If you’ve been deworming your horse on a rigid calendar schedule—paste in hand every 6 to 8 weeks like clockwork—you’re following advice that the veterinary science community has largely moved away from. Advances in parasite research have fundamentally changed how experts approach equine deworming—but many horse owners haven’t gotten the update yet and they are still relying on outdated routines.

So how often should horses be dewormed? The honest answer: it depends—and today’s science offers far more precise tools to determine what each horse actually needs.

For decades, the standard advice was simple and consistent: deworm every 6 to 8 weeks, rotate between drug classes, and you’re covered. The widely adopted “deworm horses every 2 months” approach dates back to recommendations from the 1960s, and was originally aimed at controlling large strongyles.

It was simple, easy, consistent, and predictable—and, over time, it created a serious unintended consequence.

Today, that once-standard guidance is considered outdated and is strongly discouraged by many experts. Frequent, routine deworming is now understood to accelerate the development of anthelmintic resistance—meaning the parasites gradually become less responsive to the very drugs meant to control them.

In other words, the more we used dewormer, the less effective those treatments became. The result is a growing resistance problem that horse owners and veterinarians are now managing in barns across the country.

Anthelmintic resistance—when parasites evolve to survive deworming drugs—is no longer a distant concern. It’s already a reality.

Across many regions of the world, resistance has been documented in small strongyles, ascarids, tapeworms, and pinworms. Small strongyles, in particular, have developed widespread resistance to benzimidazoles and pyrimidines, with early warning signs emerging for macrocyclic lactones as well.

That last point is especially significant. Macrocyclic lactones—the class that includes ivermectin and moxidectin, two of the most commonly used dewormers—are beginning to show reduced effectiveness in some populations. With only a limited number of drug classes available, overuse risks diminishing their usefulness even further.

In some cases, small strongyles have developed resistance to multiple drug classes. These parasites can encyst within the intestinal wall before emerging, a process that can damage tissue and impact the horse’s health.

The takeaway isn’t simply to switch products. It’s to approach deworming more strategically—treating based on evidence and need, rather than frequency alone.

Treat smarter, not more often.

The modern, evidence-based deworming approach is called targeted selective treatment (TST), or strategic deworming. Instead of treating every horse on a fixed schedule, you test each horse individually and treat based on actual need.

Current guidelines generally recommend a baseline of 1 to 2 treatments per year for most horses, with additional deworming reserved for individuals identified as “high shedders” through fecal egg count testing. This allows parasite control to be tailored to each horse rather than applied uniformly across an entire barn..

Importantly, experts also now explicitly advise discontinuing the practice of deworming all horses with fixed intervals year-round and stopping the practice of blindly rotating anthelmintic classes without evidence-based justification.

This shift is supported by the American Association of Equine Practitioners (AAEP), whose parasite control guidelines were updated as recently as May 2024, along with recommendations from leading veterinary institutions and peer-reviewed research published in journals such as Animal Frontiers and Veterinary Parasitology.

The cornerstone of this strategic modern approach is the fecal egg count (FEC) test. This simple, relatively inexpensive lab test measures the number of parasite eggs a horse is shedding in its manure, offering a direct snapshot of that individual’s parasite burden.

The best practice for deworming horses today begins with running an FEC through your veterinarian at least once or twice a year. The results help identify how heavily each horse is contributing to parasite contamination. Most horses—roughly 50 to 70 percent—are low shedders and don’t require frequent treatment. A smaller group, about 20 to 30 percent, are high shedders and are responsible for the majority of parasite spread, making them the priority for targeted deworming.

Based on FEC results, horses are typically grouped into three categories:

• Low shedders (fewer than 200 eggs per gram, or EPG): These horses tend to have strong natural immunity and usually only need 1 to 2 treatments per year.
• Moderate shedders (200–500 EPG): These horses may benefit from 2 to 3 treatments annually, timed around periods of higher parasite transmission.
• High shedders (over 500 EPG): These individuals require more frequent, carefully monitored deworming, with follow-up testing to ensure treatments are effective.

A surveillance-based program like this reduces unnecessary drug use, helps preserve dewormer effectiveness, can lower costs over time, and ensures that parasite control efforts are actually working.

For the average healthy adult horse, the answer is probably fewer treatments than many owners expect.

Most adult horses are low shedders and many only need 1 or 2 deworming treatments per year—commonly timed in the spring and fall. For the majority, twice yearly treatment is sufficient, with additional doses reserved for horses shown to carry heavier parasite burdens.

Timing matters as much as frequency. Deworming is most effective when aligned with peak parasite transmission periods, which often occur in spring and late fall in many regions (though this can vary depending on climate). Horses identified as high shedders may need additional, targeted treatments between these seasonal windows.

The fall treatment is particularly important. At this time of year, selecting the appropriate drug matters: treatment should target encysted larvae before winter, with moxidectin often recommended in many cases due to global resistance patterns. Annual tapeworm control should also be included, typically using a product that contains praziquantel.

Overall, the goal is not to deworm more often—but to deworm more effectively, using timing and evidence to guide each decision.

Young horses are not simply smaller versions of adults when it comes to parasite control—they require a completely different approach.

Foals, weanlings, and young horses under 3 years of age have immature immune systems, making them more vulnerable to parasites and less able to regulate infections on their own. They also tend to carry different types of parasites than adults, which means both the choice of deworming drugs and the treatment schedule must be adjusted accordingly.

While small strongyles and tapeworms are the primary concern in adult horses, ascarids are the most significant parasite risk in young horses and can cause serious health issues if not properly managed.

A typical deworming program for young horses often begins around 2 months of age with medications such as fenbendazole or oxibendazole. A fecal egg count is usually performed at 4 to 6 months to monitor parasite types—particularly distinguishing between ascarids and strongyles—followed by treatment with ivermectin around 5 months to address strongyles.

During the yearling stage, horses are commonly treated for strongyles multiple times throughout the year, often using ivermectin, with an additional treatment toward the end of the grazing season that includes moxidectin combined with praziquantel to address encysted larvae and tapeworms.

In many cases, foals are dewormed approximately every 2 months during their first year. However, even in young horses, best practice increasingly emphasizes regular fecal testing and tailoring treatment based on the specific parasites present rather than relying solely on a fixed schedule.

Yes—significantly. A horse’s living environment has a direct impact on parasite exposure and, in turn, its deworming needs.

A small private pasture with just 1 or 2 horses presents far lower risk than a busy boarding facility with frequent new arrivals and higher stocking density. That difference alone can influence how often deworming is necessary.

Pasture management also plays a crucial—and often underestimated—role. Regular manure removal, avoiding overstocking, and rotating grazing areas all help reduce parasite contamination. Practices like cross-grazing with cattle, sheep, or goats can further interrupt parasite life cycles. Together, these strategies lower the overall parasite burden, reducing reliance on deworming medications and improving long-term control

One of the most important—and often underused—tools is the fecal egg count reduction test (FECRT). This involves performing a fecal egg count before deworming and repeating the test about 14 days afterward, then comparing the results.

Experts recommend conducting a FECRT at least once a year to confirm that the dewormers being used in a given herd or barn are still effective.

In a successful treatment, egg counts should typically drop by around 90% or more, depending on the product used. If the reduction falls short of that benchmark, it may indicate emerging drug resistance on your farm—at which point it’s important to work with your veterinarian to reassess and adjust your deworming strategy.

The science is clear: preserving the effectiveness of the deworming drugs we still have depends on using them more carefully. Experts now emphasize selective treatment—targeting horses with higher parasite burdens, as identified through fecal egg count testing—rather than blanket, routine dosing.

For most horse owners, that translates into a few key shifts in practice:

• Stop deworming every horse on a rigid calendar schedule. Instead, run fecal egg counts at least once—ideally twice—a year, and let those results guide your decisions.
• Work with your veterinarian to identify which horses actually need more frequent treatment and which can be maintained with just 1 or 2 doses annually.
• Confirm your dewormers are still effective by performing an annual reduction test.
• Don’t overlook the non-drug side of parasite control—good pasture hygiene remains one of the most powerful tools available.

In reality, strategic deworming is often less work, more cost-effective, and ultimately more successful than the old rotational schedules.

It’s also important to recognize that the goal isn’t to eliminate every parasite. That’s neither realistic nor, according to current research, even necessary. A low level of parasites is normal, and most horses remain healthy while coexisting with them. The aim is thoughtful management—keeping horses well while preserving the effectiveness of the treatments we rely on.

Remember to always consult your veterinarian before making changes to your horse’s parasite control program. Deworming protocols vary by region, individual animal, and farm management practices.