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WILD salmon may be playing a role in ISA outbreaks in the farmed sector, according to Norway’s research institute Nofima.

Nofima say the aquaculture industry in the Troms area of Norway has been hit by annual outbreaks of ISA since 2007. Fish farmers hit by the disease can lose large quantities of fish and a mortality rate of nearly 90% has been registered in controlled challenge experiments on salmon.

Nofima add that the regularity of the outbreaks may mean that the disease is established in the affected fjord systems, and a possible explanation is that wild fish are playing a role in pathogen transfer.

Infectious salmon anaemia (ISA) is, they say, attributed to a virus (ISA virus), which is related to the influenza virus found in mammals and birds. The ISA virus is found in wild and farmed salmon, but does not cause disease in wild fish even though it is most likely infectious. On the contrary, the ISA virus in farmed salmon produces the disease and high mortality, and this may be attributed to the virus having new traits owing to changes in the genes. Even though the virus is related to influenza, only fish develop the disease.

The natural host organism for the ISA virus remains unknown, but one may not rule out the fact that the virus occurs naturally in wild salmon residing along the Norwegian coast, Nofima say.

In light of this lack of knowledge about the reciprocal risk of infection between wild and farmed fish, scientists from Nofima in Tromsø have studied the existence of the pathogenic virus in salmon caught in the Målselv waterway. The Målselv River is connected with the Astafjord basin in South Troms, and there have been repeated ISA outbreaks in farmed salmon in this area over a period of several years.

Farmed fish in South Troms were vaccinated against ISA in 2010, but this did not prevent outbreaks of the disease. This study, as with previous studies, points to the fact that all the outbreaks in South Troms in the period 2007 - 2010 are caused by just one variant of the ISA virus and, therefore, that this variant is established in the fjord system.

In this study, the ISA virus was not detected in 28 fish caught in the Målselv waterway during the 2010 season. One of the salmon tested weak positive for the virus, but an independent analysis concluded that none of the fish had the ISA virus. It is highly likely that all the fish resided in an area in which there are many fish farms that have had outbreaks of the disease.

“However, we cannot conclude that the pathogenic ISA virus does not transfer between wild fish and farmed fish or vice versa; for that purpose the sample material analysed is too small,” says Øyvind Kileng, a scientist who participated in the project.

“In order to draw sound conclusions regarding the risk of infection between wild fish and farmed fish, examinations of a large number of samples collected over a longer period of time would be appropriate. Several waterways and other fish species should also be included,” concludes the scientist.

This project is financed by the Fishery and Aquaculture Industry Research Fund (FHF). The scientific study is presented in Nofima’s report no. 28/2011 “ILA-virus i villfisk og oppdrettsfisk i Troms” (Norwegian language only).

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