Detection of Flavobacterium psychrophilum in chum salmon Oncorhynchus keta and virulence of isolated strains to salmonid fishes

We investigated the presence of Flavobacterium psychrophilum in the ovarian fluids of female adults (eight rivers) and the fry kidney tissues (nine hatcheries) of chum salmon Oncorhynchus keta in Hokkaido, Japan in 2005. F psychrophilum was detected by culture in 3.3-90.0% of ovarian fluid at all rivers and 5% of kidney tissue at one hatchery, whereas 33.3-100% and 5.0-42.5%, respectively, at all the sampling sites were positive in nested PCR targeting 16S rDNA of F. psychrophilum. Isolated bacteria were identified by an agglutination assay, the nested PCR assay, biochemical tests and PCR targeting gyrB gene. Intraperitoneal injection with 5.3 x 10(5) to 3.9 x 10(6) CFU of the isolates/g to chum salmon, rainbow trout O. mykiss and masu salmon O. masou juveniles (5.3-6.4 g) resulted in cumulative mortalities of 10-70%, 4-26%, and 20-95%, respectively. These results suggest that the bacterium is widely distributed at high frequency in adult female chum salmon in Hokkaido and virulent to salmonid fishes.