Avermectins, New Family of Potent Anthelmintic Agents: Efficacy of the B_1a Component

ABSTRACT

When given to sheep as a single oral dose at 0.1 mg/kg, the B_1a component of the avermectins caused a reduction of >95% in the numbers of Haemonchus contortus, Ostertagia circumcincta (including inhibited L₄ larvae), Trichostrongylus axei, Trichostrongylus colubriformis, Cooperia oncophora, and Oesophagostomum columbianum. When given to cattle as a single oral dose at 0.1 mg/kg, avermectin B_1a was >95% effective in reducing the numbers of Haemonchus placei, Ostertagia ostertagi (including inhibited L₄ larvae), T. axei, T. colubriformis, C. oncophora, Cooperia punctata, Oesophagostomum radiatum, and Dictyocaulus viviparus. Avermectin B_1a was similarly effective, with the exception of a detectable loss in activity against adult C. oncophora, when administered to cattle as a parenteral injection. Some of these ruminant parasites were fully susceptible to dosages of avermectin B_1a at 0.025 mg/kg, e.g., D. viviparus, O. radiatum, O. ostertagi, and H. contortus. Avermectin B_1a removed 83 to 100% of Ancylostoma caninum from dogs given a single oral dose of 0.003 to 0.005 mg/kg. The poultry nematodes Capillaria obsignata and immature Ascaridia galli were effectively removed by avermectin B_1a at 0.05 and 0.1 mg/kg, respectively, but 0.1 mg/kg was not effective for Heterakis gallinarum. Thus, the avermectins would appear to have unprecedented potency and spectrum of biological activity.