Infectious salmon anaemia

From Fish
Lesions on the gills of an Atlantic salmon with infectious salmon anemia.
Appearance of the abdomen of a fish with ISA

Infectious salmon anemia (ISA) is a viral disease of salmon, caused by an orthomyxovirus.

It is an emerging disease in USA aquaculture. It is classified as a significant disease by OIE, but the USDA has listed it as a federally notifiable disease in the USA. The first report was from farmed Atlantic salmon on the west coast of Norway in 1984.

Clinical signs

Affected fish were lethargic and severely anemic (PCV <5% in moribund fish). Acute outbreaks result in high mortality. Initial signs include lethargic fish hanging around the edges of the cage. As the disease progresses, moribund animals lie on the bottom. The most obvious external lesions are pale gills and hemorrhage in the anterior chamber of the eye. Internally, the liver appears dark and hemorrhagic, an important indication of infectious salmon anemia. Other lesions may include a fibrinous capsule around the liver, a distended stomach filled with viscous mucus, and sometimes hemorrhagic areas on the mucosa. Infected fish often have obvious ascites, and hemorrhage may be present in skeletal muscle.

Histologically, the most important lesion is multifocal, hemorrhagic hepatic necrosis, which may appear zonal; hepatocytes may be dark and swollen and necrotic areas are eosinophilic. Circulating RBC are small, and evidence of cytoplasmic vacuolation, nuclear degeneration, and cell fragmentation may be seen. Affected fish may develop lymphocytopenia and thrombocytopenia and an apparent increase in immature RBC in the peripheral circulation. Signs of chronic infection are more subtle but may include hemorrhage in the swim bladder and skin[1].


Diagnosis is based on clinical signs, with emphasis on anemia (PCV <10%), the gross appearance of a dark liver, and hepatic necrosis. Confirmation can be by viral isolation using the SHK-1 cell line. Virus may be visualized in endothelial cells of cardiac blood vessels using transmission electron microscopy. The agent is enveloped, slightly pleomorphic, and ~100 nm in size. Suspected cases can also be verified using an immunofluorescent antibody technique on frozen tissue. Transmission is horizontal and virus is shed in skin, mucus, feces, and urine.

Sea lice (Lepeophtheirus salmonis) may be a vector; disease outbreaks seem worse when sea lice are present. Currently there is no evidence of vertical transmission. Sea trout have been proposed as a possible reservoir of infection. Protective immunity has been demonstrated in salmon that survive an outbreak. The disease is heavily regulated in Norway and now in the USA, where the USDA should be notified immediately of any suspected cases.