Oxytetracycline residues in meat and kidney tissue after intramuscular or intramammary treatment as determined by a chemical-physical method and compared to a microbiological method.

Two adult cows and three calves were treated intramuscularly with an oxytetracycline (OTC) preparation. The dose was about 5 mg/kg. The animals were slaughtered eight days after the treatment and their kidneys and muscle samples were taken for analysis. Twelve cows were treated intramammarily with an OTC preparation 12 hours before slaughter. Six of the cows were treated in two udder quarters with a preparation containing 200 mg OTC chloride per dose; the other six were similarly treated in four quarters with four doses. When the kidneys of all the animals were studied for the presence of inhibitory substances using the official Finnish microbiological meat inspection method, all were found negative. The OTC concentration of the different tissue samples was determined by a chemical-physical method, based on thin-layer chromatography and on the UV-fluorescence of OTC. The muscle samples of the cows given OTC i.m. were negative except near the site of injection, where the concentrations were 4 mg and less than 0.5 mg (traces) per kg. The corresponding kidney concentrations of OTC were less than 0.5 and about 0.5 mg per kg respectively. All except one kidney sample of the calves given OTC i.m. were found to be negative. Traces of OTC (less than 0.5 mg/kg) were found in all but one kidney sample of the cows treated intramammarily with OTC. Traces of OTC were found in two muscle samples of the cows treated intramammarily with 2 X 200 mg OTC and in two muscle samples of the cows similarly treated with 4 X 200 mg OTC. All the other samples were negative. The authors stress the need for a change in the present Finnish directives on the use of antibiotics in food animals.