When the American Association of Equine Practitioners updated deworming guidelines in June 2024, it reflected a significant departure from the rotational deworming that has been standard practice for many horse owners.
In fact, the revised guidelines state that horse owners should “discontinue deworming all horses with fixed intervals year-round (e.g., every two months), and stop blindly rotating anthelmintic classes.” Instead, the targeted selection and use of dewormers should be made based on the use of fecal egg count tests, with some horses only requiring treatment once or twice a year. Experts say that this new approach to parasite management is necessary due to a rising incidence of anthelmintic resistance across all drug classes.
“Resistance isn’t just something that magically goes away, and we’ve seen this with both antibiotics and dewormers,” explained Ashley Steuer, DVM, Ph.D., an equine parasitologist and practitioner at Sheridan Animal Hospital in Sheridan, Michigan. “In one of the longest studies I know of, horses were not dewormed for nearly 20 years. Then they went back to check for resistance, and it was still there; it’s not reverting back like we had hoped it would.”
“In one of the longest studies I know of, horses were not dewormed for nearly 20 years. Then they went back to check for resistance, and it was still there; it’s not reverting back like we had hoped it would.”
Dr. Ashley Steuer
Even more troubling, there are currently no new equine anthelmintics in development—meaning that horse owners and caretakers must carefully manage the efficacy of the few drug classes we currently have available.
“It is one of those things we all kind of thought was not going to be a real problem in our lifetime,” said Julia Wilkinson, DVM, IVCA, M.Sc, clinical assistant professor at the Cummings School of Veterinary Medicine at Tufts University in North Grafton, Massachusetts. “But the studies done in recent years have shown there is already widespread resistance in some of our most common parasites, to some of our most common dewormers. We are already starting to see this as a problem.”
Fortunately, experts do see a path forward, which should prolong the efficacy of current anthelmintics, while also promoting herd health. However, it will require horsemen to rethink their current best practices when it comes to parasite management—and resist the urge to simply grab a box of the closest dewormer from the local feed supply.
“One size doesn’t fit all anymore,” said Steuer. “Knowing what we are treating, and why, and what the resistance is to that, is the most important message.”
It is also important to note that the guidelines being discussed in this article pertain to healthy adult equines over the age of 2; consult your veterinarian for guidance regarding the management of younger animals, or those dealing with significant parasite-related illness.
Understanding Resistance
The term “resistance,” when applied to the management of internal parasites in horses, can be interpreted in two ways.
First, there are some hosts (in this case, the individual horse itself) that have a higher degree of resistance to parasites; these animals show limited or no evidence of parasite-related disease and carry a lower worm burden. But the more concerning—and perhaps more prevalent—form of resistance is found in the parasite itself. Simply put, a “resistant parasite” is one that can survive exposure to a drug that formerly killed other members of that species.
For many years, horse people were told to religiously deworm all animals on their farm every six to eight weeks on the same day, with the same product. On the next cycle, they would “rotate” to a new class of dewormer. Unfortunately, this practice had the unintended outcome of increasing resistance across drug classes, because each farm’s parasite population was being systematically exposed to each category of medication. (See “Classes of Equine Dewormers.”)
“Every time you deworm, you will have some parasites who survive the deworming,” said Steuer. “It might start off with one worm; the next time, it might be two worms, then it is four. It builds, and it takes a long time, depending on which parasite we’re talking about and its life cycle.
“The individuals who have those genes that allow them to survive that treatment will pass them along to the next generation,” she continued. “So when we are talking about these resistant worms, it’s because we’ve selected for them.”
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As a parasite population develops resistance to a particular drug or drugs, their numbers will increase within an individual horse, and owners may see an increase in parasite-related disease. For veterinarians and caretakers of small ruminant species, such as sheep and goats, widespread anthelmintic resistance is already a reality, and animals are now dying from parasite infections that once could have been prevented.
“This is the long-term ramification,” said Wilkinson. “As our equine dewormers stop working, the potential for us to lose horses to parasites becomes a very real possibility.”
When it comes to parasite-related disease in the modern horse, experts are most concerned about three families of parasites: strongyles, both large and small (also known as bloodworms or cyathostomins), parascaris species (roundworms), and tapeworms. But when the concept of rotational deworming was introduced, it was with the goal of disrupting the life cycle of the most common parasite at the time, S.vulgaris, one of the large bloodworms. To that end, rotational deworming was successful; in the U.S., it is extremely uncommon to see disease caused by this parasite in a well-managed horse.
“But then small strongyles became the biggest issue,” said Steuer. “They took over, and they produce eggs in as little as six weeks. What we thought was insignificant became much more of a problem, and there are reports in the U.S. of all three drug classes having resistance to these parasites.
“When ivermectin came out in the 1970s, it was considered a miracle drug—that we could do all of these parasites with one product,” she continued. “We all want a one-and-done treatment. It makes it simple, and so we created these broad-spectrum products. It’s now expected, but it didn’t used to be that way, and now we have to shift our thinking back.”
Managing Your Herd
To combat the increased incidence of parasite resistance to dewormers, parasitologists have re-framed their recommendations when it comes to best practices in equine parasite management.
The first step is to evaluate the “shedding” status of each animal on the farm through a fecal egg count test. If test results reveal that deworming is indicated, barn managers should administer a class of dewormer specific to the type of parasite eggs seen in the sample. Finally, treated animals should be tested again two weeks later; if the number of eggs per gram has reduced by more than 95 percent, the deworming is considered successful.
“The goal of parasite management is two phase: to reduce the risk of the individual horse getting disease from parasites, and to slow down resistance,” said Wilkinson. “That’s where a fecal egg count comes into play, because it has been shown time and time again that there is a big individual variation in horses as far as how many eggs they shed in their manure.”
The egg count measures the number of eggs passed per gram in a horse’s feces.
“It is not a reflection on how many parasites the horse has, which is a big misconception,” said Steuer. “But what it does tell us is how many parasites may potentially be on the pasture.”
All healthy adult horses should be dewormed a minimum of once or twice annually, regardless of shedding status, with a macrocyclic lactone; at least one of those treatments, typically done in the fall, should also include praziquantel to target tapeworms. But beyond this baseline, additional treatment will be determined by the results of each horse’s fecal egg count. Those individuals determined to be high shedders (greater than 500 eggs per gram) will require more frequent treatment with a product specific to the types of eggs found in their sample; your veterinarian may recommend these animals be dewormed three to four times per year.
“If we can target those horses who are really high shedders, then we can control the egg burden in our fields and pastures,” said Wilkinson. “By not deworming everybody on a six- to eight-week basis, we should slow down resistance, because there would always be a group of parasites that didn’t see or get exposure to that dewormer.”
When several high shedders share a pasture, it can increase the overall egg count on the pasture itself, bumping up “infection pressure,” or the potential for other animals sharing that space to ingest the eggs. Selectively treating those high shedders with an appropriate product, and then checking its efficacy, can help reduce the risk of infection.
A fecal egg count reduction test, which compares the numbers calculated in a pre-treatment fecal sample with one collected 14 days later, can verify that the selected product was effective.
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Ideally, the second sample will show a reduction of at least 95 percent in the number of eggs per gram; any value less than that is suggestive of resistance. If the second sample doesn’t show adequate reduction, it is important to verify that the specific tube or tubes of dewormer used were unexpired, and both stored and administered correctly. Once these variables have been excluded, an additional sample should be tested; if the result is repeated, resistance is assumed.
“We would have to worry that your herd of horses, and the parasite burden, already has some resistance to that particular dewormer,” said Wilkinson.
In such a case, the horse should be treated with a product from a different class, and another fecal egg count reduction test performed.
“If we haven’t rotationally dewormed, we have a class to fall back on when one fails,” said Steuer. “Our goal is to help manage those parasites, to minimize the risk of parasitic disease; we don’t want our horses to be sick. But we want to do so smartly, and that includes trying to delay, as much as we can, the resistance from happening.”
Horse people need to understand that no parasite management program is intended or expected to wholly eliminate internal parasites from the horse.
“That is a difficult concept for the equine owner to become comfortable with—the thought of your horse having some parasites inside of it,” said Wilkinson. “But we need to become comfortable with the idea that a low level is actually OK. As long as they are low shedders, and we deworm them once or twice a year, we are minimizing the risk of them getting a disease from it.”
The Take-Home Message
Although it may be easier (and cheaper) in the short run to simply grab any box of dewormer and be done, experts caution that doing so is not in the best interest of the horse—now, or in the future.
“There needs to be a mind shift and change among equine owners as a whole,” said Wilkinson. “If we keep doing it that way, in five to 10 years, we run the real risk of having those dewormers not work, and potentially losing horses.”
“We need owners to know there is no broad-spectrum deworming; no dewormer will eliminate all stages of all parasites,” added Steuer. “And the only way we can tell if a dewormer is effective is to do a fecal egg count reduction test.”
And in the still (currently) rare circumstance that an individual horse’s parasite load shows resistance to more than one class of dewormer, it is critical to work closely with both your veterinarian and an equine parasitologist to devise a treatment strategy.
“It is important for horse owners to realize the guidelines probably will continue to change, as we are starting to see how the horse population deals with global warming, ease of transport, and the start of resistance to our dewormers,” said Wilkinson. “It is super important to work with your veterinarian, in your area, because geography and weather plays a role in the parasite life cycle.
“It’s incumbent upon all of us as horse owners to not just do what is easiest or cheapest for our horse, but to do what is going to be the best for the entire horse population long term,” she continued. “Fecals are completely non-invasive to the horse, and they are relatively cost-effective.”
This article originally appeared in the May 2025 issue of The Chronicle of the Horse. You can subscribe and get online access to a digital version and then enjoy a year of The Chronicle of the Horse. If you’re just following COTH online, you’re missing so much great unique content. Each print issue of the Chronicle is full of in-depth competition news, fascinating features, probing looks at issues within the sports of hunter/jumper, eventing and dressage, and stunning photography.
