Atypical myopathy in horses: sycamore poisoning signs and prevention

Atypical myopathy is a severe muscle disease in horses caused by hypoglycin A, a toxin in sycamore seeds, seedlings and leaves. The Royal Veterinary College says it kills around three quarters of horses affected. If you suspect it, call your vet immediately.

It is October, so this is the time of year it matters most. The helicopter seeds are coming off the trees now, and every windy night puts more of them on the ground. Most owners have heard of sycamore poisoning. Fewer know what it looks like in the first hours, or that a horse with it will often carry on eating.

This post covers what it is, when the risk peaks, the signs to watch for, what to do if you see them, and the practical steps that reduce the risk on your own fields.

What is atypical myopathy in horses?

Atypical myopathy, also called sycamore poisoning or seasonal pasture myopathy, is a disease that breaks down a horse’s muscles quickly. The Royal Veterinary College’s fact file describes it as a severe and often fatal muscle disorder caused by eating sycamore seeds, leaves or seedlings.

The toxin responsible is hypoglycin A, usually shortened to HGA. It slows or stops energy production in muscle, and that includes the heart, which is part of why the disease is so dangerous.

The numbers are stark. The RVC puts it at fatal for around three quarters of affected horses. A joint statement from the British Horse Society and the Arboricultural Association gives mortality rates of between 61 and 90 percent. Those are figures from sources you can trust, and they are the reason every vet treats a suspected case as an emergency.

There is a hopeful side to it too. The RVC notes that horses which survive the first few days of treatment usually go on to recover completely, although that recovery can take several months. Getting a vet there early is what gives a horse its best chance of reaching that point.

Not every horse that eats the toxin falls ill. The RVC reports that some apparently unaffected horses carry high levels of HGA in their blood, which suggests some horses are more susceptible than others. That is why one horse in a field can be desperately ill while its companions look perfectly well.

Are sycamore seeds poisonous to horses?

Yes. The seeds of the common sycamore, Acer pseudoplatanus, contain hypoglycin A. So do the seedlings that sprout from those seeds in spring, and the leaves can carry it too.

A few points make the risk harder to judge than it first sounds.

• Not all maples are the same. The RVC notes there are more than 25 species of Acer and not all of them carry the toxin. The common sycamore does.
• Trees vary, and so do years. HGA levels differ between trees and between seedlings from them. A 2025 study in the Equine Veterinary Journal that measured seeds from trees across central England found the toxin level varied significantly from year to year in the same trees, and that trees close to each other tended to have similar levels.
• You cannot tell by looking. A field that has been grazed safely for years is not proof the tree is harmless. Redwings told the story of Millie, a cob who died of atypical myopathy in a paddock that had been grazed by horses for over a decade without a problem. Her fieldmates were unaffected.
• Water can carry it. The same 2025 study detected HGA in water that had sycamore seeds in it for 48 hours. A trough under a sycamore, or a stream running past one, is worth thinking about.

If you want certainty about a particular tree, the RVC’s Comparative Neuromuscular Diseases Laboratory offers a sample test for owners and yards that measures HGA in sycamore material.

When is the risk of atypical myopathy highest?

The disease follows the sycamore’s year. The RVC describes horses eating seeds that fall onto pasture in autumn and winter, and the germinating seedlings in spring. Those are the two peaks.

Two other things push the risk up.

Poor grazing. A horse with plenty of grass and forage has less reason to eat seeds and seedlings. A horse on a bare field will hoover up what it finds. This is why the British Horse Society’s advice leans so heavily on forage and stocking rates.

A big seed year. The 2025 study found that estimated sycamore seed production was positively linked to the number of atypical myopathy cases in the region. When the trees fruit heavily, more horses get ill. The authors also suggested, as a possibility rather than a finding, that unripe seeds brought down by strong winds might pose a greater risk.

What are the signs of atypical myopathy?

The disease comes on quickly. The signs the RVC lists are:

• General weakness. The horse struggles to walk, to stand and to breathe.
• Muscle trembling.
• Dullness, with the head hanging low.
• Signs that look like severe colic, but unlike most colic, the horse usually still has an appetite.
• Brown or dark red urine. This is the sign most owners have heard of, and it comes from muscle breaking down.
• Being unable to stand, in severely affected horses.

The RVC also notes that many horses develop heart problems, which you will not see from the gate.

In practice, the horse might look stiff and reluctant to move, as though every step hurts, or might be lying down more than usual and struggling to get up. If you are taking readings, the pulse and breathing may be well above that horse’s normal; the TPR guide covers how to take them.

The one that catches people out is the appetite. A horse that is off its food gets attention quickly. A horse that is weak and trembling but still grazing can be put down to a cold morning or a tweaked muscle. In autumn or spring, on a field with sycamores nearby, a weak or stiff horse that is still eating is not reassuring.

A stiff, reluctant horse can have plenty of other causes, from laminitis to tying up to colic. Telling them apart is your vet’s job, not yours. Your job is to notice and to call.

What should you do if you think your horse has atypical myopathy?

Call your vet immediately. This is an emergency. Do not wait to see whether it improves, and do not wait for dark urine to appear before you ring.

When you call, tell them:

1. That the horse grazes near sycamore trees, or on a field where seeds or seedlings could have blown in. Vets consider the horse’s environment as well as its signs, so this matters.
2. Exactly what you have seen and when it started: weakness, trembling, stiffness, whether it is lying down, whether it is still eating.
3. The colour of any urine you have seen.
4. Any readings you have taken, such as pulse, breathing and temperature, and the time you took them.

Then follow your vet’s instructions. The RVC notes that confirming the diagnosis can take several days, so vets often start treatment straight away, and a horse may need to go to a specialist equine hospital for round the clock care.

Ask your vet, too, about the other horses sharing that field. Several horses on the same pasture can be exposed at once, and what to do with them is a decision to make with your vet on the day.

How do you reduce the risk of sycamore poisoning?

The British Horse Society and Arboricultural Association statement and the RVC agree on the core of it: keep horses away from sycamore material, and make sure they never need to eat it. A pasture checklist for this time of year.

• Walk every field and note any sycamores, including those in hedges and on neighbouring land. The RVC says some helicopter seeds can travel up to 200 yards, and the BHS statement notes seeds have reached paddocks by being blown in or carried downstream.
• Fence off the area under and around the trees, wide enough to cover where the seeds land, if moving the horses is not an option.
• Clear fallen seeds and leaves from grazing areas, and in spring pull up seedlings as they appear.
• Provide extra forage when grazing is short. The RVC specifically advises supplementary forage during autumn, and hay or haylage on a bare field removes the reason to forage for seeds.
• Do not overstock. The BHS statement calls grazing that is adequate for the number of horses the most effective protection of all.
• Consider bringing horses in overnight, which the BHS suggests to stop pasture being overgrazed, or reducing turnout on the highest risk fields.
• Check water troughs and streams near sycamores and keep troughs clear of seeds.
• Recheck after windy weather. A blustery night can put a fresh layer of seeds on a field you cleared the day before.

On felling: cutting a sycamore down is not the first answer. The BHS and Arboricultural Association advise testing first and making decisions from the result. Felling does not guarantee protection, because seeds blow in from elsewhere, and a felled mature tree tends to leave a flush of seedlings from seeds already in the ground. Trees may also be protected by law, so take advice before anyone picks up a chainsaw.

Where the technology fits

The honest answer is: not much, and it would be wrong to dress it up.

Stride has no pasture checker. It cannot tell you whether a tree is a sycamore, whether your field has seeds on it, or how much risk your horse is under. It does not send seasonal warnings, and it does not diagnose. None of the prevention above can be done from a phone. It is done in wellies, with a bucket.

What the app can do is keep a dated record. You can log observations against a horse, such as stiff, reluctant to work, sweating or signs of colic, each with a mild, moderate or marked severity, and they land on that horse’s health timeline. A dated note can record which field the horse was in, that you cleared seeds from it that morning, or that you saw seedlings coming up in March. If your vet visits, the record of that visit and any results can sit on the same timeline, with photos or PDFs attached.

That is useful in a narrow way. If a horse does fall ill, being able to tell your vet which field it was on and what you saw, and when, is better than trying to remember. And next spring, a note from this October reminding you which trees dropped the most seed tells you where to look for seedlings.

It is a notebook, not a safety net. The thing that protects your horse this month is walking the fields, clearing the seeds and making sure there is enough hay out that nobody goes hungry enough to eat them. If you see weakness, trembling or dark urine in a horse that grazes near sycamores, do not open an app. Ring the vet.