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The effect of the route of administration (oral, intraruminal and intra-abomasal) on the efficacy and pharmacokinetics of the new anthelmintic monepantel in sheep was investigated. The target nematodes were fourth-stage Haemonchus contortus, Teladorsagia circumcincta, Trichostrongylus colubriformis and Cooperia curticei. A clear difference in efficacy was identified between the routes of administration, although the difference did not consistently reach statistical significance; oral treatment was most effective, followed by intraruminal and then intra-abomasal administration. The same pattern was observed in the pharmacokinetic analysis, with lambs treated orally having more favourable exposure to monepantel and its sulfone metabolite (albeit in all but one instance not significantly different) than the lambs treated by the other routes of administration, which were very similar for most parameters.

Eighteen, six- to seven-month-old lambs were infected experimentally with larvae of Haemonchus contortus, Teladorsagia circumcincta, Trichostrongylus axei, Trichostrongylus colubriformis, Cooperia curticei and Nematodirus spathiger, and allocated to three equal groups. The infections were timed to ensure that fourth-stage larvae were present when groups 1 and 2 were treated orally with monepantel. Group 1 was not fed for 24 hours before the treatment, group 2 was fed two hours before the treatment and group 3 was fed at the same time as group 2 but not treated with monepantel. All the sheep had access to water. Worm burdens were determined 15 days after the treatments. Fasting or feeding had no statistically significant effects on the efficacy of the monepantel solution against the nematodes, and the period of fasting had no adverse effects.