However, the fish may show no external signs of illness and maintain a normal appetite, until suddenly dying. 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 burst easily, and the numbers of immature and damaged blood cells are increased.

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