The persistence of the anthelmintic activity of ivermectin in sheep

The persistent anthelmintic effect of subcutaneously and orally administered ivermectin given at a dose rate of 0.2 mg/kg was evaluated against experimentally induced infections of Haemonchus contortus, Ostertagia spp., Trichostrongylus spp. and Cooperia curticei in sheep. While both ivermectin treatments apparently resulted in some reduction in the establishment of most of the parasite species examined, only for C. curticei were these reductions statistically significant. For C. curticei, statistically significant antiparasitic activity was evident in animals five and ten days after treatment with ivermectin given by injection and five days after treatment with ivermectin given orally. The possible relevance of these results to parasite control and the potential for selecting for anthelmintic resistance are briefly discussed.