Tissue concentrations of a long-acting oxytetracycline formulation after intramuscular administration in cattle

The tissue distribution of a long-acting oxytetracycline formulation after intramuscular administration to calves at a dose rate of 20 mg/kg was studied. Oxytetracycline concentrations were determined in lung, bone marrow, mammary gland, uterus, uterine horn, ovary, liver, synovial fluid, joint tissue, kidney, spleen, brain, muscle, fat, urine, bile, saliva, ruminal content and serum. Observed concentrations were higher than the minimal inhibitory concentration for the majority of pathogens in all of the analysed tissues for at least 72 h post-injection. The shortest theoretical permanency time was 1.88 days (in saliva) and the longest was 19.06 days (in kidney). Taking the maximal permitted residue level as 0.1 microgram/g in muscle, 0.3 microgram/g in liver and 0.6 microgram/g in kidney, the calculated withdrawal time was 20 days.