Preparation and Evaluation of a Combined Inactivated Vaccine Against Corynebacterium pseudotuberculosis, Trueperella pyogenes, and Staphylococcus aureus

Background: Caseous lymphadenitis (CLA) is a significant infectious disease in sheep and goats, traditionally associated with Corynebacterium pseudotuberculosis. However, recent evidence highlights the polymicrobial nature of CLA, with Trueperella pyogenes and Staphylococcus aureus frequently co-isolated from lesions. Current vaccines mainly target single pathogens, and combined inactivated vaccines against all three bacteria are lacking. Methods: We developed a trivalent inactivated vaccine using strains CpS1, TpS1, and SaS9, inactivated with 0.2% formalin and formulated with either PBS, Freund’s adjuvant, or aluminum hydroxide adjuvant. Vaccine quality, safety, and immunogenicity were evaluated in experimental goats. The aluminum hydroxide-adjuvanted vaccine was further assessed for protective efficacy via challenge with individual pathogens and in field trials on goat and sheep farms. Results: All formulations induced specific antibodies against the three pathogens, with Freund’s adjuvant eliciting the highest level, while aluminum hydroxide provided a favorable safety profile and induced antibody levels that were numerically lower than but not statistically distinguishable from the Freund’s adjuvant group at most time points; however, the small group size (n = 6) limited the statistical power to detect differences between adjuvants. The aluminum hydroxide-adjuvanted vaccine conferred protection rates of 66.7% against C. pseudotuberculosis, 83.3% against T. pyogenes, and 83.3% against S. aureus in challenge trials. In field studies, vaccination was associated with a lower observed incidence of CLA, from 12.5% in unvaccinated controls to 2.5% in vaccinated goats and from 8% to 2% in vaccinated sheep within 90 days post-immunization. Conclusions: The trivalent inactivated vaccine with aluminum hydroxide adjuvant showed promising immunogenicity and preliminary protective efficacy against experimental challenge and was associated with a lower observed overall abscess incidence under the investigated field conditions.