A recent study by University of Kentucky researcher Mary Rossano, MS, PhD, assistant professor in Animal and Food Sciences, suggests that two commonly-used dewormers (fenbendazole and moxidectin) might no longer be as effective against small strongyles as once thought. Rossano conducted her study on 15 horses at UK’s research farm, but her results mirror those of other, larger studies.

Rossano’s study tested the effectiveness of a popular five-day, double dose fenbendazole treatment for reducing strongyle type fecal egg counts in yearlings. When originally introduced, the treatment was highly effective against adult, developing, and encysted small strongyles. However, the results indicated the five-day, double dose caused no significant reduction in the number of small strongyle eggs found in fecal matter. The study also examined average times for reappearance of strongyle type eggs after treatment with moxidectin. They found that while moxidectin did reduce fecal egg counts to zero after treatment, the time it took for egg counts to rebound after treatment was shorter than expected–only six weeks. When first introduced, moxidectin suppressed fecal egg counts for 10-12 weeks, or about twice as long.

A similar study was published by another UK researcher, Eugene Lyons, PhD, Department of Veterinary Science, who found that parasite recovery times after horses are dewormed using ivermectin have become shorter and seem to be based on the larvae growth stage at the time of administration.

Rossano said recent research conducted by Ray Kaplan, DVM, PhD, Dipl. EVPC, associate professor at the University of Georgia’s College of Veterinary Medicine, indicated that U.S. resistance is highest in Kentucky and Florida, where large populations of high management status (thus frequently dewormed) horses reside. The other contributing factor to parasite resistance is the mobile nature of horses in the modern world