Ovine
pneumonia is an economic important disease worldwide for the sheep industry. Multiple serotypes (S) of Mannheimia haemolytica are involved in the disease and S2 and S1 are the most frequent isolates associated with lung lesions in sheep.
Vaccines based on some M. haemolytica S2 strains have been shown to have poor immunogenicity. The objective of this study was to determine the cross-protection effect of an S1 strain based
vaccine, Bovilis MH, in sheep against an experimental challenge with an S2 strain. Lambs (n=12) in the
vaccine group were injected subcutaneously with 1 ml of the Bovilis MH
vaccine, and revaccinated 4 weeks later, while the control group (n=12) received 1 ml of saline at each occasion. Two weeks after revaccination, all lambs were challenged intratracheally with
parainfluenza virus 3, and with an M. haemolytica S2 strain at day 7 post-viral challenge. The proportion of animals having
pyrexia in the first 2 days post-bacterial challenge was significantly less in the
vaccine group compared to the control group (P<0.05). The animals in the
vaccine group had significantly lower dyspnoea scores and lung/bodyweight ratio than those in the control group (P<0.05). The
vaccine provided 49.1% overall protection. Prior to the challenge, the vaccinated animals had significantly higher titres of
antibodies to S1 and S2 whole cell
antigens and to leukotoxins produced by S1 and S2 strains compared to the control animals (P<0.05). The S1 strain
vaccine provided considerable cross-protection against the S2 strain challenge.