Effectiveness of an experimental subunit ovine Mannheimia haemolytica respiratory vaccine in reducing pneumonia in lambs.

Background: Pneumonia is the largest cause of mortality in Canadian lambs. Currently there are no licensed ovine vaccines in Canada to reduce economic losses from this production-limiting disease.

Objective animals and procedure: The effectiveness of an experimental subunit Mannheimia haemolytica leukotoxin A (LtxA) and transferrin binding protein B (TbpB) vaccine was evaluated in lambs for reduction of clinical disease in an experimental challenge study and in a controlled randomized field trial in a large commercial sheep operation.

Results: Following an experimental challenge of parainfluenza 3 virus and M. haemolytica, the subunit vaccine induced significantly higher LtxA and TbpB antibody titers at 48 d post-challenge compared to the adjuvant and Ovipast Plus bacterin (Merck Animal Health), but there were no significant differences in clinical signs or mortality among vaccine groups. Following vaccination of commercial ewes and their lambs at weaning, the only significant difference in health, growth, and carcass traits between vaccinates and non-vaccinates was a slightly higher pneumonia treatment rate in vaccinated preweaned lambs (25.7%) compared to unvaccinated preweaned lambs (23.4%) (P = 0.04).

Conclusion and clinical relevance: Although vaccination with the experimental subunit M. haemolytica vaccine induced high LtxA and TbpB antibodies, it did not reduce clinical disease in lambs following an experimental challenge study or in a controlled randomized field trial in a commercial sheep operation. Further research is required to identify additional protective antigens for a safe and effective ovine respiratory vaccine to reduce pneumonia losses in commercial sheep flocks.