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.

The problem of testing horse kidneys for the presence of antibiotics at meat inspection: how to avoid a false positive reaction.

When 33 horse kidneys were tested for the presence of inhibitory substances by the Bacillus subtilis BGA method at pH 8 and the Micrococcus luteus ATCC 9341 method, 24 were positive and 9 negative. The pH of the seeded M. luteus test medium changed from pH 6.6 before incubation to 8.7 after 24 hours incubation at 30 degrees C. When the same 33 kidneys were tested by the B. subtilis BGA method, medium pH 6, and 15 of them also by the M. luteus method using a medium buffered to pH 6, all were negative. The cadmium concentration of the 33 horse kidneys was found to be 70.17 +/- 81.28 mg/kg wet weight (m +/- s, range 10.40-355.67). The authors attributed the positive results to the presence of cadmium. The diffusion of cadmium from the horse kidney samples to the test media was found to differ at pH 6 and pH 8. It is recommended that the testing of horse kidney samples for the presence of antibiotics and chemoterapeutic substances be done at substrate pH 6 to avoid false positive reactions.