Immune responses in Atlantic salmon (Salmo salar) following protective vaccination against Infectious salmon anemia (ISA) and subsequent ISA virus infection

Infectious salmon anemia (ISA) is an orthomyxoviral disease that has had devastating effects on farmed Atlantic salmon. ISA is still a disease resulting in continued loss of revenues and therefore development of effective vaccines is of great importance. Commercial vaccines against ISA are available, but the efficacy is poorly described. There is little information about vaccine-induced immune factors preventing ISA virus (ISAV) infection today. In this study we assessed the protective effects and immunogenicity of vaccines containing three different quantities of the inactivated ISAV antigen. Our findings indicated that immunization induced effective protection in Atlantic salmon with a relative percent survival (RPS) as high as 86. The level of protection was correlated to the amount of ISAV antigen in the vaccine, and fish immunized with high antigen amounts produced detectable ISAV-specific and neutralizing antibodies. While ISAV infection was detectable in non-vaccinated control fish challenged by cohabitation, no infection was detected in fish immunized with high antigen amounts. After challenge, transcriptional analysis of selected immune-related genes demonstrated activation of innate immune responses in ISAV-infected control fish, but not in vaccine protected fish. This study furthers the knowledge about vaccine efficacy and vaccine-induced immunity to ISAV challenge in Atlantic salmon.