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Abstract
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°C throughout the medication and depletion periods in the study. Following a 20-day acclimatisation period in the holding tanks, the trout (approximately 150–200 g bodyweight at the start of the study) were subjected to two separate medication and withdrawal periods: (A) 5 day medication/5 day withdrawal and (B) 5 day medication/16 day withdrawal. This simulated a potential multiple dosing in an aquaculture setting. In both medication periods, the trout were dosed with medicated feed containing 3 g MNZ kg–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 27 000 ± 10 000 ng g–1 for MNZ and 830 ± 570 ng g–1 for MNZ-OH on day 1 of withdrawal period A to a low of 1.8 ± 2.3 ng g–1 for MNZ and < 0.4 ng g–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 16 days of withdrawal.
Acknowledgements
We are grateful for the donation by the Nova Scotia Provincial Government of speckled trout from the McGowan Lake Fish Hatchery, Caledonia, NS, Canada. We would also like to acknowledge Cory Murphy, Canadian Food Inspection Agency, for helpful discussions related to the study.