Abstract

Enrofloxacin residues in poultry tissues may be exerting sub-inhibitory selective pressure on bacteria and contributing to antimicrobial resistance (AMR). The European Union has established Maximum Residue Limits (MRLs) for enrofloxacin in poultry of 100µg/kg in muscle, 200 µg/kg in liver, and 300 µg/kg in kidney (Commission Regulation (EC) No 508/1999). In this study, the residues of enrofloxacin in tissues of broilers after discriminate and indiscriminate use were measured for compliance with the EU MRLs as well as sub-inhibitory thresholds. Eighteen Cobb-500 broilers were distributed into three groups: Control (no antibiotic), Discriminate (10 mg/kg enrofloxacin for 7 days plus 7-day withdrawal) and Indiscriminate (continuous enrofloxacin until sacrifice). Residues of enrofloxacin in the liver, kidney, breast and thigh muscle were determined by a validated HPLC-UV method. Growth performance and blood parameters were assessed. The developed HPLC method showed good performance (r² > 0.999; recovery 86.7-91.2 %). The indiscriminate use resulted in violative residues above the EU MRLs in all tissues (liver: 1845 ± 212 µg/kg). Selective use resulted in no detectable residues in muscle but persistent residues in liver (45.2 ± 6.1 µg/kg) and kidney (18.5 ± 2.4 µg/kg), which were in compliance with EU MRLs but above the sub-inhibitory threshold of 10 µg/kg. No significant differences in growth and hematological parameters were found between groups. Following the recommended 7-day withdrawal time for poultry has removed any detectable residues from muscle, but sub-inhibitory concentrations of enrofloxacin will remain in the liver and kidney. Although all residues remaining after this period comply with EU MRLs, there may still be sufficient quantities of these residues present in liver and kidneys to create a selective pressure on bacteria. It is concluded that the results of this study could help formulate new guidelines about withdrawal times for critically important antimicrobials and develop guidelines for residue surveillance on the liver and kidney in broiler chickens.

Key words: Enrofloxacin, EU MRL, Fluoroquinolone, Food safety, Residues withdrawal, Sub-inhibitory concentration