Combined Vaccination and <i>Bacillus circulans</i> Supplementation in Rainbow Trout Challenged with <i>Yersinia ruckeri</i> : A Case Study

Abstract Enteric redmouth disease (ERM), caused by Yersinia ruckeri , remains a major constraint to rainbow trout ( Oncorhynchus mykiss ) aquaculture worldwide. The aim of this study was to evaluate whether dietary supplementation with the probiotic Bacillus circulans is associated with enhanced protection when combined with immersion vaccination against Y. ruckeri . A total of 230 fish were allocated into four groups: vaccinated, vaccinated + probiotic, probiotic only, and control. Fish were maintained in a recirculating aquaculture system and challenged with Y. ruckeri . Clinical signs, mortality, growth performance, and physiological indices were recorded. Y. ruckeri infection was confirmed by bacteriological culture, PCR, and MALDI-TOF MS. Survival rates were the highest in the vaccinated + probiotic group (96.7%), followed by vaccinated only (90%), probiotic only (73.3%), and control (63.3%). Relative percent survival (RPS) mirrored these results, indicating the enhanced protection associated with the combined treatment. Growth performance and feed efficiency were numerically improved in probiotic-supplemented groups, with the highest specific growth rate and lowest feed conversion ratio observed in the combined group. Condition factor and organ indices reflected better physiological status in treated fish. These findings indicate that integrating vaccination with probiotic supplementation was associated with enhanced survival under the conditions of this case study. Information © The Authors 2026