Strength of association between isolation of Pasteurella multocida and consolidation lesions in ovine pneumonic pasteurellosis.

This study investigated the association of Pasteurella multocida isolation and the molecular characteristics of the isolates with the presence of pneumonic lesions in lambs at slaughter to assess its importance as a causative agent of pneumonic pasteurellosis compared with Mannheimia haemolytica. P. multocida was isolated from the 13.9% and 2.7%, and M. haemolytica from the 36.4% and 26.8%, of lungs with and without lesions, respectively (P < 0.05). Both microorganisms were frequently coisolated (23.2% and 12.5% from lungs with and without lesions, respectively). Isolation of P. multocida alone exhibited greater strength of association with pneumonic lesions (OR 11.4; 95% CI 3.2-40.6) than that exhibited by M. haemolytica alone (OR 3.0; 95% CI 1.6-5.4). Cluster analysis grouped the lungs into four clusters characterized by the isolation of M. haemolytica or P. multocida alone (clusters 1 and 4), coisolation of both microorganisms (cluster 3), and isolation of neither (cluster 2). Cluster 4 lungs exhibited higher frequencies of pneumonic lesions (87.5%) and severe (20.8%) and moderate (25.0%) lesions. Lungs coinfected with both pathogens (cluster 3) did not exhibit a higher frequency of severe and moderate consolidation lesions (6.1% and 14.3%, respectively), suggesting that P. multocida and M. haemolytica do not act synergically to cause more severe pneumonic infections. The greater strength of association of P. multocida isolation with pneumonic lesions together with the higher severity of the lesions caused could indicate a greater role played by this pathogen in the aetiopathogenesis of pneumonic pasteurellosis in sheep than is commonly assumed.