What is equine atypical myopathy?

What is equine atypical myopathy?

Ever heard of EAM?  I hadn’t until I was alerted to it on social media this week as there’s been lots of discussion about it.  Want to know more about this fatal equine disease, then read what I’ve found out about it so far…

What is it?

Equine Atypical Myopathy, or EAM as it is more commonly known, is a fast and frequently fatal condition, affecting grazing horses mostly in the Spring and/or Autumn.  A staggering 80+% of cases are fatal in as little as 11 hours from onset. It occurs in fields, affecting the horse’s muscles in a similar way to Azetoria (tying up). 

The Cause

Hypoglycin A, is the toxin that is believed to be responsible for this disease.  It’s present in the seeds of some Acers such as Acer pseudoplatanus, which to you and me is simply known as the Maple tree or Sycamore tree.

Risk Season

The clinical signs seem to appear after the occurrence of specific climatic conditions.  The peak seasons when these conditions are most prevalent are Autumn and Spring when rain, showers and wind blow the Acer and Sycamore seeds or seedlings on to the fields.

Areas at Risk

Any fields with Sycamore or Maple trees on or near them present a risk to equines.  The wind can blow the seeds a surprisingly long way.  To avoid the risk of any horses or ponies contracting EAM, it’s recommended that the seeds are removed in the autumn and also to look out for and remove seedlings in the Spring. 

Young horses and ponies that are out at grass 24/7 and not in work are the most commonly affected.  Those not immunized, rugged, fed supplementary feed or provided with shelter are also at greater risk.

Warning Signs

Warning signs could be one or more of the following:

  • Lethargy, reluctance or inability to move
  • Head held low, an almost laminitic type stance (if still standing); it can look similar to colic but the gut sounds often normal
  • Reduction in appetite but will still eat and pass urine and/or droppings
  • Quarter muscles tense, possibly with some trembling, shoulders the same;  the trembling seems to come in waves
  • Red nostrils
  • Vocal
  • Depressed
  • Dark Urine
  • Increased respiratory rate.


Have you experienced the fatal effects of this fatal disease? Do you have any tips you'd be willing to share to help others? If so, please do add them to our comments below.


Kim Horton

Co-Founder of EQUUS and a keen equestrian, when Kim's not at her desk she's with her horse, Waldo.


  • rebecca posted on October 31 2017 at 07:10 PM

    Pictures of the leaves, seedlings, trees would be helpful. Are all Maple varieties poisonous?

  • Tracey Harris posted on October 31 2017 at 10:10 PM

    Hi Kim- my pony has had what sounds like these exact symptoms 2weeks ago, and a blood test has revealed chronic kidney disease.
    The vet never mentioned EAM, and I’ve never heard of it before.
    He has had a course of antibiotics and pain killers and has picked up a bit the last few days. He’s had another blood test today, so i’ll keep you posted with the outcome. Fingers crossed ?!

  • Andres Deane posted on October 09 2018 at 01:10 PM

    Very interesting just had a mare signs of colic had vet out for two nights.

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