Excretion of tetracycline and chlortetracycline in eggs after oral medication of laying hens.

After dosing laying hens orally with tetracycline (TC) through either drinking water (0.25 and 0.5 g/l for 5 days) or feed (300 and 600 ppm for 7 days), and chlortetracycline (CTC) through feed (600 ppm) residues were determined by an agar plate diffusion technique in cylinders with Bacillus cereus as test-organism, separately for albumen and for yolk. The sensitivity threshold was 0.07 micrograms/g in albumen and 0.15 micrograms/g in yolk for TC and 0.01 micrograms/g in albumen and 0.06 micrograms/g in yolk for CTC. Drug excretion via egg was 3-fold higher for TC than for CTC. The drug was excreted preferentially into the yolk (about 75% of the total amount) and the elimination period lasted between 6 and 11 days for TC and 9 days for CTC, after treatment. Tetracycline use in laying hens is discussed, taking into consideration the proposals presented by the joint FAO/WHO Expert Committee on Food Additives.