Infectious salmon anaemia virus

Infectious salmon anemia virusInfectious salmon anemia (ISA) is a viral disease of Atlantic salmon (Salmo salar) caused by Salmon isavirus. It 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. Infectious salmon anemia (ISA) is a viral disease of Atlantic salmon (Salmo salar) caused by Salmon isavirus. It 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. Infectious salmon anemia appears to be most like influenza viruses. Its mode of transfer and the natural reservoirs of infectious salmon anemia virus are not fully understood. Apart from Atlantic salmon, both sea-run Brown trout (Salmo trutta) and Rainbow trout (Onchorhyncus mykiss) can be infected, but do not become sick, so it is thought possible that these species may act as important carriers and reservoirs of the virus. Research shows that while several species of Pacific salmon can be carriers of the virus, even highly pathogenic strains, Pacific salmon currently show high relative resistance and no ISAV-related symptoms. However, the potential for ISAV adaptation to Pacific salmon exists. In the autumn of 1984, a new disease was observed in Atlantic salmon being farmed along the southwest coast of Norway. The disease, which was named infectious salmon anemia, spread slowly, but caused the death of up to 80% of salmon stock in some hatcheries. By June 1988 it had become sufficiently widespread and serious to require the Norwegian Ministry of Agriculture, Fisheries and Food to declare it a notifiable disease. In the summer of 1996, a new disease appeared in Atlantic salmon being farmed in New Brunswick, Canada. The death rate of the fish on affected farms was very high and, following extensive scientific examination of the victims, the disease was named 'hemorrhagic kidney syndrome.' Although the source and distribution of this disease was not known, the results of studies by Norwegian and Canadian scientists showed conclusively that the same virus was responsible for both infectious salmon anemia and hemorrhagic kidney syndrome.