Serological evidence of infectious salmon anaemia virus (ISAV) infection in farmed fishes, using an indirect enzyme-linked immunosorbent assay (ELISA)

Antibody detection tests are rarely used for diagnostic purposes in fish diseases. Infectious salmon anaemia (ISA) caused by ISA virus (ISAV) is an emerging disease of Atlantic salmon Salmo salar L. The virus has also been isolated from diseased coho salmon Oncorhynchus kisutch in Chile. An indirect enzyme-linked immunosorbent assay (ELISA) that should facilitate serodiagnosis of ISAV infection, the study of epidemiology, and the control of ISA in farmed fishes has been developed using purified ISAV as the coating antigen, and monoclonal antibodies that detect fish immunoglobulins bound to the antigen on the plate. Application of the test to a random sample of farmed Atlantic salmon from the Bay of Fundy, New Brunswick, Canada, positively identified 5 of the 7 ISAV RT-PCR-positive fish, and all 10 RT-PCR-negative fish were also negative in the ELISA. Some RT-PCR-negative fish had an elevated non-specific antibody reactivity suggestive of chronic infection or resistance to ISAV. This test was also able to detect 11 of the 14 coho salmon pooled serum samples from a clinically affected farm in Chile that were positive by the virus neutralization (VN) test, and 2 of the 4 VN-negative samples. We conclude that this ELISA would be suitable as a routine test for ISAV infection or for assessing ISAV vaccine efficacy before placing smolts in sea cages, and for testing fishes in sea cages to detect level of resistance to ISA. The assay enables vaccination in combination with depopulation control methods.