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 L ₄ 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 L ₄ 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.