ISA

Aetiology

Infectious salmon anaemia (ISA) is an orthomyxovirus infection of sea-farmed Atlantic salmon (Salmo salar) inducing a systemic and lethal condition characterised by severe anaemia and variable haemorrhages and necrosis in several organs. ISA virus can also be isolated from disease free rainbow trout. The virus belongs to the family Orthomyxoviridae and has recently been classified as the type species of the new genus Isavirus. ISAV is related to influenza viruses, but has a number of unique characteristics that distinguish it from other known orthomyxoviruses from warm-blooded animals. ISA virus is serologically fairly homogenous and can be identified using MAb.

ISA was first detected in Norway in the early 1980s. The viral aetiology was first established in the early 1990s. The disease has subsequently been detected in Canada, Scotland, Faroe Islands, USA and Chile. ISA, defined as infection with HPR-deleted ISAV, is classified as Category C disease in Commission Implementing Regulation (EU) 2018/1882. 

Symptoms and pathology

ISA outbreaks occur mainly in spring at temperatures between 5 and 15 °C. Sick fish are lethargic, they sink to the bottom or cling to the net sides using they mouth, often sick fish place themselves vertically with their head in the water surface. Under experimental conditions the incubation period is usually 10-20 days. In infected populations under field conditions the infection can remain hidden for months before the disease breaks out. A disease outbreak can have either an acute course with high mortality or be more long-drawn with increase in daily mortality, which may continue for several months.

Typical findings are pale gills, bulging eyes and distended belly. Late in the disease course bleeding in the skin, loss of scales and bloody, swollen anus can be observed. Spleen and liver are often strongly swollen, dark and with petechial haemorrhages. Such may also occur in the perivisceral adipose tissue and peritoneum. The heart is pale because of anaemia and haematocrit values are very low (haemolytic anaemia). By histopathology haemorrhagic necrosis in the liver can be observed. The macroscopic changes are darkening, bleeding, especially at the abdomen and anaemic gills. When opening the fish typical observations are dark liver, pale heart and pale gills.   

Diagnosis and differential diagnosis

Diagnosis is based on a combination of clinical, pathological and clinical chemical findings. ISA virus is isolated from organ material from infected fish by cultivation in specific cell cultures and identified by RT-PCR or immunofluorescence. The disease might be mistaken for bacterial septicaemia (vibriosis, furunculosis) and viral haemorrhagic septicaemia (VHS).