Persistence of the virulence of Aeromonas salmonicida strains kept in river sediments.

The persistence of pathogenicity in two strains of Aeromonas salmonicida maintained in river sediments was studied at different times. Cultures were kept about nine months, and the virulence was compared to its initial value exhibited by the same strain, subcultivated in rainbow trout fingerlings. Trials involved intramuscular injection of comparable doses of bacteria to batches of 30 fish held at 15 degrees C, and recording of subsequent mortalities. No difference was found during the first six months, after which a decrease of pathogenicity resulted in its disappearance before the ninth month. Progressive decrease in incubation temperatures was also exerted to simulate an artificial wintering, but did not alter the evolution of the virulence. The possible role of mud as a source of contamination for fish is discussed.