Evidence that the causal agent of bacterial cold-water disease Flavobacterium psychrophilum is transmitted within salmonid eggs

To examine the possibility that Flavobacterium psychrophilum, cause of bacterial coldwater disease (BCWD), may be vertically transmitted, eggs and embryos at selected stages of development were taken from 17 steelhead trout. Some of the broodstock steelhead trout used for this study had been injected with erythromycin prior to spawning; some had been injected with oxytetracycline, and others had not been injected. Surface disinfected eggs/embryos were incubated in tryptone yeast extract (TYE) broth, and egg surfaces were determined to be sterile when no bacteria were isolated from the broth after 72 h incubation at 17 degrees C. These eggs/embryos were then homogenized and cultured in TYE broth, and growth was identified as F. psychrophilum by biochemical and immunological assays. F. psychrophilum contamination was identified from the surface of 28% of eggs/embryos. F. psychrophilum was also detected within the ovarian fluid samples from 10% of the broodstock trout, and the bacterium was isolated from the contents of 13% of newly spawned, surface-disinfected eggs as well as from 7% of eyed eggs and 4% of newly hatched alevins. No difference was observed in the proportion of eggs infected with F. psychrophilum from antibiotic-injected versus non-injected fish. At the hatchery, however, the remaining progeny of broodstock injected with erythromycin experienced significantly lower mortality due to BCWD up to 10 wk after pending than the progeny of fish not injected or injected with oxytetracycline. In vitro experiments indicated that F. psychrophilum is resistant to lysozyme concentrations of up to 2 mg ml(-1) (greater than concentrations found within a salmonid egg) and that 2% of F. psychrophilum cells survive after exposure of 100 ppm povidone/iodine for 30 min. The results of this study indicate that F. psychrophilum may be transmitted both horizontally and vertically within salmonid hatcheries.