Serum levels and oxytetracycline residues in milk after the administration of long acting preparations to ewes

Blood serum concentrations and excretion of oxytetracycline (OTC) into milk after the i.m. administration of long-acting preparations in adult ewes were studied. Four ewes were treated with L.A. Engemycin and four - with L.A. Tetravet. OTC was administered to animals in a single dose of 20 mg/kg of live weight. OTC concentrations in blood serum were determined 1, 6 and 24 hours and then 2 to 8 days after the treatment. The milk samples were collected from the morning and evening milking up to 166 hours. OTC was determined by the HPLC method with solid phase extraction. Significantly higher serum concentrations of OTC (p < 0.05) were recorded after Tetravet administration from the 24th hour to the 5th day. OTC was detected in blood serum up to the 5th day after Engemycin administration, and up to the 6th day after Tetravet administration. The OTC residues in milk reached their maximum mean concentrations after 24 hours (2.525, and 2.186 mg/I, respectively). Then the mean OTC concentrations rapidly decreased and the residues were below the maximum residual limit (0.1 mg/l) 96 hours after the administration. OTC residues were detected in the first group up to 144 hours (0.021 mg/l), and up to 168 hours (0.031 mg/I) in the second group. The results indicated on the influence of the vehicle (polyvinylpyrrolidone in Engemycin and dimethylacetamide in Tetravet) on the serum kinetics and time of excretion of OTC into milk of treated ewes.