Efficacies of different doses of ivermectin against male, female and L4 Oesophagostomum dentatum in pigs.

Efficacies of ivermectin against larval stages and adult males and females of Oesophagostomum dentatum were investigated in two slaughter assays. In Experiment A, 20 pigs were each infected with 6000 third-stage larvae on Day 0 and Day 24. Pigs were ivermectin treated on Day 28 at dose rates of 0, 75, 150, and 300 micrograms kg-1 bodyweight (bw) and slaughtered 6 days after treatment. In Experiment B, 20 pigs each received 6000 third-stage larvae and were treated 35 days after infection at dose rates of 0, 150, 300 and 600 micrograms kg-1 bw. Pigs were slaughtered 14 days after treatment. In Experiment A, the adult worm burden was reduced by 69.1% at a dose rate of 300 micrograms kg-1 bw and the larval burden was reduced by 68.7 and 90.9% at 150 and 300 micrograms kg-1 bw, respectively. In Experiment B, the adult worm burden was reduced by 88.8, 96.2 and 99.6% at dose rates of 150, 300 and 600 micrograms kg-1 bw, respectively. In the control group of Experiment A, the mean proportion of females among adults worms was 57.6%, but this decreased to 19.7% after ivermectin treatment at 300 micrograms kg-1 bw. In Experiment B, at 300 micrograms kg-1 bw, this proportion was reduced from 46.0% to 0.8%. The fecundity of female worms was reduced at dose rates of 150 and 300 micrograms kg-1 bw in Experiment A, but not in Experiment B. It is concluded that in O. dentatum, ivermectin not only reduces the egg output of female worms, but also is more effective against female than male worms.