Prevalence of multi drug antimicrobial resistance in Mannheimia haemolytica isolated from high-risk stocker cattle at arrival and two weeks after processing

Bovine respiratory disease (BRD) is the leading cause of morbidity and mortality in North American beef cattle. Mannheimia haemolytica (Mh) is the bacterial pathogen most frequently isolated from cattle with BRD and the prevalence of antimicrobial resistance in this pathogen has been increasing. Administration of antimicrobials to prevent BRD is commonplace in stocker cattle, but the impact of this practice on emergence of resistance in Mh is unknown. High risk, sale barn origin bull and steer calves (n = 169) were transported to a stocker facility in central Georgia and sampled via deep nasopharyngeal swab (NPS) at arrival processing. All calves received the macrolide antimicrobial tulathromycin (2.5 mg/kg subcutaneously) at arrival processing. A second NPS was collected from each calf 10 to 14 d after arrival. The occasional calves diagnosed and treated for BRD prior to 10 to 14 d were swabbed and cultured prior to treatment. Swabs were submitted for culture and antimicrobial susceptibility testing using the Kirby-Bauer disk diffusion method. Of the 169 cattle enrolled, 27 (16.0%) were culture positive for Mh at arrival processing and of these, a multi-drug resistant (MDR) strain of Mh was detected in 1 (3.7%). In contrast, 123 (72.8%) cattle were culture positive for Mh at second sampling and of these, a MDR strain of Mh was detected in 122 (99.2%). The proportions of cattle culture positive for Mh and positive for MDR Mh at arrival processing and at second sampling were significantly different (P < 0.001). At the level of the individual bacterial isolate, 366 individual Mh isolates were collected from the calves at the time of the second sampling. Of these isolates, 361 (98.6%) were intermediate or resistant to all macrolides tested (tilmicosin, gamithromycin, tulathromycin) and the fluoroquinolone enrofloxacin. In addition, 254 isolates (69.4%) were intermediate or resistant to florfenicol and 4 (1.1%) were intermediate or resistant to ceftiofur. There was a significant difference in the proportion of isolates resistant to all of the drug classes except cephalosporins at arrival processing versus second sampling (P < 0.001). Our results show that there was an increase in the proportion of calves positive for Mh from arrival processing to second sampling, and that there was an increase in the proportion of calves that had MDR strains of Mh detected from arrival processing to second sampling. More research is needed to understand the role of metaphylaxis on MDR in Mh and the impact of MDR on morbidity and mortality in stocker cattle.