Hmm, interesting. I didn't know the -azoles were SO ineffective. I've been using oxibendazole, but I assume it's in the same boat as fenbendazole and benzimendazole.
Sounds like I could eliminate this -azole deworming from my rotation. My horse will still be getting his needed Ivermectins and Praziquante and I'll save myself 5 bucks. =)