Development and evaluation of a P. multocida A-M. haemolytica A6-rLkt combined vaccine for enhanced control of bovine respiratory disease complex.

Bovine Respiratory Disease Complex (BRD) poses a major threat to the cattle industry, primarily caused by the bacterial pathogens Pasteurella multocida (P. multocida) and Mannheimia haemolytica (M. haemolytica). Current management strategies heavily rely on vaccines and antibiotics; however, the overuse of antibiotics has led to significant resistance issues, which underscores the urgent need for more effective vaccination approaches. This study aimed to develop and evaluate a novel combined vaccine that incorporates P. multocida A, M. haemolytica A6, and recombinant leukotoxin (rLkt), with the goal of enhancing immunogenicity and protective efficacy. In this study, BALB/c mice were immunized with different vaccine formulations, among which the P. multocida - M. haemolytica - rLkt combined vaccine demonstrated the best efficacy. Following immunization and a booster dose, the combined vaccine elicited robust antibody responses, achieving 100% protection against P. multocida and 80% protection against M. haemolytica. Histopathological evaluations revealed significantly reduced lung lesions and inflammation in the group receiving the P. multocida - M. haemolytica - rLkt formulation, exhibiting lung tissue integrity similar to that of the blank control group. The study highlights the effectiveness of the P. multocida - M. haemolytica - rLkt combined vaccine as a promising strategy for BRD control, emphasizing the advantages of multivalent vaccine designs in veterinary immunology. Future research should focus on large-scale trials to assess long-term efficacy and optimize vaccine delivery methods, ensuring enhanced protection and health outcomes in cattle populations.