Abstract
The antibacterial efficacy of gamithromycin administered once 1, 5, or 10 days prior to a challenge
infection with Mannheimia 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 x 10^7 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 euthanized in 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 for M. haemolytica. The three groups of animals treated with gamithromycin
before challenge had significantly lower lung M. 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 to M. haemolytica A1 challenge resulted in a reduction in bacterial isolation
from the lungs and a reduction in the severity of clinical disease.
Year
2011
Category
Refereed journal