The pharmacokinetics and milk residual behaviour of tylosin in lactating Najdi ewes

The objective of this study was to evaluate kinetics and the residual decline of tylosin in milk and plasmaof lactating Najdi ewes following single intramuscular injection of tylosin at the dose of 10 mg/kg. Blood andmilk samples were collected from the ewes before and at different time intervals after treatment. Tylosinconcentrations were determined by microbiological agar plate assay using Bacillus subtilis ATCC 6633 asthe test organism. The pharmacokinetic parameters were processed using the methods of least square andstatistical moments. The plasma levels of tylosin against time were adequately described by a onecompartmentopen model. The mean obtained values indicated a fairly low area under the plasmaconcentration-time curve (AUC) (3.0 μg.h/ml) and the maximum plasma concentration (Cmax) of 0.63 μg/mlwith Tmax of 1.33 h. The plasma elimination half-life (t1/2el) and the mean residence time (MRT) were 2.3 hand 3.9 h, respectively. A different pattern was shown for milk, in which measurable residual levels arefound in all animals up to 72 h after treatment. The mean value of milk AUC was 88.1 μg.h/ml and the t1/2elwas 3.3 h. In vitro mean plasma and milk proteins binding of tylosin were 19.3 and 30.2%, respectively. Themilk withdrawal period of tylosin in lactating Najdi ewes should be at least 72 h to avoid risks in consumers.