Aquaculture for all

Disease guideInfectious salmon anaemia (ISA)

What is it?

Infectious salmon anaemia (ISA) is an infectious viral disease of Atlantic salmon (Salmo salar L.). The disease was first reported in Norway in 1984, but has since been reported in Canada, the USA, the Faroe Islands, Ireland and Scotland. The outbreak of ISA in Scotland in 1998-99 was successfully eradicated. Atlantic salmon is the only susceptible species known to develop clinical disease, but ISA virus can replicate in rainbow trout (Oncorhynchus mykiss) and sea trout (Salmo trutta L.).

Where and when might it occur?

In Norway, cases of ISA have occasionally been reported in fresh water farms but generally in hatcheries which use part sea water. The overwhelming majority of cases occur in farmed fish in sea water. The virus has been detected in wild fish but cases of clinical disease have only been reported in farmed fish.

The disease has also been detected in the UK, Canada and Chile.


Cases of clinical disease may be characterised by severe anaemia, ascites (accumulation of fluid in the body cavity), haemorrhage in internal organs and darkening of the liver. Fish also tend to appear lethargic with pale gills and blood spots in the eyes.

ISA-infected tissue
ISA-infected tissue

Indirect fluorescent antibody test (IFAT) showing the presence of ISA virus in the kidney cells of an Atlantic salmon

The causative agent of ISA is an orthomyxovirus. The presence of the virus is confirmed by isolation and identification techniques, indirect fluorescent antibody test (IFAT) and reverse transcriptase polymerase chain reaction (RT-PCR).


The virus can be transmitted through water, but the highest risk factors for spread of disease are movement of live fish, discharge of untreated blood and contact with infected vehicles and equipment.

ISA is exotic to the European Union. It is a notifiable disease under UK legislation and a List I disease under European Directive 91/67/EEC. Under EU legislation action must be taken to contain any outbreak, to eradicate sources of infection and to protect other fish farms by:

  • Compulsory slaughter and disinfection of infected farms
  • Strict movement controls on suspect farms
  • Placing farms in the vicinity of an outbreak under surveillance


There are no treatments for ISA and no licensed vaccines in the EU. Vaccine trials in Canada have yielded equivocal results.

Source: Fisheries Research Services

Crown copyright 2007

FRS is an agency of the Scottish Executive

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