Pharmacokinetics and residual behaviour in milk of oxytetracycline in cowsfollowing administration of uterine pessaries

The plasma kinetics and residual depletion in milk of cows treated by the i ntrauterine route with pessaries containing oxytetracycline (OTC) were eval uated. The antibiotic was administered to five healthy Friesian cows at a d osage of 3g/head in the early post partum phase. Blood samples were collect ed before and at different time intervals (3, 6, 12, 24, 48, 72, 84, and 96 h) after treatment. Milk was drawn before treatment and at 12-h intervals for 4 consecutive days. Samples were analysed by a high-performance liquid chromatography method an d the pharmacokinetic parameters were processed using the minimum Akaike in formation criterion estimation (MAICE) test. The mean values obtained indic ated a relatively low area under the concentration time curve (25.19 +/- 12 .61 mug/mg per h) and maximum plasma concentration (C-max) (0.549 +/- 0.278 mug/mL) with delayed time to C-max (11.71 +/- 4.15 h) and elimination half -life (21.96 +/- 4.42 h). A similar pattern could be shown for milk, in which measurable residual lev els are found in two out of five animals until the 72nd hour after treatmen t. Data obtained demonstrate that OTC administered as a solid form is poorl y and slowly absorbed from the uterus of cows.