Efficacy of an in-feed formulation of ivermectin against adult worms and somatic larvae of Strongyloides ransomi.

The efficacy of an in-feed formulation (IVOMEC premix) containing 0.6% ivermectin was tested against Strongyloides ransomi in swine. The efficacy of ivermectin against patent infections of S. ransomi when given via the feed at 2 ppm for 7 days (Days 0-7) to provide 100 mcg ivermectin kg-1 body weight day-1 was evaluated in a study with 16 3-month-old male castrated piglets. Seven days prior to treatment each piglet was infected subcutaneously with 2500 infective larvae of S. ransomi. Fecal egg counts were carried out on Days -7, 0, 7 and 14, and worm counts on Day 14. Efficacy was 100% in all treated piglets. Two trials involving 40 pregnant gilts were carried out to evaluate the efficacy of ivermectin against the somatic larval stages of S. ransomi when given at a daily dose of 100 mcg kg-1 body weight for 7 days starting on Days 66, 78, 92 or 103 of pregnancy. The gilts were each experimentally infected with three subcutaneous injections of 250,000 infective larvae, with the last infection given between 12 and 30 days prior to commencement of treatment. Gilts were confirmed free of pre-existing intestinal stages of S. ransomi prior to ivermectin treatment. Fecal nematode egg counts were carried out in gilts/sows and piglets subsequently born. The Strongyloides larvae present in sow milk 1, 2 and 7 days post partum were counted. Fourteen days post natum, worm counts were performed in four randomly selected piglets for each litter. IVOMEC premix given to pregnant gilts prevented shedding of larvae in sow milk, egg output in feces and the establishment of S. ransomi in piglets.