PCR and probe-PCR assays to monitor broodstock Atlantic salmon (Salmo salar L.) ovarian fluid and kidney tissue for presence of DNA of the fish pathogen Renibacterium salmoninarum.

A simple, rapid PCR assay for the identification of Renibacterium salmoninarum in Atlantic salmon (Salmo salar L.) tissues detected DNA extracted from between 4 and 40 bacterial cells. PCR was at least as sensitive as culture when it was used to identify subclinically infected fish experimentally challenged with R. salmoninarum. However, PCR identified much higher numbers of kidney tissue and ovarian fluid samples from commercially reared broodstock fish to be positive for R. salmoninarum than did culture. This difference may be due to the antibiotic chemotherapy of broodstock fish used by the industry in 1994 to control the vertical transmission of R. salmoninarum. A much closer relationship between PCR and culture results was observed for ovarian fluid samples collected from broodstock fish in 1993. Also, PCR scored a much higher percentage of kidney tissue samples than ovarian fluid samples from 1994 broodstock fish positive for R. salmoninarum, which may reflect the uneven distribution of the pathogen in different fish tissues. Inclusion of a nested probe to identify the PCR-positive 1994 ovarian fluid samples increased the sensitivity of detection to between one and four cells and the number of samples that scored positive by almost threefold. These data indicate that many infected ovarian fluid samples contained very low numbers of R. salmoninarum cells and, because almost all these samples were culture negative, that PCR may have detected dead or otherwise unculturable bacterial cells.