LAUSR

Abstract Arctic charr (Salvelinus alpinus) farming is an emerging component in today's aquaculture sector; however, intensive aquaculture sites provide optimal conditions for disease. Although vaccines have been developed and tested for other major cultured salmonids, such as Atlantic salmon (Salmo salar), it is unknown if such vaccines provide protection in Arctic charr. Therefore, the present study aimed to evaluate the efficacy of two vaccines, Forte Micro® and Renogen®, administered solely or in combination, in the prevention of atypical furunculosis in Arctic charr after challenge with atypical Aeromonas salmonicida (RPC strain FFA-43). Furthermore, vaccination efficacy was assessed after short or long post-vaccination periods prior to bacterial challenge. Improved survival was observed in charr vaccinated with both single and combined vaccines in the short-term efficacy study, and this positively correlated with increased circulating IgM specific for atypical A. salmonicida. However, circulating IgM was reduced in the long-term study. Forte Micro® provided improved protection when Arctic charr were exposed to infection in the short-term vaccine efficacy compared to the long-term vaccine efficacy, where no significant protection was observed. Gene expression analysis revealed upregulation of complement, acute phase, and cellular signalling post-vaccination and post-challenge for fish in the short-term vaccine efficacy study, with similar to muted responses observed in fish in the long-term vaccine efficacy study. This study provides insight on the efficacy and molecular mechanisms of protection of two commercial salmonid vaccines in Arctic charr during infections with atypical A. salmonicida.