Assessment of immune response and protection against bacterial coldwater disease induced by a live-attenuated vaccine delivered orally or intraperitoneally to rainbow trout, Oncorhynchus mykiss (Walbaum)

Flavobacterium psychrophilum is the causative agent of bacterial coldwater disease (BCWD), which has a major global impact on salmonid aquaculture. A candidate live-attenuated F. psychrophilum vaccine strain, CSF259-93B.17 (B17), has been developed and shown to provide significant protection when delivered by intraperitoneal (IP) or immersion routes. This study examined immune response in rainbow trout and protection induced by B17 grown in iron-limited conditions against BCWD, when administered either orally or via injection. B17 was administered via both routes with and without alginate microencapsulation, and comparative protective efficacy was assessed. The microencapsulation method used successfully encapsulated viable B17 bacteria. Fewer microencapsulated B17 cells were available per unit volume of vaccine compared to unencapsulated bacteria. However, protection did not significantly differ for microencapsulated and unencapsulated B17 bacteria regardless of the route of delivery. Despite substantially elevated challenge pressure, both oral and IP delivery achieved significantly better protection than controls, and orally administered B17 achieved protection levels comparable to those achieved by IP immunisation. Serum antibody production response was slower in orally immunised fish, but achieved similar titres to IP immunised fish prior to bacterial challenge. The study indicated that with further optimisation, oral delivery of B17 may form a beneficial component of vaccination strategies, and contribute to the overall management of BCWD at salmonid hatcheries.