Determination of the Duration of Antibacterial Efficacy following Administration of Gamithromycin Using a Bovine Mannheimia haemolytica Challenge Model

The antibacterial efficacy of gamithromycin administered once 1, 5, or 10 days prior to a challenge infection withMannheimia haemolytica serotype A1 was evaluated. Forty calves were randomly allocated on day −11, restricted by body weight, to one of three treatment groups given gamithromycin at 6 mg/kg of body weight 10, 5, or 1 days before challenge or to an untreated control group.M. haemolytica A1 challenge infections were induced on day 0 by depositing 7.4 × 107 CFU at the bifurcation of the main bronchus using a bronchoscope. Clinical observations were made daily from the day of allocation to day 10, when necropsy was scheduled; three calves died or were euthanizedin extremis on welfare grounds prior to scheduled necropsy. At necropsy the lungs were removed, pneumonic lesions were scored, and samples of lung tissue were cultured forM. haemolytica . The three groups of animals treated with gamithromycin before challenge had significantly lower lungM. haemolytica counts and fewer clinical signs of respiratory disease than did the saline-treated group. For most of the clinical parameters, the pattern of responses differed significantly (P < 0.05) between the gamithromycin-treated groups and the control group. There were no statistically significant differences between groups in the mean lung lesion scores, partly as a result of high individual variability, particularly within the control group. The administration of gamithromycin 1, 5, and 10 days prior toM. haemolytica A1 challenge resulted in a reduction in bacterial isolation from the lungs and a reduction in the severity of clinical disease.