Comparison of diagnostic tests for bacterial kidney disease in juvenile steelhead trout (Oncorhynchus mykiss).

In order to accurately diagnose bacterial kidney disease caused by Renibacterium salmoninarum in steelhead trout, kidney tissue from experimentally infected fish was evaluated using a commercially available enzyme-linked immunosorbent assay (ELISA) test kit, fluorescent antibody (FA) testing, bacteriologic culture, and histopathology. Seventy-five steelhead trout were randomly assigned to 1 of 4 groups and intraperitoneally inoculated with 0.15 ml saline (n = 20), 1 x 10(10) organisms/ml (n = 18), 1 x 10(8) organisms/ml (n = 18), or 1 x 10(6) organisms/ml (n = 19) of R. salmoninarum. ELISA, FA and bacteriologic culture were positive for R. salmoninarum from the kidney tissue of the 2 groups infected with the highest doses. Although the ELISA and FA tests were accurate when compared to the bacteriologic culture from the 2 groups infected with higher doses of the organism, they were less sensitive at the lowest level of inoculum. Histopathology was not specific for this disease; however, all infected fish had a marked proliferative histiocytic interstitial nephritis, characterized by marked expansion of the renal hematopoietic tissue by histiocytes without tissue necrosis. Other microscopic findings included splenitis and myositis (at the injection site) of some fish.