Resistance of salmonids to Aeromonas salmonicida: Relation between agglutinins and neutralizing activities

Serum of furunculosis‐resistant rainbow trout Salmo gairdneri, when injected into brook trout Salvelinus fontinalis, increased resistance of the brook trout to virulent challenges of Aeromonas salmonicida, the furunculosis pathogen. Serum from susceptible Atlantic salmon Salmo salar did not confer passive resistance on brook trout although natural agglutinin titers in Atlantic salmon serum were equal to those in rainbow trout. Further analyses indicated that rainbow trout serum neutralized toxic components produced by A. salmonicida, but that Atlantic salmon serum did not. When 11 strains of rainbow trout were challenged with 10⁹ virulent cells of A. salmonicida, a range of mortality from 0 to 83% resulted. Although no correlation was observed between the natural serum agglutinin level of a particular strain and its relative resistance to experimental pathogen challenge, an inverse relation was apparent between cytotoxic serum‐neutralization activity and resistance to furunculosis.