Recent Study Indicates Possible Parasite Drug Resistance

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.

"Horses move around, and they take their parasites with them. That's how parasites spread," said Rossano.

And so does drug resistance. There are some drugs no longer on the market because they eventually stopped working on equine parasites. The most commonly-resistant parasites appear to be small strongyles, but ascarids are now widely resistant to ivermectin. Rossano cautioned that the applications of her recent research are limited given the small sample size of horses.

"It's a small study—-I don't want to draw broad conclusions from it," she said.

She does not recommend that owners immediately rework deworming programs to exclude these drugs, but she does suggest they test their horses’ fecal egg counts periodically through the year. Farm managers should consider testing all horses once to identify which ones tend to shed the greatest number of eggs and retest them two weeks after deworming to see whether the drug is effective on their farm.

If a product does not reduce the egg counts by at least 80%, it isn't very effective. If a drug does not work, there is no reason to continue using it. Performing follow-up tests on some of the treated horses will also provide information on the egg reappearance time for a dewormer, or the time it takes for the egg counts to rebound. This provides information on how frequently the high egg shedding horses will need to be treated. There might be some variability in prevalence of different parasite species and drug resistance across regions, or even operations, which is why testing provides farm managers with the most accurate information.

Rossano also cautioned that horse owners should target the types of parasites most likely to affect their horses.

"By testing, you will know what parasites are present on your operation, and from there, you can choose an appropriate dewormer," she said.

Natalie Voss is a recent graduate in equine science and management and an equine communications intern for the UK Equine Initiative.


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