Weekly Diclazuril to Prevent EPM in Horses by a Vet – 2025 Update on Preventive Therapy 🧠🐴
In this article
Weekly Diclazuril to Prevent EPM in Horses by a Vet – 2025 Update on Preventive Therapy 🧠🐴
By Dr Duncan Houston, BVSc
Introduction
Equine Protozoal Myeloencephalitis (EPM) is one of the most serious neurological diseases affecting horses in North America. Caused by the protozoan Sarcocystis neurona, EPM often strikes horses under stress and with weakened immunity. While several medications exist to treat the disease, preventive strategies have remained elusive—until now. A new 2025 update sheds light on a promising use of weekly diclazuril dosing to potentially protect horses before infection sets in.
What Is EPM? 🧬
- Caused by ingesting protozoa found in opossum feces
- Not all exposed horses become sick—but those that do may develop:
- Muscle atrophy
- Incoordination (ataxia)
- Facial nerve paralysis
- Behavior changes
Who Is at Risk? ⚠️
- Young horses
- Performance or show horses under stress
- Horses recovering from illness or injury
- Horses exposed to opossums or contaminated feed/water
Challenges in EPM Prevention 🧪
- Not all horses exposed to Sarcocystis neurona develop clinical disease
- That makes it difficult to test preventive strategies in live trials
- Studies must rely on blood levels of medication and known inhibitory thresholds
New Study: Once-Weekly Diclazuril 💊
Recent research evaluated whether once-weekly dosing of diclazuril—an FDA-approved treatment for EPM—might be protective.
Key Findings:
- Diclazuril was administered once a week at the standard label dose
- Plasma concentrations were measured after dosing
- Results showed that drug levels were 130 times higher than what is needed to inhibit the protozoal organism
Why This Matters 🧠
- This new approach may allow at-risk horses to maintain constant therapeutic levels with less frequent dosing
- It could offer a cost-effective way to reduce infection risk during show season or after stressful events
What’s Next in Research 🔍
- This was a pharmacokinetic study—not a clinical trial
- More data is needed to confirm whether once-weekly dosing actually prevents clinical disease
- Larger, longer-term studies are required before making it standard protocol
How Diclazuril Works 🧫
- Targets and inhibits the protozoal enzyme needed for reproduction
- Used in labeled form for treatment of active EPM over 28 days
- Oral pellet or paste formulations available
When to Consider Preventive Diclazuril 💭
- During high-risk times—intensive training, travel, show season
- On endemic farms with previous EPM cases
- When horses show low-grade or recurring neurologic signs and exposure is suspected
Vet Recommendations in 2025 🩺
- Do not begin preventive diclazuril without vet consultation
- Use only in appropriate candidates—not all horses need it
- Weekly administration may reduce cost and labor, but effectiveness still under review
Additional Preventive Strategies 🛡️
- Keep opossums away from feed and water areas
- Use rodent-proof bins for grain and supplements
- Limit pasture access during periods of wet ground or wildlife activity
- Reduce stress exposure by adjusting workload and nutrition
Case Example: Preventive Dosing Trial
- A barn with past EPM cases administered once-weekly diclazuril to high-risk performance horses during competition season
- Blood tests showed levels 100x over the inhibitory threshold
- No new cases of EPM occurred, but formal efficacy still needs confirmation
FAQs About Preventive Diclazuril
Q: Is once-a-week diclazuril approved for prevention?
A: Not yet—it is an off-label use based on pharmacokinetic data, not clinical outcome trials.
Q: Will this prevent all EPM cases?
A: No preventive guarantees 100% protection, but this approach may reduce risk significantly in high-exposure horses.
Q: What about side effects?
A: Diclazuril is generally well tolerated. Your vet can monitor for any changes in appetite or GI function.
Conclusion
EPM remains a serious threat to horse health—but new research into once-weekly diclazuril is a promising step toward prevention. By maintaining high blood levels of an FDA-approved treatment drug, we may be able to protect horses during peak stress and exposure. Until further studies are published, always consult your veterinarian about preventive options tailored to your horse's needs.
Considering EPM prevention this season? Visit AskAVet.com or use the app 📱 to consult directly with Dr Duncan Houston and the neurology team. 🩺🐴
