Abstract

To determine the virulence of Piscirickettsia salmonis in coho salmon Oncorhynchus kisutch and rainbow trout O. mykiss, juvenile fish (16-17 g) of both species were held in freshwater and inoculated intraperitoneally (0.05 mL) with seven 10-fold serial dilutions of this rickettsia (from 10 3 TCID50/mL, the tissue culture infective dose producing a cytopathic effect in 50% of inoculated cultures). Duplicate groups of 30 fish for each dilution and appropriate controls were used. Deaths began during the 3rd week and clinical disease occurred in both species. Deaths were fewer in rainbow trout, although they appeared earlier. At the highest concentration of the pathogen, there was a 60% cumulative mortality in coho salmon and 28.3% in rainbow trout. The infectious dose (50%), measured by immunofluorescence in kidney smears and estimated by the Probit procedure, was 10 1.9 in coho salmon and 10 2.4 in rainbow trout. Although rainbow trout were affected by piscirickettsiosis, they were less susceptible than coho salmon.