Parasite Resistance in Horses by a Vet – 2025 Strategic Deworming & Rotation Guide 🪱🐴

By Dr Duncan Houston, BVSc

Introduction

Intestinal parasites are a constant threat to your horse’s health—but in 2025, one of the biggest challenges is dewormer resistance. Older products that were once reliable, like fenbendazole (Panacur or Safeguard), are now failing in many horse populations. This guide covers how parasites develop resistance, what dewormers still work, and how strategic rotation can restore effectiveness with your veterinarian’s guidance.

The Shift from Large to Small Strongyles 🐛

• Historically, large strongyles were the biggest problem in horses
• These have been effectively controlled through routine deworming
• Now, small strongyles (cyathostomins) are the major concern—especially in young horses

How Resistance Develops 🧬

Resistance occurs when worms survive treatment and pass on genes that allow their offspring to survive deworming too.

• Overuse of a single product (e.g., Safeguard) leads to survival of resistant strains
• Once resistance is established, that product may no longer reduce egg counts

Case Study: Resistance from Overuse

• A herd dewormed exclusively with Safeguard for just 18 months showed complete resistance in small strongyles
• Resistance was higher in younger horses compared to older adults

Can Rotation Help? 🔄

Yes—with veterinary guidance and the right schedule. A study examined a group of horses with proven resistance to Safeguard.

Rotation Plan (Every 90 Days):

• Ivermectin (macrocyclic lactone)
• Pyrantel (Strongid – tetrahydropyrimidine)
• High-dose Safeguard (benzimidazole)
• Moxidectin (Quest – macrocyclic lactone)

Outcome:

• Each product, including Safeguard, became effective again after being used in rotation
• Parasite loads dropped and fecal egg count reduction improved

Why Vet Input Matters 🩺

• Vets know which parasites are common in your area
• They can guide when to rotate and what to use based on fecal egg count (FEC) data
• They’ll help with Fecal Egg Count Reduction Tests (FECRT) to assess dewormer effectiveness

Recommended Dewormers by Class 💊

When to Rotate 🔁

• Rotate every 3–4 months only under veterinary supervision
• Perform fecal testing before and after deworming to track efficacy
• Do not rotate “just to rotate”—base changes on lab results and effectiveness

Customized Control Program 🗂️

1. Vet performs fecal egg counts on all horses in spring
2. High shedders receive appropriate dewormer based on rotation plan
3. Repeat FEC in fall; re-evaluate rotation based on resistance trends

Young Horses Need Special Attention 🧒🐎

• Parasites in foals and yearlings tend to resist faster
• They shed higher numbers and respond inconsistently
• Require more frequent monitoring and adjusted protocols

FAQs About Parasite Resistance

Q: Should I still use Panacur or Safeguard?

A: Only if your vet confirms it’s effective through egg count reduction testing.

Q: Is rotation better than sticking with one dewormer?

A: Yes—when done based on fecal data, rotation can slow resistance and restore product effectiveness.

Q: Can resistance be reversed?

A: Not reversed, but careful rotation and monitoring can manage it and reduce shedding.

Conclusion

Resistance to dewormers—especially among small strongyles—is a growing concern in 2025. Fortunately, strategic rotation, combined with fecal egg count monitoring and vet involvement, gives horse owners a reliable way to control parasites while preserving dewormer effectiveness. Resist the urge to buy over-the-counter products blindly—partner with your vet to protect your horses and your pasture.

Need help assessing resistance or building a rotational plan? Visit AskAVet.com or our app 📱 to speak with Dr Duncan Houston and the equine parasite management team. 🩺🐴