# Comparative efficacy of Bacillus probiotics and formalin-killed bacterin against Vibrio anguillarum in European eel elvers

## TL;DR

This study compares the effectiveness of probiotics and a vaccine in protecting European eel elvers from a bacterial infection, finding the vaccine to be more effective.

## Contribution

The study demonstrates that immersion vaccination is more effective than probiotic feeding for vibriosis control in eel aquaculture.

## Key findings

- VACC group showed significantly higher immune responses compared to PRO and CON groups.
- Relative percent survival was highest in the VACC group (41.67%) compared to PRO groups.
- Immersion vaccination is a superior strategy for managing vibriosis in eels.

## Abstract

This study evaluated and compared the effectiveness of Bacillus species probiotics and formalin-killed bacterin to control vibriosis caused by Vibrio anguillarium in European eel (Anguilla anguilla) elvers. Fish were allocated into four triplicate groups defined as follows: a control non-vaccinated (CON) group that fed on a basal diet without additives, two groups fed on diets supplied with Bacillus species probiotics (PRO; Sanolife PRO-F) in a dose of 0.5 g /kg diet (PRO-F1) and 1.0 g /kg diet (PRO-F2), and the fourth group was vaccinated with immersion formalin-killed bacterin and got a booster vaccination at 14th day (VACC). The experiment lasted for 28 days. In the end, the immune responses, including antibody titers, lysozyme activities (in serum and skin mucous), and bactericidal activity, were evaluated. The relative percent survival (RPS) after intraperitoneal challenge with V. anguillarium was calculated after an additional 10-day observation period. A one-way ANOVA followed by Dunnett’s multiple comparison test was used to compare the experimental groups (VACC, PRO-F1, and PRO-F2) with the CON group. The results revealed significantly higher antibody titers, lysozyme activities (in serum and skin mucous), and serum bactericidal activity in the VACC and PRO-treated groups over the CON group. Of interest, the lysozyme activities (in serum and skin mucous) and antibody titers were significantly higher in the VACC group over both PRO groups. After the experimental infection, it was noticed that both VACC and PRO significantly protected eel elvers against V. anguillarium compared to the CON group. In addition, the RPS were 41.67%, 29.17%, and 37.50% in the VACC, PRO-F1, and PRO-F2 groups, respectively. This study unveils the vital roles of PRO and VACC and their effective roles in enhancing innate and mucosal immunities and protecting European eel elvers against vibriosis caused by V. anguillarium. Based on our findings, immersion vaccination emerges as the superior strategy for managing vibriosis in eels. This approach offers greater protection than feed-based probiotics, underscoring its potential as a viable vaccination method in eel aquaculture.