Use of Diagnostic Assays to Evaluate Transmission of Bacterial Coldwater Disease

Flavobacterium psychrophilum, the causative agent of Bacterial Coldwater Disease (BCWD), affects several economically important species of fish in the Pacific Northwest. F. psychrophilum has been shown to survive both on the exterior of eggs and within the egg indicating it may be vertically transmitted. To evaluate this, 30 Coho salmon (Oncorhynchus kisutch) broodstock were screened for F. psychrophilum using various diagnostic assays including ELISA and membrane filtration fluorescent antibody test, both of which use a monoclonal antibody specific to F. psychrophilum. Five families showing F. psychrophilum infection levels in the females ranging from below detection limits to 10⁶ CFU ml^-1 were selected for further experiments. Eyed eggs from these families were transported to the University of Idaho and disinfected in 100 ppm Ovadine® for 10 minutes upon arrival. Eggs were then randomly sampled from each family, surface disinfected in 400 ppm Ovadine® for 15 minutes, and yolk material aseptically withdrawn. Presence of F. psychrophilum in fluid from within the egg was confirmed by nested PCR assay for four of the five families. Progeny were sampled weekly for seven weeks and F. psychrophilum was detected in all families at least once during this time period. To determine if progeny from infected broodstock have a greater risk of developing BCWD, fry (mean weight approximately 0.35 g) were moved to a high density, low flow rearing system and are currently being monitored for disease outbreaks. We will also test if progeny from heavily infected broodstock are more susceptible than progeny from broodstock with a limited infection when directly challenged with F. psychrophilum. Results from these experiments will provide insight on the use of these assays as tools for managing BCWD and setting a threshold level for screening broodstock and culling eggs.