Infectious salmon anemia

Infectious salmon anemia or anaemia (ISA) is a viral disease of Atlantic Salmon (Salmo salar) that affects fish farms in Canada, Norway, and Scotland, causing severe losses to infected farms.

The aetiological agent of ISA is the infectious salmon anaemia virus (ISAV). ISAV is the only species in the genus "Isavirus" which is in the family Orthomyxoviridae.

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Pathology

As the name implies, it causes severe anemia of infected fish. 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.

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.

Spread of the disease

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. 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.

In May 1998, a salmon farm at Loch Nevis on the west coast of Scotland reported its suspicions of an outbreak of infectious salmon anemia. The suspicions were confirmed, and by the end of the year, the disease had spread to an additional fifteen farms not only on the Scottish mainland but also on Skye and Shetland.

Transmission

Transmission of the virus has been demonstrated to occur by contact with infected fish or their secretions. Contact with equipment or people who have handled infected fish also transmits the virus. The virus can survive in seawater and, not surprisingly, a major risk factor for any uninfected farm is its proximity to an already infected farm.

More recently the sea louse, a small crustacean parasite that attacks the protective mucous, scales and skin of the salmon has been shown to carry the virus passively on its surface and in its digestive tract, although transmission of the disease by sea lice has not been demonstrated. It is not known whether the Infectious salmon anemia virus can reproduce itself in the sea louse, although this is a remote possibility as viruses are usually very host specific unlike bacterial diseases that can replicate in ticks such as Lyme disease and Rocky Mountain Spotted Fever.

Control and treatment

ISA is major threat to the viability of salmon farming and is now the first of the diseases classified on List One of the European Commission’s fish health regime. Amongst other measures, this requires the total eradication of the entire fish stock should an outbreak of the disease be confirmed on any farm. The economic and social consequences of both the disease and the measures used to control it are thus very far reaching.

Infectious salmon anemia is currently regarded as a serious threat not only to farmed salmon, but also to dwindling stocks of wild salmon. Anecdotal evidence suggests that fish which survive the first infection become immune to the virus. Work is now underway to develop a vaccine against ISA. A recent report suggests that the North American virus may be slightly different to the Norwegian virus. This makes it unlikely that the sudden appearance of the disease, at least in Canada, was due to the importation of infected Norwegian fish. The possibility then is that a single vaccine might only be effective in a limited area and maybe only for a limited time.

References

• Ely, B. Infectious Salmon Anaemia. Mill Hill Essays, National Institute for Medical Research.
• Raynard RS, Murray AG, Gregory A (2001) Infectious salmon anaemia virus in wild fish from Scotland, Diseases of Aquatic Organisms, Vol. 46: 93-100