Aeromonas salmonicida subsp. salmonicida
is the causative agent of furunculosis in several fish species including Atlantic salmon (Salmo salar). Although g enetic susceptibility and resistance to A. salmonicida have been documented in salmon, the mechanisms are not well understood. Here, we compared naïve and immunized Atlantic salmon from farmed, wild and hybrid crosses to A. salmonicida susceptibility and immune response. Naïve fish were intraperitoneally
injected with 2.5×104 CFU/100g and the immunized fish were challenged with 4×106 CFU/100g at 10 weeks post-primary immunization. Samples of tissues were taken at different time points . We found that naïve wild salmon were significantly more susceptible to A. salmonicida infection than the farmed and hybrid crosses (p <0.001). Colonization of lymphoid tissues by A. salmonicida correlated with fish susceptibility. Expression of il-1β , il-10 , and tlr5
were up-regulated after 10 days post-infection with higher expression in farmed fish in contrast to wild fish. Expression of tnfα , igM were up-regulated in farmed fish, but down-regulated in wild fish. In contrast to naïve fish, no differences in susceptibility were found between vaccinated fish challenged with A. salmonicida . This study provides new insights into Atlantic salmon immune response and susceptibility to A.
salmonicida infection and immunization.