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Atypical Myopathy

Atypical Myopathy (AM), also referred to as Seasonal Pasture Myopathy (SPM), is a disease associated with horses eating sycamore seeds and  seedlings1

  • Last reviewed: 3rd October 2023
Sycamore Seed Sycamore Seed

Sycamore material has 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 eaten, or during the spring when the sycamore seedlings are growing. 

Atypical Myopathy can affect individual horses or several horses within the same group.  It is currently unknown why some horses remain unaffected by HGA and others develop life threatening AM. 


The onset of AM is rapid, and horses can quickly deteriorate within 6-12 hours. The prognosis for the horse is often poor with mortality rates around 56-61%2.

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 


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

Joseph, (see photo below) was a healthy horse first thing in the morning, but by the afternoon he was in an equine hospital being treated for AM.

Further information on Joseph’s story, ‘Recovering from Atypical Myopathy’



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

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 onto the pasture.



Mowing could potentially decrease the risk due to reducing the amount of toxic material available to eat (but the mowed grass and seedlings must be collected and not left on the pasture). However, by leaving the root in the ground seedlings are more likely to grow back and you may not be able to remove the whole of the seedling.

There are sprays available which kill off the seedlings however studies have found that the HGA toxin was still present in stored seedlings 6-8 months after being sprayed. Dependant on the toxicity level of the individual plant this could be longer. Even though the seedling has ‘died-off’, the left-over plant material still contains the HGA toxin, and therefore would still need to be removed from the pasture3.

Therefore, both mowing and spraying doesn’t eliminate the risk of poisoning and seedlings need to be removed from the pasture. Unfortunately, there is no easy method to remove the seedlings and it's understandable that the logistics, time and resources required to complete this presents a challenge for many horse owners.

Provide supplementary forage

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 against this.  

Pasture Management

In 64% of AM cases reported to the Atypical Myopathy Alert Group between 2006-2019, the pasture had minimal grazing1. Good pasture management is advised to help to reduce the risk of AM.  

Be aware that the harrowing of pastures has found to increase the risk of AM. This practice may disperse the sycamore 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 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.

Sycamore Seed
During autumn, owners should try and reduce the risk of their horses consuming sycamore seeds
Seedling 7
Neither mowing or herbicidal sprays are effective at removing the HGA content of sycamore seedlings

Further reading


Joseph's Story

Thursday 2 November started out like any other day for three-year-old cob Joseph. His owner, Sarah, keeps Joseph at her home, where he currently lives out with several other horses. Sarah’s groom was making the morning rounds, when she noticed that Joseph didn’t look quite right, standing away from the group and she immediately notified Sarah.

Continue reading
Poorly Joseph Small

Get in touch – we’re here to help 

The Horse Care and Welfare Team are here to help and can offer you further advice with any questions you may have. Contact us on 02476 840517* or email – You can also get in touch with us via our social media channels. 

Opening times are 8:35am - 5pm from Monday – Thursday and 8:35am - 3pm on Friday. 

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