Sulfamethoxazole (SMX) and trimethoprim (TMP) have been widely used to treat bacterial infections in aquaculture. Little is known about the elimination of SMX and TMP in striped catfish (Pangasianodon hypophthalmus) and hybrid red tilapia (Oreochromis mossambicus × Oreochromis niloticus). The aim of this study was therefore to validate a LC-MS/MS method and use it for establishing the withdrawal period for SMX and TMP following treatment of striped catfish and red tilapia.

A LC-MS/MS method was developed and validated in terms of linearity, specificity, precision (repeatability and within-laboratory reproducibility), recovery and decision limit (CCα). Liquid chromatography coupled to mass spectrometry in combination with Agilent Bond Elut QuEChERS was used to determine SMX and TMP. Withdrawal time experiments were done with striped catfish (40.3 ± 7.2 g) and red tilapia (10.7 ± 4.5 g) in six experimental tanks of 500 L and on-farm trials (red tilapia only). Catfish and red tilapia were treated with medicated feed containing SMX (674 ± 10.1 mg/kg) and TMP (113 ± 11.0 mg/kg) for five consecutive days. Samples were collected one day before provision of medicated feed as well as at days 1 and 5 during medication and 3, 7 and 15 days after the application of medicated feed was terminated. As expected, antimicrobial residue levels were the highest on the last day of treatment, e.g. SMX and TMP concentrations in striped catfish muscle were 577 ± 179 and 92.5 ± 83.7 μg/kg. The concentration of TMP was 10.0 ± 1.2 μg/kg in catfish muscle three days after medication was terminated and 30.1 ± 15.7 μg/kg in skin seven days after medication, i.e. below the maximum residue limit (MRL; 50 μg/kg) assigned by the Commission of the European Communities (EU). The concentration of SMX in muscle and skin was below the EU-MRL (100 μg/kg) seven days (40.5 ± 15.9 μg/kg) and 15 days (2.4 ± 0.5 μg/kg), respectively, after the last day of medication. In red tilapia, SMX (38.5 ± 11.4 μg/kg) and TMP (28.2 ± 13.9 μg/kg) concentrations were below the EU-MRLs three days after the last day of medication; findings which were supported by on-farm tilapia trials.

In conclusion, the validated LC-MS/MS combined with Agilent Bond Elut QuEChERS extraction performed well for the analysis of SMX and TMP and is recommended for routine analysis in striped catfish muscle. A withdrawal time of 15 days for striped catfish and three days for red tilapia treated with TMP and SMX is sufficient to meet the EU MRLs for fish intended for human consumption.