Atypical Myopathy in Horses: Box Elder and Sycamore Seed Toxicity
- See your vet immediately. Atypical myopathy is a life-threatening muscle disease linked to hypoglycin A in box elder seeds in North America and sycamore maple seeds in Europe.
- Signs often start suddenly in horses on pasture, especially in autumn or early spring after windy, rainy weather. Weakness, stiffness, trembling, dark urine, fast breathing, and trouble standing are major red flags.
- Diagnosis usually relies on pasture exposure history, a physical exam, and bloodwork showing very high muscle enzymes such as CK and AST. Your vet may also run urine testing and specialized metabolic testing.
- There is no antidote. Treatment is supportive and may include stall rest, IV fluids, pain control, correction of electrolyte problems, nutritional support, and intensive monitoring for kidney or heart complications.
- Typical US cost range is about $800-$2,500 for urgent field evaluation and initial testing, and roughly $3,000-$10,000+ if hospitalization and critical care are needed.
What Is Atypical Myopathy in Horses?
Atypical myopathy, also called seasonal pasture myopathy or atypical myoglobinuria, is a severe muscle disease that can affect horses turned out on pasture. It is most strongly linked to eating seeds or seedlings from certain maple trees, especially box elder (Acer negundo) in North America and sycamore maple (Acer pseudoplatanus) in Europe. The toxin involved is hypoglycin A.
After a horse eats enough of this toxin, the body can no longer use certain fats normally for energy. That causes rapid damage to skeletal muscle and can also affect the heart and breathing muscles. Horses may look weak, painful, shaky, or suddenly unable to stand. Dark or coffee-colored urine can appear when damaged muscle releases myoglobin.
This is an emergency because horses can decline quickly, sometimes within hours. Even with treatment, the condition can be fatal. Fast recognition, immediate removal from the pasture, and prompt veterinary care give your horse the best chance.
Symptoms of Atypical Myopathy in Horses
- Sudden weakness or profound lethargy, often in a horse that was recently grazing
- Muscle stiffness, trembling, or reluctance to walk
- Low head carriage and a dull, depressed attitude
- Fast heart rate and fast breathing
- Dark brown or red-brown urine from myoglobin release
- Sweating, especially with weakness or muscle pain
- Difficulty standing, stumbling, or progressing to recumbency
- Difficulty swallowing or choke-like signs in some horses
- Signs of respiratory distress if breathing muscles are affected
- Sudden death in severe cases
Mild early signs can look like colic, tying-up, or general weakness, so this condition is easy to underestimate at first. What makes it especially concerning is how quickly it can progress. A horse that seems stiff or tired in the morning may be unable to stand later the same day.
See your vet immediately if your horse has weakness, dark urine, trembling, trouble breathing, or trouble standing, especially after turnout near box elder or sycamore-type trees. Keep the horse calm, remove access to the pasture, and avoid forcing exercise while you wait for veterinary guidance.
What Causes Atypical Myopathy in Horses?
Atypical myopathy is caused by hypoglycin A toxicity. In North America, the main concern is box elder seeds and seedlings. In Europe, the best-known source is sycamore maple seeds and seedlings. Horses are usually exposed while grazing, especially when seeds have blown into pasture or when spring seedlings are growing in turnout areas.
Cases are reported most often in autumn and early spring. Risk appears to increase after windy, rainy, or cool weather, when more seeds or seedlings are available on pasture. Horses kept out for long periods, especially with limited supplemental forage, may be more likely to nibble seeds or seedlings.
The toxin is converted in the body into compounds that block multiple enzymes needed for normal fat metabolism. That energy failure leads to widespread muscle injury. Not every exposed horse becomes sick, and the exact toxic dose can vary, which is one reason pasture mates may be affected differently.
This is not a contagious disease. One horse cannot spread it to another. The shared risk is the pasture environment, so any horse exposed to the same field should be considered at risk and discussed with your vet.
How Is Atypical Myopathy in Horses Diagnosed?
Your vet usually starts with the combination of history, season, pasture exposure, and exam findings. A horse with sudden weakness, stiffness, dark urine, and access to box elder or sycamore-type seeds raises immediate concern. Because this disease can worsen quickly, treatment may begin while testing is still underway.
Routine bloodwork often shows markedly increased CK and AST, which are muscle enzymes released during muscle damage. Your vet may also check electrolytes, kidney values, blood glucose, and acid-base status, because complications can develop as muscle breaks down. Urine may contain myoglobin, which explains the dark color.
In some cases, your vet may recommend specialized testing such as acylcarnitine profiling in blood or organic acid testing in urine to support the diagnosis. These tests help identify the metabolic pattern caused by hypoglycin A exposure. There is not always a widely available commercial hypoglycin A test in the United States, so diagnosis is often based on the overall clinical picture plus supportive lab findings.
Your vet may also consider other causes of weakness or muscle disease, including exertional rhabdomyolysis, ionophore toxicity, neurologic disease, or other toxic plant exposures. That is why a careful pasture history and a full exam matter so much.
Treatment Options for Atypical Myopathy in Horses
Spectrum of Care means you have options. Here are treatment tiers at different price points.
Budget-Conscious Care
- Emergency farm call or urgent exam
- Immediate removal from pasture and strict stall rest
- Baseline bloodwork such as CK, AST, kidney values, and electrolytes
- Oral or limited supportive fluids if appropriate and safe
- Pain control and anti-inflammatory support as directed by your vet
- Warmth, close observation, and easy access to water and forage
- Evaluation of exposed pasture mates
Recommended Standard Treatment
- Hospitalization or day-hospital level monitoring
- IV fluids to support circulation and help protect the kidneys
- Serial bloodwork to track muscle enzymes, electrolytes, and kidney function
- Pain control, nursing care, and careful nutritional support
- Monitoring for heart and respiratory complications
- Urine monitoring and repeat exams
- Pasture risk review and guidance for herd management
Advanced / Critical Care
- 24-hour equine hospital or ICU care
- Aggressive IV fluid therapy and correction of electrolyte or acid-base abnormalities
- Continuous monitoring for arrhythmias, respiratory compromise, and kidney injury
- Advanced diagnostics such as blood gas analysis, ECG, and specialized metabolic testing
- Nasogastric or assisted nutritional support when needed
- Intensive nursing for recumbent horses, including pressure sore prevention and assisted standing when appropriate
- Frequent reassessment of prognosis and welfare
Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.
Questions to Ask Your Vet About Atypical Myopathy in Horses
Bring these questions to your vet appointment to get the most out of your visit.
- Do my horse’s signs and pasture history fit atypical myopathy, or are there other likely causes of weakness?
- What blood and urine tests do you recommend today, and what results would make hospitalization the safer option?
- Is my horse stable enough to stay on the farm, or do you recommend referral to an equine hospital now?
- What complications are you most worried about, such as kidney injury, heart problems, or breathing muscle involvement?
- What supportive treatments are available at each care level, and what cost range should I plan for?
- Should my other horses be removed from the pasture and tested or monitored even if they look normal?
- Are there box elder trees, seedlings, or windblown seeds on my property that need to be fenced off or removed?
- What signs at home mean I should call back immediately or transport my horse urgently?
How to Prevent Atypical Myopathy in Horses
Prevention focuses on pasture management. Walk turnout areas in fall and spring and look for box elder trees, seeds, and seedlings. If these trees are present, talk with your vet and farm team about fencing off the area, restricting turnout during high-risk periods, and removing fallen seeds where practical. Wind can carry seeds into fields that do not have trees directly inside the fence line, so check neighboring areas too.
Risk may be higher when pasture is sparse and horses are grazing for many hours with little supplemental forage. Offering adequate hay and avoiding overgrazed turnout may reduce the chance that horses nibble seeds or seedlings. Bringing horses in during high-risk weather periods, especially after strong winds or storms, may also help.
If one horse on a property is diagnosed, treat pasture mates as exposed until your vet says otherwise. Remove them from the field, monitor closely for weakness or dark urine, and discuss whether screening bloodwork is appropriate. Early recognition matters.
There is no vaccine or home antidote for hypoglycin A toxicity. The most effective prevention is reducing exposure and acting quickly when signs appear.