A study of chloramphenicol distribution and residues in dairy cows.

Distribution and residues of chloramphenicol were examined in 29 normal and 15 emergency-slaughtered dairy cows after parenteral injection of 3 chloramphenicol base formulations, using microbiological and chemical assay procedures. Five qualitative test methods for antibiotic residues failed to detect the presence of the drug even in samples collected within 18 hours of treatment at a dose level of 37 mg/kg, and in emergency-slaughtered cows known to have been treated with chloramphenicol before slaughter. The limited usefulness of these methods for the detection of chloramphenicol residues is stressed by observations indicating the presence of chemically measurable concentrations of the drugs (or its metabolites) in the muscle, kidney, liver, bile, and urine 65 hours after treatment. Quantitative microbiological assay procedures for chloramphenicol (sensitivity limits 1.0 microgram/ml tissue fluid) conducted on specimens of kidney and liver samples of cows treated with chloramphenicol (doses approximately 10 to 40 mg/kg intramuscularly) 1/2 ; 1; 3 1/2; 12; 18; 46; and 65 hours earlier were all negative. However, chemical assay procedures (sensitivity limits 0.2 to 3 microgram/ml or mu/g tissue) for chloramphenicol and some of its metabolites detected the drug in these samples as well as in the meat. Chloramphenicol is apparently extensively metabolized in adult bovines. The extended persistence of "chloramphenicol" residues in the carcass is due to the relatively slow elimination rate of the metabolized drug and the poor and slow absorption rate from the intramuscular injection site after treatment, with preparations containing chloramphenicol base dissolved in organic solvents. Examples are given of the predicting of of pre-slaughter carcass withdrawal times on the basis of arbitrary drug tolerance or detection levels.