Atypical myopathy: what you need to know right now to keep your horses safe
- This article has been edited and approved by Karen Coumbe MRCVS, H&H’s veterinary advisor since 1991.
Atypical myopathy in horses (also known as seasonal pasture myopathy, sycamore myopathy or sycamore poisoning) is an often fatal illness, usually found in grazing equines — mostly in the autumn and spring.
With the first of the UK autumn storms of 2025 having passed, H&H has been contacted by owners who have lost horses to atypical myopathy after grazing around sycamore trees. This year is a mast year for many types of trees (including oak), which means there is a very large number of seeds being produced, which increases the risk. Periods of high winds can result in large dumps and carry the shedding seeds and leaves over long distances, so the risk isn’t limited to sycamore trees in or on the fence lines of paddocks.
The British Equine Veterinary Association (BEVA) has previously reminded owners of the dangers. “With leaves still on trees now is the time to identify sycamores in the vicinity of your horse’s fields and predict the future risk,” said Lucy Grieve, veterinary projects officer at BEVA. “Once the leaves start to turn and then drop it will be much more difficult to spot sycamores.”
This serious threat to horses’ health weakens the muscles of the horse’s body, including the heart, and can present with sudden stiffness, muscle tremors, collapse and colic-like signs, accompanied by a low temperature. Often dark urine is seen because the damaged muscle cells release the pigment myoglobin, which can damage the kidneys.
Atypical myopathy has a fatality rate of around 70% so should always be considered a veterinary emergency if it is suspected.
Atypical myopathy is not infectious and can affect horses of all ages and types, although young horses may be more vulnerable. Some horses appear to be more susceptible than others, which may be due to genetic differences.
Atypical myopathy: Causes | Risks | Signs | Diagnosis | Treatment | Prognosis | Prevention
Never miss out on the latest happenings in the equestrian world
What causes atypical myopathy in horses?
Studies have established that atypical myopathy is caused by ingestion of the hypoglycin A toxin from the seeds, seedlings and possibly flowers of certain trees of the genus Acer. The most common of this type in the UK is the sycamore tree, but different trees contain variable amounts of the toxin. It is important to note that the toxin is not present in every seed, seedling or tree.
The Royal Veterinary College provides a hypoglcyin A test that can be used on both plant samples and equine serum to identify the level of the toxin. This can vary within an individual tree from year to year, although a 2025 study found that trees in the same area typically have similar concentrations of hypoglycin A in their seeds. The study also found that toxicity could occur from water sources containing sycamore seeds and leaves.
Hypoglycin A prevents energy being produced within the muscle cells.
Much research has been done by the University of Liege and involved horses from Belgium, Germany and the Netherlands. High concentrations of hypoglycin A were found in all the sick horses. Botanists visited the pastures that had been grazed and the sycamore was found to be nearby in every case.
In the United States research has linked seasonal pasture myopathy (the US equivalent) to toxins from the box elder tree. Both trees produce seeds containing the agent hypoglycin A.
The “helicopter” wing-shaped seeds are thought to be the primary source of the toxin, but sycamore seedlings have also been found to contain the toxin, being a particular concern in the spring. It is recommended that seedlings are mowed and the cuttings collected and removed from the pasture afterwards as the toxic hypoglcyin A remains in the plant material after it has been cut.
When are horses most at risk?
Outbreaks of the fatal condition tend to be seasonal, with cases typically occurring in the autumn and spring. Victims are usually kept in sparse pastures, where seeds or seedlings are on the ground and are eaten when there is not much other grazing available. Windy autumn weather is particularly risky as it can deposit large numbers of sycamore seeds on to grazing land, which are then eaten.
What are the signs of atypical myopathy in horses?
The onset of the disease can be extremely rapid, with some horses being found collapsed or dead in their fields. Signs include:
- Stiffness
- Weakness
- Reluctance to move
- Lethargy
- Muscle tremors
- Muscle pain
- Sweating
- Difficulty breathing
- Shivering
- Dark red or brown urine
- Fast, irregular heart beat
- Colic signs
- Unable to stand
- Sudden collapse and death
How is atypical myopathy diagnosed?
Your vets will be able to make a likely diagnosis based on the clinical signs as well as the horse’s environment, along with lab tests to confirm the levels of toxicity.
What treatment is available?
Horses diagnosed early will need treatment with intravenous fluids, pain relief and intensive care at an equine hospital, but once the signs are present it is already serious. Affected animals often deteriorate for the first 24 to 48 hours so should be moved to an equine hospital as soon as possible after the diagnosis is made as the period the horse is able to remain standing while in transit is going to be limited.
There is a compromise in that the horses do not want to be moved far, however effective intensive care is not really possible in the field, where they may be ingesting more toxin.
Supportive therapy including vitamins, minerals and antioxidants may be helpful.
A 2018 study showed that activated charcoal may bind the toxin, so your vet may consider stomach tubing with this following removal of any cases from pasture.
Atypical myopathy in horses: what is the prognosis?
With some 70% of cases proving fatal, the prognosis for horses with atypical myopathy is cautious at best. However, if a horse can survive the first few days of treatment, they can go on to make a full recovery over a period of some months with no long-term effects of the disease.
How can owners reduce the risk?
- Identify trees both around grazed fields as well as those in close proximity
- Fence off areas where sycamore seeds and/or leaves have fallen
- Regularly inspect fields to ensure seeds have not blown in from nearby sycamore trees
- Remove any seeds and leaves that have dropped into water troughs
- Mow areas where seedlings are growing and collect the cuttings
- Supply extra forage (hay or haylage), especially where pasture is poor
- Reduce stock density, so there is enough good grazing for every horse
- Turn out horses for short periods (ideally less than 6hrs.)
- Pick up and remove sycamore seeds, if possible
- Test for the presence of hypoglcyin A in your pastures
- Monitor horses carefully and if concerned, contact your vet immediately
If a case occurs and your horse is in the same field, ask your vet for advice and consider moving your horse.
Further reading:
Assessment of tree-associated atypical myopathy risk factors in Acer pseudoplatanus (sycamore) seeds and leaves – January 2025
Royal Veterinary College’s atypical myopathy fact sheet for horse owners [PDF] – 2022
Answers to the Frequently Asked Questions Regarding Horse Feeding and Management Practices to Reduce the Risk of Atypical Myopathy – February 2020
The Story of Equine Atypical Myopathy: A Review from the Beginning to a Possible End – November 2012
You might also be interested in:
The dangers of acorns to horses you need to know about
As autumn rolls on, confirmed cases of acorn poisoning increase. Sarah Smith MRCVS outlines the dangers for horses grazing in
5 common plants that could kill your horse
While many plants can be poisonous if eaten to excess, there are some poisonous plants that horses should avoid at
Are buttercups poisonous to horses? We ask the experts…
Subscribe to Horse & Hound magazine today – and enjoy unlimited website access all year round
Carol is an experienced journalist and editor, who is also a passionate amateur rider and dedicated horse owner, having ridden since childhood. She has a particular interest in the emotional well-being and ethical training of horses, alongside veterinary matters, having garnered extensive first-hand experience of numerous equine health conditions through more than 30 years of horse ownership.
- Karen CoumbeMA VetMB CertEP CertVA MRCVS