Parasite Egg Shedding in Horses by a Vet – 2025 Strategic Deworming Guide 🪱🐴

By Dr Duncan Houston, BVSc

Introduction

In 2025, intestinal parasite resistance to dewormers is one of the most urgent health concerns in equine care. As older deworming protocols continue to fuel drug resistance, horse owners must now use targeted strategies to reduce parasite loads without overusing medications. The key tool? Fecal egg counts (FEC). This guide explains how egg shedding works, why selective deworming matters, and how to test smarter for long-term parasite control.

What Is Parasite Egg Shedding? 🧬

• Horses infected with intestinal parasites shed eggs in their manure
• These eggs contaminate pastures and infect other horses through grazing
• Shedding levels vary from horse to horse—some are heavy shedders, others are naturally resistant

Why Deworming Every 8 Weeks Is Outdated ⚠️

• Decades ago, deworming every 6–8 weeks was common practice
• This blanket approach killed off susceptible worms—but allowed resistant parasites to thrive
• Now, the most important parasite in adult horses—small strongyles—shows high resistance to most dewormers
• No new dewormer classes are being developed—so preserving current drugs is critical

The 80/20 Rule of Egg Shedding 💩

Research shows that in most horse herds:

• 20% of horses shed 80% of the eggs into the environment
• Only this 20% need to be dewormed more than 1–2 times per year
• Low-shedding horses do not contribute significantly to parasite burdens

How Fecal Egg Counts Help 📊

• FECs measure eggs per gram (EPG) of manure
• Results help identify high, moderate, and low shedders
• Allows your vet to design a custom deworming schedule for each horse

Shedding Categories:

Consistency of Egg Shedding 🧩

A study from England revealed:

• Adult horses tend to have consistent shedding patterns over time—even if untreated
• Young horses (<5 years) show more variation—so they require more frequent FECs

This means in adult horses, once a high or low shedder is identified, they usually remain in that category year to year.

When to Perform FECs ⏰

• Spring and fall are ideal times to check
• Test before deworming to determine if it’s needed
• Perform Fecal Egg Count Reduction Test (FECRT) 10–14 days after treatment to confirm effectiveness

Custom Deworming Program Example 📋

1. Vet collects manure from each horse in early spring
2. Results: 3 high shedders, 5 low, 2 moderate
3. Only high and moderate shedders are treated in spring
4. Repeat testing in fall to monitor changes

Additional Parasite Control Tools 🛡️

• Manure removal: Clear paddocks 1–2 times weekly
• Pasture rotation: Rest grazing areas and avoid overgrazing
• Don’t drag pastures unless weather is hot and dry
• Keep feed off the ground to reduce exposure

Dewormer Classes & Resistance 📉

Some dewormers are no longer effective in many areas:

• Fenbendazole (Safe-Guard): High resistance
• Pyrantel (Strongid): Resistance increasing
• Ivermectin & Moxidectin: Still effective—but egg reappearance times are shortening

FAQs About Egg Shedding

Q: How often should I test adult horses?

A: Once or twice per year—unless shedding level is unclear or changing.

Q: What about foals and yearlings?

A: Test and deworm more frequently—they are more vulnerable and shed inconsistently.

Q: Should I rotate dewormers?

A: No. Rotate only if needed based on FEC and FECRT results—not by the calendar.

Conclusion

Parasite control in 2025 must be science-based. Fecal egg count testing helps you avoid unnecessary treatments and reduce resistance by targeting the true shedders in your herd. Most adult horses do not need frequent deworming. Let your vet guide your farm toward sustainable, effective parasite control using fecal monitoring as your foundation.

Ready to test your herd or develop a strategic deworming plan? Contact Ask A Vet at AskAVet.com or use the app 📱 to speak with Dr Duncan Houston and the equine parasite control team. 🩺🐴