Investigation of the persistence of levamisole and oxyclozanide in milk and fate in cheese.

In this study, dairy cows (n = six) were treated with an oral combination product containing levamisole (5 mg/kg body weight, (bw)) and oxyclozanide (10 mg/kg bw). Animals were milked twice daily up to day 16 post-treatment. Milk samples were subsequently analyzed by ultraperformance liquid chromatography coupled to tandem mass spectrometry (UPLC-MS/MS). The highest levels of levamisole (<600 μg/kg) and oxyclozanide (<25 μg/kg) were determined at first and third milking, respectively. Residues of levamisole and oxyclozanide were typically below reporting limits of 0.83 and 1 μg/kg respectively at the 11th and 13th milking, respectively. Soft (3 days ripening), hard (35 days ripening) and whey cheeses were produced from the milk samples collected from the first two milkings. Levamisole residues were found to concentrate in all cheese types. There was a 3-fold concentration effect for levamisole in mature cheese. Oxyclozanide residues were found to occur at lower levels in soft and hard cheese than milk with a 10-fold concentration in whey cheese compared to milk. The results of this study demonstrate that levamisole and oxyclozanide residues are rapidly excreted in dairy cows and milk is compliant after a few days. Oxyclozanide and levamisole residues were shown to be stable during the fermentation process and the whey heat treatment to persist in cheese.