Abstract: Mannheimia haemolytica and Pasteurella multocida capsular serotype A both are Gram-negative, bipolarity staining cocobacilli, the main bacterial pathogens that lead to the bovine respiratory disease, resulting in significant economic losses to the cattle industry every year. At present, the lack of vaccines is still the key points of non-effective control to both pathogens. Here, bovine M. haemolytica (Mh) strain Mh422 and bovine P. multocida capsular serotype A (Pm) strain PmCQ2 isolated from calves were used as strains for inactivated bacterins. Overnight culture of Mh422 or PmCQ2 was seeded in Martin broth medium, and incubated at 37℃ for 12 h with shaking at 220 r·min^-1, the bacterial cells were collected and inactivated with 0.4% formalin, the monovalent inactivated bacterins with two concentration of Mh and Pm were prepared by the routine method, and Mh-Pm combined inactivated vaccines were made by using the mixture of Mh and Pm monovalent inactivated bacterin with a ratio of 1:1, 2:1 and 3:1. The mice were inoculated subcutaneously with monovalent and combined vaccines at a dose of 0.2 mL, respectively, the control group mice were inoculated with PBS emulsifier, and the mice had no adverse reactions after immunization with all inactivated vaccines. Booster immunizations were conducted with a half dose of the primary immunization on the 7th and 17th day, blood was sampled via tail vein on the 7th day and every 5 days thereafter and serum antibody titers were detected by ELISA, the specific antibodies were generated to high levels at the 10th day after the first booster and continue increase to the peak at the 15th day after the second booster and maintained 25 d then decreased slowly. Mice were challenged intraperitoneally with Mh422 and PmCQ2 on the 20th day after the third immunization, the results showed that the immune protective rates of Pm monovalent and combined inactivated vaccines against PmCQ2 strain were all 100%, but the immune protection rates of Mh monovalent inactivated vaccines against Mh422 were 0%, there were no cross-immune protections between Mh and Pm. Interesting here, the immune protection rates of Mh-Pm combined inactivated vaccines against Mh422 strain were up to 53%-71%. The results indicate that there is no mutual inhibition between Mh and Pm in its special antibody production inducing and PmCQ2 can promote the immune protection of Mh inactivated vaccines against Mh422. This study provides a theoretical basis for the further research on the combined vaccine of bovine M. haemolytica and bovine P. multocida.

Key words: bovine Mannheimia haemolytica, bovine Pasteurella multocida, inactivated bacterin, immune protection