Efficacy of oral reimmunisation after immersion vaccination against Vibrio vulnificus in farmed European eels

Abstract

Vulnivaccine, a licensed vaccine against Vibrio vulnificus , protects eels against vibriosis after vaccination by triple prolonged immersion at glass eel stage. Protection lasts for at least 6 months, but later, protection decreases, and eels can suffer stress-related vibriosis. The main objective of this work was to design an oral vaccine that can be used for reimmunisation at any developmental eel stage. With this aim, the efficacy of Vulnivaccine mixed with food as an oral booster was tested in an eel farm. The protection and the immune response (innate and acquired) in serum, mucus (from skin and intestine) and bile were evaluated in reimmunised and control animals (non-reimmunised) for a 60-day period. Reimmunisation significantly increased protection (relative percentage of survival, RPS) and antibody titres. In addition, a significant stimulation in lysozyme and antibody production was observed in reimmunised animals after bath infection challenges with the pathogen. On the basis of these results, we propose that a whole immunisation schedule against V. vulnificus is useful in eel farming. Such a schedule would comprise a primary vaccination with Vulnivaccine, administered by prolonged immersion at glass eel stage, and an oral booster with Vulnivaccine-supplemented food when RPS values against the pathogen lowered to below 50%.