We know that anthelmintic (or dewormer) resistance in equine internal parasites—meaning the drugs we use are becoming ineffective against the parasites they’re designed to control—is changing the way researchers and veterinarians recommend that we deworm our horses. “But what are the odds that I’ll have this problem on my little farm,” you wonder. According to recent study results, the odds are pretty good: They suggest that some commercially available dewormers—fenbendazole, oxibendazole, and pyrantel pamoate—showed poor efficacy on the majority of farms sampled during the project.

In the study, nearly 1,000 horses from 67 farms in the mid-Atlantic region of the United States received one of four different deworming products. At the same time a fecal sample was collected for analysis for parasite eggs. The horses with strongyle fecal egg counts totaling over 200 eggs per gram were retreated with fenbendazole, oxibendazole, or pyrantel pamoate. Of those horses, only 6%, 21% and 43%, respectively, showed fecal egg count reductions greater than 90%.

“On the majority of farms sampled, these three products performed poorly,” said study author Meagan A. Smith, DVM, Dipl. ABVP, staff veterinarian for the University of Pennsylvania’s School of Veterinary Medicine’s Field Service.

Currently, ivermectin and moxidectin—two macrocyclic lactone medications—still offer good control against small strongyles. “Their continued efficacy is significant because so far they are the only available dewormers for horses that still have consistently good activity against strongyles,” she said