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Title: Enhanced efficacy of an attenuated Flavobacterium psychrophilum strain cultured under iron-limited conditions

Author
item LONG, AMY - University Of Idaho
item FEHRINGER, TYSON - University Of Idaho
item SWAIN, MARISSA - Eckerd College
item Lafrentz, Benjamin
item CALL, DOUGLAS R. - Washington State University
item CAIN, KENNETH - University Of Idaho

Submitted to: Fish and Shellfish Immunology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/14/2013
Publication Date: 8/26/2013
Publication URL: http://handle.nal.usda.gov/10113/58452
Citation: Long, A., Fehringer, T.R., Swain, M.A., Lafrentz, B.R., Call, D., Cain, K.D. 2013. Enhanced efficacy of an attenuated Flavobacterium psychrophilum strain cultured under iron-limited conditions. Fish and Shellfish Immunology. 35:1477-1482.

Interpretive Summary: Flavobacterium psychrophilum is a Gram-negative bacterium that causes bacterial coldwater disease (BCWD) and rainbow trout fry syndrome (RTFS). Both of these diseases can cause high mortality in aquaculture reared salmonid fish such as rainbow trout and coho salmon and negatively impact production. Previously, an attenuated strain of this bacterium (CSF259-93B.17) was developed which was capable of infecting rainbow trout following immersion exposure, but not capable of causing disease. Research demonstrated that immersion exposure of rainbow trout to this attenuated strain resulted in protective immunity against the bacterium. The protection conferred was below desired levels indicating further effort should focus on enhancing the efficacy of the CSF259-93B.17 vaccine. Previous research has shown that growing bacteria in the absence of iron can stimulate the bacteria to increase the production of proteins that are important for colonizing and invading hosts. In this study, the vaccine strain was grown in iron-limited media to determine if this strategy may enhance the effectiveness of the vaccine. Additionally, the vaccine was tested in coho salmon to determine if it is effective in this fish species. The results demonstrated that the effectiveness of the vaccine was enhanced by culturing the strain in iron-limited media and also demonstrated that the vaccine is effective in coho salmon. Although confirmation of these results under field conditions is needed, production of the vaccine using this strategy may represent the most appropriate method for immersion vaccination of salmonid fry. Further development of this vaccine has the potential to impact the rainbow trout industry as well as conservation aquaculture by reducing losses associated with BCWD and RTFS.

Technical Abstract: An attenuated strain of Flavobacterium psychrophilum (CSF259-93B.17) has shown potential as a vaccine for prevention of bacterial coldwater disease (BCWD) in rainbow trout, Oncorhynchus mykiss (Walbaum). Because BCWD outbreaks can result in high mortality in other salmonid species, specifically coho salmon, O. kisutch (Walbaum), the live attenuated strain was tested as a vaccine in this species. Additionally, we hypothesized that culture of the vaccine strain under iron-limited conditions would lead to improved protection against BCWD. To test this hypothesis, coho salmon were either injection or immersion immunized with CSF259-93B.17 cultured in iron-replete or iron-limited medium. Resultant antibody titers were low and not significantly different between the two treatments regardless of vaccine delivery method (P > 0.05). Following injection challenge with a virulent F. psychrophilum strain, mortality for injection vaccinated fish was significantly reduced compared to the control but did not differ by treatment (P > 0.05). Relative percent survival (RPS) was high in both treatments (90% in iron-replete, 98% in iron-limited medium). Fish immunized by immersion with CSF259-93B.17 grown in iron-replete medium exhibited lower mortality (29.3%; RPS 46%) when compared to mock immunized fish, but this was not significant. However, mortality was significantly lower in fish immunized with CSF259-93B.17 grown in iron-limited medium (14.7%; RPS 73%) when compared to mock immunized fish. The results demonstrate that the live-attenuated F. psychrophilum strain can confer protection to coho salmon and vaccine efficacy is enhanced by culturing the strain under iron-limited conditions.