Experimental selection for ivermectin resistance in Ostertagia ostertagi in cattle.

Recent reports of suspected ivermectin (IVM) resistance in Ostertagia ostertagi have highlighted the need for research into the mechanisms of IVM resistance. However, there are no reports of resistant field isolates of O. ostertagi, which have been characterized for molecular research. Therefore, an anthelmintic susceptible O. ostertagi population was selected for IVM resistance by repeatedly exposing the population to subtherapeutic and therapeutic levels of IVM over 10 generations. In each selection round, a group of calves was infected with the progeny of the previous IVM-selected O. ostertagi population. In the last selection round a therapeutic IVM dose (0.2 mg/kg BW) only reduced the faecal egg counts by 57% and 65% on days 7 and 14 after treatment, respectively. In contrast, the therapeutic IVM dose was 100% effective at eliminating the parental IVM-susceptible isolate.