Mortality after IPNV challenge of Atlantic salmon (Salmo salar L.) differs based on developmental stage of fish or challenge route

The mortality of Atlantic salmon parr and post-smolts was studied after experimental challenge using a virulent strain of infectious pancreatic necrosis virus (IPNV). No mortality was obtained in parr challenged in freshwater, but challenge three weeks after seawater transfer resulted in high mortalities (up to 64%) in cohabitants compared to the intraperitoneally injected fish (up to 19%). No non-infected control fish died. In parr, challenge was performed using intraperitoneal injection (ip) or bath, but even after stressing the fish three times by lowering the water level no mortality occurred. The parr had low virus titres and almost undetectable levels of anti-IPNV antibodies five weeks post-challenge. Post-smolts of the same origin as the parr and challenged by ip injection and/or cohabitation with the same batch of virus, died after challenge. The surviving post-smolts had low virus levels and low antibody levels. Clinically diseased post-smolts had higher IPNV titres in internal organs and virus was detected in all fish that died during the experiment. The IPN virus re-isolated from challenged fish had the same VP2 gene virulence motif as the virus used for challenge. The mortality profiles show that the cohabitant challenge is a reliable challenge model for post-smolts.The parr and post-smolts were fed four different diets, but no difference in mortality was observed between the diet groups. The feed consumption varied between the diets, and fish fed with immunostimulant had eaten less than the others and did not grow well. As there were no differences in mortalities between the diet groups, this verifies the stability of the challenge model for post-smolts.