Asal Ahmadi

The bovine respiratory disease (BRD) complex is a major health and welfare challenge in the cattle industry worldwide, with substantial economic and antimicrobial resistance implications. Pasteurella multocida (PM) is a key bacterial agent of BRD; however, knowledge gaps remain regarding its transmission dynamics, genomic diversity and virulence-associated gene repertoire in cattle. In this cross-sectional study, we used population genomics to characterize PM isolates circulating in Norwegian calves and compare them with global bovine PM lineages. By integrating genomics with epidemiological and clinical data from both healthy and BRD-affected animals across different herd production systems, we identified associations between PM genotypes, herd-level factors and disease status. Fattening herds harboured greater PM diversity than dairy herds, and herd-specific population structures were common, with limited evidence of recent inter-herd transmission. Sequence type (ST) 173 was strongly associated with clinical signs of respiratory disease and exhibited a distinct genomic profile, including genes linked to iron acquisition and quorum sensing. A possible tropism for the lower respiratory tract was also observed for this ST. In contrast, ST79 was more prevalent among healthy animals and appeared to act predominantly as a commensal. Nasal swabs often, but not always, reflected the PM strain present in the lungs, highlighting both the utility and limitations of non-invasive diagnostics. Collectively, these findings demonstrate that herd management and bacterial genetic diversity play key roles in BRD epidemiology. Accordingly, targeted control strategies should account for the heterogeneity and pathogenic potential of circulating PM strains.