Vaccination studies of lambs against experimental Mannheimia (Pasteurella) haemolytica infection

In this study, efficacy of a commercial vaccine (One Shot Ultra 8; Pfizer Animal Health, USA) against experimental Mannheimia (Pasteurella) haemolytica infection in sheep was investigated. Antibody responses following vaccinations were detected by leukotoxin neutralization test (LNT) and ELISA tests, respectively.The trial was performed with 45 lambs, divided into three groups comprised of 15 lambs, each. One of the groups constituted the control group (group 1, designated as T1), while the other two (group 2, T2 and group 3, T3) were the trial groups. Lambs of group 2 were vaccinated only once with One Shot Ultra 8 vaccine and group 3 was vaccinated twice, with 2 weeks intervals. Three weeks after the second vaccination all of the lambs were challenged. Following the challenge, all of the lambs were necropsied and lung lesions were scored. Antibody titers following vaccinations were determined serologically.In the trial, lung lesion scores were comparatively examined and statistically important differences (P < 0.001) were observed between the control and the two trial groups. However, there were statistically insignificant differences (P > 0.05) between the two trial groups. After the evaluation of results obtained by serological tests, after LNT test, antibody titers of lambs in group 2 were determined to be between 1/20 and 1/40, while they differed between 1/20 and 1/80 in group 3. Serum antibody titers obtained after ELISA tests differed between 1/80 and 1/320 in group 2, while they differed between 1/160 and 1/320 in group 3.In conclusion, when the control group and the trial groups were compared, vaccinating the lambs with One Shot Ultra 8 lowered the lung lesions in the trial groups, but no significant differences were observed between the two trial groups concerning the lung lesions.