Plasma levels of chloramphenicol after oral administration in calves during the first weeks of life

Three experiments were carried out to investigate the mechanisms whereby adequate plasma levels of chloramphenicol may be obtained following oral administration in young calves but not in older animals. In the first experiment, plasma levels of chloramphenicol following an oral dose of 50 mg/kg were followed in six calves, given weekly doses for the first 11 weeks of life. A plasma chloramphenicol level of 5 μg/ml, taken as the minimum therapeutic level, was attained only for a few hours in the 1 week old calves. Thereafter plasma levels decreased very rapidly until the fourth week, and rather more slowly between the fourth and eleventh weeks. At 11 weeks the plasma chloramphenicol level fell to around 0.3 μg/ml, which was the lower limit of sensitivity for the assay technique used. In the second experiment, the same single dose was administered to calves in the twelfth or eighteenth weeks of life which had not previously been exposed to the antibiotic. Plasma levels of 1 μg/ml were barely reached. This suggests that the non-absorbtion of chloramphenicol is unlikely to be related to repeated administration of the antibiotic. In the third experiment, the same single dose was administered orally to two cows. Chloramphenicol could not be detected in the plasma following such administration, and some side-effects were observed in the 48 h following dosing. It is suggested that rumen function may interfere with the absorbtion of chloramphenicol following oral administration to ruminants, even in relatively young animals.