Infectious Salmon Anaemia (ISA) – Gross Pathology

Infectious salmon anaemia (ISA) is a disease caused by infectious salmon anaemia virus (ISAV), genus Isavirus, family Orthomyxoviridae that affects primarily marine-farmed Atlantic salmon (Salmo salar). Recorded for the first time in 1984 in Norway, it still causes recurrent epidemic outbreaks in Chile. The disease is present in most countries that farm Atlantic salmon: Norway, Scotland, Ireland, Faroe Islands, Canada, USA, and Chile.

The virus is adapted to cold-water salmonid fish and has an optimum growth at 15°C. Atlantic salmon is the only species known to develop clinical disease, but ISAV can replicate in sea trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss).

An ISAV outbreak develops slowly but most fish in an infected population may succumb during the production cycle; mortality during an outbreak can vary significantly. Daily mortality ranges between 0.05% to 0.1%, although cumulative mortalities may reach 90% over prolonged periods. Outbreaks in farmed Atlantic salmon occur mainly in seawater but clinical disease and mortality have been reported in freshwater.

It is a contagious disease that can retain infectivity outside the host for long periods and may spread by water-borne transmission. The virus may shed into water via mucus, skin, faeces, urine, blood, and within waste from dead fish. Shedding of the virus from infected fish may happen through natural secretion or excretions. The disease can spread to other farms, using routes such as passive transmission in water or via contaminated equipment, boat traffic or movement of fish. Proximity to farm sites with ongoing ISAV outbreaks is a risk of primary importance, and this risk increases the closer a susceptible farm is to an infected farm. The virus binds to red blood cell membranes, so controlling spread of infection by limiting contact with, or disinfecting bloodwater or effluent from fish processing plants is critically important.  

Gills and skin are the most likely portals of entry into the fish, although infection via the intestine cannot be excluded. Individuals may harbour the virus for several weeks before the development of the disease.

The disease presents as a systemic condition mainly characterized by severe anaemia and haemorrhage, with necrosis in several organs, but notably the liver. Fish appear lethargic, can display abnormal swimming behaviour, and may sink to the bottom of the cage or keep close to the wall of the net pen. External signs include pale gills, distended abdomen, petechia in the eye and exophthalmia, scale pocket oedema, and skin haemorrhages.

Gross post-mortem findings are mainly anaemia and circulatory disturbances in several organs, such as liver, kidney, gills, and gut. Generally, fish appear with no feed in the digestive tract.

Changes consistently observed in ISA include patchy congested liver, swollen dark red spleen and kidney, congestion of intestine, petechial haemorrhages of skeletal muscle and peripiloric fat and peritoneum, and blood-tinged or yellow fluid in peritoneal and pericardial cavities.


Figure 1. Salmo salar with ISA. Moribund fish presenting abnormal swimming behaviour.
Figure 2. Salmo salar with ISA. Skin haemorrhages.
Figure 3. Salmo salar with ISA. Severe jaundice.
Figure 4. Salmo salar with ISA. Fin base with haemorrhage.
Figure 5. Salmo salar with ISA. Petechia in the eye chamber.
Figure 6. Salmo salar with ISA. Exophthalmia and pallor of distal gill filaments.
Figure 7. Salmo salar with ISA. Blood-tinged fluid in peritoneal cavity.
Figure 8. Salmo salar with ISA. Dark red liver and petechial haemorrhages in peripiloric fat.
Figure 9. Salmo salar with ISA. Dark liver caused by diffuse and severe haemorrhage. Image by Marcelo Vera G.
Figure 10. Salmo salar with ISA. Petechial haemorrhages in peripiloric fat accompanied with dark haemorrhagic liver. Image by Cristian Sauterel R.
Figure 11. Salmo salar with ISA. Characteristic dark liver. Image by Cristian Sauterel R.
Figure 12. Salmo salar with ISA. Haemorrhagic pyloric caeca and petechial haemorrhagic in peripiloric fat. Image by Jaime Santana.
Figure 13. Salmo salar with ISA. Petechial haemorrhages in peripiloric fat. Image by Cristian Sauterel R.
Figure 14. Salmo salar with ISA. Mildly congested gastric mucosa.
Figure 15. Salmo salar with ISA. Enteritis and haemorrhagic intestine.
Figure 16. Salmo salar with ISA. Petechial haemorrhages of peripiloric fat and dark haemorrhagic liver. Image by Jaime Santana.


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