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.

Bovine respiratory disease (BRD) is the most common cause of morbidity and mortality in North American beef cattle. Mannheimia haemolytica (Mh) is the bacterial pathogen most often isolated from cattle with BRD and the prevalence of antimicrobial resistance (AMR) in this organism has increased in recent years. Antimicrobials are commonly used to prevent BRD in cattle at high-risk of developing BRD, however, recent work would suggest that this practice might be one factor contributing to the increased prevalence of AMR in Mh. We hypothesized that the administration of the short-acting fluoroquinolone, enrofloxacin, would be just as effective as the long-acting triamilide, tulathromycin, in preventing BRD but would be less likely to select for AMR Mh in stocker calves at high-risk of developing BRD. Three hundred forty-one stocker calves were enrolled in the study with 172 calves in 4 pens being randomly assigned to treatment with enrofloxacin and 169 calves in 4 pens randomly assigned to treatment with tulathromycin. Calves within each treatment group were allocated to one of 4 replicate pens based on the week of enrollment. Of calves receiving enrofloxacin, 33.7% required treatment for BRD at least once within 45 days after arrival, compared to 18.3% of calves receiving tulathromycin (P = 0.040). The percentages of calves that required more than one treatment for BRD within 45 days after arrival did not differ statistically for those receiving enrofloxacin compared to those receiving tulathromycin (10.5% and 4.7%, respectively; P = 0.107) Likewise, the percentages of calves that died during the 45 day follow-up period did not differ for those receiving enrofloxacin compared to those receiving tulathromycin (12.2% and 10.1%, respectively; P = 0.592). Mannheimia haemolytica was cultured from 11% of calves sampled at arrival, and from 50% of calves sampled at revaccination 14-17 days later. There was a significant effect of sampling time on the proportion of calves carrying multi-drug resistant (MDR) isolates, with calves having a higher prevalence of MDR isolates at revaccination than arrival (100% vs. 13%; P < 0.001). Future research evaluating the impact of MDR on response to antimicrobial therapy is necessary.

Background: Acute puerperal metritis (APM) affects 30% of postpartum dairy cattle. Bacteremia negatively impacts survival in cattle with coliform mastitis. However, the prevalence of bacteremia in dairy cattle with APM is unknown. Hypothesis: Bacteremia is detectable in a large proportion of cattle with APM. Animals: Seventeen dairy cows with APM and 17 healthy dairy cattle. Methods: Prospective case-control study. Cases were identified by daily monitoring of cattle in the first 10 days after calving. Controls were matched to cases by parity and days in milk. Cows were examined at the time of identification of APM. A complete blood count, serum biochemical analysis, and bacteriologic culture of blood and lochial fluid were performed on each animal at the time of diagnosis. The same samples were collected from healthy herdmates of a similar parity and days in milk. Blood culture results and clinicopathologic variables were compared between groups. Conditional logistic regression was used to evaluate factors associated with APM, whereas multivariate logistic regression was used to evaluate factors associated with bacteremia. Results: Bacteremia occurred in 53% (9/17) of cattle with APM and 53% (8/15) controls. Bacillus spp. was the organism most commonly isolated from the bloodstream in cattle of both groups. Bacteremic cattle in both groups were significantly less likely to have basophils in the peripheral circulation (P = .02) and more likely to have higher serum globulin concentrations (P = .02). Conclusions and Clinical Importance: Bacteremia is a common occurrence in postpartum dairy cattle. Further study is warranted to investigate the modes by which bacteria colonize the bloodstream in this population of animals and the importance of bacteremia on health and productivity of affected animals.

Bovine respiratory disease (BRD) is the most common cause of morbidity and mortality in North American beef cattle. In recent years, isolation of strains of Mannheimia haemolytica that are resistant to multiple different classes of antimicrobials has become commonplace. New research would suggest that the routine use of antimicrobials by some cattle operations might be driving emerging resistance patterns, with the majority of the spread observed due to propagation of strains of M. haemolytica that have acquired integrative conjugative elements. To date, there is little information evaluating the impact of antimicrobial resistance on clinical outcome in cattle with BRD.