Depletion study of fenbendazole in rainbow trout ( Oncorhynchus mykiss) after oral and bath treatment

Abstract

The tissue depletion of the anthelmintic fenbendazole (FBZ) was studied in rainbow trout ( Oncorhynchus mykiss). FBZ was administered orally at a dose of 6 mg kg −1 body weight. A bath treatment at a dose of 1.5 mg l −1 for 12 h at a water temperature of 12 °C was also carried out. Muscle tissue and skin samples were collected at 12 and 24 h intervals following drug administration and analysed for FBZ and the metabolites oxfendazole (FBZ-SO) and oxfendazole sulphone (FBZ-SO 2) by high-performance liquid chromatography (HPLC). Following oral treatment, FBZ was detected in muscle tissue until 96 h after treatment. The highest median concentration was found at the 24 h sampling time (40 μg kg −1), thereafter a decline occurred. The levels of FBZ in the skin were higher than those found in the muscle tissue, with the highest content found at the 24 h sampling time (median 249 μg kg −1). FBZ-SO could not be detected in muscle tissue. In skin, FBZ-SO was detected in four samples out of five at the 24 and 48 h sampling points. FBZ-SO 2 was not detected in all samples examined. Following bath treatment, FBZ was found in muscle tissue and skin at the 12 and 24 h sampling points only, the median concentrations being 96 and 17 μg kg −1 for muscle tissue and 282 and 109 μg kg −1 for skin, respectively. FBZ-SO was present in muscle tissue at the 12 and 24 h sampling points only. In the skin, this compound was detected until 96 h after treatment. FBZ-SO 2 was not detected in all samples examined. This study clearly demonstrates that, in trout, FBZ is metabolised to FBZ-SO and that both FBZ and FBZ-SO accumulate in trout skin after administration of the drug. Furthermore, the data obtained indicate that FBZ-residues in rainbow trout following oral and bath treatments are largely depleted within 96 and 24 h, respectively.