First Report of Bacterial Kidney Disease (BKD) Caused by Renibacterium salmoninarum in Chum Salmon (Oncorhynchus keta) Farmed in South Korea

In 2021, a prominent increase in mortality was observed in juvenile and subadult cultured chum salmon (<i>Oncorhynchus keta</i>) on a mariculture farm in Jeollanam-do Province, South Korea. The affected fish displayed distinct symptoms: pale gills, petechial hemorrhages in the muscles, and white nodules on the kidneys. Infectious pancreatic necrosis virus (IPNV) was cultured from some fish samples using fish cell lines. Bacteria were isolated from various fish tissues using kidney disease medium-two (KDM-2) culture medium. By detecting and sequencing the 16S rRNA gene using DNA extracted from the kidneys of the infected fish via PCR, the isolated bacteria were identified as <i>Renibacterium salmoninarum</i>. Histopathological examination primarily focused on hematopoietic tissues of kidneys and revealed clear evidence of severe necrosis and granulomatous changes. Additionally, nuclei with peripherally displaced chromatin were abundant in the kidneys of affected fish. These findings suggest that mass mortality of chum salmon was caused by <i>R. salmoninarum</i>, which induced typical bacterial kidney disease (BKD) symptoms, without IPNV infection. This represents the first outbreak of BKD attributed to <i>R. salmoninarum</i> infection in farmed chum salmon in South Korea.