Depletion of metronidazole in brook trout (Salvelinus fontinalis).

Metronidazole (MNZ), which is effective in the treatment of intestinal infections in fish, is also a suspected carcinogen and has been banned in numerous jurisdictions for use in any food-producing animal, including fish. Few reports have been published on the depletion of MNZ in fish. A depletion study was therefore undertaken using MNZ in feed provided to trout under controlled conditions. The water was maintained at 17.5 +/- 0.9 degrees C throughout the medication and depletion periods in the study. Following a 20-day acclimatisation period in the holding tanks, the trout (approximately 150-200g bodyweight at the start of the study) were subjected to two separate medication and withdrawal periods: (A) 5day medication/5day withdrawal and (B) 5day medication/16day withdrawal. This simulated a potential multiple dosing in an aquaculture setting. In both medication periods, the trout were dosed with medicated feed containing 3g MNZkg(-1) fish. Fish were sacrificed in accordance with accepted animal care protocols and tissue samples were analysed by UPLC-MS/MS. Analyte concentrations in trout muscle ranged from a high of 27000 +/- 10000ngg(-1) for MNZ and 830 +/- 570ngg(-1) for MNZ-OH on day 1 of withdrawal period A to a low of 1.8 +/- 2.3ngg(-1) for MNZ and <0.4ngg(-1) for MNZ-OH on day 16 of withdrawal period B. The results demonstrate that when using the UPLC-MS/MS method, residues of MNZ may be detected in fish treated with MNZ after 16days of withdrawal.