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

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 mgl(-1) for 12 h at a water temperature of 12 degrees 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-SO2) 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 mu gkg(-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 mu gkg(-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-SO2 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 mu gkg(-1) for muscle tissue and 282 and 109 mu gkg(-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-SO2 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. (C) 1997 Elsevier Science B.V.