Researchers in Argentina replaced ivermectin-resistant worms (Cooperia spp.) with susceptible worms. After two production cycles, the clinical efficacy of ivermectin improved from 73% to 99.4%, while the absolute efficacy improved from 54.1% to 87.5%.
No clinical signs of parasitism were observed in the beef herd, after parasite-free heifers had been inoculated with infective worm larvae (40,000 or 30,000 L3 Cooperia).
"Low parasite refugia at the end of the summer is key to this approach," according to the researchers.
Read abstract in Veterinary Parasitology, November 2017