Its mode of transfer and the natural reservoirs of infectious salmon anemia virus are not fully understood.

[8] 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.

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.

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.

In 2011, two wild Pacific salmon taken from the central coast of British Columbia were suspected to have ISA after preliminary tests showed possible evidence of the virus.

[13] In January 2016, it was announced that the virus had been discovered in farmed and wild salmon British Columbia for the first time.

[14] In Chile, ISA was first isolated from a salmon farm in the 1990s and described for the first time in 2001,[15] although the initial presence never resulted in widespread problems.

The Lepeophtheirus salmonis sea louse, a small crustacean parasite that attacks the protective mucus, scales and skin of the salmon, can carry the virus passively on its surface and in its digestive tract.

[19] ISA is a 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.

[21] This research points out that starting in 1879 when rainbow trout were first brought to Europe from North America, there were many transfers of fish across the Atlantic ocean which may have carried the ISA virus.

Another study suggests that this virus was introduced into Norway between 1932 and 1959 and that the original strain was the European subtype found in North America.