Experimental induction of bacterial kidney disease in Chinook salmon by immersion and cohabitation challenges

Abstract

Two challenge methods for inducing bacterial kidney disease (BKD), caused by Renibacterium salmoninarum (Rs), in salmon were evaluated using chinook salmon Oncorhynchus tshawytscha. The first method involved immersing the fish in various concentrations of Rs cells (10(4) to 10(6) ml-1) for 15 or 30 min (the immersion method); the second method was based on the enforced cohabitation of uninfected fish with others that had been injected intraperitoneally with various doses (10(3), 10(5), and 10(7)) of Rs cells (the cohabitation method). Both methods were successful in establishing BKD in the test fish but, as expected with this slow-acting pathogen, the challenges took considerable time to complete. With the immersion method, a 2-fold increase in exposure time and a 100-fold increase in pathogen concentration did not significantly affect the mean time to death of the fish (ca 203 d; average water temperature 12.6-degrees-C). However, mortalities at 180 and 305 d d post challenge were dose-related. With the cohabitation method, mean time to death in the non-injected fish was 145 d; fish that had been injected with 10(3), 10(5), and 10(7) Rs cells and that served as the source of the infection for the non-injected fish had mean times to death of 60, 83, and 104 d, respectively (average water temperature 9.9-degrees-C). The advantages and disadvantages of the challenge methods are discussed.