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Atypical Myopathy

November 2020

Atypical Myopathy (AM), also referred to as Seasonal Pasture Myopathy (SPM), is a disease associated with horses eating sycamore seeds or seedlings. Sycamore seeds and seedlings have been found, in varying concentrations, to contain a substance called Hypoglycin-A (HGA). When eaten HGA is converted into a toxin within the horse’s body. The toxin has a rapid negative effect and can slow or even stop energy production in the horse’s muscle fibres.

Most cases of AM occur during the autumn when seeds dropped on the pasture are consumed or during the spring when the sycamore seedlings are growing. It is advised that the preventative measures outlined below, are put in place for 3 months, twice yearly. Starting in March for spring cases and October for autumn cases1, although depending on the weather, this may have to be implemented earlier for example strong winds causing higher numbers of sycamore seeds falling.


Sycamore seed

Sycamore seedling 
 Sycamore seed Sycamore seedling

AM can affect individual horses or several horses within the same group suggesting that some horses may be more susceptible to the disease than others. Research is currently on-going into the reasons why some horses seem not to be affected by HGA and others develop life threatening AM.

Clinical Signs

The onset of AM is rapid and horses can quickly deteriorate within 6-12 hours. Prognosis for the horse is often poor as mortality rates are around 70%2. Clinical signs include:

  • Muscle stiffness
  • Muscle tremors
  • Sweating
  • High heart rate
  • Depressed with their head hung low
  • Brown or dark red urine
  • Weakness, struggling or reluctance to walk and/or have difficulty standing
  • Breathing difficulties
  • The horse may still want to eat


Seek urgent veterinary assistance if you are concerned about your horse.


Prevention - Seeds

During the autumn, owners should try and reduce the risk of their horses consuming sycamore seeds by clearing them from the pasture.

We urge all horse owners to be vigilant and watch out for seeds, as even paddocks free of sycamore trees may still be at risk from seeds being blown on to the pasture.
Provide supplementary forage to horses on poorer grazing to help minimise the risk of them foraging for alternative food such as the seeds. Feeding hay from the floor or close to sycamore trees can increase the risk of accidentally ingesting sycamore material, so hay feeders could help to prevent this. 


Prevention - Seedlings

Neither mowing or two specific herbicidal sprays were effective at removing the HGA content of sycamore seedlings which means that they could still pose a risk to your horse3.

The current recommendation from leading researchers is to mow, collect and dispose of the cut grass and seedlings.


Pasture Management

In 64% of AM cases reported to the Atypical Myopathy Alert Group between 2006-2019, the pasture grass had minimal grazing1. Good pasture management is advised to help to reduce the risk of AM. Read our advice on the steps you can take to improve your pasture management.

Be aware that the spreading of manure and harrowing of pastures has found to increase the risk of AM. This practice may disperse the toxic material throughout the pasture1.

HGA is a water-soluble toxin that may pass from plants to water through direct contact. For this reason, pastures that contain a natural water source, such as a river or stream, should not be used during the high-risk seasons and an alternative water supply should be provided1.

Pastures contaminated with sycamore material should not be used to produce hay, haylage or silage as research has found that seeds and seedlings present in bales after 6-8 months storage still contained appreciable amounts of HGA3.


Management of Grazing Time

The length of exposure to the toxins appears to play a role in the risk of AM. Where sycamore trees are present near horse’s pasture, it is recommended to limit grazing time to less than six hours a day1.

Testing for HGA

HGA levels can differ between individual sycamore trees possibly due to the time of year or due to different climatic conditions; however, we are unsure to the reason why and risk factors have not yet been confirmed, but research is on-going.

There is a test available from the Comparative Neuromuscular Diseases Laboratory at the Royal Veterinary College for the testing of seeds and seedlings for HGA; further details are available from The Royal Veterinary College website.


Further Reading

Case Study

Joseph was a healthy horse first thing in the morning but by the afternoon he was in an equine hospital being treated for Atypical Myopathy. Find out more about Joseph’s story here


1. Votion D-M et al. (2020) Frequently Asked Questions Regarding Horse Feeding and Management Practices to Reduce the Risk of Atypical Myopathy. Animals 10, 365.

2. Richard Piercy. (2019). EVJ In Conversation Podcast No. 37- atypical myopathy. [Accessed online 09.09.2019].

3. González-Medina S et al. (2019) Atypical myopathy-associated hypoglycin A toxin remains in sycamore seedlings despite mowing, herbicidal spraying or storage in hay and silage. Equine Veterinary Journal 0 pg.1-4. 

Our Policy

Read The British Horse Society's policy on felling of Sycamore trees in relation to Atypical Myopathy.


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