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 s almonicida by both injection and bath models. The groups were injected intr aperitoneally with 100 mu l of logarithmically decreasing dilutions of A. s almonicida (range 10(8)-10(3) cells/fish) or bathed in 10(5) CFUs/ml of A. salmonicida for 24 h. Halibut were significantly more resistant to infectio n, compared to Atlantic salmon, with 10(6) and 10(7) 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 cha llenge 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 s urviving halibut were culture positive, but only from the intestinal lumen. All surviving salmon but none of the surviving halibut, showed histologica l evidence of infection with A. salmonicida. Stress tests of both halibut a nd Atlantic salmon showed that recovered salmon still had a high carrier ra te of A. salmonicida while none of the surviving halibut were carrier test positive. A significant rise in agglutinating antibody titre was observed i n surviving Atlantic salmon. However, no rise in antibody titre was observe d in the surviving halibut, suggesting that the processing of the bacteria during an acute infection may be fundamentally different between these two species. (C) 1999 Elsevier Science B.V. All rights reserved.