A multiple-technique approach to investigating the presumptive low-level detection of Renibacterium salmoninarum at a broodstock hatchery in Maine.
Tissues from 160 fish were evaluated with a membrane filtration-fluorescent antibody technique (MF-FAT) and cultured with metabolite-supplemented media to determine if R. salmoninarum (Rs) or unrelated cross-reacting microorganisms were present in a population of hatchery-reared, adult brook trout (Salvelinus fontinalis). Fluorescing bacteria were detected in kidney (8.8%), spleen (11.9%), and gut (13.1%) samples; however, no Rs was confirmed by culture. Many fluorescing cells were morphologically consistent with Rs; others were not. Non-Rs bacteria, isolated by culture from 3 gut samples, reacted strongly by a direct fluorescent antibody test (DFAT) with polyvalent anti-Rs serum. Previously, a very low prevalence of fluorescing cells were detected in kidney smears by DFAT, and the hatchery was considered presumptively positive for Rs. It was concluded that systemic cross-reacting bacteria, detected in spleen and gut samples, could have contributed to previous questionable positive kidney smears. However, these cells did not account for all the fluorescing cells morphologically similar to Rs detected by MF-FAT. No tissue sample was confirmed Rs-positive by culture; therefore, the presumptive Rs-positive classification of the hatchery remains questionable. Even in the absence of conclusive results, this study illustrates a practical, multiple technique approach, with a combination of MF-FAT and metabolite-supplemented media, to evaluate fish populations suspected of low-level infections of Rs or in which cross-reacting bacteria are suspected of interfering with DFAT results.