ORIGIN OF CHLORTETRACYCLINE IN PIG-TISSUE

Violative chlortetracycline (CTC) residues in excess of the maximum re sidue limit (MRL) have been identified in 0.9% of all pigs tested at s laughter in Northern Ireland. Three experiments were carried out to in vestigate the possible reasons. In Experiment 1, pigs were fed CTC at therapeutic levels (300 mg kg(-1) head(-1) d(-1)) for 7 days and were slaughtered at day 0, 2, 4, 6, 8 and 10 following CTC withdrawal, Musc le, liver and kidney cortex and medulla samples were analysed for CTC residues by using high-performance liquid chromatography (HPLC). In al l tissues, residues were less than the MRL from day 2 onwards. Mean CT C concentrations in kidney cortex were greater than medulla concentrat ions (p < 0.05). In Experiment 2, feed spiked with a sub-therapeutic d ose of CTC (40 mg kg(-1)) was fed to six pigs that were slaughtered af ter 4, 8 and 12 days of medication. Violative CTC residues were not de tected. In Experiment 3, short term supra-therapeutic administration w as examined. Eight pigs received 500 mg kg(-1) CTC head(-1) d(-1) for 1 or 2 days and were killed in pairs, 24 and 48 h following the final medicated feed. Violative levels were detected in both groups after 24 h only. It is concluded that: firstly, recommended CTC withdrawal per iods are sufficient to avoid violative carcass residues; secondly, vio lative residues do not arise from sub-therapeutic dosing but can arise From supra-therapeutic dosing which could occur with inadequately mix ed feed; and thirdly, the difference in kidney cortex and medullary CT C concentrations is significant from a regulatory perspective.