Withdrawal time for sulfamethoxazole and trimethoprim following treatment of striped catfish (Pangasianodon hypophthalmus) and hybrid red tilapia (Oreochromis mossambicus × Oreochromis niloticus) Tran Minh Phu a,d, ⁎, Marie-Louise Scippo b , Nguyen Thanh Phuong a , Cao Thi Kieu Tien a , Co Hong Son c , Anders Dalsgaard d a College of Aquaculture and Fisheries, Can Tho University, 3/2 Street, Campus II, Can Tho, Vietnam b Department of Food Sciences, Laboratory of Food Analysis, Faculty of Veterinary Medicine, CART (Center for Analytical Research and Technology), University of Liège, B-4000 Liège, Belgium c National Agro-Forestry-Fisheries Quality Assurance Department, Branches 6, Cach Mang Thang 8 Street, Can Tho, Vietnam d Faculty of Health and Medical Sciences, University of Copenhagen, Stigböjlen 4, 1870 Frederiksberg Denmark abstract article info Article history: Received 17 June 2014 Received in revised form 3 December 2014 Accepted 8 December 2014 Available online 13 December 2014 Keywords: striped catfish sulfonamides LC-MS/MS red tilapia maximum residue limit Vietnam Sulfamethoxazole (SMX) and trimethoprim (TMP) have been widely used to treat bacterial infections in aquacul- ture. 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 treat- ment 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. With- drawal time experiments were done with striped catfish (40.3 ± 7.2 g) and red tilapia (10.7 ± 4.5 g) in six ex- perimental 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. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Striped catfish (Pangasianodon hypophthalmus) aquaculture in the Mekong Delta, Vietnam has grown rapidly and produced 1.15 million tons in 2013 (De Silva and Phuong, 2011; Directorate of Fisheries, 2014). The intensive production practices and open farming systems are seen as the main factors associated with frequent disease outbreaks, mainly caused by Edwardsiella ictaluri and Aeromonas spp. with mortal- ities ranging from 30% to 60% (Crumlish et al., 2002; Phan et al., 2009; Rico et al., 2013). Vaccination of catfish against these bacterial diseases is not commonly practiced and farmers instead apply medicated feed containing antimicrobials (Rico et al., 2013). In 2010, production of farmed tilapia in the Mekong Delta reached 60,000 tons, equivalent to 60% of total tilapia production in Vietnam. Nile tilapia (Oreochromis niloticus) is reared in earthen ponds and cages whereas red tilapia (Oreochromis mossambicus × O. niloticus) is Aquaculture 437 (2015) 256–262 ⁎ Corresponding author at: College of Aquaculture and Fisheries, Can Tho University, 3/2 Street, Campus II, Can Tho, Vietnam. Tel.: +84 908512101; fax: +84 7103830323. E-mail address: tmphu@ctu.edu.vn (T.M. Phu). http://dx.doi.org/10.1016/j.aquaculture.2014.12.009 0044-8486/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Aquaculture journal homepage: www.elsevier.com/locate/aqua-online