The effect of level of feed intake on the pharmacokinetic disposition and efficacy of ivermectin in sheep.

The kinetic disposition of orally administered [3H]-Ivermectin (IVM) was examined in sheep in which the feed intake was maintained at either 800 or 400 g/day. The [3H]-metabolites were almost completely associated with particulate digesta in the rumen. In the low feed intake group the digesta flow was slower than in sheep on high feed intake. This resulted in an extended residence time and greater availability of IVM and its metabolites. The anthelmintic efficacy of IVM was then examined in sheep in which feed intake was reduced from 800 g/day to 400 g/36 h prior to and 36 h after IVM administration. In sheep with reduced intake 97% of IVM-resistant Haemonchus contortus were removed, compared with 53% in sheep maintained on high feed intake.