Dewormer Resistance


Whether you use a daily dewormer or rotate paste dewormers, parasite control has remained a vital part of equine care. Many horse owners remember the devastating colic from the past caused by large strongyles. But with the introduction of ivermectin in the 1980’s, health problems from large strongyles (Strongylus vulgaris) have become rare. While the battle against equine parasites may appear diminished, other parasites once considered a small problem have risen to challenge caregivers. Current strategies of deworming were developed to kill S. vulgaris, but implementing these regimes has created resistance in small strongyles (cyathostomes).

Horses become infected with cyathostomes by ingesting infective larvae present in pasture that can either mature in the large colon or invade the intestinal wall. If they invade the intestinal wall, they undergo a period of arrested development (like hibernation), but problems arise when the larvae develop and emerge into the intestine. Common clinical signs of cyathostome infection include decreased level of performance, decreased growth rates, weight loss, colic, rough hair coat, diarrhea and protein loss.

Even though it may appear that there are a plethora of dewormers, all of them fall into only three drug classes: the benzimidazoles (e.g. Fenbendazole), macrocyclic lactones (e.g. ivermectin and moxidectin) and pyrantels. Unfortunately, when a parasite becomes resistant to a drug in one class it usually becomes resistant to all others in that class, making resistance trends particularly worrying to veterinarians and horse owners. Additionally, pharmaceutical companies are not developing anything new, anytime soon!

At least 40% of farms in southeastern USA now have cyathostomes that are resistant to both benzimidazoles and pyrantel drugs. Even more alarming, recent research in Kentucky has shown potential for resistance to ivermectin, a drug belonging to the last class of drug which cyathostomes haven’t developed resistance to until now.

With all this in mind, what can you as horse owners and caregivers do to determine whether this problem is occurring in your horses and prevent the development of parasite resistance? The simplest answer is a fecal examination. Fecal egg counts before deworming and 14 days afterwards can easily confirm whether treatment was effective and whether resistance is present. In some cases, fecal exams can determine whether your horse needs deworming more frequently than 2-3 times a year. If daily Strongid is your primary method of deworming, fecal exams may also be beneficial in the spring and summer, ideally before giving ivermectin. A 2007 study in Canada demonstrated that daily Strongid-C was highly effective against small strongyles, but made a point of noting that all horses on the farm should receive treatment to minimize the likelihood of resistance emerging. To date there has not been any scientific publication demonstrating the emergence of resistance in horses fed daily Strongid.

Understanding the life cycle of these parasites and how dewormers control them is key to a successful deworming program without generating drug resistance. This is only a glimpse into the bigger picture of parasites that are becoming resistant to frontline drugs: Ascarids (Parascaris equorum) are another parasite developing resistance to dewormers in heavily populated areas of weanlings and yearlings. As with all medical treatment, every case is different and your regular veterinarian should remain involved for proper parasite management.

i. Kaplan et al. JAVMA 2004 v.225 (6): 903-910.

ii. Lyons et al. Parasitol. Res 2008 v.103(2):287-91.

iii. Slocombe et al. JEVS 2007 v. 27(10): 439-444.