Efficacy of Mannheimia haemolytica A2, A7, and A2 and A7 combined expressing iron regulated outer membrane protein as a vaccine against intratracheal challenge exposure in sheep

This experimental study was done on a total of 40 male lambs with the objectives of developing experimental vaccines from Mannheimia haemolytica serotypes A2 and A7 that express iron regulated outer membrane protein and in vivo evaluation of their efficacy. Lambs were categorized in to four experimental groups and vaccinated with 1 ml of vaccine containing 5 × 108 CFU/ml. Group 1 was vaccinated with M. haemolytica A2, group 2 with A7, group 3 with serotype A2 and A7 combination, and group 4 received saline as control. They were challenged intratracheally by the respective homologous serotype after 35 days of vaccination. Post challenge clinical investigation showed that significant higher rate of morbidity was seen in control group which was demonstrated by raised rectal temperature (by 0.5-1°C) and respiratory signs. From the total of 26 lambs challenged with live M. haemolytica A2 and A7, 6 (23.1%) and 4 (15.3%) lambs were found dead and sick, respectively. Higher mortality and morbidity were observed in unvaccinated control group; however, lesser was recorded in combined vaccinated group. Lung lesions of variable severity were observed in 13 (50.0%) lambs following challenge. From vaccinated groups, 5 (27.8%) lambs were found to have a +1 lung lesion score. All of the lambs in unvaccinated control group had scores between +2 and +3. There was a statistically significant difference (p 0.05) was seen among vaccinated groups concerning lung lesion scores. Furthermore, the respective serotypes of M. haemolytica were successfully re-isolated from pneumonic lungs at a mean titre range of 102.2 – 108.1 CFU/g. In conclusion, lambs which received combined vaccine confer relatively good protective efficacy than M. haemolytica A2 or A7 vaccinated groups. Therefore, further study should be done on evaluation of antibody titer at different time points.   Key words: Challenge, efficacy, lambs, lung lesion score, Mannheimia heamolytica, morbidity, mortality.