Epidemiological Study Of Aeromonas salmonicida, Causative Agent Of Furunculosis, In Salmonid Fish In Japan Nomura1 Hiromi Honma2, Hisae Kasai and Mamoru Yoshimizu1 National Salmon Resources Center, Japan; 2Graduate School of Fisheries Sciences, Hokkaido University, Japan Furunculosis, caused by the bacterium Aeromonas salmonicida, is serious disease in salmonid fish and this study was carried out for the purpose of establishing control methods for the disease. In a recent epidemiological study, we determined the distribution and prevalence of A. salmonicida in asymptomatic mature chum (Oncorhynchus keta), pink (O. gorbuscha) and masu salmon (O. masou) in Hokkaido and Honshu. A. salmonicida was distributed widely in the populations of mature salmonids in Hokkaido and Honshu. From 1979 to 1999, a total of 21,389 chum, pink and masu salmon were collected from the rivers in Hokkaido and Honshu. We isolated A. salmonicida from the chum salmon in 21 of the 34 rivers examined and the overall prevalence was 12.2 % in the infected stocks. The prevalence of the agent in pink salmon and masu salmon were 4.6 % and 1.4 % respectively. The prevalence of A. salmonicida on gill surface, kidney and intestine of chum salmon was studied using CBB medium, made by dissolving 0.1mg/ml Coomasie Brilliant Blue R250 in Nutrient agar. Using CBB medium, we also examined the chum salmon caught at the Chitose River catching site. The prevalence of the agent on gill surface was high (50 %) at catching site on the Chitose River, and decreased in fish in the holding pond in the river.The agent was isolated from the gill surfaces, but could not be found in the kidneys and intestines. A. salmonicida was also isolated from the gill surfaces of masu salmon that were kept directly behind the chum salmon holding pond. The bacteria are spread during fish migration within the river and during transportation of the fish from capture site to holding pond. The agent was isolated from the gill surface of chum salmon caught off the coast of Shibetsu, Rausu, but the prevalence was low (1.7%). To control furunculosis in salmonid fish, fish should mature in ponds under conditions of low density and should be disinfected to prevent spread of furunculosis before keeping in the pond for maturation.