Dewormer Resistance in Horses by a Vet – 2025 Smart Parasite Control Guide 🐎🪱

By Dr Duncan Houston, BVSc

Introduction

Intestinal parasites remain a major health threat for horses—but in 2025, the bigger danger might be ineffective deworming. Decades of overuse and frequent treatment schedules have caused many parasite species, especially small strongyles, to become resistant to common dewormers. This veterinary guide outlines how to beat resistance with smart, evidence-based strategies tailored to your horse's needs.

What’s Happening? The Resistance Problem ⚠️

• In the 1980s–1990s, horse owners were advised to deworm every 6–8 weeks.
• This frequent, blanket treatment killed susceptible worms, leaving behind drug-resistant populations.
• Today, many dewormers from past decades—like fenbendazole or pyrantel—are ineffective against the most harmful worms.

Why This Matters in 2025 🧬

The primary parasite of concern is the small strongyle (cyathostomin). These worms can:

• Encyst in the horse’s gut wall and emerge all at once—causing colic, diarrhea, and weight loss.
• Lead to chronic inflammation and poor nutrient absorption.
• Survive ineffective treatments, furthering resistance across your entire herd.

Current Dewormer Effectiveness 🧪

The Old Protocol Doesn’t Work Anymore 😵‍💫

Routine deworming every 6–8 weeks is outdated and harmful. Instead, a 2025 strategy includes:

• Annual or biannual fecal egg count testing (FEC) by your vet.
• Only treating horses who shed significant parasite eggs.
• Using effective products like ivermectin or moxidectin at the right intervals—based on your herd and environment.

Understanding Fecal Egg Counts 🔬

• Measures parasite egg output in manure (eggs per gram = EPG).
• Horses with low counts (<200 EPG) may not need treatment.
• Moderate (200–500 EPG) and high (>500 EPG) shedders should be strategically dewormed.
• Follow up with a Fecal Egg Count Reduction Test (FECRT) to check if the dewormer worked.

Strategic Deworming: What to Do ✅

1. Schedule a fecal egg count for each horse once or twice a year—spring and fall ideal.
2. Identify high shedders for targeted treatment.
3. Use effective products only when needed—avoid automatic deworming.
4. Rotate pasture, remove manure, and avoid overcrowding to reduce parasite load naturally.

Vet-Approved Deworming Protocol for 2025 📋

• Foals: Deworm at 2–3 months, then again at 6 months, with fecal checks before 1 year.
• Adult horses: Fecal check twice/year. Treat high shedders or symptomatic horses only.
• Use only ivermectin or moxidectin (as resistance to other classes is widespread).
• Don’t use combined products unless recommended by your vet (e.g. ivermectin + praziquantel for tapeworms).

Why You Need Your Vet in 2025 👩‍⚕️

• Your vet can interpret fecal results correctly and recommend specific deworming intervals.
• They know regional parasite patterns and product resistance trends.
• They’ll help you build a program that balances prevention with resistance management.

Common Mistakes to Avoid ❌

• Over-treating all horses “just in case”
• Using old or expired products from the feed store
• Skipping fecal testing and guessing treatment needs
• Relying on one product too often—especially pyrantel or fenbendazole

Case Study: Smart Deworming Saves the Farm 🐴

1. A Texas farm with 10 horses was deworming every 6 weeks with store-bought pyrantel.
2. Fecal tests showed persistent shedding, and two horses had colic symptoms despite treatment.
3. Vet recommended strategic use of moxidectin, rotational pasture management, and reduced deworming frequency.
4. After 1 year, fecal counts were 75% lower, colic signs vanished, and resistance risk dropped.

FAQs About Resistance

Is there a new class of dewormers coming?

A: Not yet. No new dewormer classes have been developed for horses in decades.

Can herbal or natural products help?

A: Most lack scientific validation. Stick with vet-approved protocols and confirmed diagnostics.

How long does resistance take to develop?

A: Decades of misuse have caused today's problems. Resistance can develop faster in small herds with frequent treatment.

Conclusion

Dewormer resistance is one of the biggest threats to equine health in 2025—but it's also one of the most preventable. By switching to evidence-based fecal testing, using only effective dewormers, and working with your veterinarian, you can reduce resistance and protect your horses for the long haul.

Need a custom parasite plan? Contact Ask A Vet at AskAVet.com or download our app 📱 to speak with Dr Duncan Houston and our team about testing, treatment, and prevention. 🩺🐎