Diseases of wild and farmed Finfish

Some fish and shellfish diseases of particular significance in Scotland.

What Ii 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. Both outbreaks of ISA in Scotland in 1998-99 and 2008-09 were 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 causative agent of ISA is an orthomyxovirus. The presence of the virus is confirmed by histopathological changes consistent with ISA, immunohistochemistry, isolation and identification techniques and real-time reverse transcription polymerase chain reaction (RT-PCR) followed by nucleic acid sequence analysis.


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 a notifiable disease within Great Britain. ISA – infection with genotype HPR-deleted of the genus Isavirus (ISAV) is listed as a non-exotic disease under Annex IV, Part II of Council Directive 2006/88/EC (as amended in 2014/22/EU). Great Britain is an approved zone for this disease, and to maintain this disease-free status, all farms holding susceptible species of fish are routinely inspected for clinical signs of the disease. 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.

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.

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