💊 Vet Strategies to Prevent Dewormer Resistance in Horses | 2025 Parasite Control Guide 🐴

By Dr Duncan Houston, BVSc

For decades, deworming has been one of the most common and trusted ways to keep horses healthy. But today, thanks to overuse and outdated protocols, a major challenge has emerged: dewormer resistance. 🧫🐛

Intestinal parasites are still a threat, but the tools we once relied on to control them—many used for 30+ years—are rapidly losing their effectiveness. In this 2025 vet guide, I’ll explain what resistance is, how it developed, and what you can do today to protect your horse and your farm from parasite overload. 🧠🐴

🦠 The Role of Parasites in Horse Health

All grazing horses carry some level of intestinal parasites. Small strongyles (cyathostomins) are now considered the most important equine parasite due to their prevalence and the potential for severe health effects, including:

• ⚖️ Weight loss or poor condition
• 🥱 Lethargy and poor performance
• 💩 Diarrhea and colic risk
• 🩸 Anemia or protein loss

In the past, routine deworming every 8 weeks was standard. But this strategy has led to a dangerous shift: the most resilient parasites have survived and multiplied, while the susceptible ones were wiped out. 📉🦠

🧬 What Is Dewormer Resistance?

Dewormer resistance occurs when parasites develop the ability to survive treatments that previously killed them. Over time, resistant worms become the dominant population in your horse’s gut—and eventually, the medications stop working altogether. 😱💊

This is not theoretical—it’s already happening. Many of the drugs that worked well in the 1990s are now virtually ineffective against small strongyles. ⚠️

📉 The Decline of Dewormer Effectiveness

Let’s look at how drug performance has changed over time due to resistance:

These shortened reappearance periods show how resistant parasites are rebounding much faster post-treatment. 🧬🔁

🧪 Fecal Egg Count Testing: The Modern Approach

Rather than automatically deworming every 8 weeks, vets now recommend a smarter approach: Fecal Egg Count (FEC) testing. This lab test evaluates how many parasite eggs are present in a horse’s manure. 💩🧫

📋 Benefits of FEC Testing:

• 📉 Reduces unnecessary deworming
• 🔍 Identifies high-shedding horses
• 🧬 Tracks resistance development
• 💰 Saves money on meds you don’t need

Your vet will collect a manure sample and count the number of eggs per gram (EPG). Horses with high egg counts will be treated, while low-shedding horses may not need deworming at all. 🧪✅

📊 Interpreting Egg Counts

Here’s how equine fecal egg counts are typically interpreted:

• 🔵 Low Shedders: < 200 EPG – may not need deworming
• 🟡 Moderate Shedders: 200–500 EPG – treatment depends on farm history
• 🔴 High Shedders: >500 EPG – deworming recommended

Some horses consistently shed more eggs than others due to genetics and immune function. Identifying those individuals helps tailor your deworming program. 🧠📈

💡 Strategic Deworming in 2025

Effective parasite control doesn’t mean treating every horse all the time. Instead, use strategic deworming—treat only the horses that need it, based on testing and season. 🌿📆

🌼 Spring & Autumn Are Key:

• 🌸 Spring – test all horses as they come off winter pasture
• 🍂 Fall – treat high shedders and target tapeworms with praziquantel

🚫 Avoid “Calendar Deworming”:

Automatic treatments every 6–8 weeks lead to resistance. Deworm based on results—not habit. 🔁💊

🚫 Dewormers to Avoid for Strongyles

Due to resistance, the following dewormers are often ineffective against small strongyles:

• 🟤 Fenbendazole (Panacur)
• 🟠 Oxibendazole (Anthelcide)
• ⚪ Pyrantel (Strongid)

Stick with ivermectin and moxidectin for now—but even these need careful timing and testing to stay effective. 🧪🛡️

🏇 Farm Management: Reducing Parasite Pressure

You can reduce worm load without medication by improving pasture and hygiene practices. 🧹🌱

Best Practices Include:

• 💩 Pick up manure from paddocks weekly
• 🌾 Rotate pastures to break parasite life cycles
• 🐎 Avoid overcrowding and overgrazing
• 🚫 Don’t spread fresh manure where horses graze

Combining these methods with strategic deworming gives you the best long-term control. 🧠🏆

🧬 Why Resistance Matters: No New Drugs in Sight

The development of new dewormers is slow and expensive. Unlike antibiotics, antiparasitic drugs for horses receive far less investment. That means the tools we have now need to last. 🧪🛠️

Once we lose ivermectin and moxidectin to widespread resistance, there may not be an alternative. That’s why proactive testing and smart treatment are so critical today. 🛑🧬

📲 Use Ask A Vet to Manage Deworming Smarter

The Ask A Vet app helps horse owners like you:

• 📆 Set reminders for fecal testing and deworming
• 📲 Get vet-approved parasite control plans
• 📸 Upload manure sample images for review
• 📘 Track each horse’s deworming history

It’s a smarter, data-driven approach to parasite prevention—right at your fingertips. 🐎📱

✅ Quick Takeaways for 2025

• 🧫 Resistance to dewormers is rising—especially for small strongyles
• 📉 Many drugs from the 80s/90s are now ineffective
• 🧪 Fecal egg count testing is the new standard of care
• 📊 Treat only horses that truly need it, based on testing
• 🌾 Combine testing with pasture management for best results

📲 Final Thoughts from Dr Duncan Houston

It’s tempting to reach for the dewormer tube out of habit—but doing so without testing may be doing more harm than good. Dewormer resistance is a serious, global issue—but together, with smarter strategies, we can preserve these critical tools for future generations of horses. 🐴💙

Start with fecal testing, talk to your vet, and use the Ask A Vet app for tailored parasite control advice in 2025. 🧬📲