Susceptibility of Atlantic halibut, Hippoglossus hippoglossus (L.) to infection with typical and atypical Aeromonas salmonicida

Halibut, Hippoglossus hippoglossus (L.) and Atlantic salmon, Salmo salar (L.) were challenged with virulent typical or atypical strains of Aeromonas salmonicida by both injection and bath models. The groups were injected intraperitoneally with 100 μl of logarithmically decreasing dilutions of A. salmonicida (range 108–103 cells/fish) or bathed in 105 CFUs/ml of A. salmonicida for 24 h. Halibut were significantly more resistant to infection, compared to Atlantic salmon, with 106 and 107 typical and atypical A. salmonicida cells/halibut being the minimum lethal dose. No halibut died in the bath challenge although approximately 80% of salmon died by this challenge method. All animals that died during the challenge were positive for A. salmonicida on culture A. salmonicida was cultured from internal organs of approximately 60% of surviving Atlantic salmon while only 2.3% of the surviving halibut were culture positive, but only from the intestinal lumen. All surviving salmon but none of the surviving halibut, showed histological evidence of infection with A. salmonicida. Stress tests of both halibut and Atlantic salmon showed that recovered salmon still had a high carrier rate of A. salmonicida while none of the surviving halibut were carrier test positive. A significant rise in agglutinating antibody titre was observed in surviving Atlantic salmon. However, no rise in antibody titre was observed in the surviving halibut, suggesting that the processing of the bacteria during an acute infection may be fundamentally different between these two species.