Equine Atypical Myopathy

Sycamore Poisoning

Equine Atypical Myopathy

Equine Atypical Myopathy (also known as Sycamore poisoning) is a devastating disease caused by the consumption of a toxin found in the leaves, seeds and seedlings of Sycamore and Box Elder trees. Although most cases are seen in the autumn, it’s important to remember that seeds, leaves and seedlings can be consumed all year round.

Atypical Myopathy is caused by the toxin Hypoglycin A, which irreparably damages muscle cells, particularly postural muscles that enable the horse to stand, cardiac muscle of the heart and those in the chest wall that are needed for respiration. Breakdown of the muscle releases large amounts of protein into the blood, which the kidneys struggle to clear into the urine. Ultimately this can result in collapse with respiratory, cardiac, and renal failure. There is no antidote to the toxin and as many as 70% of severely affected horses will die.

It is essential to seek veterinary advice as soon as possible because early intervention can radically improve the chances of survival.

Clinical Signs

The onset of AM is rapid, and horses can quickly deteriorate within 6-12 hours.

  • 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.

Many cases will need hospitalisation and extensive fluid therapy and it is easier to move a horse that can still stand unaided. The majority of affected horses that are still alive 5 days after the initial signs of Atypical Myopathy are likely to fully recover.

Which horses are at risk?

A lot of research is going on to better understand Atypical Myopathy but currently it is thought to affect any breed and any age of horse. There have been cases where co-grazers of the same paddock as an Atypical Myopathy case have not developed symptoms and where the presence of Hypoglycin A has varied between individuals in the same group. This is thought to reflect the fact that individual horses graze differently from each other rather than to provide any indication of some horses being more likely to contract the disease than others.

What can I do to protect my horse?

Although cases are more likely in autumn, a peak also occurs in the spring due to consumption of Sycamore seedlings that are also high in the toxin Hypoglycin A.

There are some steps you can take:

  • Checking fields carefully for Sycamore leaves and seeds
  • Fencing off areas where Sycamore seeds and leaves have fallen
  • Hoover-up/pick up sycamore seeds off the pasture
  • Turning horses out for shorter periods
  • Provide extra forage (hay or haylage) especially where pasture is poor, or grazing is tight
  • Reducing stocking density so there is plenty of good grazing for every horse

If you have any concerns regarding your horse please call us immediately.