| Summary: | Prevalence of Aeromonas salmonicida was studied in mature chum (Oncorhynchus keta), pink (O. gorbuscha), and masu salmon (O. musou) without signs of furunculosis. From September 1979 to November 1990, a total of 15,264 chum, pink and masu salmon were collected from 33 rivers. We isolated A. salmonicida from chum salmon in 18 of 28 rivers and the prevalence ranged from 1.8 to 34.4%. For pink salmon populations, A. salmonicida was isolated in 9 of 14 rivers with prevalence ranging from 1.7 to 14.6%. In masu salmon, A . salmonicida was isolated in 6 of 10 rivers and the prevalence ranged from 0.5 to 3.7%. The above results indicated that A. salmonicida distributed widely in the populations of mature salmonids, except those in the rivers on the coast from Tsugaru Strait to Uchiura Bay. The annual change in prevalence of A. salmonicida in salmon was closely related to the fish density in the holding ponds for maturation. The viable numbers of A. salmonicida in the kidney were found to be from 10^3 to 10^5 cfu/g.
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