Residue analysis and determination of withdrawal period of florfenicol upon in-feed administration to striped catfish Pangasianodon hypophthalmus (Sauvage, 1878).

Florfenicol (FFL) is a broad-spectrum antibiotic to treat bacterial diseases of fish with a recommended dose of 10 mg/kg biomass per day for 10 successive days. This study investigates the accrual depletion of the antibiotic residue in fish tissue when administered at varying dosages and durations in Pangasianodon hypophthalmus. The concentrations of FFL and its major metabolite, florfenicol amine (FFA) exhibited dose- and tissue-dependent accumulation patterns, with peak levels occurring at different sampling points during the dosing period rather than uniformly on a single day. In plasma, maximum FFL concentrations were observed on day 20 in the 1× and 3× groups, whereas in muscle tissues peak levels occurred on day 20 in the 3× and 5× groups. In contrast, kidney and liver tissues generally showed higher accumulation on day 10 of dosing. The kidneys of the 1× group exhibited FFL residues of 372.88 ± 13.20 ng/g on day 10 of FFL administration, which subsequently decreased to 92.00 ± 1.30 ng/g by day 30 of dosing, and further dropped to 8.34 ± 0.70 ng/g on post-FFL dosing day 10. At a therapeutic dosage of 10 mg/kg biomass, the muscle residues in P. hypophthalmus also dropped below the maximum residual limit of 100 µg/kg within 1-day post-dosing. In this investigation, supporting the sanctioned dosage of 10 mg/kg biomass per day for 10 successive days, the noted withdrawal time period was significantly shorter than the recommended 15 days. Following the CVMP protocol, the residues in the muscle of P. hypophthalmus decreased below the MRL value of 100 µg/kg within 1 day (27-degree days, water temperature 27 °C) for the therapeutic dose group. Nonetheless, considering the tolerance limit and a confidence level of 95%, a 5-day (135-degree days) withdrawal period was projected at the therapeutic dosage.