Effects of diet on plasma concentrations of oral anthelmintics for cattle and sheep.

Groups of parasite-free lambs and calves which were either housed and fed hay and concentrates or were grazing on pasture were dosed separately with the oral anthelmintics fenbendazole and ivermectin (lambs only). The plasma concentrations of the drugs and their major metabolites were monitored during the period of their metabolism and excretion. The peak plasma concentrations and the availability of the drugs, as estimated by the areas under the plasma concentration-time curves, were significantly less in the grazing animals. When similar groups of lambs were dosed orally with the inert marker chromium EDTA, which has a particle size similar to the anthelmintics, it was observed that a higher percentage of chromium was excreted by the grazing lambs during the first 40 hours after dosing, suggesting that the extent of absorption in the grazing animals was less than in the housed animals.