Immunization Efficacy of a Commercialized Inactivated Mycoplasma bovis (Strain HM) Vaccine in Calves at a Large-Scale Dairy Farm in Yancheng, Jiangsu Province

Mycoplasma bovis (M. bovis)-positive clinical Bovine Respiratory Disease (BRD) seriously threatens the survival and growth of calves. Against the global backdrop of escalating antimicrobial resistance, effective vaccines represent a promising strategy for the prevention and control of M. bovis-positive clinical BRD. This study aimed to characterize the epidemiological features of BRD on a large-scale dairy farm in Yancheng, Jiangsu Province, and to assess the efficacy of an inactivated M. bovis strain HM vaccine using a randomized, double-blind, controlled field trial conducted during the high-incidence season. In total, 200 healthy 3–4-week-old Holstein calves seronegative for M. bovis-specific antibodies were randomly allocated to either a vaccine group or a control group. Calves in the vaccine group received two intramuscular vaccine doses on day 0 and day 14, while control calves received an equal volume of placebo at identical time points. The entire trial spanned 194 days. A comprehensive assessment was carried out, including vaccine tolerability, growth performance, serum antibody and cytokine profiles, BRD incidence, infection rates of common pathogens, and vaccine protective efficacy. The results showed that the farm-level annual BRD incidence was 9.27%, consistent with typical temporal–spatial-herd distribution patterns. Calves exhibited good tolerability after vaccination, and the vaccine had no adverse effect on body-weight gain. Importantly, vaccination markedly elevated serum specific antibody levels, seropositivity rates, and concentrations of inflammatory cytokines including IL-4, IL-6, and IL-10 (p < 0.05). Moreover, vaccination reduced the risk of M. bovis-positive clinical BRD by 61% and decreased infection risks for several respiratory pathogens. Collectively, this inactivated vaccine demonstrated satisfactory tolerability and protective effects, providing critical technical evidence for the prevention and control of M. bovis-positive clinical BRD in dairy farms.