Atypical myopathy

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Atypical Myopathy

Sycamore Poisoning, Seasonal Pasture Myopathy

Atypical myopathy (AM) also known as seasonal pasture myopathy (SPM), is a frequently fatal disease in horses caused by eating sycamore seeds ("helicopers") or seedlings from the sycamore (Acer pseudoplatanus) and/or box elder (Acer negundo) trees. The seeds contain hypoglycin A, which is metabolized into a toxic substance known as MCPA (methylenecyclopropylacetic acid). MCPA prevents energy from being produced within the horses' muscle cells which result in damage to the muscles in the body. The severity of the disease varies from horse to horse, as it depends on the amount of toxin within the seeds and seedlings, amount eaten, and individual susceptibility of the horse. Affected horses may develop a wide range of clinical signs and may initially include lethargy, loss of appetite and reluctance to work.

AM affects full-time pastured horses and is more frequently reported in the autumn, immediately following inclement weather such as cold, humidity and rain. Horses that develop AM are usually kept in sparse pastures with an accumulation of dead leaves, dead wood and trees in or around the pasture and are often not fed any supplementary hay or feed.


Red or brown urine
Recumbency or reluctance to move
Congested mucous membranes
Difficulty swallowing


  • History
  • Clinical signs
  • Physical exam
  • Laboratory tests

While waiting for your veterinarian

  • Horses should be removed from the pasture as soon as possible and stabled and kept as quite and comfortable as possible and should avoid physical effort and stress. The health of other horses in the same pasture should be evaluated.



Supportive care: individually adapted fluid therapy and monitoring of digestive, respiratory, cardiac, and urinary systems.


  • Do not keep horses out in pastures with access to sycamore or box elder trees at times of risk
  • Ensure horses have access to good quality uncontaminated pasture grass.
  • Provide supplementary feed in the field to minimize the risk of horses being tempted to ingest seeds.
  • Avoid leaving wet hay on the ground where it will rot and potentially trap seeds.
  • Be aware that a field without sycamore trees can still contain seeds spread by high winds or flood water.
  • Do not prune seed laden trees as this can lead to massive pasture contamination and further increase the risk to horses.
  • Restrict access to seeds by using temporary fencing.
  • Avoid feeding fats or oils
  • Provide horses with a fresh water source such as from a water trough
  • Fence off horses from drinking from streams or ponds under trees


Guarded to poor. The estimated mortality rate is 89%.

Scientific Research

General Overviews

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Age Range

Younger horses less than 3 years of age are more susceptible.

Risk Factors

  • Sycamore trees on property
  • Turning horses out in sparse pastures with contain lots of dead leaves and trees
  • Not knowing what tree species are in horse pastures
  • Not providing horses with enough grass to eat while turned out in pasture