Subclinical Infection of Renibacterium salmoninarum in Fry and Juveniles Chum Salmon Oncorhynchus keta in Hokkaido, Japan

Genomic and mRNA sequences of Renibacterium salmoninarum, the causative agent of bacterial kidney disease (BKD), were detected in the kidney tissue and surface mucus of asymptomatic fry and juveniles chum salmon Oncorhynchus keta, using nested PCR, quantitative real-time PCR and RT-nested PCR. Fry were obtained from a hatchery belonging to the Hokkaido Salmon Resources Association from March 25, through May 10, 2016. In addition, the fry were transferred from the hatchery into a wet laboratory at the Salmon and Freshwater Fisheries Research Institute, reared in running well water at 8°C-10°C, and monitored for the bacterium from April 19, 2016 through January 4, 2017. The fish showed no mortality from BKD, and live bacteria were not detected in the kidney tissue by culture. Yet prevalence of R. salmoninarum genome and positive rate of the mRNA in the kidney tissue samples were 0%-100% and 0%-70%, respectively, with number of genome of 1.0 × 101-1.7 × 104 copies/mg. Surface mucus samples, which were obtained in November 21, 2016 and January 4, 2017, showed similar detection levels to the kidney tissue based on prevalence and intensity of the genome. The results verify subclinical infection with R. salmoninarum in chum salmon fry in Hokkaido, Japan.