What is Pasture Myopathy?

What is Pasture Myopathy?

What is Atypical Pasture Myopathy?

Atypical pasture myopathy is a fatal disease in horses caused by the toxin hypoglycine A. It results in a disturbance of muscle metabolism, which can be fatal within only three days.

Pasture myopathy is caused when horses eat the seeds of certain maple species.

Younger horses and those that are grazed for more than 12 hours a day are more susceptible to this disease, although any horse can get pasture myopathy.

Symptoms of Seasonal Pasture Myopathy

The symptoms of pasture myopathy progress very quickly. In fact, the disease is fatal within 72 hours if not detected and treated immediately. The following signs are the most common:

  • Weakened muscles
  • Apathy
  • Fatigue
  • Darkened mucous membranes
  • Trembling of the muscles
  • Refusal to move
  • Inability to lie down (inability to get up after lying down)
  • Fast and irregular heartbeat
  • Dark coloured urine
  • Occasional gagging
  • Difficulty breathing
  • Respiratory failure
  • Collapse
  • Coma

Causes of Atypical Pasture Myopathy in Horses

The toxin hypoglycin A is found in the seeds of certain maple seeds. Not all maple species contain hypoglycin A, but it has been detected in the seeds of sycamore and ash maple.

After consumption, hypoglycine A is converted into methylenecyclopropylacetic acid (MCPA), which prevents the absorption and metabolism of fatty acids. The muscles are no longer supplied with sufficient energy and are destroyed in the process. The muscles needed for standing and breathing are also affected, and even the heart muscles are affected. This leads to increasing weakness, which progresses to coma and eventually death.

If you think your horse has eaten the seeds of one of these trees, you should take your horse to a vet immediately, even if you haven't noticed any symptoms yet.

Treatment of Atypical Pasture Myopathy in Horses

Your horse's treatment will depend on how many seeds were ingested and how long ago that was. If the horse has only eaten small amounts and no symptoms appear, there is nothing you can do but observe the animal for 12 - 24 hours. If no symptoms have appeared yet, but you know that your horse has eaten a large number of seeds, the vet will probably treat your horse immediately with infusions (electrolytes and sugars), the administration of painkillers and the administration of vitamins and antioxidants. The prognosis depends on the severity of the condition and when it is detected. As long as the horse can still stand, a complete cure can be achieved.


Clearly, the best chance is prevention. Check around the grazing area for maple trees and remove any sprouts. It is best to play it safe. It is better to close a pasture or even cut down a tree once than to lose one or more horses.

When temperatures are cold, stabling (especially at night) is good, and feeding concentrate, mineral feed and good quality hay is also important. Then, when the fields are grazed, the horses do not start chewing on leaves, bark or dead wood. Always remove leaves if possible and fertilise the paddocks with calcium cyanamide.

The danger is also reduced if horses do not spend more than six hours a day in the pasture. Incidentally, most cases of pasture myopathy occur in spring between 1 March and 31 May and in autumn between 1 October and 31 December. So you should be especially careful here.

If you suspect that your horse might have pasture myopathy, you should seek veterinary help immediately.