The efficacy and persistent anthelmintic effect of ivermectin in sheep.

The direct efficacy and the long-term persistent anthelmintic effect of an oral suspension and an injectable formulation of ivermectin at a dose rate of 0.2 mg kg-1 was studied in sheep. Lambs were infected experimentally with Haemonchus contortus, Trichostrongylus vitrinus and Cooperia curticei. After 3 weeks they were treated with an oral suspension or with an injection, and reinfected with the same dose of larvae 3, 6 or 10 days after treatment. Post-mortem worm counts showed no persistent effect of the oral suspension. The injectable formulation showed an excellent persistent effect for up to 10 days against H. contortus. Reinfection with C. curticei 3 days after treatment resulted in a 64% reduction of the worm burden, but reinfection after 6 and 10 days was 100% reduced. The reduction of T. vitrinus was 46% after 3 days and 92% after 6 and 10 days.