Hirame novirhabdovirus (HIRRV) is a significant viral pathogen of Japanese flounder ( Paralichthys olivaceus ). In this study, seven monoclonal antibodies (mAbs) against HIRRV (isolate CA-9703) were produced and characterized. Three mAbs (1B3, 5G6, and 36D3) were able to recognize nucleoprotein (N) (42 kDa) and four mAbs (11-2D9, 15-1G9, 17F11, and 24-1C6) recognized matrix (M) protein (24 kDa) of HIRRV. Western blot, Enzyme-linked immunosorbent assay, and indirect fluorescent antibody technique (IFAT) results indicated that the developed mAbs were specific to HIRRV without any cross-reactivity against other different fish viruses and epithelioma papulosum cyprini cells. All the mAbs comprised IgG1 heavy chain and κ light chain except 5G6, which has a heavy chain of IgG2a class. These mAbs can be very useful in development of immunodiagnosis of HIRRV infection.

In 2021, a prominent increase in mortality was observed in juvenile and subadult cultured chum salmon (Oncorhynchus keta) 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 Renibacterium salmoninarum. 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 R. salmoninarum, which induced typical bacterial kidney disease (BKD) symptoms, without IPNV infection. This represents the first outbreak of BKD attributed to R. salmoninarum infection in farmed chum salmon in South Korea.