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Atypical Myopathy

 September 2019


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 toxin called Hypoglycin-A (HGA). When eaten this toxin 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.

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%. 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.

Prevention - Seedlings

New research has shown that 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 horse.

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

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 is 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

Recent Research

Read about the latest research and tests on this continued threat posed to horses. Learn more.

Our Policy

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

Download our leaflet for further information about Atypical Myopathy.


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