The focus of equine parasite control has changed quite dramatically since we first started using dewormers in the 1960s, especially in recent decades. The problem, however, is that veterinarians and researchers have had a challenging time getting the news out. And let’s face it—when it comes to our horses’ overall care, doing extensive research on the latest updates in parasite control isn’t usually a first priority.

Widely regarded as the world’s leading equine parasitologist, Martin K. Nielsen, DVM, PhD, DipEVPC, DACVM and former Schlaikjer Professor of Equine Infectious Disease at the University of Kentucky’s Gluck Equine Research Center, has been trying to spread the word that rotational deworming’s goal of eradicating all worms is an outdated approach to parasite control.

For the past four decades, the long-held practice of treating horses every eight to 12 weeks with products of different chemical classes has not prevented resistance the way veterinarians once thought it would. “This method of over-deworming caused widespread drug resistance and left veterinarians and horse owners with increasingly fewer treatment options for horses that do suffer from parasitic disease,” Nielsen explained.

To further complicate the situation, no new dewormers have been introduced since 1981 and none are in development—a process that usually takes a minimum of five years. “We are concerned that there won’t be any effective deworming medications left in the near future,” he added.

While it’s unpleasant to consider the fact that slimy, slithering worms could be taking up residence in your horse’s intestines preparing to hatch thousands of eggs that will soon contaminate your pasture, parasitic disease is even less appealing. It can wreak havoc on your horse’s intestines and put his health at serious risk.

Here, we’ll help you evaluate the effectiveness of your current deworming program to ensure your approach is successful and that you’re following new protocols recommended by researchers to keep your horse and his herdmates healthy.

New Objectives

Put in the most basic of terms, your deworming program should target the right worms with the right deworming medications at the right time of year. Unfortunately, it’s not quite as simple as it sounds because there is no single, universal parasite-control prescription that applies to all horses on all properties.

Nielsen says any modern-day deworming program—no matter the size and scale of operation—should subscribe to three primary goals:

1.) Minimize your horse’s risk of parasitic disease.

2.) Control parasite egg shedding.

3.) Maintain drug efficacy and avoid further development of drug resistance through minimal treatments; this is especially important among the medications still effective against the equine parasites posing the greatest risk.

Parasite-Control Guidelines

As experts gained more scientific knowledge about equine parasite control over the years, the American Association of Equine Practitioners (AAEP) formed a task force to provide a more modern and comprehensive set of recommendations. Nielsen explained the task force’s goal is to help veterinarians and owners determine the minimum number of deworming treatments all horses should have with certain medications. The guidelines also recommend using specific diagnostic tools to measure treatment effectiveness and to help owners determine whether horses need further treatment.

AAEP’s Internal Parasite Control Guidelines were revised in 2024. Click here to read the entire guidelines; you can also download a copy for easy reference here.

“The new guidelines provide veterinarians with the fundamental parameters for deworming,” Nielsen explained. “Once you understand those parameters, you can deworm according to your horse’s particular situation. Diagnostic testing results will tell you whether the treatments were effective. From there, you can make any necessary changes to ensure your plan works for your horses and property.”

Work With Your Vet

To create an effective deworming program, you need to take a number of factors into account, including:

• What part of the country you live in
• Your property and your horse’s living situation
• Your horse’s age
• His current workload

Therefore, it’s essential to work with your veterinarian to ensure you’re targeting the right worms with the right medication at the right time, because the deworming recipe will inevitably vary depending on these external factors. This is why it’s difficult to recommend particular deworming products, classes of medication or a timeline for when owners should deworm with specific medications.

“Your veterinarian will know what species are the biggest threat based on your geography, pasture density, your horse’s age and the amount of traffic on your property,” Nielsen noted. “So, he or she can advise you on the right products and when to use them.”

Fecal Egg Counts Are Crucial

To find the ideal balance between administering too much and too little deworming medication, monitor treatments through fecal egg testing. “The goal is to deworm only as often as necessary for your particular horse,” Nielsen said. “Deworming too much, as we’ve done for decades, leads to drug resistance. But deworming too little won’t reduce worm burdens.”

The best way to determine what’s appropriate for your horse is to first measure the levels of parasite eggs—primarily small strongyles, which are the most common—existing in his digestive system, or “shedding” in his manure, using a fecal egg count. Then repeat the test two weeks after treatment to evaluate how well it worked.

Your veterinarian will perform a fecal egg count from a sample of your horse’s manure under a microscope. Several methods are used, but the results—expressed as eggs per gram—measure the type of strongyle shedder a horse is:

• Low: less than 200 EPG
• Moderate: 200 to 500 EPG
• High: more than 500 EPG

“The overwhelming majority of mature horses are in the low-shedding category,” Nielsen said, and he offers statistics from his research: “Typically, 50 to 70 percent of the horses in a herd will be in the low-shedding category. The moderate shedders will always be a small proportion of the herd, usually about 10 to 20 percent. The remaining horses are the high shedders—often around 10 to 20 percent.”

What accounts for the differences? “We see this in adult horses and only for the strongyle parasite category. Horses seem to find their general level of egg shedding, which they maintain throughout their lives, and a majority of horses will have very low or negative egg counts,” Nielsen said. “We believe immune status has a lot to do with strongyle shedding status. Horses who move a lot, are training hard or trailered to shows every weekend tend to have higher egg counts. Horses that are hospitalized for non-intestinal conditions like lameness or reproduction can also have increased egg counts.”

What does this mean for how often a horse should be dewormed? Shedding status correlates with frequency and timing. “For low shedders, two annual dewormings—one in the spring and one in the fall, depending on the length of the grazing season—is likely all that’s needed,” Nielsen explained. “Some horses might need even fewer treatments, while high shedders might require more, especially in climates where the grazing season extends beyond five or six months.”

Follow-up testing: Unfortunately, a majority of horse owners who do fecal egg count tests, tend to only do so when they suspect parasites are threatening their horses’ health and rarely follow AAEP’s recommendation to repeat the fecal egg test after treatment. “You can’t just look at a horse a say, ‘Well, he’s got that spark back in his eye so it must be working,’” Nielsen explained. “A lot of people are using products that don’t work. But they will never know if they don’t check. You have to do a second fecal egg count to verify the medication is effectively controlling the parasites you’re targeting, and you have to routinely perform fecal egg counts to continue monitoring a dewormers’ efficacy.”

Nielsen also advises horse owners not to rely solely on the information provided on a drug’s label to rate a dewormer’s effectiveness. “It’s historic information—in other words, how the product worked when it was first introduced. But often, it does not represent the current situation as resistance is ignored,” he noted. “So, the only way to know, is to test.”

Unfortunately, research shows that fecal egg counts are not yet widely used by horse owners. “We are seeing owners deworm less now compared to 20 years ago, but they aren’t really using egg counts more and they almost never test for treatment efficacy,” Nielsen said. “In the end, the explanation is simple: Reducing the number of treatments does not cost anything; you actually save money. But collecting fecal samples and running egg counts comes with a cost. So, we researchers still have a lot of work to do to get the word out.”

Know Thy Enemies

Nielsen says it’s entirely normal for horses to have parasites. “For the most part, these worms—regardless of species—don’t do anything harmful. Horses can harbor substantial parasite burdens without being affected,” he explained. “Equine disease caused by parasites is the exception to the norm. In these rare instances, there are almost always other contributing factors that render a horse more susceptible to disease from parasites.”

Over the past several decades, certain equine parasite populations have shifted, meaning parasites that once posed the greatest risk to horse health aren’t such a threat anymore, and others that we previously believed weren’t a problem now pose a risk. In adult horses, the primary parasites of concern are small strongyles and tapeworms. In foals and weanlings, ascarids remain the most important parasite to target.

Ascarids

When newborn foals first sniff around the pasture, they ingest ascarid eggs, acquire this worm infection and then naturally eliminate the resulting parasite infection within a year. Nielsen says all foals experience a phase of ascarid infection but after eliminating the worms, horses usually never get this parasite again.

Signs if disease: Rarely, ascarids can cause disease, discomfort or malnutrition and stunted growth in foals. Migrating larvae can also cause airway inflammation, and in the most severe cases, ascarid infestations can clog and even impact the small intestinal tract. Small-intestinal impactions are difficult to manage, very painful for the foal and typically requires hospitalization, veterinary supervision, pain medication and sometimes surgery.

Small strongyles

As foals transition to yearlings, your deworming plan shifts its emphasis from ascarids to small strongyles and tapeworms. Once believed to be nothing more than a nuisance, small strongyles (also called cyathostomes) are now regarded as the primary intestinal parasite threatening the health of adult horses. Ubiquitous among horses on pasture, these worms reside in a horse’s large intestine.

Signs of disease: Small strongyle infestations can cause mild weight loss, a coarse or dull hair coat, exercise intolerance, decreased performance and in rare cases these parasites can cause a life-threatening condition that results in severe diarrhea, weight loss and edema. Unlike large strongyles, the small strongyles’ larvae don’t migrate as extensively and damage intestinal blood supplies, which actually makes them a much more subtle and sneaky parasite.

“As part of their life cycle, the small strongyle larvae burrow into the wall of a horse’s large intestine where they go into a hibernation-like state for long periods of time until they all of a sudden wake up and emerge,” Nielsen explained. “When a horse has a high accumulation of these encysted larvae all coming out of hibernation at the same time, it creates a vast inflammatory condition in the large intestine that causes diarrhea.”

Tapeworms

For many years, these parasites weren’t known to cause a lot of disease in horses, but tapeworms have emerged as a growing concern—especially in certain parts of the country. Horses can only ingest tapeworm larvae through an intermediary host—most commonly, a nearly invisible mite that resides in grass and is more prevalent in lush grass and moist areas. There are pockets of equine tapeworm infestations throughout the U.S. because the life cycle of this worm is different from that of other parasites.

Signs of disease: Once ingested, Nielsen says tapeworms tend to hoover around the junction between the small and large intestine and eventually attach to the large intestinal wall near this junction. Large infestations in this area can result in physical obstruction and severe colic requiring surgery.

It can be difficult to measure treatment efficacy for tapeworms because its larvae aren’t always identified in traditional fecal egg counts. While there are specialized tests that can identify tapeworm eggs, many veterinarians advise owners to treat for tapeworms once or twice a year, regardless of verified egg counts. Since these parasites tend to be regional, it’s always important to work closely with your veterinarian to determine if this is the right course for your horse.

The Lesser Culprits

In addition to the primary parasites of concern, there are a handful of less-prevalent worms that occasionally cause problems or disease in horses.

Large strongyles: While these worms were once considered the most threatening equine parasitic pathogen, there has been a significant reduction in large strongyle population and disease over the last 40 years—thanks to a lack of drug resistance. If your horse does get this type of roundworm, however, it can wreak havoc on his digestive system. The adult worms take up residence and remain in the large intestine, but the larvae migrate through blood vessels that supply the intestines.

Pinworms: These parasites rarely lead to severe disease in horses, but they do cause acute itching and tail rubbing. Pinworm eggs can be identified using a “Scotch Tape test,” in which a veterinarian takes an impression of the horse’s skin near the rectum and then examines the tape under a microscope.

Summer Sores: More prevalent in dry parts of the country, this fly-borne parasite normally establishes itself in the stomach after a horse ingests a dead fly carrying the larvae. Occasionally, however, flies deposit the larvae into an open wound, causing slow-to-heal skin sores or ulcers that can become chronic. While this is eventually a dead end for the parasite, it can still cause significant and chronic irritation for the horse.

Neck threadworm: As its name suggests, this parasite—typically, a single adult female—resides in the large ligament of a horse’s neck. Without an abundant blood supply, this is an ideal place for the threadworm to hide from a horse’s immune system. The adult worm is not known to cause horses any discomfort, whereas the tiny microfilaria can spread and cause skin inflammation. “It’s kind of like the Alien movies where the Mother Alien is hiding somewhere hatching her baby aliens,” Nielsen said. “That’s how this parasite operates.” The threadworm’s microfilaria can be treated, but there are no effective treatments against the adult worm.

Bots: Botflies lay eggs on a horse’s hair; when the larvae hatch, they migrate toward the mouth to enter the digestive tract or the horse ingests the larvae when he licks or bites his coat. The larvae then attach to the horse’s stomach wall or the first few inches of the small intestine. Botflies are relatively benign but in large numbers they can cause damage to the stomach lining or block passage of food from the stomach to the small intestine.

Deworming Arsenal

The AAEP guidelines list four chemical formulations commonly used to treat internal parasites in horses though incidence of resistance is increasing.

Benzimidazoles: These have been a mainstay of equine parasite control for more than 50 years and act by interfering with parasites’ metabolism. Agents in this class include:
• fenbendazole
• oxfendazole
• oxibendazole.

All three are effective against ascarids, large strongyles and pinworms, but small strongyles are widely resistant. Oxibendazole also works against threadworms.

Tetrahydropyrimidines cause paralysis in parasites. Small strongyles are commonly resistant to the agents in this class:
• pyrantel pamoate, which works against ascarids, large strongyles, pinworms and tapeworms
• pyrantel tartrate, which is formulated in alfalfa pellets as a feed-through dewormer. It is effective against ascarids and large strongyles, but only as a preventive. It will not purge parasites from the intestines.

Macrocyclic lactones block nerve transmissions to paralyze worms. Agents in this class include:
• ivermectin
• moxidectin.

Both work against most of the common equine parasites, but ascarids are widely resistant. In addition, ivermectin does not kill encysted small strongyles.

Isoquinoline-pyrozines is a class that contains one agent:
• praziquantel, which is effective only against tapeworms.

Additional Management Tips

Dewormers are just one facet of an effective parasite control plan. There are several strategies, including good pasture management, that can also reduce a horse’s risk of exposure and possible infection that researchers recommend:

Provide sufficient space: The ratio of animals you keep on a particular pasture in relation to the pasture’s health and size—is one of the most simple yet effective parasite-control strategies. “If you stock too many animals on a pasture given its size, it will quickly become overgrazed, leaving horses more exposed to parasites,” Nielsen explained. “When horses eat down every last blade of grass, they end up ingesting a lot of parasites.”

Horses tend to establish separate eating areas (“lawns”) and waste areas (“roughs”) in their pastures. As a general rule, provide about two acres of pasture space per horse to help prevent overgrazing and reduce the chances that parasite eggs and larvae in the roughs will be consumed.

Remove manure: Pick up and remove manure on a regular basis—at least once a week.

Mow and harrow pastures: Do this periodically to break up manure piles and expose parasite larvae to the elements. Frequency will depend on the weather and the length of time the pasture can be rested afterward. Larvae can survive freezing temperatures, but they can only tolerate heat and arid conditions for a few weeks.

Age matters: Be sure to consider the ages of all the horses on your property and treat youngsters as high shedders.

Test new herdmates: Evaluate the health—including shedding status—of new horses before turning them out with a herd.

Pasture rotation: Rotate pastures when possible or section off portions to give parasites time to die off while the horses graze elsewhere. This can take three to four weeks in hot, dry weather—when the temperature is above 85 degrees Fahrenheit—and as long as six to nine months during colder times of the year.

Mixed-animal grazing: Use mixed or alternate grazing with other animals like cattle and sheep—species that aren’t susceptible to exactly the same parasites as horses and vice versa. “Mixed grazing leads to better pasture utilization, as the cattle will eat the rough areas horses typically don’t eat,” Nielsen explained. “The cattle not only ingest a high volume of equine parasites, they also help eliminate many of the protections parasites seek on pasture, including moisture in vegetation and shade created by high grass. More exposure to direct sunlight will create drier pasture conditions and a lower parasite infectivity.”

Future of Deworming

Time will tell how horse owners and veterinarians put the scientific community’s latest recommendations on equine parasite control to use. As Nielsen sees it, one of the greatest challenges for researchers is simply getting their message out. “We have to communicate as effectively as possible and make good use of the platforms available,” he said. “Scientists in general have to become much better at using social media.”

Nielsen also points to ongoing research and product development that will shape the future of equine parasite control. Among the advances in use and on the horizon:

Parasight System: A smartphone-based fecal egg diagnostic and intestinal parasite management tool developed by Nielsen and colleagues that allows veterinarians to perform rapid, reliable, quantitative, on-site and in-house fecal egg counts in five minutes.

Ultrasound exam: This method helps detect heavy ascarid burdens in foals and identify the risk of impactions.

Deworming agent: This utilizes a protein produced by the soil bacterium Bacillus thuringiensis to kill parasites. Nielsen says the research has moved beyond the lab and holds promise.

Computer simulation model: These models have the potential to offer insights on the timing of deworming treatments by using weather data—primarily temperature and precipitation rates—to identify the grass-growing/parasite-transmission season in virtually any locale.

Deworming Takeaway

These efforts and more will influence how internal parasites are dealt with in horses in the months and years to come. For now, a strategy that identifies and treats according to a horse’s individual needs is likely to yield the most desirable results.  

For more tips on smart pasture management, click here.

Learn how to administer paste dewormer the right way—and without making a mess—here.