Tilmicosin was administered intravenously and subcutaneously at a dose rate of 10 mg/kg bwt to determine its concentration in blood and bronchial secretion as well as its kinetic behavior in healthy and Pasteurella haemolytica type A1-infected calves. Sever acute bronchopneumonia was induced in 10 calves by inoculating them intra-tracheally with P. haemolytica type A1. The calves were treated with tilmicosin;5 of these received the drug intravenously and the other 5 were injected subcutaneously. After a slow intravenous injection, the serum concentration-time curve indicated a two compartment open model with a mean elimination half-lives (t1/2bs) of 22.09 and 22.14 hours before and after infection, respectively. The mean residence time (MRT) corrected for a bolus injection was 2.25 and 2.20 hours and the mean MRTinf was 25.27 and 25.46 hours in healthy and P. haemolytica-infected calves, respectively. After subcutaneous injection, the drug was eliminated more slowly (before and after infection) from serum and bronchial secretions, with t1/2bs of (24.60 and 25.85 hours) and (33.74 and 31.78 hours), respectively. The apparent volume of distribution (Vd(area)) of tilmicosin was more than 1 litre·kg-1. The peak serum and bronchial secretions of tilmicosin concentration were (1.33 and 1.36 mg·ml-1) and (1.40 and 1.70 mg·ml-1) attained at (7.21 and 7.15 hours) and 7.11 and 7.10 hours) after subcutaneous injection, respectively. Tilmicosin was good secreted into bronchial secretions having AUCbronchial secretion/ AUCserum ratio of approximately 1:1.24 and 1:1.22 in healthy and P. haemolytica-
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