Smarter, Not More: The New Era of Equine Deworming
Anthelmintic resistance in horses is no longer a looming threat — it’s here, widespread and often underestimated. According to Dr. Martin Nielsen, professor of equine clinical sciences at Aarhus University in Denmark and one of the authors
of the 2024 AAEP parasite control recommendations, resistance levels are particularly high in the U.S., a consequence of decades of intensive deworming.
“Anthelmintic resistance is everywhere in equine parasites,” Dr. Nielsen said.
The 2024 AAEP guidelines marked a continued shift away from traditional rotational deworming programs, emphasizing the need for evidence-based strategies. But changing the approach requires more than following new protocols — it requires a mindset shift. “It’s not just veterinarians who need to change,” Dr. Nielsen explained. “Horse owners and farm managers need to test for resistance, too.”
common deworming misconceptions. Dr. Martin Nielsen highlights several persistent myths that need to disappear:
- Deworm at the first frost – Timing treatments to the weather isn’t
always effective. - High egg counts = many parasites – Egg counts don’t always reflect actual parasite burden.
- The parasite-free horse – No horse is truly free of all parasites.
- Half-dosing “wormy” horses – Underdosing promotes resistance, not cure.
- Relying solely on the label – Product labels may not reflect current resistance patterns.
- The “good all-round dewormer” – No single drug works for every parasite.
- Drug rotation prevents resistance – Rotation alone won’t stop resistance from developing.
- “More sensitive egg counting techniques” – Advanced tests don’t replace strategic treatment decisions.
Takeaway: Smarter, evidence-based deworming beats tradition or guesswork every time.
From recommendations to best practices
While the 2024 AAEP guidelines have made it clear that the days of blanket, interval-based deworming are over, translating those recommendations into practice has been a challenge for many horse owners and veterinarians.
Dr. Nathan Voris, DVM, MBA, director of equine technical services at Zoetis, explains that the core of effective parasite control isn’t particularly complicated, but it can feel overwhelming. “The AAEP Internal Parasite Control Guidelines is a very comprehensive document with a lot of information for veterinarians and horse owners to absorb. At the heart of it, effective parasite control isn’t that complicated, but a trip down the dewormer aisle at the local feed store can be overwhelming, which can lead to a reluctance to change,” he said.
Dr. Voris points out that the historical protocols of frequent, often every-other-month deworming were necessary in an era when large strongyles were the primary concern, causing severe colic and other serious health issues. “Since the discovery of ivermectin and moxidectin, the practice of every-other-month deworming is no longer necessary,” he noted.
Modern protocols are instead tailored based on the age of the horse and the time of year. For adult horses, one to two treatments per year are generally sufficient, targeting small strongyles and tapeworms. Foals, however, require four treatments during their first year to combat roundworms, and horses under three may need up to four treatments annually, as they are commonly infected with all three parasite types.
Over the last decade, Dr. Voris observes, veterinarians have become increasingly central to the deworming process. “The number of horse owners reporting that deworming is a routine procedure performed by their veterinarian has more than doubled. While many horse owners still purchase dewormers from farm stores or online catalogs, they rely on their veterinarian as their primary source for up-to-date recommendations on all aspects of equine health, including which dewormer to buy.” The adoption of fecal egg count testing has also helped identify the roughly 20% of adult horses that require more than the standard one to two annual treatments, ensuring treatments are targeted where they are most needed.
Yet despite these advances, gaps remain between the AAEP recommendations and real-world practices. “The biggest gap is horse owner awareness,” Dr. Voris says. “At the heart of it, the new recommendations are pretty easy to understand, but both equine veterinarians and horse owners have a lot on their plates and might have missed some of the initial communications. The more we can get this information in front of those who care for horses, the more we can build confidence that most adult horses just don’t need to be treated more than one to two times per year if using the right products at the right time.”
Communicating this shift to owners accustomed to frequent deworming requires both clarity and a willingness to challenge long-standing habits. Dr. Voris underscores the financial and biological implications of over-treating. “Horse owners who deworm more than necessary are overspending on this aspect of their horse’s needs and subjecting parasites to significant selection pressure, leading to the development of resistance.”
The message from Zoetis —“Buy less dewormer” — is intentionally strong, designed to provoke a rethink of outdated routines. Dr. Voris elaborates, “A recent industry survey (Brakke Equine Market Mega Study VI) reported that over 60% of horse owners deworm their horses more than three times per year and spend over $100 per year on dewormers. For 80% of adult horses, this frequency of treatment isn’t just a waste of horse owner financial resources, but it also subjects parasites to intense selection pressure, leading to the development of resistance.”
Long-term benefits
Reducing treatment frequency, however, is not just about saving money; it is also a strategy for long-term parasite control. “Horse owners can reduce deworming treatments and improve long-term parasite control by using the correct products for today’s threats,” Dr. Voris said. “By using the most effective products in the spring and fall, horse owners can protect their horses against the most dangerous parasites and reduce selection pressure for the development of resistance. By reducing selection pressure for resistance, we can maintain effective treatment options that enable us to protect horses with just one or two treatments per year.”
The need for strategic treatment is particularly urgent when considering anthelmintic resistance in the U.S., especially among small strongyles. “Small strongyles are ubiquitous in horse populations in the United States. Nationwide, they have been found to be highly resistant to benzimidazole (fenbendazole) anthelmintics, making this class of dewormer unsuitable for the effective treatment of adult horses,” Dr. Voris explains.
If resistance continues at its current pace, the consequences could ripple across equine health management and veterinary practice. “The reports of parasite resistance are a wake-up call for all of us who care for horses. In the drug discovery and development world, there are hundreds of ideas that fail for every successful product that comes to market, and these efforts take decades. Developing a new product in the face of a rapidly changing resistance pattern makes these efforts even more challenging. While we shouldn’t feel that the sky is falling, there are some simple changes we simply must make to the way we manage equine parasites.”
For practices looking to modernize, the guidelines themselves, along with resources like zoetisequine.com, offer practical starting points. “The AAEP Internal Parasite Control Guidelines are a great resource for anyone wanting a deep dive into equine parasite control. In addition, zoetisequine.com hosts a variety of parasite control educational materials and a dewormer questionnaire to help horse owners determine what product to use at the most appropriate time of year to help maintain their horse’s health,” Dr. Voris said.
Finally, Dr. Voris emphasized that changing the conversation around deworming isn’t simply a matter of issuing new guidelines — it’s about reframing what responsible care looks like. “The annual spring wellness visit is a perfect time to bring up this topic. Everyone wants to do what’s best for their horse, and when they can do what’s best while using less time, energy and money, it’s a triple-win.”
By focusing on smarter, not more frequent, treatments, veterinarians and horse owners alike can preserve the efficacy of available anthelmintics, safeguard equine health, and reduce unnecessary costs. The path forward is clear: one guided by testing, strategy, and a willingness to let go of outdated routines, ensuring that horses remain protected today and for decades to come.
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istockphoto.com/Fly View Productions
istockphoto.com/mgstudyo
istockphoto.com/Zbynek Pospisil
