The effect of the dye-marking of mastitis remedies on the incidence of antibiotic residues in Pretoria's market milk supplies.

A 1973 survey on the incidence of inhibitory substances (mostly antiobiotic residues) in market milk supplied in Pretoria, on 3 195 herd milk samples, 65 tanker milk samples and 252 samples of pasteurised milk using the disc assay procedure with Bacillus stearothermophilus C 953 as test organism, revealed inhibitory substances equivalent to 0,005 IU penicillin/ml in 7.8% of the herd samples, 29,2% of the tanker samples and in 38.5% of the samples of pasteurised milk. In 38.9% of the positive herd milk samples and 73% of the samples of pasteurised milk, penicillin was indentified with the aid of the penicillinase test. Some of the pasteurised milk contained inhibitory substances equivalent to more than 1.0 IU penicillin/ml; in some of the herd milk samples this figure exceeded 5,0 IU penicillin/ml. A repeat survey was undertaken in 1977/78 to evaluate the effect of compulsory dye-marking of non-prescription mastitis remedies on the situation. In a total of 1 081 herd milk samples, 60 tanker milk samples and 112 samples of pasteurised milk, antibiotic residues were found in 2,13% of the herd milk, 11,7% of the tanker milk and 2,1% of the pasteurised milk samples, with a much lower average concentration of antibiotic residues. The compulsory dye-marking of mastitis remedies had a beneficial effect on the occurrence of antibiotic residues in milk but did not ensure their complete absence, presumably because dye-marking was not made compulsory in scheduled preparations.