🐴 Seasonal Pasture Myopathy in Horses: Prevention & Awareness for 2025 🍂🧬

By Dr Duncan Houston BVSc

Seasonal pasture myopathy (SPM) is a rare but often fatal muscle disease that affects horses grazing on pastures, particularly during the fall. Although it was once mysterious, recent research has revealed that the culprit is a toxin found in the seeds of certain trees—especially the box elder. In 2025, this condition remains highly deadly, but it's also preventable with proper awareness and pasture management. Here’s what every horse owner needs to know. 🍁🐎

🧬 What Is Seasonal Pasture Myopathy?

SPM is a type of acute rhabdomyolysis, or rapid muscle breakdown. It primarily affects skeletal, respiratory, and cardiac muscles. Horses affected by SPM often show severe clinical signs and deteriorate quickly. 🧠💥

Key traits of the disease:

• 📆 Occurs seasonally—mostly in the fall
• 🌾 Associated with pastures lacking grass or forage
• 🍃 Linked to ingestion of box elder tree seeds (Acer negundo)
• 💀 Fatal in nearly 90% of diagnosed cases

📍 Where Is It Found?

While SPM was originally identified in the Midwest, it is now being reported in other parts of the U.S., including East Texas, due to the spread of the box elder tree. 🌳📉

The disease is most common in regions where horses graze near these trees—especially in the fall when seeds drop to the ground. 🧠

🚨 Signs of Seasonal Pasture Myopathy

The clinical signs of SPM are often severe and rapidly progressive:

• 🐌 Stiff gait and reluctance to move
• 🛌 Difficulty standing or frequent lying down
• 🧬 Dark brown or red urine (due to myoglobinuria)
• 💨 Difficulty breathing as respiratory muscles weaken
• 💓 Rapid heart rate and signs of cardiovascular collapse

Because of how quickly the condition progresses, early detection is crucial—but difficult. Many horses die within 72 hours. 🕒💀

🧪 The Role of the Box Elder Tree

Thanks to research by Dr. Stephanie Valberg and her team at the University of Minnesota, we now know that the seeds of the box elder tree contain a toxin called hypoglycin A, which interferes with muscle metabolism. 🌳🔬

• ⚠️ Hypoglycin A disrupts energy production in muscle cells
• 💥 Leads to rapid, widespread muscle death

Unfortunately, not all horses exposed to box elder seeds develop the disease, and we still don’t know why some are more vulnerable than others. 🧬

🎯 Risk Factors

Several conditions increase the likelihood of SPM:

• 📅 Time of year: Primarily fall (October–November)
• 🌳 Presence of box elder trees in or near pasture
• 🌱 Sparse or overgrazed pasture, increasing chance of seed ingestion
• ⏱️ Horses turned out for more than 12 hours/day
• 🐎 Young or curious horses may be more at risk

🛑 How to Prevent Seasonal Pasture Myopathy

Prevention is the only effective defense against SPM. There is no specific cure once clinical signs appear. 🔐

Here’s what you can do:

✅ Step 1: Inspect Pastures

• 🔍 Look for box elder trees—identify by their compound leaves and helicopter-shaped seeds
• 🍂 Remove or fence off areas near box elder trees in fall

✅ Step 2: Provide Adequate Forage

• 🌾 Keep hay or pasture grass available to reduce foraging for seeds
• 🥕 Consider dry lot turnout if pasture is sparse in fall

✅ Step 3: Reduce Turnout During Risk Periods

• 📆 Limit turnout during fall seed drop (October–November)
• 🧽 Use paddocks or stalls during high-risk days

✅ Step 4: Educate and Monitor

• 👀 Watch for early signs—stiffness, dark urine, lethargy
• 📢 Inform all barn staff and caretakers about SPM and its risks

🧠 What to Do If You Suspect SPM

If you suspect a horse is developing SPM:

1. 📞 Call your vet immediately—this is an emergency
2. 💉 Bloodwork can check muscle enzymes (e.g., CK, AST)
3. 🩸 Urine may show dark color from myoglobin
4. 💊 Treatment is supportive—IV fluids, anti-inflammatories, antioxidants

Prognosis is poor once symptoms appear, so prevention is critical. 🛡️

📲 Ask A Vet for Pasture Safety Guidance

Not sure if your pasture is at risk? Visit AskAVet.com or use the Ask A Vet App to send photos of trees, get forage tips, and develop a seasonal turnout plan. 📱🌿

Dr Duncan Houston and the team can help you create a fall pasture safety checklist to reduce risk and increase peace of mind. 🧠🌳

🏁 Final Thoughts

Seasonal pasture myopathy is a tragic but avoidable condition. In 2025, we know more than ever about the toxic link between box elder seeds and deadly muscle failure. With proper management, most cases can be completely prevented. 🧬❤️

If you live in or near areas with box elder trees, don’t take chances—plan ahead each fall. And when in doubt, keep your vet close and forage options safer. 🐴

Need a personalized pasture review? Visit AskAVet.com to prevent seasonal pasture myopathy in your herd 🐎🌿