Comparative efficacy of enrofloxacin to that of tulathromycin for the control of bovine respiratory disease and prevalence of antimicrobial resistance in Mannheimia haemolytica in calves at high risk of developing bovine respiratory disease

Bovine respiratory disease (BRD) is the most common cause of morbidity and mortality in North American beef cattle. Mannheimia haemolytica 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 M. haemolytica. 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 M. haemolytica 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 d after arrival, compared with 18.3% of calves receiving tulathromycin (P = 0.040). The percentages of calves that required more than one treatment for BRD within 45 d after arrival did not differ statistically for those receiving enrofloxacin compared with those receiving tulathromycin (10.5% and 4.7%, respectively; P = 0.107) Likewise, the percentages of calves that died during the 45-d follow-up period did not differ for those receiving enrofloxacin compared with 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 to 17 d later. There was a significanst effect of sampling time on the proportion of calves carrying multidrug-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.

Agreement Among 4 Sampling Methods to Identify Respiratory Pathogens in Dairy Calves with Acute Bovine Respiratory Disease

Background

Four sampling techniques commonly are used for antemortem identification of pathogens from cattle with bovine respiratory disease (BRD): the nasal swab (NS), guarded nasopharyngeal swab (NPS), bronchoalveolar lavage (BAL), and transtracheal wash (TTW). Agreement among these methods has not been well characterized.

Objective

To evaluate agreement among TTW and NS, NPS, or BAL for identification of viral and bacterial pathogens in dairy calves with BRD.

Animals

One hundred dairy calves with naturally acquired BRD.

Methods

Calves were sampled by all 4 methods. Viral agents were identified by real‐time RT‐PCR, bacteria were identified by aerobic culture, and Mycoplasma bovis (M. bovis) isolates were speciated by PCR. Agreement among TTW and NS, NPS, or BAL was evaluated by calculating the kappa statistic and percent positive agreement. McNemar's exact test was used to compare the proportions of positive results.

Results

Agreement among TTW and NS, TTW and NPS, and TTW and BAL, was very good for identification of P. multocida, M. haemolytica, and M. bovis. For bovine respiratory syncytial virus (BRSV), agreement with TTW was moderate for NS, good for NPS, and very good for BAL. For bovine coronavirus (BCV), agreement with TTW was moderate for NS and NPS, and good for BAL. McNemar's test was significant only for BCV, indicating that for this pathogen the proportion of positive results from NS and NPS could not be considered comparable to TTW.

Conclusions and Clinical Importance

This study provides guidance for veterinarians selecting diagnostic tests for antemortem identification of pathogens associated with BRD.