Comparison of the membrane-filtration fluorescent antibody test, the enzyme-linked immunosorbent assay, and the polymerase chain reaction to detect Renibacterium salmoninarum in salmonid ovarian fluid.
Ovarian fluid samples from naturally infected chinook salmon (Oncorhynchus tshawytscha) were examined for R. salmoninarum by the membrane-filtration fluorescent antibody test (MF-FAT), an antigen capture ELISA, and a nested polymerase chain reaction (PCR). On the basis of the MF-FAT, 64% (66 of 103) samples contained detectable levels of R. salmoninarum cells. Among the positive fish, the R. salmoninarum concentrations ranged from 25 cells/ml to 4.3 × 109 cells/ml. A soluble antigenic fraction of R. salmoninarum was detected in 39% of the fish (40 of 103) by the ELISA. Although considered one of the most sensitive detection methods for bacterial kidney disease in tissues, ELISA did not detect R. salmoninarum antigen consistently at bacterial cell concentrations below about 1.3 × 104 cells/ml according to the MF-FAT counts. When total DNA was extracted and tested in a nested PCR designed to amplify a 320-base-pair region of the gene encoding a soluble 57-kDa protein of R. salmoninarum, all of 100 samples tested were positive. The results provided strong evidence that R. salmoninarum may be present in ovarian fluids thought to be free of the bacterium on the basis of standard diagnostic methods.