Adult C. oncophora recovered at necropsy post treatment. Boxplot of estimated numbers of adult C. oncophora worms recovered at necropsy of calves 10 days after injection (45 d.p.i.) with IVM. Calves were previously infected with a mixture of C. oncophora and O. ostertagi, representing isolates with different deworming histories. Isolate A was propagated IVM-susceptible, whereas B and C were two different field isolates showing phenotypic clinical IVM resistance in field trials. Boxplots show larger variation within groups than between groups.