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  • Atypical Myopathy

Atypical myopathy is a severe and frequently fatal disease, affecting horses across Europe and the USA. Despite the first case being reported in Wales in 1939, and there being large outbreaks in Europe since the 80s, what is believed to be the cause has only just been discovered.

Last year the toxin Hypoglycin A, found in seeds of the Box Elder tree in the USA was found to be the cause – it was subsequently discovered that the Sycamore tree, which belongs to the same family of trees, is the cause in Europe. Research is still ongoing to clarify risk factors, and toxin levels in other plant species.

Cases are at their highest during the Autumn, especially after windy weather when many seeds have fallen on pasture. Experts in Belgium have also previously warned that an Autumn with a high number of cases is often followed by a bad Spring.

The disease causes severe damage to muscles, which causes weakness, pain, and difficulty breathing. The muscles also release a chemical as they breakdown, which can cause kidney damage as it is filtered out of the blood and into urine. Approximately a quarter of cases in Europe survive, with prompt recognition and action essential.

Clinical Signs

  • Initial dullness and lethargy
  • Severe generalised weakness – may be found lying down and unable to stand
  • Dark (red/brown) urine
  • Tremors
  • Some may show colic signs
  • Increased respiratory rate and effort


Horses with atypical myopathy are treated symptomatically – which means that we cannot directly attack the causal agent of the disease, but we take steps to support the horse and keep them comfortable. This is mostly through providing pain relief and intravenous fluid therapy.

Preventing Atypical Myopathy

Avoiding Sycamore seeds is paramount to preventing atypical myopathy. Fencing off affected areas under trees, picking up seeds, and providing extra hay to reduce foraging seeds may all be helpful strategies.