The Modern Guide to Horse Deworming

Current Best Practices in Equine Parasite Control

Updated July 15, 2024 | Special thanks to equine parasitologist Martin Nielsen, DVM, Ph.D., DipEVPC of the University of Kentucky for his thoughtful and thorough review of this article.

Today’s most effective strategy for safely deworming your horse involves a targeted parasite control program. According to guidelines from the American Association of Equine Practitioners (AAEP), a parasite control program should be designed with these goals in mind:

1. Minimize the risk of parasitic disease
2. Maintain efficacious drugs and avoid further development of anthelmintic (deworming drug) resistance as much as possible

By working with your veterinarian and taking a strategic approach to fighting parasites, you’ll do a better job of protecting your horse, your property, and the entire herd. Here, we’ll walk you through each step – from assessing fecal egg counts to picking the right type of dewormer – towards planning an evidence-based parasite control program.

Why Deworming is a Smart Choice

If you’re like any other owner who wants to take great care of their horse, the idea of them carrying a high load of parasites is enough to make you cringe. However, it is important to remember that it is completely normal for horses to have parasites. The overwhelming majority of horses are perfectly happy and healthy living with their parasites. However, in rare cases, a high parasite load can lead to poor quality coat, weight loss, diarrhea, or colic.

Horses get infected with parasites, but it doesn’t happen exactly like getting infected with a contagious disease, such as flu (Equine Influenza) or rhino (Equine Herpesvirus). Generally, horses acquire worms when they’re turned out in a contaminated pasture. A horse can even re-infect himself with his own intestinal worms.

Fortunately, there is more you can do than cringe when it comes to parasite control. Along with proper environmental management to reduce parasite transmission (like regularly removing manure from pastures – not spreading it!), you can help reduce the parasite population with strategic deworming protocols.

Types of Equine Intestinal Parasites

Small Strongyles, aka Cyathostomins

The most common type of internal parasites in horses around the world are strongyles. Indiscriminate deworming practices have led small strongyles to developing a high resistance against multiple classes of dewormers. They encyst, or burrow, into the horse’s intestinal wall and then emerge, which damages tissue. Cyathostomins are almost always present in horses that graze on green pastures, however, they’re rarely associated with causing obvious symptoms.

Large Strongyles, including Bloodworms (Strongylus vulgaris)

Large strongyles are extremely rare among managed horse populations today. They migrate into the arteries of the horse’s abdomen causing damage to blood vessels. Although widespread anthelmintic use had nearly eradicated large strongyles, this parasite may resurge with inappropriate deworming practices [3].

Tapeworms (Anoplocephala perfoliata)

Tapeworms attach at the junction of the ileum and cecum in the horse’s hindgut, potentially obstructing the gastrointestinal tract and leading to colic [4]. They’re considered common in horses kept on green pastures, yet most infected horses tend to have a fairly small count of these worms. Although tapeworms are difficult to detect through fecal testing, blood and saliva antibody tests can show if your horse has been exposed.

Ascarids, aka Roundworms (Parascaris spp)

Roundworms migrate from the intestine through the circulatory system. Once roundworms enter the lungs, horses can cough them up and re-swallow them, completing their life cycle. They’re more often found at breeding farms and pose a serious threat to foals or other horses with immature immune systems.

Bots and Pinworms (Gasterophilus spp and Oxyuris equi)

Both bots and pinworms are generally just a nuisance. Bots migrate through the oral cavity to burrow into the stomach, and an infestation could cause signs of colic. Pinworms lay eggs around the anus, causing irritation. In severe cases of pinworms, horses may rub their tails or hindquarters intensely, causing them to spread the worms onto fence posts, stall walls, grooming tools, etc.

Neck Threadworms (Onchocerca cervicalis)

Infestations of neck threadworms can cause intense itching and skin irritation along the ventral midline (underneath the belly), face, neck, chest, withers, or forelegs. This irritation is caused by an immune response to immature Onchocerca (microfilariae) in the skin. While there is no current treatment available for adult Onchocerca, ivermectin and moxidectin have been shown to be effective against the microfilariae. Treatment with these drugs generally improves the itchy skin irritation dramatically.

Stomach Worms (Habronema and Draschia spp.)

As their name implies, these adult worms live in a horse’s stomach where they rarely cause an issue. A female stomach worm’s eggs and larvae are passed in manure, which are ingested by flies (an intermediate host). Then, flies will deposit the infective larvae around a horse’s lips, which are then ingested, completing the stomach worm’s normal life cycle.

When flies break this normal cycle and deposit the stomach worm larvae onto wounds or mucosal membranes, it can cause cutaneous habronemiasis (known as summer sores). These persistent skin lesions can be challenging to treat and are commonly seen in warm climates.

Fecal Egg Counts and Anthelmintic Resistance

Deworming may be something you’ve been doing the same way for as long as you can remember—most likely rotating between anthelmintic paste products about every eight weeks or so. However, as drug-resistant parasites and the lack of new dewormers in the pipeline become increasingly concerning, horse owners, barn managers, and veterinarians are rethinking how to deworm horses.

Have a Fecal Egg Count Done for Your Horse

Your first step in developing your horse’s deworming program should be to consult your veterinarian for advice on having a Fecal Egg Count (FEC) performed. In adult horses, the purpose of a FEC is to measure the number of strongyle eggs your horse is passing in each gram of manure.

You can either have your veterinarian perform the FEC for you or, if your vet doesn’t offer FECs, SmartPak offers a mail-away Equine Fecal Test Kit. The results will help you and your vet determine if a targeted treatment is needed, and if so, the most appropriate anthelmintic to use. Experts recommend having FECs performed at least twice per year on all adult horses.

Fecal Egg Count Reduction Tests (FECRT)

It’s recommended to have a second fecal egg count test performed 14 days after deworming to check how effective the anthelmintic treatment was. This is called a Fecal Egg Count Reduction Test (FECRT), and it measures the percentage of reduction in egg counts after deworming.

If your veterinarian determines the test results indicate anthelmintic resistance, then that specific deworming drug should not be used. Scientists are discovering increasing resistance of small strongyles, pinworms, tapeworms, and ascarids to commonly used dewormers. So, it’s critical to make sure that the number of eggs being shed in your horse’s manure fall within the targeted range after the deworming. If they do not, you may be using an ineffective dewormer.

Prioritize having FECRTs performed at least once per year for every herd or barn, with at least 5 horses included in the test, if available.

Video on Parasite Fecal Egg Counts

In this video, equine parasitologist Dr. Martin Nielsen explains why owners should have fecal egg counts done for their horses and what important information these test results tell us.

Assessing Your Horse’s Egg-Shedding Level

When you send the FEC sample to your veterinarian or an independent laboratory, you will get back a number, such as 50 eggs per gram (EPG) or 500 EPG. That number tells you and your vet whether your horse is a low, moderate, or high parasite egg-shedder, which in turn helps both of you in developing your horse’s deworming plan.

If your horse’s FEC results come back less than about 200 EPG, then your horse is likely a low egg-shedder. If the FEC results show more than 500 EPG, then your horse is likely a high shedder and needs additional treatments.

Horses may not show any signs of parasitic disease. However, they could still be shedding a high number of eggs across their environment that will infect other horses.

Horses and Parasites Will Always Cohabitate

Even if your horse’s FEC comes back negative, that does not mean your horse is parasite-free. Horses are grazers, so they’ll never be completely free of parasites. Your horse with a negative fecal test still has intestinal parasites. However, those parasites simply aren’t shedding enough eggs to make it above the detection limit of the test.

How Often to Deworm a Horse

A two-tiered parasite control approach from the AAEP involves:

1. One or two treatments per year administered to all adult horses in the herd. These annual or bi-annual treatments are put into place as a basic foundation to target large strongyles, ascarids, and tapeworms (the non-cyathostomin parasites).
2. Additional targeted treatments given to moderate to high egg-shedders to bring down the parasitic population of cyathostomins.

Different Types of Dewormers

Once you have the results of your horse’s FEC, you can speak with your vet about which dewormer you should use and how often. There are a variety of deworming drugs, or anthelmintics, each targeting specific types of parasites with their own methods of killing them. Overuse and misuse of anthelmintics has played a major role in increasing resistance of parasites to dewormers

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Planning a Targeted Deworming Program

It should be clear by now that there’s no “one size fits all” approach to deworming. Factors that play a role in your individual horse’s parasite control program and the deworming schedule of the entire herd include:

• Anthelmintic resistance status
• The level of parasitic egg shedding
• Your geographic location, climate, and seasonality
• The age distribution amongst the herd members
• The stocking density and pasture quality of turnout
• Manure management on the farm

Parasite Transmission Season

Low strongyle egg-shedders may only need to be dewormed once or twice a year, typically at the beginning and end of the grazing season. Depending on region and climate, moderate to high shedders should be dewormed additionally at appropriate times of the year. Pay attention to when parasite transmission season is in your region and use that to your advantage.

For example, if your horse is a high shedder but it’s winter and temperatures are below freezing, those conditions will stop larval development, so deworming is unnecessary at that time. On the other extreme, excessively hot temperatures in the summer (over 104 degrees Fahrenheit or 40 degrees Celsius) will cause strongyle eggs to die.

Speaking with your vet about your horse’s specific requirements and seasonal factors at play will shine the most light on their potential deworming schedule.

Manure Matters

A core part of your horse’s parasite control program is the amount of manure in their environment. Adult parasites lay eggs in the horse’s intestinal tract, which are then passed in manure that can contaminate pastures and the stable. As the eggs develop into larvae (under the right environmental conditions), they can be ingested by a grazing horse. Once ingested, the larvae can continue to mature and then reproduce, completing its life cycle. Strong manure management practices can stop this cycle.

Horses typically try to avoid grazing close to areas where they defecate, but when grazing is limited, they’re forced to find grass closer to manure piles. This proximity puts them at a higher risk of ingesting parasite larvae. Strongyles also exist on the lower portion of grasses, so horses are more at risk as they deplete the taller grass.

Strategies for Mitigating Parasites:

• Be proactive in cleaning paddocks and picking up manure.
• Avoid overstocking the pasture with too many horses so the land does not become overgrazed.
• When implemented correctly, rotational grazing can allow pastures to rest and regrow.
• Turn out other animals such as cows, sheep, goats, llamas, or alpacas on the pasture. These species aren’t susceptible to equine strongyles and will eat the grass around the manure piles. This will, in turn, decrease larval survival on the pasture as there will be less protection from shade and moisture.
• Properly compost manure and bedding. Never spread non-composted manure on pastures as this will allow parasites to flourish across the entire pasture.

Caution in Using Natural Deworming Alternatives

There are many companies that market natural, organic, or herbal dewormers as alternatives to commercial products. These alternatives are not considered drugs, and therefore, do not have to prove themselves effective by the Food and Drug Administration’s (FDA) requirements. Use caution with these products as there is little to no oversight on their label claims, safety, or efficacy.

In contrast, commercial deworming drugs must prove their label claims unequivocally to the FDA through safety and efficacy testing before advertising the product. Watch this video as Dr. Lydia Gray explains the AAEP’s guidelines on alternative methods to deworming and ingredients commonly advertised as natural anthelmintics.

Still Not Sure When You Should Deworm Your Horse?

Dr. Gray explains the importance of deworming and when the best times of year are to deworm in this video.

Please note that foals, weanlings, and young horses less than about three years of age require special deworming protocols as they have immature immune systems. They can carry different parasites than adult horses, requiring different chemical classes and frequency of dewormer treatments. Speak with your vet about steps to take for safely deworming your filly or colt.

Put Your Parasite Control Program into Action

And that’s it! Creating a targeted parasite control program for your horse really is easier than it sounds at first. In actuality, it is less work, less expensive, and more effective than the rotational deworming schedule of the old days.

By following the simple steps outlined above and working with your veterinarian, you’ll be doing a better job of protecting your horses, their living environment, and the few effective dewormers left!

Already armed with the results of your horse’s fecal egg count test and the advice of your veterinarian? Check out our selection of dewormer products.

Key Takeaways

In summary, follow these dos and don’ts to design an effective parasite control program:

• Do have fecal egg count reduction tests performed at least once per year to check the effectiveness of your dewormers.
• Don’t deworm horses on fixed schedules or blindly rotate between anthelmintics.
• Don’t use fecal egg counts as a method of diagnosing diseases.
• Do use fecal egg counts once or twice a year to categorize horses as low, moderate or high shedders.
• Don’t ignore active parasite transmission season. Take seasonality into consideration when deworming.
• Do deworm every horse at the baseline recommended one to two times per year with additional treatments to high shedders (identified using FECs).
• Don’t rely on dewormers that aren’t approved by the FDA.
• Do understand that no dewormer will remove all parasites from your horses.
• Do work with your veterinarian to pick an appropriate type of dewormer based on your individual horse’s age, deworming history, FEC, environment, and other factors.

Additional Resources and References

These resources and scientific journals were used in the writing of this article and may be helpful to horse owners:

1. “AAEP Internal Parasite Control Guidelines.” American Association of Equine Practitioners, May 2024, https://aaep.org/wp-content/uploads/2019/01/Internal-Parasite-Control-Guidelines_May-2024.pdf.
2. Kaplan, Ray M., et al. “World Association for the Advancement of Veterinary Parasitology (W.A.A.V.P.) Guideline for Diagnosing Anthelmintic Resistance Using the Faecal Egg Count Reduction Test in Ruminants, Horses and Swine.” Veterinary Parasitology, vol. 318, June 2023, p. 109936. https://doi.org/10.1016/j.vetpar.2023.109936/
3. Nielsen, M. K., et al. “Strongylus vulgaris Associated With Usage of Selective Therapy on Danish Horse farms—Is It Reemerging?” Veterinary Parasitology, vol. 189, no. 2–4, Oct. 2012, pp. 260–66. https://doi.org/10.1016/j.vetpar.2012.04.039.
4. Nielsen, M. K. “Equine Tapeworm Infections: Disease, Diagnosis and Control.” Equine Veterinary Education, vol. 28, no. 7, June 2015, pp. 388–95. https://doi.org/10.1111/eve.12394.