Influence of tilmicosin on quantified pulmonary concentrations of three bacterial pathogens in calves with naturally-occurring bovine respiratory disease

Abstract

A randomized trial was conducted to determine the lower respiratory tract pathogen load in cattle at risk of developing bovine respiratory disease (BRD). The objectives were to characterize and quantify concentrations of respiratory pathogens in the main-stem bronchi before and after administration of tilmicosin as a treatment for BRD.Ninety-three beef steers and heifers were selected from a group of 421 high-risk calves based on a BRD case definition that included clinical score and rectal temperature parameters. Three calves meeting the BRD case definition constituted a cohort; 1 animal within a cohort served as an untreated control, whereas the remaining 2 received 6.75 mg/lb (14.85 mg/kg) of body weight tilmicosin. An aliquot of solution obtained via lavage of the main-stem bronchi upon enrollment and at 72, 144, and 216 hours post-enrollment was serially diluted and cultured for respiratory bacteria.Treated animals had significant reductions (P<0.05) in Mannheimia haemolytica concentrations at each post-enrollment time point compared to untreated controls, while concentrations of Histophilus somni at 144 and 216 hours post-enrollment were lower than untreated controls. Rectal temperature was also reduced (P<0.05) at all post-enrollment sampling times in calves treated with tilmicosin. An improvement (P<0.05) in clinical score during the post-enrollment period was noted for calves receiving tilmicosin.