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Equine Atypical Myopathy

Atypical Myopathy is a disease most commonly associated with horses kept on grass pastures with the majority of cases occurring during autumn and less commonly spring time.  All horses (and according to more recent reports also Donkeys) can be susceptible but very young animals or those over the age of 20 are more at risk of developing the disease.   The Equine Veterinary Journal published a paper in 2013 that linked toxins found in the seeds of the sycamore tree (Acer pseudoplantanus) which is pictured below to the disease.  There is a similar disease recorded in America called seasonal pasture myopathy which is caused by the seeds of the box elder tree (Acer negundo) which is from the same family as the sycamore. 

Hypoglycin A is the toxin found in these seeds. It affects the muscles, particularly skeletal muscles and muscles involved in respiration, by causing degeneration of the muscle fibres and the accumulation of fat within the muscle cells.  This leads to stiff muscles, muscle tremors and collapse. 

Clinical signs

The onset of clinical signs of Equine Atypical Myopthy are extremely rapid and it is often fatal.  If the horse is lucky enough to be found alive, important signs to look for include:

  • Muscle stiffness
  • Unwilling to walk
  • Excessive Sweating
  • Shivering and trembling
  • Colic-like symptoms
  • Depressed appearance or appearing as if they have been sedated
  • Dark Urine
  • High heart rate
  • Low temperature
  • Weakness and difficulty standing
  • Difficulty Breathing
  • Horses often still want to eat


What Can Owners do?

  • Avoid fields with sycamore trees.  If this is unavoidable then fence off the trees and any areas where the seeds, seedlings and leaves may blow.
  • Fence off areas where sycamore seeds and/or leaves have fallen
  • Regularly inspect fields to ensure seeds have not blown in from nearby sycamore trees
  • Provide hay or haylage especially where pasture is poor
  • Ensure adequate stocking density so there is enough grazing for each horse
  • Pick up and remove sycamore seeds, if possible
  • If possible stable horses overnight
  • If concerned contact us immediately.


What to do if you suspect Equine Atypical Myopathy?

  • CALL (01270) 628888 and speak to one of our vets.
  • Try not to move or stress your horse.  However if they will move try and get them somewhere warm and dry to be treated.
  • Offer water and small amounts of sloppy feeds.
  • Try and collect a urine sample if possible. 
  • Move any other horses that are grazed in the same paddock.