Measurement of Benzimidazoles and their Metabolites in Animal Tissues

A high performance liquid chromatographic method for the separate detection of benzimidazole anthelmintics and their principal metabolites has been developed, and an evaluation was made of the reproducibility of the method using spiked tissue, milk and plasma samples. In addition, an investigation of benzimidazole concentrations in tissues, milk and plasma of treated animals was carried out, from the day of administration until residues could no longer be detected, to assess if the recommended withdrawal periods were adequate and, by means of an abattoir survey, if benzimidazole residues pose a problem to the consumer. The animals used were cattle and sheep, and the drugs used were febantel (a pro-benzimidazole), and the benzimidazoles, albendazole, fenbendazole, oxfendazole and thiabendazole. For all of the drugs studied residues were found in tissues. Relatively high concentrations of benzimidazoles and metabolites were found in liver compared with plasma or other tissues. Detectable residues of febantel, fenbendazole, oxfendazole and thiabendazole in liver persisted beyond the recommended withdrawal periods. In muscle, residues of all the drugs studied were low, being similar to those found in plasma. None of the sulphide benzimidazoles (albendazole and fenbendazole) were detectable in milk at any time after administration to cows and only sulphoxides (oxfendazole and albendazole sulphoxide) and sulphones (oxfendazole sulphone and albendazole sulphone) were detectable. In an abattoir survey of cattle and sheep carcasses, no residues of any of these drugs were detected in one hundred livers analysed. The relevance of these results to the safety of benzimidazoles in humans is discussed.