Atypical Myopathy


Atypical myopathy (also known as Seasonal Pasture Myopathy) is a serious muscle disease caused by horses ingesting the toxin hypoglycin A which is found in tree seeds, particularly sycamore and box elder seeds. Whilst we see most cases of this disease in the autumn, when the seeds are present in high numbers on the ground and can be eaten by grazing horses, we also see cases in the spring when tree seedlings are growing.

The disease can affect horses of any age or type but horses that are kept in overgrazed fields with a large quantity of dead leaves and dead wood are particularly at risk. This is exacerbated when they are not provided with adequate hay or other supplementary forage as this drives horses to seek other foodstuffs like dead leaves. Even so sometimes well-fed animals can still be affected.


The onset of atypical myopathy is rapid and affected horses are often found out at pasture unwilling to move. Clinical signs of the disease include:

  • Lethargy
  • Stiffness
  • Colic- like symptoms
  • Muscle weakness
  • Passing very dark urine. If a horse is suspected of having atypical myopathy urgent veterinary assistance should be sought as early, intensive treatment is essential to give the best chance of survival. The toxin directly targets aerobic energy metabolism and treatment includes intravenous fluids and intensive care with diagnosis confirmed by either a blood or urine test. Whilst vets have been successful in treating some cases if caught early enough, the disease is sadly still frequently fatal, with BEVA estimating a survival rate of under 25%, and we have yet to discover a definitive cure. Prevention Due to the lack of specific treatment and the high mortality rate, prevention of atypical myopathy is crucial. The most effective way of preventing the disease is by identifying sycamore trees in grazing fields and limiting the exposure of horses to these trees. Ideally this would mean removing the horses from the field completely during the times of year when seeds or seedlings are present on the ground and are easily accessible. However, this is easier said than done and even fields with no population of sycamore trees can still contain seeds and leaves spread on the wind. Preventative advice for horse owners includes:
  • If moving horses is not possible, fence off areas surrounding the sycamore trees
  • Remove all seeds and dead leaves from the ground prior to turnout
  • Consider stabling horses overnight where possible
  • Where grazing is sparse, offering additional fresh hay or forage is necessary to prevent horses from searching for alternative food such as dead leaves and seeds
  • Avoid leaving wet hay on the ground where it will rot
  • Ensure the grazing pasture is not over populated and there is enough pasture for every horse
  • Remove sycamore saplings in the spring
  • Check your field even if there are no sycamore trees nearby as a field without sycamore trees can still contain seeds and leaves spread on the wind or potentially by flood water
  • If concerned contact your vet immediately

Contributor: B&W Equine Vets provide nationally and internationally renowned cutting edge veterinary care which would be the backbone of our referral service. They provide the very highest level of equine veterinary care as well as performing innovative procedures. We strive to provide the highest level of professional and personal care for you and your horse or pony, whether that be be via our vastly experienced mobile team or at our equine hospital.


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