H&H explains how to identify this deadly condition and what measures you can take to protect your horses.

What is atypical myopathy?

Atypical myopathy (seasonal pasture myopathy) is a mysterious yet often fatal illness, usually found in grazing horses — mostly in the autumn and spring.

The illness weakens the muscles of the body and can present with sudden stiffness, muscle tremors, collapse and colic-like signs, with a low temperature. Often dark urine is seen. The fatality rate is around 70%.

Sycamore seeds are liked to Atypical MyopathyA study published in 2013 in the Equine Veterinary Journal revealed that toxins from the seeds (pictured right) of the tree acer pseudoplatanus — more commonly known as the sycamore — is the likely cause.

The research was done by the University of Liege and involved 17 horses from Belgium, Germany and The Netherlands.

High concentrations of hypoglycin A were found in all the horses. The pastures of 12 of the horses were visited by botanists and the sycamore was found to be present in every case.

In America research has linked seasonal pasture myopathy (the US equivalent) to toxins from the box elder tree. Both trees produce seeds containing the agent hypoglycin A.

“This toxin is not always present in every seed, or in seeds from every tree. This makes it difficult to predict whether a particular horse will become ill when exposed,” says H&H vet Karen Coumbe.

It is not contagious and can affect horses of all ages and types, but young horses may be more vulnerable.

When are horses at risk?

Outbreaks of the fatal disease tends to be seasonal, with most cases occurring in the autumn. Victims are usually kept in sparse pastures, where seeds are on the ground and are eaten when there is not much grazing.

What are the signs?

The onset of the disease can be extremely rapid, with some horses being found dead in their fields.

Signs include muscular weakness and stiffness, dark urine, fatigue, colic-like signs, shivering, sweating and trembling.

What should owners do?

  • 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
  • Supply extra forage (hay or haylage) especially where pasture is poor
  • Reduce stock density, so there is enough good grazing for every horse
  • Turn out horses for short periods (ideally less than 6hrs.)
  • Pick up and remove sycamore seeds, if possible
  • If concerned contact your vet immediately

What treatment is available?

Horses diagnosed early by blood and urine tests can be treated with intravenous fluids and intensive care, but once the signs are present it is already serious.


  • Brewin Bear

    Please Please Please keep your horses from broadleaf trees and seeds.
    If not you will see your horse die within 12hours.
    It is devastating if not heart breaking