Efficacy of antimicrobial treatment for bovine respiratory disease: a systematic review and meta-analysis for European Network for Optimization of Veterinary Antimicrobial Therapy guidelines

<ns3:p> Bovine respiratory disease (BRD) is a leading cause of illness and death in cattle and a major driver of antimicrobial use. This systematic review and meta-analysis assessed the clinical effectiveness of five commonly used antimicrobial classes (penicillins, tetracyclines, phenicols, macrolides, and fluoroquinolones) in the treatment of cattle with natural BRD. Cattle from all production systems (feedlot, dairy, slaughter, and stocker) were included, and metaphylaxis trials were excluded. Peer-reviewed trials were searched in several databases, and all reports were screened in duplicate for data extraction and risk of bias. The Grading of Recommendations Assessment, Development, and Evaluation (GRADE) methodology was employed to evaluate the certainty of evidence. Twenty-eight randomized controlled trials were included in the final analysis, and no pairwise comparison between antimicrobial classes showed a clinically meaningful effect in reducing the need for re‑treatment. The results were consistent in the subgroup analysis of different production systems (feedlot and non-feedlot) and in trials with <ns3:italic>Mycoplasma</ns3:italic> positive animals. Mortality, relapse, and growth outcomes were measured inconsistently and could not be reliably pooled. Overall, certainty ranged from very low to moderate, largely due to indirectness of the outcomes and risk of bias. These findings suggest that commonly used antimicrobial classes have broadly comparable short-term therapeutic performances, without any apparent advantage for those classified as high-priority, critically important antimicrobials in human medicine. The results of this systematic review and meta-analysis will inform the BRD Antimicrobial Use Guidelines of the European Network for Optimization of Antimicrobial Therapy (ENOVAT). </ns3:p>