Parasite Control for Today’s Horses
Rethinking parasite control means battling resistance, embracing evidence-based deworming, dispelling long-held myths, and recognizing the role of your veterinarian
by Aimee Elyse Robinson
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Photo: iStock
“So, what is a good all-around dewormer for horses?” asks a veterinarian from the back of the room as the question-and-answer session begins.
Dr. Martin Nielsen, professor of equine clinical sciences at Aarhus University, in Denmark, has just completed a presentation to a room of veterinarians on parasite control for today’s horse. In the session he covered the ever-growing risk of parasite resistance to anthelmintics—dewormers—and the need for individualized deworming.
“I hope my presentation demonstrated this does not exist,” he responds. “There is no such thing as a good all-around dewormer that treats all of the parasites that we want; however, we do still have some dewormers that will get some of the parasites. We need to figure out, A, which parasites we are dealing with, and B, what is the resistance profile? Then C, we can choose the product that will have efficacy, accordingly.”
This single exchange warrants acknowledgment. Today’s parasite control is no longer about good “all-around dewormers,” rotation schedules, and frequent treatments. It requires evidence-based and individualized management to combat widespread anthelmintic resistance.
Martin K. Nielsen, DVM, PhD, DVSc, Dipl. ACVM, Dipl. EVPC,
graduated with his DVM degree from the Royal Veterinary and Agricultural University, in Denmark and his PhD in equine parasitology at University of Copenhagen. In 2011 he joined the M.H. Gluck Equine Research Center at the University of Kentucky, in Lexington, where he served until 2024. He is board-certified in veterinary parasitology with the European Veterinary Parasitology College (EVPC) and with the American College of Veterinary Microbiologists (ACVM). In 2023 he obtained the doctor of veterinary science (DVSc) degree from the University of Copenhagen. Nielsen is chair of the AAEP Parasite Control Subcommittee in the U.S., member of the WAAVP anthelmintic efficacy and anthelmintic resistance guideline committees, member of the CANTER Core Steering Committee in the U.K., and co-editor-in-chief for Veterinary Parasitology. Nielsen currently serves as professor of equine clinical veterinary medicine at Aarhus University, in Denmark.
A Parasite Primer
More than 150 species of parasites can infect horses. Some of the most common include:
Small strongyles (cyathostomins). Most common and often does not present clinical signs; however, highly concerning acute larval cyathostominosis can develop in severe cases. This happens when strongyle larvae emerge from the intestinal walls, where they have been encysted and dormant, and cause a major inflammatory response.
Roundworms (ascarids). The “childhood disease” common in foals, ascarids can cause intestinal impaction in severe cases.
Tapeworms (Anoplocephala perfoliata). Heavy burdens of tapeworms can cause ileocecal colic.
Large strongyles (Strongylus vulgaris). While generally uncommon, large strongyle infections can cause colic and blood clots.
Pinworms (Oxyuris equi). While not a disease threat, they do present discomfort because eggs produced near the anus cause itching and tail rubbing.
Threadworms (Strongyloides westeri). Threadworms can cause dermatitis.
Bots (Gasterophilus spp). This is an external parasite with a life cycle that takes it internal. Botflies produce the bothersome yellow eggs you see on your horse’s coat. Hatched larvae cause discomfort in the horse’s mouth and travel to their gut.
Photo: Shelley Paulson
The Problem: Widespread Resistance
A Historical View
Far predating the cordless phone and cable television, the much-outdated practice of rotational (or interval) deworming goes back to the 1960s. In essence, the practice involved treating with one product then switching classes (types) of dewormers and treating with another in two to three months’ time.
“Rotation is a strategy of the past; it has no place in (today’s) parasite control,” says Nielsen. “Rotational deworming is closing our eyes, holding our breath, and hoping everything will be fine. It is not a very sophisticated approach and is pushing resistance even more over the edge.”
Anthelmintic Resistance
Interval deworming overexposed parasite populations and quickly accelerated anthelmintic resistance risk. As owners dewormed horses more frequently, the practice controlled parasites more susceptible to the products, but resistant worms survived and reproduced. This process repeated itself with each treatment. Now, after decades of frequent dewormer use, the resistant populations often make up the majority.
“There is so much drug resistance, no horse owner can feel confident,” says , professor of parasitology and senior associate dean at St. George’s University’s School of Veterinary Medicine. “Everyone assumes their neighbor’s farm is the one with resistance, not theirs, but I can tell you with 100% certainty that’s not true.”
Reports timestamp resistance dating as far back as the 1960s, just shortly after the introduction of the first modern anthelmintics phenothiazine and thiabendazole.
“Anthelmintic resistance is reported in equine nematodes with increasing frequency in recent years, and no new anthelmintic classes have been introduced during the past 40 years,” says Nielsen.
“What we know from data that we have collected years ago already, and from smaller studies done since, is that we have to talk about drug resistance on an individual parasite-by-parasite species basis,” says Kaplan. “Resistance develops at different rates to different drugs for each parasite, so if a drug doesn’t work against one type of parasite because it has become resistant, it has no reflection on whether it will not work on a different parasite. Each of the major parasites are different in multiple ways, and which drugs will work best against each parasite is different.”
, equine parasitologist and associate professor in equine science at the University of Limerick agrees, saying, “The question is not whether we can prevent anthelmintic resistance to the currently available equine anthelmintics but, rather, how long we can delay it.”
Horse owners need to understand resistance levels vary by deworming class and internal parasite. Reviewing the table below can help clarify dewormer classes, label indications, and resistance status—all key pieces of information to consider for your horse.
Ray M. Kaplan, DVM, PhD, Dipl. ACVIM, EVPC
is a professor of parasitology in the Department Pathobiology in the School of Veterinary Medicine at St. George’s University, in Grenada, West Indies, where he also serves as the senior associate dean. Previously, Kaplan was a professor of parasitology in the University of Georgia College of Veterinary Medicine’s Department of Infectious Diseases, in Athens. He is an internationally recognized expert in parasite drug resistance and equine parasite control and serves on the AAEP’s Parasite Control Guidelines subcommittee.
Nikki Walshe, MVB, PhD, CertAVP,
is an associate professor in equine science at the University of Limerick, in Ireland, specializing in equine parasitology and sustainable parasite‑control practices. In her work she focuses on practical challenges such as parasite disease, anthelmintic resistance, and evidence‑based herd‑health management. Alongside her academic role, she works as a consultant in parasitology herd health. She previously developed an equine herd‑health service with a parasitology focus at University College Dublin, also in Ireland. She has contributed to national veterinary training through the TASAH (Targeted Advisory Service on Animal Health) program, delivered under the Department of Agriculture, Food and the Marine (DAFM) support schemes. Walshe also serves on several advisory bodies, including ESCCAP UK & Ireland, the ITBA veterinary committee, and the Veterinary Advisory Council for World Horse Welfare, and she is chair of the EPIC expert group in equine parasitology.
Equine Dewormer Classes and Resistance Status
| Benzimidazoles | Pyrimidines | Macrocyclic lactones | Quinolone pyrazines | |
| First Introduced | 1960s | 1970s | 1980s | 1990s |
| Form | Paste, liquid, or pellets | Paste, liquid, or pellets | Paste or gel | Paste |
| Members of this class | Fenbendazole, oxibendazole | Pyrantel tartrate, pamoate (embonate) | Ivermectin, moxidectin | Praziquantel |
| Mode of action | Destructs cell infrastructure | Spastic paralysis | Flaccid paralysis | Increases cell permeability |
| Labeled to treat | Strongyles, ascarids, pinworms, threadworms | Strongyles, ascarids, pinworms, threadworms, tapeworms | Strongyles, ascarids, pinworms, threadworms, bots | Tapeworms |
| Resistance status | Strongyles (very common), ascarids (reported) | Strongyles (very common), ascarids and tapeworms (reported) | Ascarids and pinworms (very common), strongyles (confirmed) | Tapeworms (reported) |
Once anthelmintic resistance has developed within a parasite population, that population will be resistant forever. In one study, researchers (Lyons et al., 2007) demonstrated resistance was still present in small strongyles 22 years after all deworming treatment had ceased. Without any new dewormer classes on the horizon, and resistance becoming an ever-growing concern, it becomes more crucial that veterinarians and horse owners implement responsible and evidence-based parasite control programs.
Closing on resistance, Walshe notes resistance on farm, doesn’t guarantee you will have parasitic disease in your horses. “If you can keep infection levels low through pasture management, ruminants (unsusceptible to most aforementioned parasites), and fecal decontamination, you shouldn’t have parasite disease—even if the wormer isn’t working,” she says.
With a look at the problem, let’s refer to the solution brought forward in the American Association of Equine Practitioners’ (AAEP) Internal Parasite Control Guidelines.
STORY CONTINUES BELOW
Photo: Thinkstock.com
The Solution: Evidence-based Parasite Control, and Pasture Management
AAEP’s Internal Parasite Control Guidelines
Parasite control “has become more complicated, for the better,” explains Nielsen. “It’s not a matter of treat every six or eight weeks. Horses don’t need to be treated that often, first of all, and second of all, no horse has the same parasites, parasite burdens, and the same treatment needs.”
With a swift shift away from rotational deworming strategies of yesteryear, what does effective parasite control look like for today’s horse?
Helping horse owners navigate parasite control, the AAEP continues to publish and update its Internal Parasite Control Guidelines. Nielsen chaired and Kaplan was a member of the parasite control task force that produced the Guidelines.
Here are key takeaways to be aware of when considering your horse’s parasite control program and building a proper protocol alongside your veterinarian.
1. Discontinue blindly rotating anthelmintic classes in a rotational or interval deworming program.
2. Understand the goal is not to eliminate all parasites from all horses; this is not possible. “No anthelmintic will eliminate all parasitic stages from a horse,” the Guideline authors share.
Scientists have developed region-specific equine parasite control guidelines for horse owners and veterinarians across the globe. In general, recommendations focus on an evidence-based program versus blanket deworming and highlight the importance of testing for resistance.
Nielsen describes a difference between highest priority parasites: In the United States the most important parasite category in adult horses is tapeworms. Most horses with a few tapeworms show no ill effects; however, when worm burdens get large, then tapeworms can cause colic in horses. “Tapeworms aggregate in a specific point in the GI tract between the small intestine and large intestine, and just inside the large intestine is the cecum,” explains Nielsen. “This is where tapeworms want to hang out.”
As a result, the horse can get ulcers in the mucosal wall of the cecum, which leads to inflammation and, ultimately, colic. “When burdens are really large, it’s a little crowded in here, but there’s room next door,” explains Nielsen. “Next door is the lowest part of the small intestine, the ileum, and sometimes we see tapeworms hang out in there, which is a much narrower passage and, mechanically, they cause impaction.”
Veterinarians in the U.K., the Netherlands, Germany, and France, however, would place strongyles ahead of tapeworms. While in the United States it’s rare to see issues with strongyles, in these European countries small strongyles more commonly cause clinical signs such as diarrhea, dehydration, and reduced weight and protein loss. “Because of protein loss, edema (fluid swelling) can form around the chest,” says Nielsen. “This creates an osmotic effect because protein should have stayed in the blood and now the blood is missing the protein particles to retain the water in the blood.”
Coast to coast and across the pond, ascarids remain a common risk for all foals. Simply, it’s “the childhood disease they have to go through,” Nielsen says. The risk for foals peaks at 5 months of age. The parasites live in the small intestine and can cause impaction because the worms are quite large. Luckily, though, most foals don’t experience resulting problems, says Nielsen. “When working in Kentucky, I would work with local vets and ask, ‘How many ascarid impactions do you see in a year?’ It’s remarkable that it’s very rare. With hundreds of foals hitting the ground every year, there would typically be only two or three recorded cases with all the vets in the area.”
With this very high-level look at parasite control priorities across the globe, let’s look toward environmental management strategies to complement a strategic parasite control program.
Photo: Adobe Stock
Pasture Management: The Roughs and the Lawns
“Parasites are a pasture problem that results in a horse disease,” says Walshe.
When you look out over your pasture, what do you see? Kaplan advised taking a “holistic view” of parasite control, beginning with pasture management and “the roughs and the lawns.” The “roughs” are the areas where a horse primarily poops, and “the lawns” are the grazing ground away from the poop. Horses have a natural aversion to grazing near manure, he says, serving as a natural means to control parasites because the roughs contain the most parasite eggs and larvae.
“In a healthy pasture that is not overgrazed, you should see tall grass in the roughs. If you don’t see these roughs, it means that the horses are grazing close to the poop,” explains Kaplan. Horses only do this when they’re hungry and forage in the lawns doesn’t meet their nutritional needs.
Watch for signs of overgrazing, which increases horses’ risk of parasite burden:
- No roughs near horses’ natural pooping destination
- Absence of tall grass along fence rows
- Grass grazed down to the nub
- Three-quarters of all the parasites on pasture reside in the lowest 2 inches of grass.
“If there are no easily recognizable areas of roughs, then horses will likely be ingesting huge numbers of parasites,” says Kaplan. “Parasites don’t have legs; they cannot move very far. Studies have shown that the roughs have approximately 15 times more parasites than the lawns do. Horses naturally control their exposure to parasites with their natural grazing behavior, but when the natural environment is disrupted by overstocking, you are disturbing their natural behaviors.”
“… roughs have approximately 15 times more parasites than the lawns do.”
You want to see the following in a lush, green pasture:
- Grass strands at least 4 inches or higher in the lawns
- Grazing pasture to below 3 inches damages the root structure, slowing regrowth
- Grazing below 2 inches increases the consumption of parasites
- Roughs: again, where the grass grows taller than the lawns
“People need to move away from asking, ‘What wormer do I give?’ and ask instead, ‘What can I do about the parasites on my pasture?’” says Walshe.
Best practices to reduce parasites on pasture can include regular paddock cleaning, composting manure (and never spreading noncomposted manure across the fields), implementing rotational grazing, and rotating different species (cattle, sheep, or goats) on the fields.
Photo: Adobe Stock
Misconceptions About Equine Parasite Control
Debunking the Myths
Given the opportunity to speak with equine parasitology experts Nielsen, Kaplan, and Walshe, we inquired about the top misconceptions they have heard. As you review the below, given the information shared so far, can you think of any other misconceptions you have encountered?
Myth 1: Deworming your horse accomplishes parasite control.
“Deworming itself is not going to accomplish the goal of good parasite control in horses because drug resistance is common and different parasites will be susceptible to different drugs,” says Kaplan. “Thus, it requires testing of dewormers with the FECRT and having a more comprehensive perspective of parasites, including how transmission occurs in the pasture environment. It’s key to note multiple parasites need to be targeted, but they each have their own cycles and the optimum time for treatment is, thus, different. Additionally, different drugs kill different parasites. It is rare that a product will kill the full range of parasites listed on the label, thus, it is increasingly rare that a single product will kill all the of the parasites that should be targeted in a worm control program. Bottom line is that there is more complexity in achieving good parasite control than most horse owners appreciate.”
Myth 2: Horses shouldn’t have any parasites.
“Our goal isn’t to get rid of parasites,” says Walshe. “Our goal is to prevent parasite disease. Parasites are meant to live with their horse buddies, and the horse is designed to live with gastrointestinal nematodes. They evolve together; they are part of their natural microbiota. We know from our own work that removing parasites (entirely) can cause a disturbance in the microbiota, and it can also trigger an inflammatory response from the horse’s immune system.”
Myth 3: You can tell a horse’s parasite burden by weight and coat condition.
“In a relatively healthy group of horses, you simply cannot pick out which horses have more parasites and which have less by visual observation,” says Kaplan. “By the time you are seeing symptoms such as weight loss and poor hair coat, either there is a severe parasite problem or there is a nonparasitic disease process going on.”
Myth 4: Deworm at first frost.
“Good parasite control does not depend on first frost,” says Nielsen. “There are external stages of eggs and larvae which can and do survive freezing temperatures.”
Myth 5: Horses should be stalled several days post-deworming.
“The horse has been shedding parasite eggs for months before treatment,” says Nielsen. “Yes, there will be some worms present in the feces following deworming, but they are dead, which is why they’re coming out. Plus, they’re not at the infective stage. The infective stage are third-stage larvae.”
Myth 6: High egg count means high worm burden.
Nielsen says: “There is no direct correlation, but there are several plausible explanations for this:
- The host immune system can suppress egg production.
- As parasites age, they become reproductively inactive and no longer make eggs.
- There are many different species producing the same egg type, but these different species each make different numbers of eggs.
- Parasites affect each other. When worm burdens are large, each worm produces fewer eggs, but when burdens are small, they increase their egg production. So, we are not doing egg counts to determine the worm burden. We’re doing them first and foremost for the most important thing—resistance testing. Everyone needs to do it.”
STORY CONTINUES BELOW
Photo: Adobe Stock
Working With Your Veterinarian
Involving Your Veterinarian
In the United States, horse owners can purchase equine dewormers without a prescription. They can find them at the local feed store or online retailers; however, this does not reduce the importance of having veterinary oversight of effective and responsible product use.
“You need to know which product you can buy that actually works,” says Nielsen. “You need to know what parasites you have in your horses, and there’s no way around having testing done, and that’s where you need to work with your veterinarian—for good advice and a good strategy for your horse.”
Fecal egg counts and FECRTs guide sound, individualized parasite management. Evaluating product efficacy and resistance levels remain critical because parasite resistance to anthelmintics occurs in nearly all managed horse populations, with largely susceptible parasites persisting only in feral herds.
“In managed horses, there is always an element of drug-resistant parasites, and you need to figure out what the situation is in your horses,” says Nielsen.
Your Veterinarian’s Role
One of the most important changes in Ireland was when anthelmintics became prescription‑only in December 2025. “This created a moment of reckoning, a legislative gateway for veterinary involvement,” says Walshe, who helped deliver a national training initiative on parasite control through The Department of Agriculture Food and Marine (DAFM). Walshe says she hopes horse owners understand the difference between deworming and parasite control: “If you just want a worming program, go get it off a calendar. But if you want (an effective) parasite‑control program, then yes—you should be working with your vet.”
When Walshe works with any horse farm, she focuses on three biological control points, including:
- Pasture contamination
- The horse’s immunity (age, health, stress, and nutrition)
- System-level management practices
“The first two things you should be looking at on farm are pasture management and stock management,” says Walshe. “Only then do you ask what parasite control they use, and after that, diagnostics.”
She shared a success story from a practice client’s stud farm, illustrating how your veterinarian might be able to identify how management—rather than medication—can be the root cause. Such was the case in the farm’s nursery paddocks.
On breeding farms the challenge often lies in the sheer number of mares and foals moving through the same limited pasture. In this farm’s case, up to 30 mares and foals would cycle through the same area over a season. The continuous turnover meant pasture contamination climbed rapidly, and parasite pressure increased. Thus, targeted fecal contamination of these specific areas can have an impactful result.
A different issue emerged on another stud farm that managed its breeding stock as one large herd unit. The early foals were fine, but the later foals were landing onto hugely contaminated pasture and suffering ?from ascarid infection?, says Walshe. The solution in this case was quite simple: By breaking the herd into smaller groups and introducing cattle grazing between pasture rotations, farm managers were able to interrupt the parasite life cycle and dramatically reduce the parasite burden in the vulnerable late foals.
As these examples show, involving your veterinarian puts you on the right track for successful parasite management.
Photo: Adobe Stock
Advocating for Change
Change takes time, education, and repetition. “We meet people who are stuck in the past who stick to old treatment programs,” says Nielsen.
So, how do you navigate conversations in the barn to spur on more responsible deworming strategies?
Kaplan provided guidance to help others advocate for the current guidelines and against the interval strategies of yesteryear.
- Frame up “the massive amount of evidence” of drug resistance.
- Encourage horse owners and farm managers to have their veterinarians perform an FECRT to see how effective their program is.
- After seeing the results, which almost certainly will indicate they have resistance to one or more drugs, they might be more accepting to changing from a rotational or interval program. Realize that, until then, they will likely “live with the fallacy that resistance must be a problem somewhere else, not a problem here,” says Kaplan.
With resistance on the rise, the future of parasite control rests on evidence-based parasite control strategies and diligent pasture management. In partnership with your veterinarian, apply these insights to support sound parasite control and long-term horse health.
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Credits
Aimee Elyse Robinson has been starstruck with horses since being gifted a pony named Sparkler when she was 3 years old. With a journalism degree from Oklahoma State University, her magazine-editorial and marketing career centers around the animal health industry. From writing educational pieces that help horse owners to new product launches including vaccines and more, in every aspect of her career she has been driven to help the horses, livestock, and pets we love.
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