The efficacy of ivermectin against nodular worms of pigs: the response to treatment using three different dose levels against Oesophagostomum dentatum and Oesophagostomum quadrispinulatum.

Anthelmintic efficacies of 3 different doses of ivermectin (IVM) were evaluated in 3 isolates of nodular worms in pigs. An isolate of Oesophagostomum quadrispinulatum (OQ) was recently obtained from a commercial farm where poor efficacy of IVM at the recommended dose (300 micrograms.kg-1 body weight) was detected. On this farm, IVM had been used for treatment of sows twice yearly for 6 years. Two other isolates, an O. dentatum (OD) and a mixed Oesophagostomum dentatum and Oesophagostomum quadrispinulatum isolate (ODQ) were obtained from a farm where anthelmintics had never been used. Efficacies of IVM against adult worms of the OQ-isolate at dose rates of 150, 300 and 600 micrograms.kg-1 body weight ranged from 40.5-78.6%. Efficacies against larval stages (L3 and L4) were superior. Efficacies against the OD-isolate were 88.7, 96.1 and 99.6%, respectively. In the ODQ-isolate the efficacies of IVM against adult stages furnished similar results. In conclusion, the efficacy of IVM against O. dentatum was high but against both isolates of O. quadrispinulatum poorer. This suggests that IVM is intrinsically less effective against O. quadrispinulatum and therefore not indicative of acquisition of anthelmintic resistance in the OQ-isolate.