Infectious Salmon Anaemia (ISA): A Guide to the Disease

Infectious salmon anaemia virus causes severe anaemia and high mortality in farmed Atlantic salmon. There is no treatment; control relies on culling and biosecurity.

Overview

Infectious salmon anaemia (ISA) is a serious viral disease of farmed Atlantic salmon. The causative agent is infectious salmon anaemia virus (ISAV), a segmented negative-sense RNA virus of the genus Isavirus, family Orthomyxoviridae. The clinical disease is caused by virulent HPR-deleted variants, while the low-pathogenic HPR0 form produces transient subclinical infection detectable only by molecular testing. ISA has been a notifiable disease of the World Organisation for Animal Health (WOAH/OIE) since 1990.

Affected species

Clinical ISA occurs almost exclusively in farmed Atlantic salmon (Salmo salar). Brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) can carry the virus without developing disease, and Pacific salmon show high relative resistance. Carrier species are epidemiologically important because they can maintain and spread the virus without obvious signs.

Clinical signs

Severe anaemia with pale gills
Lethargy and swimming near the surface, gulping for air
Exophthalmia (bulging eyes)
Ascites (abdominal fluid accumulation)
Dark, congested or swollen liver and spleen
Haemorrhages and necrosis in several internal organs

Transmission

ISAV spreads horizontally through contact with infected fish and their secretions, contaminated water, and shared equipment or people who have handled infected fish. The salmon louse (Lepeophtheirus salmonis) can passively transfer the virus between fish. Because the virus persists in seawater and can move with live fish and eggs, movement controls are central to containment.

Diagnosis

Diagnosis combines reverse transcription PCR (RT-PCR), virus isolation in cell culture, and antibody-based methods such as immunofluorescence and immunohistochemistry, supported by histopathology. Molecular sequencing of the highly polymorphic region distinguishes the avirulent HPR0 form from virulent HPR-deleted variants, which is essential for risk assessment and reporting.

Treatment, control and prevention

There is no treatment once fish are infected. Control of clinical ISA relies on official disease-management measures and strict biosecurity.

Stamping out: culling and proper disposal of infected and exposed stock
Fallowing of affected sites to break the infection cycle
Zoning and movement restrictions on live fish and eggs
Disinfection of equipment, wellboats, and facilities
Integrated sea lice control to remove a transmission vector
Surveillance and mandatory reporting under WOAH protocols