In 2008, we reported parasitologists were finding small strongyle eggs reappearing in the manure of ivermectin-treated horses as quickly as 30 days or so after deworming. There was initial concern that this might be a resistance problem, but later studies confirmed the drug was indeed killing the parasites but the horses were becoming reinfected by a strain with a short life cycle.
A new study just published in Veterinary Parasitology found exactly the same thing is happening even with moxidectin (brand name Quest), which removes encysted larval stages. Most horses treated with moxidectin were showing eggs again by 35 days and all were by 45 days. In that same study, the effectiveness of double-dose fenbendazole (brand name Panacur Power Pack) was checked. There was no difference in egg counts between fenbendazole-treated and untreated horses.
There is widespread resistance to all dewormer drugs, except ivermectin and moxidectin. If you continue to rotate using other drugs, you need to do a fecal-egg count 14 days after deworming to make sure you're not wasting your money.
Realize that the emergence of a small strongyle strain with a very short life cycle means that the usual interval between dewormings of three months after moxidectin and two months after ivermectin are no longer reliable time periods.
Very young, very old and very debilitated or immunocompromised horses may need to be dewormed as often as every four to five weeks, even if using ivermectin or moxidectin.