IMMUNIZATION OF SALMONIDS AGAINST THE FISH PATHOGEN, Aeromonas salmonicida

An avirulent strain of Aeromonas salmonicida was shown to lack “A‐layer” protein exterior to the outer cell membrane, but was still immunogenic in fish. After bacteria were inactivated with chloroform, whole cells, soluble antigen and combined whole cell plus soluble antigen vaccines were prepared. Those brook trout (Salvelinus fontinalis) that were vaccinated by a 60‐second immersion in the combined whole cell plus soluble antigen vaccine were protected against experimental challenges. Further immersion vaccinations of Atlantic salmon (Salmo salar) in the combined vaccine indicated that protection conferred by immunization was highly significant, efficacious, and reproducible. In field situations, mortality in brown trout (S. trutta) immersion‐vaccinated in this preparation was 2.1% whereas that in control fish was 28.9% after natural challenge.