Bacterial kidney disease in salmonid fish

Bacterial kidney disease (BKD) is a chronic systemic infection of salmonid fish. BKD has been a main threat to salmonid health, in aquaculture as well as in feral fish populations, all over the world during recent decades. Various disease manifestations have been reported, from the characteristic swollen kidney containing white nodules, to more diffuse signs and even of asymptomatic carriers. Regular screening programmes have been the most important possibility to prevent transmission, since no effective antibiotics or prophylactic therapy are available. BKD is caused by the Gram-positive bacterium Renibacterium salmoninarum, a slow growing and fastidious diplobacilli. Diagnosis has been complicated due to the difficulties in bacterial cultivation. R. salmoninarum can survive intracellularly in the phagocytic cells of the fish, resulting in a granulomatous reaction, similar to that in tuberculosis of mammals. Progress in the development of prophylactic methods to control BKD requires knowledge of basic immune functions in salmonid fish. In this thesis, techniques have been developed to study the immune response and immune functions in salmonid fish during the progression of BKD. An enzyme-linked immunosorbent assay (ELISA) was developed to detect R. salmoninarum antigens in fish kidney. The negative-positive threshold value in the ELISA was determined based on the results of more than 400 kidney samples negative for BKD. The sensitivity of the ELISA method was tested on samples from populations with clinical or enzootic BKD. An improvement in the possibility to detect BKD in fish populations by the ELISA was demonstrated when compared with bacterial cultivation. Monoclonal antibodies to salmonid Ig were used in an ELISA for determination of the humoral response to R. salmoninarum by a non-lethal sampling. Elevated antibody levels were measured in populations of rainbow trout and salmon with clinical BKD. No correlation, of the antibody level and the occurrence of R. salmoninarum antigens in the kidney of individual fish, was found. A cell-mediated immune response was measured in rainbow trout with experimentally induced BKD by an in vitro stimulation of the lymphocyte population by R. salmoninarum antigens. Three monoclonal antibodies (Mabs) 1C2, 7A5/20 and 8H4/23 to lymphocytes of rainbow trout were developed. The Mab 1C2-positive lymphocyte population of spleen was indicated to have an influence on the immune response to BKD