Optimization of diagnostic assays and enhancement of vaccine efficacy for Flavobacterium psychrophilum

Flavobacterium psychrophilum, the causative agent of bacterial coldwater disease (BCWD), causes significant mortality in salmonid aquaculture. Reduction of outbreaks in fry may be possible through implementation of a management strategy in which progeny of heavily infected broodstock are culled. Experiments were conducted to validate the diagnostic assays used for determining broodstock infection levels. The relationship between broodstock infection levels and disease prevalence in progeny was then evaluated. Methods for enhancing efficacy of a live-attenuated vaccine against BCWD were also assessed.;An enzyme-linked immunosorbent assay (ELISA) which detects an outer membrane protein (Fp1493) in kidney tissue was shown to be both sensitive and specific while providing an estimate of infection severity. Therefore, it is an appropriate tool to screen broodstock for F. psychrophilum. To estimate bacterial concentration in kidney tissue, a quantitative PCR (qPCR) assay was developed for fp1493. While the qPCR assay is also sensitive and specific, there was no correlation between ELISA and qPCR results for rainbow trout, Oncorhynchus mykiss (Walbaum), and coho salmon, O. kisutch (Walbaum), samples. It is hypothesized the qPCR assay provides an estimate of gene copy number which does not reflect the level of Fp1493 expression.;Female rainbow trout and coho salmon broodstock at two hatcheries were screened for F. psychrophilum by ELISA and a nested PCR. Progeny from 10 broodstock (five from each facility) exhibiting graded levels of infection were monitored through early life stages for F. psychrophilum. Prevalence in progeny was positively correlated to broodstock kidney ELISA values for coho salmon. These results support broodstock screening as a potential tool for predicting BCWD outbreaks in progeny.;Reduction of BCWD outbreaks can also be attained by use of an efficacious vaccine. Iron-limited medium (ILM) was employed to alter both immunogenicity of the live-attenuated F. psychrophilum strain and pathogenicity of the virulent strain. Significant increases in protection were observed when fish were immersion immunized in the live-attenuated strain grown in ILM. Mortality was elevated in fish immersed in the virulent strain grown in ILM. These results indicate ILM increases immunogenicity, and may be useful for development of a successful waterborne challenge model for F. psychrophilum. .