Avermectins, new family of potent anthelmintic agents: efficacy of the B1a component

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.