Effect of removing anthelmintic selection pressure on the benzimidazole resistance status of Haemonchus contortus and Trichostrongylus colubriformis in sheep

The potential for benzimidazole resistant strains of Haemonchus contortus and Trichostrongylus colubriformis to revert towards anthelmintic susceptibility when not exposed to anthelmintic treatments was investigated in sheep. Changes in anthelmintic resistance status were monitored by an egg hatch test at each generation throughout the 12 generation study and by controlled experiments at the sixth and 12th generation. No reversion towards susceptibility was recorded for either nematode species. The significance of these results in relation to the management of the resistance problem under field conditions is discussed.