Tissue distribution and residues of oxytetracycline in normal and emergency-slaughtered ruminants.

The concentration of oxytetracycline (OCT) in the blood, kidneys urine and meat was determined in a group of clinically normal dairy cows slaughtered at different times after treatment. Drug concentrations were similarly determined in a group of emergency-slaughtered dairy cows and calves. Distribution pattern of the drug in the body, expressed as muscle-drip-to-serum and kidney-cortex-to-serum ratios of the drug concentrations were determined in clinically normal cows and these ratios were compared with the corresponding ratios found in the emergency-slaughtered cows. Attempts were also made to relate changes in the expected ratios to the clinical and pathological findings in the emergency-slaughtered cows. High and persistent levels of the drug in the serum and a slower drug elimination from the body were the main characteristic findings in the sick cows. Drug persistence could not be related to specific tissue uptake or to better drug distribution, although the actual drug concentration in the muscle and the kidney of cows with different clinical pathological findings varied widely and was quite unpredictable. It is concluded that the kinetics of the oxytetracycline residues follow the same pattern as the drug residues in the serum, and these principles seem to apply in normal and in emergency-slaughtered dairy cows.