The role of absorbed drug in the efficacy of oxfendazole against gastrointestinal nematodes.

Comparisons were made of the relative efficacy of ozfendazole (OFZ), administered to sheep at 5 mg/kg either as an oral drench, single intravenous injection or 12 and 24 divided intravenous injections over 24 and 48 hours, against benzimidazole-resistant Haemonchus contortus and Trichostrongylus colubriformis. A single intravenous injection was at least equally potent as the oral drench whilst the divided dose intravenous regimes significantly increased OFZ efficacy against both parasite species. These findings demonstrate that (i) absorbed drug is important for the efficacy of OFZ against nematodes in the abomasum and small intestine and may be more important than unabsorbed drug passing down the gastrointestinal tract, and (ii) the maintenance of plasma OFZ levels of approximately 2 micrograms/ml by divided dose regime increased efficacy compared with that achieved with the same total dose given as a single administration.