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Infectious Salmon Anaemia

by Barry Ely
[from Mill Hill Essays 1999]

Over the last thirty years increased activity in the farmed fish industry has been accompanied by the emergence of several serious infectious diseases of fish. The effective management of diseases of aquatic animals presents particular difficulties. Meaningful quarantine areas may be difficult or impossible to establish since infections may be spread by flowing or tidal waters. Knowledge of carriers of the infection, animals which harbour the infection, and even the infectious agent itself may be limited or non-existent. Equally important, large natural populations of wild animals, often sharing the same water systems as farmed animals, may act as almost limitless reservoirs and sources of infection. The newest disease to emerge, Infectious Salmon Anaemia provides a typical and topical example of the problems encountered.

Everyone is familiar with scenes of wild salmon returning from the sea to the fresh water site of their hatching. Here they spawn and the complex life cycle of the salmon begins again. Today salmon is no longer the luxury food it once was and, perhaps surprisingly, it presently costs less to buy at a supermarket than does a staple fish such as cod. However, to produce it, the commercial salmon farmer is still required to reproduce the salmon life cycle at every stage. This is both a labour and a capital-intensive scheme. As a consequence, salmon farming often provides employment and infrastructure in what may otherwise be a relatively poorly developed locality. The success of the industry is therefore very important both in economic and social terms.

The commercial production of salmon begins during the winter. Salmon eggs are fertilised by artificial insemination and incubated in large holding trays at freshwater hatcheries. From mid-winter until early spring the hatchlings, or alevin, feed from their yolk sacs. As they are weaned onto food suitable to small salmon, the fry become free swimming rather than bottom feeders. They are then transferred to large fresh water tanks or cages where the parr, as they are now known, grow very rapidly. Most of them now undergo major changes in shape and metabolism that turn them into smolts, or young salmon that are able to cope with a salt-water environment. In the natural state these fish would be ready to migrate downstream to the open sea. At the salmon farm however, they are physically transferred from freshwater to sea farms using lorries, wellboats or even helicopters depending on the layout of the facilities. Fish are then grown on and usually harvested when they have reached between one and six kilograms in weight.

The many steps involved in raising salmon from the egg stage to the adult fish provide several avenues for diseases to attack them. A mixture of good husbandry and disinfection usually avoids the common diseases and infestations of farmed fish. However, in the autumn of 1984, a new disease was observed in Atlantic salmon being farmed in Nordland County along the southwest coast of Norway. The geographic extent of the disease, which was named Infectious Salmon Anaemia, spread slowly. By June 1988 it had become sufficiently widespread and serious to require the Ministry of Agriculture, Fisheries and Food to be informed if it were detected.

An outbreak of the disease, as the name implies, shows itself by a severe anaemia of infected fish. The fish develop pale gills and may be seen swimming close to the water surface where they gulp for air. Much more insidiously however, the disease often develops without the fish showing any external signs of illness, even maintaining a normal appetite, until suddenly they die. The disease can progress slowly throughout an infected farm and, in the worst cases, death rates may approach one hundred percent. Port-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. There is often evidence of altered activity in the blood-regenerating system, characterized by a large reduction in the numbers of blood cells. Those red blood cells still present often burst easily and the numbers of immature and damaged blood cells are increased.

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 Haemorrhagic 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 Anaemia and Haemorrhagic Kidney Syndrome.

Finally, in May of 1998, a salmon farm at Loch Nevis on the west coast of Scotland reported its suspicions of an outbreak of Infectious Salmon Anaemia. The suspicions were confirmed, and by early this year the disease had spread to an additional fifteen farms not only on the Scottish mainland but also on Skye and Shetland.

For nearly ten years following the first identification of Infectious Salmon Anaemia the causes and origins of the disease, and the method of its introduction to the fish remained unknown. However, based on the assumption that it was a disease transmitted by a virus, effective, albeit drastic measures were introduced in order to contain outbreaks. The disease is a serious and major threat to salmon farming and is now the first of the diseases classified as List One under 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.

Experiments on the transmission of the disease from infected fish to healthy fish, using extracts from the infected fish treated in such a way that almost the only infectious agent they could have contained was a virus, supported the idea that Infectious Salmon Anaemia was a viral disease. Then, in 1994, electron microscopy of tissues from infected fish revealed for the first time pictures of viruses budding from the surfaces of white blood cells as well as from cells of the gills and the heart.

By 1995 a new cell culture had been developed in Norway from the Atlantic salmon and for the first time the virus was produced in laboratories. It was found that the virus reproduces itself best in the cultured cells at fifteen degrees centigrade but if the temperature is raised above twenty-five degrees the virus does not reproduce at all. This suggests that the virus is confined to cold-blooded animals and, indeed, the virus will stick to and infect red blood cells from fish but not those of warm-blooded creatures such as mammals or birds.

Characterization of the virus has now begun and from its shape and its chemical make-up Infectious Salmon Anaemia appears to be most like influenza viruses. The way it is transferred and the natural reservoirs of Infectious Salmon Anaemia virus are not fully understood. Apart from Atlantic salmon, both sea-run Brown trout and Rainbow trout 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.

Transmission of the virus has been demonstrated to occur by contact with infected fish or their secretions. Contact with equipment that has been contaminated by infected fish or indeed, contact with persons handling 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. It is presumed that the tidal excursion, the distance that a particle of water will move during a tidal cycle, is an important aspect of this risk. The control procedures used to limit the extent of an outbreak of the disease are based on these observations. Currently, they rely on eradication of animal stocks and heat or chemical disinfection of waste and waters to destroy residual infection.

More recently the sea-louse, a small marine crustacean that attacks the protective mucous, scales and skin of the salmon has been shown to transmit Infectious Salmon Anaemia virus. Sea lice have often been called the greatest enemy of farmed salmon and farmers have constantly to work against lice infestations of their fish. It is not known whether the Infectious Salmon Anaemia virus can reproduce itself in the sea louse in the way that viruses seem to do within the ticks that carry other diseases.

Infectious Salmon Anaemia is currently regarded as a serious threat not only to farmed salmon, but also to the world’s dwindling stocks of wild salmon. Anecdotal evidence that fish which survive the first infection are less likely to succumb to a second infection by the virus has been supported by laboratory studies. The fish become immune to the virus and this protects them against reinfection. Experiments are now aimed at developing a fully protective vaccine against the disease, and trials will soon begin in Canada. However, much about the biology of Infectious Salmon Anaemia remains to be discovered. 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.