Infectious Salmon Anaemia

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Also Known As: ISA

Caused By: Infectious Salmon Anaemia Virus –- ISAV


Infectious Salmon Anaemia is a viral disease of fish, caused by an enveloped orthomyxovirus. Disease may be severe, systemic and lethal but is also subclinical in some fish species.

This disease is not considered zoonotic.


Norway, Canada, Scotland, Eastern USA, Chile and Ireland.

ISAV replicates in Atlantic salmon, forming a marine reservoir for the virus, and their migration allows distant distribution of the disease. The virus also replicates in other fish species (e.g. rainbow trout, Atlantic cod, chum, coho and Chinook salmon, Arctic Char) but no disease has been observed under aquaculture conditions. However, these species may serve as reservoir hosts for the virus.

Transmission is mainly via the water, virus being shed into it in mucus, faeces, urine and also via the skin.[1] It then enters vulnerable fish via the gills or broken skin. ISA also has a crustacean vector for transmission.

Spread between netpens is usually slow, and mortality and severity often varies between them.


A range of salmonid fish can be affected including rainbow trout, sea trout, Atlantic cod, chum, coho, Chinook and Atlantic salmon and Arctic Char. Both wild and farmed fish are susceptible.

Clinical Signs

Diseased fish are usually in normal nutritional condition but may swim sluggishly near the water surface or "hang" listlessly at the wall of the netpen and eventually sink to the bottom.

ISA infections eventually cause severe anaemia and multi-organ haemorrhage. This may be seen as pallor, abdominal distension, scale oedema or fluid accumulation in the abdomen and musculature. The gills are often pale first and most markedly. Fish may develop a “pop-eye” appearance (exopthalmos).

Haematocrit values below 10 are seen in advanced disease, commonly being found <25 and thus leading to multi-organ dysfunction.

Mortality usually varies between affected farms and between netpens on a farm, but can reach 90%.


Clinical signs and pathological/histopathological findings are highly suggestive of ISA. Hepatomegaly, ascites and splenomegaly are marked with white/grey patches of necrosis often visible on the liver and dark red splenic congestion. Petechiation in the adipose tissue and swim bladder is also common.[1] The major histopathological feature is multifocal haemorrhagic necrosis of the liver that may become confluent.[2] This leads to congestion and sinusoid dilation which eventually fill with blood. The sinusoid endothelium degenerates and eventually disappears entirely. In diseased fish in the USA in particular, renal interstitial haemorrhage is a dominant feature, with tubular necrosis, congestion and cast formation.

The virus can also now be isolated in cell cultures, detected by indirect immunofluorescent antibody testing (IFAT) in tissue imprints or detected in fixed sections by immunostaining for definitive diagnosis. PCR tests are also available in the form of RT-PCR.


There is no treatment for infected fish.


As transmission between farms is generally via the movement of fish or their breeding/meat products, implementation of general hygiene legislation and movement procedures may greatly reduce incidence of ISA. Slaughterhouse and transport regulations must also be set and followed.

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  1. 1.0 1.1 Thorud, K., Djupvik, H. O (1988) Infectious anaemia in Atlantic salmon (Salmo salar L.). Bulletin of the European Association of Fish Pathologists, 8(5):109-111
  2. Evensen, O., Thorud, K. E., Olsen, Y. A (1991) A morphological study of the gross and light microscopic lesions of infectious anaemia in Atlantic salmon (Salmo salar). Research in Veterinary Science, 51(2):215-222; 23

Rimstad, E., Dale, O.B., Dannevig, B.H. and Falk, K. 2011. Infectious Salmon Anaemia. In: Fish Diseases and Disorders, Volume 3: Viral, Bacterial and Fungal Infections, 2nd edition (eds. Woo, P.T.K. and Bruno, D.W.), CABI, Wallingford, UK. Pp143-165.


This article was originally sourced from The Animal Health & Production Compendium (AHPC) published online by CABI during the OVAL Project.

The datasheet was accessed on 7 July 2011.