The environmental fate of veterinary drugs: in field assessment

Recent studies have shown that a multitude of drugs are present in the aquatic systems [1]. Due to the spreading of drug-resistance there is concern about the persistence of antibiotic residues in the environment. In past times tetracyclines and macrolides were used as feed additives in food producing animals; now their use has been restricted to therapeutic purposes. A high percentage of the drug is often excreted unchanged in manure and its use as top soil dressing can contaminate water via run-off drainage or through leaching. In 1996 a “Note for guidance for the environmental risk assessment of veterinary drugs” [2] was approved and several information on the environmental persistence and safety of drugs are now requested for registration. In this study the results from an experimental model carried out “in-field” are reported. The environmental fate of tylosin (TYL) and oxytetracycline (OTC) orally administered for five days to 40 calves (20 mg/kg and 60 mg/kg, respectively) was monitored for 6 months in manure, in soil and in surface water. TYL showed a rapid degradation, falling below the limit of quantification after 45 days. For OTC drug half-life in manure was calculated (T1/2el=30 days); only traces (lower than the limit of detection, 10 ppb) were found in soil and ground water. Ten ppb is the Safety Limit recommended by the EMEA [2] for drug residues in soil. The data were fitted to a simple mathematical model to check its suitability to predict environmental concentration in manure, soil, and ground water. Result indicate that the five months period necessary for manure ripening permits an adequate degradation and appears sufficient to minimize the environmental impact of these drugs.