ESTIMATION OF VARIATION OF VIRULENCE OF RENIBACTERIUM-SALMONINARUM BYSURVIVAL ANALYSIS OF EXPERIMENTAL-INFECTION OF SALMONID FISH

The variation of virulence of Renibacterium salmoninarum, the causativ e agent of bacterial kidney disease (BKD) in salmonid fish, was studie d by infecting rainbow trout, Oncorhynchus mykiss (Walbaum), with two isolates (strains 325 and 932) from diseased Atlantic salmon, Salmo sa lar L., and one isolate (strain 4366) from an apparently healthy Atlan tic salmon. Coho salmon, Oncorhynchus kisutch (Walbaum), were injected with the strain 932 to estimate difference in fish species resistance . Fish were removed by;random sampling for other study purposes, a stu dy design possible with analysis of lifetime distribution's incorporat ing both sampling-, death- and survival-times. At the end of the exper iment, the rainbow trout infected with strains 325, 932 and 4366 had a survival probability of 33%, 51% and 72%, respectively. The coho salm on infected with strain 332 had a 26% survival probability. The strain differences were significant according to the log-rank test, and the risk ratio between the strains ranged from 1.8 to 5.4. The strain from the apparently healthy fish was least virulent. The survival of the f ish species was different over time. Rainbow trout were more likely to die early in the time course, but high numbers of coho died later, re sulting in an overall risk of mortality of 1.4 in favour of rainbow tr out. Differences in virulence may reflect changed selective pressure o n R. salmoninarum when introduced from feral stocks into the environme nt of fish farms.