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Atypical Myopathy

In recent years, the frequently fatal muscle disease Atypical Myopathy (AM) has become more prevalent in Great Britain.

Sycamore or 'helicopter' seed

Associated with horses kept predominately at grass, AM occurs predominantly during the autumn, although cases are also more rarely seen in the spring. AM can affect individual horses or several horses within the same group. All horses are potentially susceptible to AM, although youngsters and horses above the age of 20 have been found to be at greater risk.

Myopathy diseases in horses result in damage to the muscle tissue and cause significant muscular pain in affected animals.

Clinical Signs

The onset of AM is rapid and the disease may appear without warning. Affected horses will often be found out at pasture unwilling to move. Clinical signs include:

  • Muscular stiffness
  • Reluctance to walk
  • Muscle tremors
  • Sweating
  • Depression and/or the horse looks as if it is sedated
  • High heart rate
  • Dark urine
  • The horse appears weak and may have difficulty standing
  • Breathing difficulties
  • The horse may still want to eat
  • Once the clinical signs of the disease are present, the prognosis for the horse is very poor as mortality rates are between 75 and 90 percent.


Please seek urgent veterinary assistance should a case of AM is suspected.


During the autumn owners should decrease horses’ opportunities to consume sycamore seeds. This is easier said than done, as even those paddocks that are free of sycamore trees may still be at risk from seeds being blown on to the land. Therefore, the main preventative measure is to ensure that horses turned out for the majority of the time on poor quality grazing, have sufficient supplementary feed to minimise the risk of foraging for alternative food such as the seeds.

Additional actions include:

  • Ensure the pasture is not over-stocked
  • Maintain good pasture management to prevent weeds taking over
  • If moving horses is not an option, fence off areas around the sycamore trees
  • Removal of sycamore seeds and dead leaves from the paddock
  • Removal of sycamore saplings
  • Where possible, consider stabling the horses overnight

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.



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