Antibiogram use on dairy cattle for bovine respiratory disease: A repeated cross-sectional study evaluating antimicrobial susceptibility of Pasteurella multocida and Mannheimia haemolytica.

Bovine respiratory disease (BRD) remains a costly challenge to the dairy industry, as the bacterial complex involved in BRD has shown decreased susceptibility to antimicrobials, exacerbating its effects on animal health, productivity, and recovery. In human medicine, cumulative antimicrobial susceptibility testing (CAST) is used to generate antibiograms that support empirical decision-making for antimicrobial treatment. The goal of this study was to evaluate a CAST framework for BRD in dairy cattle and factors of relevance when analyzing and reporting antibiogram data for on-farm treatment decisions. For this purpose, a repeated cross-sectional study was conducted using criteria adapted from the Clinical Laboratory Standards Institute for the collection, analysis, and presentation of CAST data. From 3 dairy farms in California, calves, heifers, and cows were sampled using deep nasopharyngeal swabs for culture and antimicrobial susceptibility testing for Pasteurella multocida and Mannheimia haemolytica. Antimicrobial susceptibility testing was performed using the minimum inhibitory concentration approach for 12 antimicrobial drugs of relevance to dairy cattle. The variability in the odds of nonsusceptible isolates was evaluated for effects of age group, farm, and season using a logistic regression model in SAS. The antimicrobial susceptibility of co-occurring P. multocida and M. haemolytica isolates were evaluated using Pearson chi-squared and Cohen's kappa in SAS. Overall, P. multocida isolates were mostly nonsusceptible to tetracycline (74%) and spectinomycin (46%), whereas M. haemolytica isolates were frequently nonsusceptible to penicillin (43%) and tetracycline (34%). There were increased odds of nonsusceptible isolates across farms and age groups, with calves and heifers showing a higher number of nonsusceptible isolates than cows. For P. multocida, isolates nonsusceptible to tetracycline and spectinomycin were affected by age group, with calves showing greater odds of nonsusceptibility to antimicrobials. The farm effect was significantly associated with P. multocida nonsusceptible to all antimicrobials but penicillin, suggesting individual farm management practices may heavily affect profiles of nonsusceptible BRD bacteria. Similarly, nonsusceptible M. haemolytica was consistently associated with farm for macrolides and tetracyclines and with age, with heifers showing the greatest odds of being nonsusceptible when compared with cows. Season did not affect the nonsusceptibility of either pathogen, suggesting that either California seasonal patterns or other factors, such as farm practices and animal management, play a more prominent role in the change of susceptibility profile. Overall, these findings highlight the potential effects of farm and age when interpreting data from CAST reports for BRD, where these and other factors should be considered when analyzing and presenting antibiogram reports.