Distribution of the anthelmintic drug albendazole and its major metabolites in ovine milk and milk products after a single oral dose.

The distribution of albendazole (ABZ) and its main metabolites albendazole sulphoxide (ABZSO), albendazole sulphone (ABZSO2) and albendazole 2-aminosulphone (NH2ABZSO2) were investigated in bulk milk and milk products after administration of a single oral dose of the drug (12.5 mg/kg) to 80 Laticauda sheep. An analytical method was developed for this investigation from an existing procedure used for the determination of these compounds in plasma and digesta samples. No traces of the parent compound or NH2ABZSO2 were found in milk or milk products, with the exception of the milk collected 36 h after treatment in which 89 micrograms NH2ABZSO2/kg was detected. Results indicated that ABZ was rapidly oxidized to ABZSO and then to ABZSO2. These metabolites were found at high levels (1-4 mg/kg) in milk collected within 24 h after treatment. Products derived from such milk also contained high concentrations of the two oxidized metabolites, including up to 5 mg ABZSO/kg in Pecorino cheese. Only small quantities of these two metabolites were found in milk collected during the second day after treatment (range 50-500 micrograms/kg). They were no longer detectable in milk collected during the third day after dosing, nor were they found in products made from such milk. These findings confirm that the two polar metabolites ABZSO and ABZSO2 were efficiently excreted from the body. Considering that the established maximum residue limit for ovine milk is 100 micrograms/kg for ABZ plus its metabolites, our results confirmed the appropriateness of the currently prescribed withdrawal time (3 d) after the use of ABZ in lactating sheep. However, considerable levels of ABZSO were detected in milk collected within 24 h after treatment as well as in products and by-products derived from such milk. Owing to the known toxicity of the ABZSO, we stress the need for careful control to ensure adherence to the prescribed withdrawal time.