Pharmacokinetics and residual behaviour in milk of oxytetracycline in cows following administration of uterine pessaries.

The plasma kinetics and residual depletion in milk of cows treated by the intrauterine route with pessaries containing oxytetracycline (OTC) were evaluated. The antibiotic was administered to five healthy Friesian cows at a dosage of 3g/head in the early post partum phase. Blood samples were collected 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 and the pharmacokinetic parameters were processed using the minimum Akaike information criterion estimation (MAICE) test. The mean values obtained indicated a relatively low area under the concentration time curve (25.19+/-12.61 microg/mg per h) and maximum plasma concentration (Cmax) (0.549+/-0.278 microg/mL) with delayed time to Cmax (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 levels are found in two out of five animals until the 72nd hour after treatment. Data obtained demonstrate that OTC administered as a solid form is poorly and slowly absorbed from the uterus of cows.