Equine Parasite Control:
Deworming and Beyond
What scientists are learning about equine parasite resistance to dewormers and how to curb it.
By Christa Lesté-Lasserre, MA
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Peaceful Cohabitation
(When Controlled)
PHOTO: iStock.com
Dr. Hubertus Hertzberg
Hubertus Hertzberg, PhD, is a researcher at Switzerland’s University of Zurich Vetsuisse Faculty Institute of Parasitology and head of parasite monitoring at Health Balance, a private holistic animal management and veterinary practice, in nearby Niederuzwil.
Dr. Martin Nielsen
Martin Nielsen, DVM, PhD, DVSc, Dipl. ACVM, Dipl. EVPC, was the Schlaikjer Professor in Equine Infectious Disease at the M.H. Gluck Equine Research Center at the University of Kentucky until 2024. In late 2024 he started working as professor of equine clinical sciences at Aarhus University in Denmark.
Internal Parasites: A Visual Guide
Bloodworms
Photo Credit: Dr. Lisa Edwards
Tapeworms
Photo Credit: Dr. Martin Nielsen
Ascarids
Photo Credit: Dr. Martin Nielsen
Summer Sores
Photo Credit: Dr. Martin Nielsen
Helminths (worms) and other intestinal parasites:
- Cyathostomins, aka small strongyles. These are the most common helminth in equids today. Most horses don’t show clinical signs of infection, but if they develop acute larval cyathostominosis (a mass eruption of encysted larvae that causes an inflammatory response in the horse’s gastrointestinal tract) the fatality rate can reach 50%, though this is extremely rare.
- Tapeworms (Anoplocephala perfoliata). Heavy infections are associated with ileocecal colic.
- Ascarids, aka roundworms (Parascaris spp). These parasites are especially risky for young foals and can cause intestinal impaction.
- Pinworms (Oxyuris equi). Unassociated with serious disease, pinworms maintain their life cycle in the colon. Females lay eggs around the anus, causing itching.
- Bots (Gasterophilus spp). The larval stage of botflies, bots begin as yellow eggs on horses’ hair coats. The larvae hatch there due to warmth, moisture, and carbon dioxide, and the horses lick the larvae, which reside (and cause discomfort) in oral tissues, then travel to the stomach.
- Skin-living larvae (Habronema and Draschia). These species are stomach parasites but the larvae end up in the wrong place. “They are not supposed to end up in wounds or mucosal membranes, and they will not be able to complete their life cycles from there,” says Nielsen. “But if this happens, they will cause a foreign-body-type reaction known as summer sores.
- Neck threadworm (Onchocerca cervicalis). Adult worms can live for 15-20 years within the horse and are not susceptible to anthelmintic treatment. The females live along the nuchal ligament in the neck and release microfilariae (larval stages) into the subcutaneous tissues. These microfilariae can cause inflammation of the skin (dermatitis) in some horses.
- Bloodworms (Strongylus vulgaris). Mostly eradicated in domestic horses, migrating bloodworm larvae can cause dangerous blood clots, artery damage, and colic.
STORY CONTINUES BELOW
Resistance:
Generations of Survivors
PHOTO: The Horse
"Drug resistance is a natural biological consequence of incidentally selecting for a few individuals in these populations that happen to have the genes that allow them to survive."
“No method is 100% effective in killing all the parasites,” so there will always be survivors, says Hertzberg.
So, early parasites that survived a round of treatment probably had a natural level of resistance to it. Then, in a classic Darwinian case of survival of the fittest, they passed down that resistance through their genes to their offspring. Treatment wiped out the “unfit” worms, leaving those with the genes for resistance to reproduce, creating entire families of resistant helminths, Nielsen explains.
“Drug resistance is a natural biological consequence of incidentally selecting for a few individuals in these populations that happen to have the genes that allow them to survive,” he says.
Once parasites develop resistance, it remains, generation after generation. “It’s not like with antibiotics, where bacteria sometimes lose their resistance after several generations,” Nielsen says. “These worms are much more complex organisms, with genomes that are organized like other animals, DNA in the chromosomes, and much more genetic material. So once genes are acquired, they stay for the long haul.”
Anthelmintics (Deworming Drugs)
Anthelmintics are the drugs used to deworm horses. Here’s a guide to those drugs and how they work.
| Drug Class | Benzimidazoles | Pyrimidines | Macrocyclic Lactones | Isoquinoline-Pyrazines |
|---|---|---|---|---|
| Mode of Action | Attack worms’ cellular metabolism, depriving them of usable energy. | Cause paralysis of the worms’ neuromuscular junctions. | Target worms’ nerve and muscle cells, causing paralysis; highly potent at low doses; and also kills external parasites. | Kill only tapeworms. |
| Form | Paste, liquid, or pellets | Paste, liquid, or pellets | Paste or gel | Paste |
| Drug Names | Fenbendazole, oxibendazole, or oxfendazole | Pyrantel salts such as pyrantel pamoate and pyrantel tartrate | Ivermectin, moxidectin, or abamectin | Praziquantel |
| Resistance status | Widespread resistance in small strongyles. Cases of resistance in ascarids. | Widespread resistance in small strongyles. Cases of resistance in ascarids and tapeworms. | Widespread resistance in ascarids and pinworms. Common resistance in small strongyles. | Cases of resistance in tapeworms. |
Curbing Resistance
PHOTO: Adobe Stock
By checking FEC just before and a 14 days after giving anthelmintics, practitioners can calculate the percent reduction, as well as how long it takes for eggs to start showing up in the feces again—both of which indicate levels of resistance.
This can help veterinarians know what works and doesn’t work before making treatment decisions, says Nielsen. He points out that the American Association of Equine Practitioners’ Internal Parasite Control Guidelines and the Controlling ANTiparasitic resistance in Equines Responsibly (CANTER, based in Europe) parasite guidelines outline thresholds for each dewormer against each parasite.
This approach necessitates giving up that traditional rotational anthelmintic treatment program that veterinarians recommended in decades past, says Nielsen. It was useful when developed 50 years ago, aiming to kill bloodworms before they could mature and lay eggs—a cycle occurring approximately every two months. However, today bloodworms are rare in horses. And meanwhile, cyathostomins, roundworms, tapeworms, and pinworms are becoming increasingly more drug resistant.
While resistance can’t be undone, our sources say we can decrease the rate of further development with resistance testing. Here’s how that looks in practice: Hertzberg describes a “check-before-treat system.” The method involves assessing individual horses and whole farms to determine, among other trends, which horses “contaminate” the environment the most by shedding the most parasite eggs. It’s a method Hertzberg has been overseeing in farms across Switzerland for the past decade. The strategy involves veterinarians, stable owners, and horse owners making parasite control decisions based on “portfolios” of each farm’s parasite situation—the parasite load based on fecal egg counts, stable and pasture management approaches, farm type (sport, leisure, breeding), etc. With more than half of Swiss horse farms enrolled entirely or partly in the program, his scientific group has found that fecal egg counts are generally low and that most adult horses do very well with a single annual deworming treatment. “I am not aware of a single horse running into clinical problems due to that strategy,” Hertzberg says. While such a nationwide enrollment strategy doesn’t exist in the United States, owners and farm managers can still work with qualified veterinarians to determine individual treatment protocols based on a custom needs assessment. They should form all such protocols on basic egg counts from fecal samples, Nielsen says.Keeping Bloodworms at Bay
Intensive deworming over the past few decades has led to near elimination of bloodworms (large strongyles), which can cause serious disease in horses (blood clots, artery damage, and colic). Still, we must continue to be vigilant about bloodworms, says Dr. Hubertus Hertzberg, PhD, a researcher at Switzerland’s University of Zurich Vetsuisse Faculty Institute of Parasitology and head of parasite monitoring at Health Balance, a private holistic animal management and veterinary practice, in nearby Niederuzwil.
Reducing deworming frequency—a practice currently recommended due to developing parasite resistance against common dewormers—could lead to their reemergence, he warns. If veterinarians and owners detect bloodworms on a farm, they should suspend the “check before treat” system until they’ve eliminated those worms.
“Before we started our portfolio management program (involving veterinarians, stable owners, and horse owners making parasite control decisions based on “portfolios” of each farm’s parasite situation), a study indicated a bloodworm prevalence of less than 2%, and we have no indication that this situation changed in the last decade,” he says.
A critical step in curbing parasite resistance has been urging owners to simply “let go” of the idea of the parasite-free horse, says Dr. Hubertus Hertzberg. “We’re helping people accept the coexistence of host and parasite, which is the normal situation in all pastured animals,” he says.
STORY CONTINUES BELOW
Husbandry Practices to Manage Equine Internal Parasites
Managing internal parasites goes beyond just deworming. Incorporate the following management practices to protect your horse.
Clean paddocks of manure (Fig. 1)
Labor-intensive but effective, picking up manure daily or weekly lowers the infection pressure on the pasture.
Compost manure (Fig. 2)
Naturally heating your gathered manure by composting it kills ascarid and strongyle eggs, which won’t survive 102°F (40°C) over a few days.
Implement rotational grazing (Fig. 3)
Allow pastures to rest by using sacrifice areas and rotational grazing.
Graze multiple species on your pastures (Fig. 4)
Put cattle, sheep, goats, or even camelids (llamas, alpacas) on your horse pasture, instead of (or with) your horses. These species aren’t susceptible to equine strongyles, or tapeworms, or ascarids, and they’ll eat through grass near where horses have defecated.
Use biological controls (Fig. 5)
A feed-through product containing nematophagous fungi, which eat worms, is harmless to people and the environment. Nielsen notes that we don’t know how effective it is.
Egg Counts:
The Foundation of Parasite Control
PHOTO: The Horse
Key to checking before treating are the fecal analyses described earlier in the story, which can reveal a lot about the types of parasites each horse is hosting, as well as how much he’s contributing to the herd’s general infection risk, says Nielsen. Fecal egg counts (FEC) show how many eggs individual horses shed—which can vary considerably from horse to horse. “Egg counts are the foundation for parasite control,” he says.
Nielsen and others analyzed data consisting of over 58,000 equine strongylid fecal egg counts determined during 2019–2022. They showed that veterinarians consistently performed more fecal egg counts in the spring and fall, and egg counts tended to be higher during those times as well. Mean egg counts were generally lower in the western region of the U.S. Overall, 18% of horses contributed 80% of the total strongylid egg output across the study.
Adult horses with fewer than 200 strongyle eggs per gram (epg) of feces are considered “low shedders.” “High shedders” have loads of more than 500 epg, and “moderate shedders” fall between. However, horses in the latter categories don’t necessarily have proportional parasite burdens, Nielsen adds. “The fecal egg counts do not say anything about the intensity of infection,” he says. “They identify the high shedders, which is a different matter. ”
A horse that’s a high shedder contaminates the environment with parasite eggs more readily than other horses. These horses need treatment so they expose the herd to fewer eggs and larvae on pasture—not to improve their own individual health, says Nielsen. In other words, it’s a concept of community.
“It’s often only 15-20% of adult horses that shed about 80% of the eggs in the pasture,” says Nielsen, adding that horses usually remain consistent in their shedding rates throughout their lifetimes. In Hertzberg’s population, only 8% of samples show egg counts above 200 epg.
We can’t recognize these high-shedding horses by simply looking at them, our sources say. They’re not necessarily the weakest or thinnest, the least or most dominant, or the oldest or youngest. In fact, genetic predisposition for immunity might play a role, says Dr. Sara Ringmark. In her group’s study of 12 free-ranging Gotland Russ stallions, they noted better parasite resistance in three horses, two of which had the same sire. This seems to go along with an earlier finding by Canadian researchers studying feral horses on Sable Island, in Nova Scotia, that parasite resistance can be heritable, she says. Ringmark hopes to research the topic further.
Because horses can live healthy lives with controlled small strongyle and other parasite burdens, our sources agree it’s better to just treat them less—in a targeted way, based on FEC testing of all horses. While that might seem counterintuitive, science is revealing clear advantages: It keeps us from reducing the parasite load to only the parasites that resist anthelmintics. By allowing the light shedders to continue shedding, we’re letting the parasites maintain generations of worms that aren’t developing resistance because they haven’t been exposed to the drugs as much.
“The AAEP guidelines define a foundation of treatments that should be considered for all horses,” says Nielsen. “Additional treatments should be based on diagnostic testing and only be directed to individual horses, not entire herds.”
However, individuals’ egg counts do allow managers to look at the farm as a whole. They can identify trends, especially regarding types of parasites on the farm, and treatment efficacy. By checking FEC a few weeks after giving anthelmintics, for example, known as running a fecal egg count reduction test, they can determine the percentage of survivors, as well as how long it takes for eggs to start showing up in the feces again—both of which indicate levels of resistance.
In Hertzberg’s widely adopted system, his team uses a fecal egg count testing schedule that matches the calendar used for traditional rotational deworming—four times per year (cutting that to three times once farms show stability. Adult horses with strongyle epgs lower than 200 (with no other harmful parasite infection—roundworms, large strongyles, tapeworms—detected) receive no treatment. As a security measure, however, all horses receive a blanket anthelmintic treatment at the end of the grazing season (fall).
In a recent analysis of monitored stables, Hertzberg found that the average epg among nearly 17,000 samples from adult horses was only 60.
Dr. Sara Ringmark
Sara Ringmark, PhD, is a researcher at the Swedish University of Agricultural Sciences Department of Anatomy, Physiology, and Biochemistry, in Uppsala
Simplified Yet High-Tech Egg Counting
The search for equine parasite eggs is nothing new. In fact, scientists were counting eggs in horse feces 100 years ago, says Martin Nielsen, DVM, PhD, Dipl. ACVM, of the University of Kentucky, in Lexington. “It’s time we came up with something more sophisticated,” he says, adding that advanced, more accessible technology could encourage more owners and veterinarians to check fecal counts before treating.
In recent years fecal egg counts have become easier and less expensive because owners can pick up poop themselves and send it into laboratories, Nielsen says.
Nielsen’s team, though, took it a step further. They developed an automated system with an app for fecal egg counting. “It analyzes a picture through a process that takes less than three minutes,” he explains. While it’s not as simple as snapping a photo of your horse’s poop and uploading it, it still allows veterinarians to prepare samples with stain according to set procedures in their offices and get analyses based on the system’s predefined image analysis.
“This is only the first generation of the product from a startup company,” he says. “With time I could see a consumer product developing, as well.”
Climate Considerations
PHOTO: iStock.com
It can be useful to consider how climate affects worm life cycles when managing parasites, says Nielsen. Parasites can reproduce year-round in warm, wet climates, meaning there’s a constant mix of worms at different stages in their life cycles. In cold climates with strong winters, more pronounced life patterns tend to appear.
“Colder climates tend to synchronize parasites’ stages of life, so they all start out young in spring and get old around the same time by the end of the year,” he says.
With climate change, the Swiss researchers are recognizing changes in parasite life cycle patterns, says Hertzberg. “Higher mean temperatures are accelerating larval development on pasture,” he says. “Longer pasture periods, starting earlier in spring and lasting as late as December, are increasing the number of parasite life cycles per year.”
“While researchers have a basic understanding of the connection between climate and parasite management, developing specific climate-based recommendations will require significant further research,” says Nielsen. Meanwhile he and his colleagues have a computer simulation study predicting the impact of climate change on parasite burdens in horses.
Basic Management Strategies
Meanwhile he and his colleagues have a computer simulation study predicting the impact of climate change on parasite burdens in horses. During the study, the team combined their cyathostomin (small strongyle) simulation model with six different climate change prediction models. Their key findings findings included:
- Shifting of seasonality will have a marked impact on parasite transmission patterns.
- This in turn can lead to larger parasite burdens in areas changing from temperate to warmer climates.
- The warmer climate and longer parasite transmission season can also affect development of dewormer resistance.
- Non-drug related strategies will become increasingly important for parasite control.
The Veterinarian:
Your Parasite Management Partner
PHOTO: Adobe Stock
Involving your veterinarian in your farm’s parasite management is “critical,” says Hertzberg. Veterinarians who’ve been trained in modern management of equine parasites understand the epidemiology, related pathologies (diseases), risk factors, clinical signs of disease, parasite life cycles, effects of climate and environment, and the fight against anthelmintic resistance. However, it’s equally important that the veterinarian has received appropriate training in farm management, Hertzberg adds.
“A lot of the vets coming just out of veterinary school will get that education, and others can sign up for classes (or continuing education) after graduation,” he says. “But many of the vets who’ve been in practice for years already will not have that specific training, so it’s necessary to ask if they have.”
What’s more, it’s vital that farms stick with a single portfolio manager—one veterinarian who assesses the farm’s parasite situation and makes recommendations. “You shouldn’t have parallel strategies going on, as they will be confusing for the owners,” Hertzberg says. “Just one strategy per farm, and you keep with that strategy over the long term as long as it works successfully.”
Protecting the Environment
Excessive deworming—that is, blanket treatment of all horses more than necessary or recommended—doesn’t just encourage parasite resistance. It might also harm the ecosystem horses live in, says Dr. Sara Ringmark.
“Horses can contribute to landscape preservation, but not if their anthelmintics are destroying the local fauna and flora,” she says. Macrocyclic lactones, for example, are toxic for arthropods, fish, and soil nematodes, among others.
Take-Home Message
PHOTO: The Horse
Equine parasite management has evolved with updated strategies aimed at improving efficacy. It might take time for owners and veterinary practices to switch over to individualized portfolio-based treatment protocols. But global efforts for improvement can help prevent the serious and irreversible situation of parasite resistance, in which parasites no longer respond to treatment and our existing dewormers no longer work. By following science-based recommendations, owners and their veterinarians can ensure they can manage parasites effectively for the long term.
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About the author:
Christa Lesté-Lasserre, MA
Passionate about horses and science from the time she was riding her first Shetland Pony in Texas, Christa Lesté-Lasserre writes about scientific research that contributes to a better understanding of all equids. After undergrad studies in science, journalism, and literature, she received a master’s degree in creative writing. Now based in France, she aims to present the most fascinating aspect of equine science: the story it creates.
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