The Discovery of Ivermectin and Other Avermectins

The avermectins were initially detected in a program in which thousands of microbial fermentation products were tested in mice for activity against the nematode, Nematospiroidesdubius. Among the few preparations showing activity in this assay, was the product of a microorganism isolated from soil by workers at The Kitasato Institute. The microorganism was classified as a new species of actinomycete, Streptomycesavermitilis. Its anthelmintic activity was shown to reside in 8 closely related macrocyclic lactones, named avermectins, which were also found to possess activity against free-living and parasitic arthropods. One of the natural components, avermectin B1, is now being evaluated as a pesticide for the control of mites of citrus and cotton crops and control of the Red Imported Fire Ant. A chemical derivative, 22,23-dihydroavermectin B1, or ivermectin, has been developed as an antiparasitic agent. It is being marketed for use in cattle, horses and sheep and is expected to become available for swine and dogs.