Instability of benzimidazole resistance in nematode eggs.

The resistance of the eggs of benzimidazole resistant Haemonchus contortus and Trichostrongylus colubriformis is not stable and will decrease in the absence of drug challenge. Dosing animals harbouring resistant H contortus with either thiabendazole or mebendazole selects for nematodes producing thiabendazole resistant eggs. The eggs of a thiabendazole selected strain of H contortus were more resistant to thiabendazole, parbendazole and oxibendazole but less resistant to cambendazole and mebendazole.