A modified-live Mannheimia haemolytica expressing Mycoplasma bovis EFTu and Hsp70 induced systemic and mucosal antibody responses in American bison but failed to fully protect against Mycoplasma bovis challenge.

Mycoplasma bovis (M. bovis) is an emerging pathogen in American bison (Bison bison) responsible for high mortality epizootics of severe pneumonia and systemic disease. Though M. bovis poses a significant threat to bison conservation and ranching, there are no commercial vaccines licensed for use in this species. To this end, novel modified-live Mannheimia haemolytica (M. haemolytica) vaccine strains, serotypes 1 and 6, secreting inactive leukotoxin fused to truncated M. bovis Elongation Factor Thermal unstable (EFTu) and Heat shock protein (Hsp) 70 (EFTu-Hsp70-ΔlktCAV4) were evaluated for efficacy in bison. Modified-live M. haemolytica were administered intranasally and both serotypes were sporadically recovered from bison nasal swabs. Bison inoculated with the two M. haemolytica EFTu-Hsp70- ΔlktCAV4 strains produced systemic and mucosal antibodies to the M. bovis antigens and M. haemolytica surface antigens prior to intranasal challenge with M. bovis. Following intranasal M. bovis challenge no differences were observed in lung bacterial counts nor in lung lesion formation between bison that received the M. haemolytica EFTu-Hsp70-ΔlktCAV4 strains or the ΔlktCAV4, lacking the M. bovis antigens. Despite this, fewer M. bovis positive tissue swabs were collected from animals administered the M. haemolytica EFTu-Hsp70-ΔlktCAV4. These results demonstrate that modified-live M. haemolytica vaccine strains expressing M. bovis antigens failed to provide full protection against M. bovis infection in bison.