Bacterial kidney disease

Bacterial kidney disease (BKD), caused by the Gram-positive bacterium Renibacterium salmoninarum (Rs), is one of the most important diseases affecting cultured salmonids. The geographic distribution of Rs is almost worldwide, both in marine and freshwater habitats. All salmonids are considered susceptible to BKD. In Europe, it is mostly a problem in aquaculture, while fish populations of genus Oncorhynchus in N-America are extremely susceptible and severely affected by the disease. Rs is found both extra- and intracellular and can be transmitted both horizontally and vertically. BKD is a slowly developing chronic disease, with a long incubation time. External gross BKD signs are characterized by pale gills, skin darkening, distension of the abdomen, exophthalmia, and small sores on the skin. Common internal signs include diffuse swelling and discoloration of internal organs and the occurrence of white nodules in organs, especially the kidney, which is usually initially affected. Usually, the disease becomes systemic, affecting all organs. The histopathology is characterized by diffuse, systemic, chronic granulomatous inflammatory reaction. Some host-related (genetic traits and fish age) and environmental factors (salinity, temperature, water quality, and biomass) can enhance the development of BKD, and nutrition might also be an influential factor. Unlike many common bacterial fish diseases, no sufficiently effective treatment and vaccines are available, which calls for alternative approaches for BKD management, in particular, preventing RS from entering an aquaculture facility by good hygiene and husbandry practices and to reduce stressful rearing conditions as well as quarantine of infected stocks. A vital part of that are specific and sensitive diagnostic tools. Reliable tests are available, including several molecular methods (polymerase chain reactions), immunological tests (enzyme-linked immunosorbent assay, immunofluorescence), as well as conventional methods like culture on selective media and histopathology.