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 salm oninarum in Atlantic salmon (Salmo salar L.) tissues detected DNA extr acted from between 4 and 40 bacterial cells, PCR was at least as sensi tive as culture when it was used to identify subclinically infected fi sh experimentally challenged with R. salmoninarum. However, PCR identi fied much higher numbers of kidney tissue and ovarian fluid samples fr om commercially reared broodstock fish to be positive for R. salmonina rum than did culture. This difference may be due to the antibiotic che motherapy of broodstock fish used by the industry in 1994 to control t he vertical transmission of R. salmoninarum. A much closer relationshi p between PCR and culture results was observed for ovarian fluid sampl es collected from broodstock fish in 1993. Also, PCR scored a much hig her percentage of kidney tissue samples than ovarian fluid samples fro m 1994 broodstock fish positive for R. salmoninarum, which may reflect the uneven distribution of the pathogen in different fish tissues, In clusion of a nested probe to identify the PCR-positive 1994 ovarian fl uid samples increased the sensitivity of detection to between one and four cells and the number of samples that scored positive by almost th reefold, These data indicate that many infected ovarian fluid samples contained very low numbers of R. salmoninarum cells and, because almos t all these samples were culture negative, that PCR may have detected dead or otherwise unculturable bacterial cells.