Pathological and molecular evidence for the role of Pasteurella multocida in respiratory disease of lambs.

Pasteurella multocida is an opportunistic pathogen associated with respiratory disease in sheep, although its relevance has not yet been fully defined. The objective of the present study was to investigate the association between P. multocida and pulmonary lesions through an integrated pathological and molecular approach. A comparative cross-sectional study was conducted in southern Spain using 201 lung samples collected from apparently healthy fattening lambs (50-60 days old) during winter and summer. This study aimed to molecularly characterize pulmonary lesions in fattening lambs and assess their association with the capsular type, lipopolysaccharide (LPS) serovar, virulence-associated genes (VAGs), and sequence types (STs) of P. multocida isolates. Bronchopneumonic lesions of different grades (I-III) were identified in 48.8% of the animals. Bronchiolitis obliterans, an underrecognized chronic lesion in fattening lambs, was detected in 27.36% of the lungs, most of them associated with diffuse bronchopneumonia (81.8%). Seasonal variation was significantly associated with lesion severity and bronchiolitis obliterans, with both outcomes occurring more frequently during the summer season. P. multocida was isolated in 17.41% of the 201 samples analyzed, and its presence was associated with increased odds of type III bronchopneumonia (OR = 2.46, 95% CI: 1.10-5.51, p = 0.029). Three capsular types and two LPS serovars were identified, with D:L3 and A:L3 serogroups detected more frequently in diffuse bronchopneumonia and bronchiolitis obliterans. Eight VAGs profiles were identified, although no association with the lesion severity could be demonstrated. A high genetic heterogeneity (20 STs; Genetic Diversity-GD-0.93) was observed among the P. multocida isolates. The results show that P. multocida isolation was significantly associated with severe cases of bronchopneumonia, while seasonality may also contribute to differences in pulmonary lesion severity.