Silje Enge Falkeid

Respiratory tract infection (RTI) is a common diagnosis in cattle, particularly in calves, and contributes substantially to antimicrobial use. This study describes the incidence and patterns of RTI treatments in Norwegian cattle, with emphasis on antimicrobial, analgesic and anti-inflammatory drug use.Registry data from 2015 to 2022 were extracted from two national cattle databases, covering more than 67,000 cattle treated for RTI.The annual proportion of herds with at least one RTI treatment increased from 24% in 2015 to 44% in 2022. The median annual number of treatments per affected herd was two. On calf level, the probability of receiving at least one RTI treatment by 180 days of age increased from 1.8% to 2.4% over the study period. Median age at first treatment was 52 days, with a clear seasonal peak during winter. RTI treatments usually involved three or fewer calves (96%), and most animals received only one RTI treatment. Antimicrobials were administered in 96.5% of treatments, most commonly penicillins (β-lactamase sensitive; 54.5%), followed by amphenicols (15.2%) and tetracyclines (7.9%). Short-acting formulations predominated (89.2%). Antimicrobial class changes within 10 days of initial treatment occurred in 13% of calves, typically after four days. Non-steroidal anti-inflammatory drugs (NSAIDs) were co-administered with antimicrobials in 85% of treatments.In conclusion, RTI treatments in Norwegian cattle were widely distributed across herds and increased over time. Treatment practices remained dominated by penicillin, and repeated treatments and changes in antimicrobial class were uncommon. This suggests that narrow-spectrum first-line therapy was generally sufficient in routine RTI management.

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.