Atypical myopathy, also called pasture myopathy, is an acute muscle disease in horses that live outdoors. The disease often moves fast and often ends badly. So act at the first sign. This guide explains what it is, which maples pose a risk, when the danger is greatest, and what you can do in the pasture. If you suspect it, call the vet right away.
What Atypical Myopathy Is
The Utrecht University Animal Hospital (UDI, Universitair Dierenziekenhuis) describes the disease as a pasture-related muscle disease: it occurs in horses that live in the pasture and get little or no supplementary feed. Fat metabolism is disrupted, so muscle cells break down. It mainly affects the muscles used for standing and breathing. The name "atypical" comes from the fact that the disease differs from the muscle stiffness seen after exertion.
The cause is a toxin, hypoglycin A. It is found in the seeds and seedlings of the sycamore maple (Acer pseudoplatanus), and to a lesser extent in the leaves. So this is poisoning, not infection. According to Utrecht University, it has not been proven beyond doubt that maples cause every case. The disease occasionally occurs in pastures with no maples at all.
Which Maples Are a Risk
Utrecht University and RIKILT (Wageningen) tested 278 samples of the three most common maple species in the Netherlands. Only in the sycamore maple was the toxin found, in every sample. That doesn't mean every tree is equally dangerous: the amount varies widely from tree to tree.
| Maple | What research shows |
|---|---|
| Sycamore maple (Acer pseudoplatanus) | Toxin found in all Dutch samples, in varying amounts |
| Field maple (Acer campestre) | No toxin found in the Utrecht study; Paardenpunt Vlaanderen calls it proven non-toxic |
| Norway maple (Acer platanoides) | No toxin found in the Utrecht study; according to Paardenpunt Vlaanderen none to very little |
| Box elder (Acer negundo) | Contains the toxin; according to Paardenpunt Vlaanderen, cases are linked to this species mainly in the United States |
| Japanese and silver maple | According to Paardenpunt Vlaanderen, none to very little toxin detected |
There are hundreds of maple species and varieties that also cross with each other, writes Paardenpunt Vlaanderen (the Flemish horse sector information point), and too little research has been done. The sycamore and Norway maple look very much alike. If you don't know which maple stands near your pasture, treat it as a sycamore maple.
When the Risk Is Highest
- Fall: seeds and leaves drop. Paardenpunt Vlaanderen names the period from October until the first hard frost. According to the Merck Veterinary Manual, the disease occurs most often in fall.
- Spring: the seedlings come up. Many seeds in fall mean many seedlings in spring.
- Weather: the disease is mainly seen during or after cold, wet, windy weather.
- Pasture: little grass, no supplementary feed or feed that is placed on the ground, or a damp pasture covered in dead leaves.
- Horse: all ages can fall ill; according to Paardenpunt Vlaanderen, young horses under three carry more risk. Good body condition doesn't rule the disease out.
- Distance: the tree doesn't have to stand in the pasture. According to Paardenpunt Vlaanderen, the wind can carry maple leaves up to 100 meters.
Seedlings in Hay
Dried seedlings and leaves stay toxic. Paardenpunt Vlaanderen writes that seedlings in year-old hay were still found to be toxic. If your hay comes from a field next to maples, check when you open the bale whether there are shoots or leaves in it.
The Symptoms
Signs appear quickly. Watch for:
- not wanting to walk, moving stiffly;
- lying down a lot, or being unable to get up;
- hard, painful muscles, trembling, and sweating;
- dark, coffee-colored urine;
- listlessness, holding the head low;
- trouble swallowing and heavier breathing.
The picture varies. Some horses first show dullness, stiffness, or mild colic signs; others are found dead in the pasture or lying on their side. Several horses in a group can fall ill at the same time, while others show no signs. If it looks like colic but the horse is in a pasture with maples, tell the vet.
What to Do If You Suspect It
- Call the vet right away. Say you suspect atypical myopathy and where the horse was standing.
- Move the horse as little as possible. According to the UDI, walking and transport cause extra muscle damage. Put the horse inside, as close as possible to where it was standing. If there is no stable, Utrecht University advises setting up an emergency shelter.
- Keep the horse warm. Use a blanket, and rub a horse that is sweating heavily dry.
- Provide rest and comfort. As general measures, the UDI names deep bedding, good hay, and turning a horse that is lying down onto its other side every four hours. Ask the vet what fits your situation.
- Take the other horses out of the pasture and watch them for the next three days, says Paardenpunt Vlaanderen. Only let them back once seeds, seedlings, and leaves have been removed.
- Don't give any medication yourself. The vet examines the horse, takes blood and urine, and gives painkillers and an IV drip. According to the Merck Veterinary Manual, the treatment is supportive.
Once the horse has recovered, blood tests for muscle enzymes show the vet when it can slowly start training again.
How Likely Is Recovery
Utrecht University calls the disease usually fatal. According to Paardenpunt Vlaanderen, more than 75 percent of affected horses die within 72 hours. The sources used give no Dutch survival rate. What you can do: call the vet quickly and keep the horse moving as little as possible.
Prevention in the Pasture
There is no guarantee, but these measures lower the risk:
- Walk the pasture in fall and spring, and after every windy spell.
- Add forage when there is little grass, in a dry spot or a hay rack, and not on the ground. Provide shelter.
- Limit access to seeds, seedlings, and leaves. Make the pasture smaller with electric tape or a fence, so the distance to the trees gets larger.
- Remove leaves and seeds with a leaf blower and pull up seedlings. Take them away, because dried they stay toxic. Keep horses inside after a windy spell until everything is cleared.
- Limit time outdoors where that fits. According to Utrecht University, horses that spend less than six hours a day outside almost never get the disease. Full stabling is also sometimes advised, but Utrecht University considers it better for welfare to keep the pasture free of seeds, seedlings, and leaves.
- Use a risk pasture only with enough grass or supplementary feed. If you can use another, maple-free pasture, Paardenpunt Vlaanderen calls that the best measure.
Cutting down trees is usually not necessary. Utrecht University advises against it around horse pastures, because not all maples are toxic. Paardenpunt Vlaanderen calls cutting only worthwhile for one or a few large trees in the pasture itself, so the advice differs a little. Don't plant any new sycamore maple next to a horse pasture.
Frequently Asked Questions
Is atypical myopathy contagious?
No. It is poisoning by a toxin, not an infection. A sick horse doesn't infect another horse. Horses in the same pasture can eat the same seeds or seedlings, though, so take them out of the pasture and watch them.
Which maple is toxic?
In Dutch research, only the sycamore maple contained the toxin, in varying amounts. The field maple and Norway maple turned out to contain no toxin. If you don't know the species, treat the tree as a sycamore maple.
When is the risk highest?
In fall, when seeds and leaves drop, and in spring, when the seedlings come up. Cold, wet, windy weather and little grass in the pasture raise the risk.
Can a horse survive it?
Sometimes, but the disease is usually fatal. According to Paardenpunt Vlaanderen, more than 75 percent of affected horses die within 72 hours. So call the vet right away.
Should I cut down the maples near my pasture?
Usually not. Utrecht University advises against cutting around horse pastures, because not all maples are toxic. It is better to keep the pasture free of seeds, seedlings, and leaves.
Are seedlings in hay dangerous?
Yes. Dried seedlings stay toxic, even after a year. Check hay from fields near maples for leaves and shoots.
In Short
Atypical myopathy is poisoning with hypoglycin A from the seeds and seedlings of the sycamore maple. The risk is highest in fall and spring, especially in cold, wet, windy weather and with little grass. If you recognize stiffness, lying down a lot, trembling, sweating, or dark urine, call the vet right away, move the horse as little as possible, and keep it warm. Prevention means a pasture free of seeds and seedlings, supplementary forage, less time outside, and attention to hay.
Sources (verified November 2, 2026): Utrecht University Animal Hospital: Myopathy · Utrecht University Animal Hospital: Many cases of atypical myopathy in horses · Utrecht University: Many cases of atypical myopathy in horses · Paardenpunt Vlaanderen: Maple poisoning in horses (atypical myopathy) · Merck Veterinary Manual (MSD): Plants Causing Myopathies in Horses
See also: Toxic Plants for Horses · Colic in Horses: What to Do · When to Call the Vet · Pasture Management and Parasites · How to Feed Your Horse Well · Level: Considering Your Own Horse