Infectious Salmon Anemia Virus

The Infectious Salmon Anemia Virus or Salmon Influenza is a viral disease that, as the name implies, causes severe anemia of infected fish. Unlike mammals, the red blood cells of fish have DNA and those cells can become infected with viruses. The anemia results in inadequate supllies of oxygen and the fishes develop pale gills and may swim close to the water surface, gulping for air.

The Infectious Salmon Anemia Virus (shortened to ISAV or ISA virus) is currently the only known species in the newly formed Isaviruses, within the family Orthomyxoviridae. This makes it closely related to the Influenza viruses. There are a number of districts strains of the virus known: a European strain (Genotype I) and a North American strain (Genotype II). Various clades occur within these genotypes, suggesting that the virus undergoes genetic drift,while reassortment seems a frequent event among the dominating Infectious Salmon Anemia Viruses in farmed Atlantic salmon. The genome consists of eight segments coding for at least 10 proteins. These aquatic viruses show many of similarities to influenza A viruses but differ in many important aspects such as the number of hosts, the host population structure and the route of transmission.

Infection occurs mainly in Norway, Scotland, Canada, the USA and Chili in farmed trout and salmons. It causes high mortality among affected species[1]. Subclinical infections have been recorded in herring, Atlantic cod and Pollock.

Calculation of the time of most recent common ancestor (TMRCA) suggests that the Norwegian Infectious Salmon Anemia Viruses separated from the European subtype found in North America between 1932 and 1959. The TMRCA data also suggest that the Infectious Salmon Anemia Viruses in Chile were transmitted from Norway in the period from 1995 to 2007[2].

[1] Cottet et al: Infectious salmon anemia virus--genetics and pathogenesis in Virus Research - 2011 
[2] Plarre et al: Evolution of Infectious salmon anaemia virus (ISA virus) in Archives of Virology - 2012