Comparison of changes in resistance of Haemonchus contortus eggs following withdrawal of thiabendazole selection.

Two strains of Haemonchus contortus with different levels of thiabendazole resistance were developed from the same parent strains by being subjected to different selection intensities. These two strains were each subdivided into two; one selected by thiabendazole in each generation, the other unselected for four generations. Levels of thiabendazole resistance were monitored in each generation by an in vitro egg hatch technique. During this time the unselected strain with the higher level of resistance maintained an LC50 similar to its selected control. The LC50 of the unselected strain with the lower level of resistance actually increased until at the end of the fourth generation it had an LC50 similar to the thiabendazole selected strains.