Is Your Deworming Schedule Effective?

A study investigates if horseowners are overworming and under protecting

As conscientious horse owners, we all look to increase our horse’s health by decreasing the number of intestinal parasites. The larvae of these parasites cause damage to the large intestine of the horse and can result in diseases as mild as subclinical inflammatory bowel disease to life-threatening diarrhea and weight loss. Most caregivers have been taught that the best way to eliminate worms is through a regular six- or eight-week rotation of several select deworming agents or anthelmintics. Ongoing studies, however, indicate that this philosophy may not, in fact, be the correct one. 

Meagan A. Smith, DVM, Diplomate of the American Board of Veterinary Practitioners, and member of the William Boucher Field Service team at New Bolton Center (part of the University of Pennsylvania’s School of Veterinary Medicine) has been investigating the effectiveness of certain dewormers against small strongyles (also known as cyathostomes), the most common intestinal parasite now found in horses. Her research is the first of its kind in the Mid-Atlantic states and the results could change thinking about deworming practices in this area.

Dr. Smith used a fecal egg-count reduction test on patients she visited. She collected a sample of fresh manure, immediately administered a dewormer and then collected a second fecal sample 10 to 14 days later. The number of parasite eggs was counted in each set of fecal samples and the percentage reduction in eggs after treatment was determined. The five commercially available dewormers used in this study were randomly assigned to each horse. Just over 1,000 horses were sampled during the duration of the study, and roughly 25 percent of them had high enough fecal egg counts to be included in the testing. 

“My results reflect the findings of similar studies in other parts of the country and world,” says Dr. Smith. “Small strongyles appear to be becoming resistant to some of the dewormers we are using. Certain dewormers are, at most, minimally effective at reducing the parasite load.” This is of concern, she says, because eventually these worms could become resistant to everything that we have to kill them. And it may very well be the result of our good intentions: The practice of exposing the parasites to different chemicals several times a year may have actually contributed to the resistance.

And those who have chosen to go the route of daily dosing may also have problems. Dr. Smith has observed no difference in fecal egg counts, for example, on farms where a daily dewormer is used and on those where it has not. She has even seen that horses on a daily dewormer have host parasites that are 100-percent resistant to that daily dewormer. 

“Veterinarians have been getting the word out to horses owners that we need to change our philosophy,” says Dr. Smith. “It’s time to consider deworming less frequently and only with dewormers that are effective.” She adds that when there is a need to administer a dewormer, there is no reason to be concerned about its effect on your horse’s performance. “There is no scientific evidence to support the notion that deworming your horse will have a negative impact on your horse’s ability to perform.” 

Efficacy of dewormers can differ from farm to farm, and the frequency of deworming necessary will vary from horse to horse. The best way to know how often to deworm your horse starts with a fecal egg count, a simple test that your veterinarian can do. Talk to your veterinarian to develop a program that works for your horse and your farm.


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