Environmental detection and the use of non-lethal sampling for diagnosis and detection of Renibacterium salmoninarum

Bacterial kidney disease (BKD) is a serious and notifiable disease caused by the gram-positive bacterium Renibacterium salmoninarum (RS). The disease is chronic and insidious, making it difficult to detect before the infection is widespread in the population. Effective detection of infected fish and populations is essential to manage the disease. In the autumn of 2024, BKD was detected at a land-based fish farm in Iceland that raises Atlantic salmon from roe to slaughter. The disease has been detected intermittently since 2017, and RS appears to persist in the facility. The facility uses both freshwater and seawater from several intakes and testing of roe have never yielded positive results. Repeated sampling of fish, water and the environment were performed from 25.09.2024 and throughout 2025 with the aims of determining which sample material and analysis methods will improve the sensitivity for detecting R. salmoninarum in Atlantic salmon with various stages of infection and disease, to what extent detection of bacteria in water corresponds to infection and disease status of the fish, and how the bacterium potentially enter, spreads and persists in the farming environment. 63 salmon from 6 fish groups and 8 tanks have been necropsied. Tissue was taken for RT-qPCR and histological analysis from the liver, gills, skin, spleen, kidney, and heart. PCR-analysis of homogenate of the head kidney and swabs from skin, cloaca, gills, and kidney were performed. Water and swabs from locations considered likely infection areas were analysed with RT-qPCR, with the aim of tracing a potential source of infection. So far, 56 swabs have been taken from the production environment, in addition to 22 water samples from 7 different locations. Preliminary results showed that 5 fish from the same tank and fish group had grossly observable multifocal white nodules in one or more tissues. RS was detected in most examined samples, including swabs from the gills, skin, and cloaca in all fish. Genetic material from RS was also detected in 16 fish without macroscopic changes consistent with BKD (Ct value: 18.04-32,91). Only effluent water from the tank containing diseased fish were positive for the bacterium (Ct value: 31.93). RS was widely detected in the production environment (27/56 swab samples, Ct value: 29.03-33.25). Further sampling, including bacterial culture of environmental samples is ongoing. We will present the preliminary results from this study, including tissue distribution of bacteria and lesions and correlation between fish, water and environmental detection of RS.