Asymptomatic Infection of Renibacterium salmoninarum in Hatchery-reared Juvenile Chum Salmon Oncorhynchus keta Resulted in Mass Mortalities after Long-term Rearing

Abstract

Mass mortalities due to bacterial kidney disease (BKD) occurred after 10-15 months rearing in two groups of chum salmon Oncorhynchus keta which had been raised from asymptomatically infected eggs and fry transferred from private hatcheries. Mortality reached 56.1% and 19.0% in the two groups, respectively, 146 days after the first death. A Renibacterium salmoninarum-specific gene was detected from various organs of the subclinical and dead fish. In the dead fish, the number of gene copies was higher than those in the subclinical fish in all the eight organs examined. These results suggested that the bacterium distributed in the bodies of asymptomatic fish proliferated systemically, resulting in an outbreak of BKD.