Atypical Myopathy in Horses: Signs, Causes and Prevention
Atypical myopathy is a serious and potentially fatal muscle disease linked to sycamore poisoning. Learn how to recognise the warning signs, when to call your vet and how to reduce the risk to horses at pasture.
- What is atypical myopathy?
- When are horses most at risk?
- What are the signs of atypical myopathy in horses?
- What should I do if I think my horse has atypical myopathy?
- How is atypical myopathy diagnosed and treated?
- How can I reduce the risk of atypical myopathy?
- Can a field without a sycamore tree still be risky?
- Can a horse recover from atypical myopathy?
- Frequently asked questions
As sycamore seeds fall across fields and paddocks in autumn, horse owners are reminded of the risk of atypical myopathy. The disease can develop quickly and cause severe muscle damage, making early recognition and urgent veterinary care essential.
Although autumn is a key time for concern, the risk does not end when the leaves have fallen. Sycamore seedlings emerging in spring can also contain the toxin associated with the disease, and seeds can blow into fields from trees some distance away.
Understanding where the risks lie and checking grazing areas regularly can help owners make informed decisions about turnout throughout the year.
If you suspect your horse has atypical myopathy, contact your vet immediately. Do not wait for every sign to appear.
What is atypical myopathy?
Atypical myopathy is a severe muscle disease associated with horses eating material from certain trees. In the UK, it is particularly linked to seeds and seedlings from the sycamore tree, Acer pseudoplatanus.
Sycamore seeds, seedlings and leaves can contain a substance called hypoglycin A. Once eaten, it is converted into a toxin that interferes with the horse’s ability to produce energy in its muscles. This can cause rapid and serious muscle damage, affecting the muscles used for movement and, in severe cases, those involved in breathing and heart function.
The condition is also known as sycamore poisoning or seasonal pasture myopathy. According to the British Horse Society’s guidance on atypical myopathy, the disease can affect one horse or several horses grazing together. It is not yet fully understood why some horses become seriously ill while others exposed to the same material do not.
When are horses most at risk?
Most cases occur in autumn and spring, although owners should remain aware of the risk outside these seasons.
Autumn and winter: fallen seeds
In autumn, sycamore trees drop their distinctive winged seeds, often called “helicopters”. These can collect in grazing areas, around field boundaries and in water sources. Wind can carry them into fields that do not have a sycamore tree within their boundaries.
Check pastures after windy weather, particularly where nearby trees or neighbouring land could be a source of seeds.
Spring: emerging seedlings
In spring, fallen seeds can germinate and produce small seedlings among the grass. These may be difficult to spot, especially in longer or uneven pasture.
The Royal Veterinary College’s atypical myopathy fact file identifies fallen seeds and germinating seedlings as key sources of the toxin.
What are the signs of atypical myopathy in horses?
Atypical myopathy can develop rapidly. Signs may vary between horses, but can include:
- Muscle stiffness or soreness
- Trembling or muscle twitching
- Sweating, even when the horse is resting
- Weakness or reluctance to move
- Difficulty walking, standing or getting up
- A lowered head and a dull or depressed appearance
- A raised heart rate or difficulty breathing
- Dark red, brown or coffee-coloured urine
- Signs that may resemble colic
Do not wait to see dark urine before calling the vet. It is one possible warning sign, but a horse may show other signs first, and the condition can deteriorate quickly.
What should I do if I think my horse has atypical myopathy?
Atypical myopathy is a veterinary emergency. If your horse is showing signs or you are concerned they may have eaten sycamore material:
- Call your vet immediately and explain the signs you have seen and any possible exposure to seeds, leaves or seedlings.
- Keep the horse calm and do not exercise them. Ask your vet whether and how the horse should be moved. Do not force a weak horse to walk or stand.
- Discuss the other horses in the field with your vet. They may have been exposed even if they appear well. Follow your vet’s advice on moving them away from the suspected source and monitoring them.
- Let your vet know about any sycamore material you have found, including seeds or seedlings that may have blown into the field.
Prompt veterinary attention gives the horse the best opportunity to receive assessment and supportive care as early as possible.
How is atypical myopathy diagnosed and treated?
Your vet will consider the horse’s signs, grazing history and potential exposure to sycamore material. Blood and urine tests can help assess muscle damage, and further testing may be used to support the diagnosis.
Treatment depends on the individual horse and the severity of the illness. It may include intravenous fluids, pain relief, nutritional support and close monitoring. Some horses need referral to an equine hospital for intensive care.
Atypical myopathy can be fatal, even with treatment. The BHS reports mortality rates of around 56% to 61%, although an individual horse’s outlook will depend on its circumstances and response to care.
How can I reduce the risk of atypical myopathy?
It may not be possible to eliminate every risk, particularly when seeds can travel on the wind. However, there are practical steps owners and yard managers can take to reduce horses’ exposure.
Identify sycamore trees near your grazing
Look beyond your own fence line. Trees in neighbouring fields, hedgerows, gardens and woodland may still be close enough for seeds to reach your pasture.
The UK sycamore has broad, lobed leaves and produces paired, winged seeds. If you are unsure about a tree’s identification, seek advice from someone suitably experienced. Avoid assuming that a tree is safe simply because it looks different from a familiar sycamore.
Check fields regularly
Walk your grazing areas and look for fallen seeds, leaves and seedlings. Pay particular attention to field edges, sheltered corners and areas where wind-blown material gathers. Check again after strong winds.
Seedlings can appear among the grass in spring, so a field that looked clear in winter may need checking again.
Remove or restrict access to contaminated areas
Remove sycamore material from pasture where practical and consider fencing off affected areas. Bear in mind that fencing around a tree will not necessarily stop seeds reaching the rest of the field.
Where contamination is widespread or cannot be managed reliably, speak to your vet about alternative turnout or management arrangements.
Provide suitable forage
Horses grazing on sparse or overgrazed pasture may be more likely to encounter unwanted plant material while searching for food. Providing appropriate hay or haylage can help reduce reliance on poor grazing, although supplementary forage does not guarantee that a horse will avoid sycamore material.
Place forage and water away from areas where seeds or leaves may collect, and check them regularly for contamination.
Take care when managing seedlings
Mowing or spraying seedlings does not necessarily remove the toxin. Research cited by the RVC has found that hypoglycin A can remain in sycamore seedlings after mowing, herbicide treatment and storage in hay or silage. Discuss safe removal and disposal with an appropriate professional if needed.
Can a field without a sycamore tree still be risky?
Yes. Sycamore seeds can be blown into a field from trees outside its boundaries. This is why it is important to check the pasture itself, as well as identifying nearby trees.
The amount of hypoglycin A in sycamore material can also vary. You cannot tell whether a seed or seedling contains a harmful amount simply by looking at it.
The RVC offers a plant sample testing service for owners who want to investigate sycamore material. If you are considering testing, check the laboratory’s current instructions and discuss what the result means for your field with your vet.
Can a horse recover from atypical myopathy?
Some horses do recover, but recovery can take time and should be guided by a vet. Depending on the severity of the illness, a horse may need a period of rest, monitoring and a carefully managed return to exercise.
Your vet can advise on follow-up blood tests, nutrition and a suitable plan for your horse. Do not rush a return to work because the horse appears brighter or more comfortable.
Frequently asked questions
Yes. Sycamore seedlings can contain hypoglycin A, and cases occur in spring as well as autumn. Continue checking fields as seedlings emerge.
Horses grazing together may all have been exposed to the same material, although they will not necessarily all develop signs. If one horse is suspected to have atypical myopathy, speak to your vet promptly about the others.
No. It is associated with exposure to a toxin rather than an infection spreading between horses. However, horses sharing the same grazing may be exposed to the same source.
Yes. Seeds can be carried into a field from trees elsewhere, so inspect grazing even if there are no sycamores within the field boundary.
Want to learn more about equine muscle disorders? Our guide to equine myopathies explores recurrent exertional rhabdomyolysis, polysaccharide storage myopathy and myofibrillar myopathy, including the role nutrition can play in their management. Read our guide to equine myopathies and nutrition here.