Atypical myopathy - How you can help protect your horse

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Caused by a toxin found in sycamore seeds, Atypical Myopathy is a devastating disease that destroys a horse’s skeletal and heart muscles.

The severity of the disease and the prognosis for the affected horse depends on which muscles are affected and to what extent they have been damaged. It is often fatal and can be misdiagnosed as colic. Atypical Myopathy can affect horses of any age and outbreaks occur most commonly in the autumn.

A toxin present in the seeds of the tree Acer pseudoplatanus is the likely cause of Atypical Myopathy (AM) in Europe. The common name for this tree is sycamore in the UK, but it is also known as the sycamore maple in some countries. In the past ten years, approximately 20 European countries have reported the disease.

The toxin is called hypoglycin A and it is present, in varying concentrations, in the leaves and the helicopter-shaped seeds of the sycamore tree; there is evidence that the weather can affect the level of toxin produced by the tree and not all seeds contain the same level. Horses that suffer from the disease are often kept on bare pasture surrounded by trees, without access to supplementary forage, which encourages them to eat the seeds.

A horse suffering from the condition presents with muscle trembling and stiffness, it will be weak and lethargic and may be found lying flat out in the paddock. If the disease has affected the heart and respiratory muscles, the horse will exhibit laboured breathing and an elevated pulse. If muscle breakdown is extensive the urine will be dark in colour.

There is no antidote for hypoglycin A poisoning – treatment consists of managing the effects of the poison on the body, flushing out the horse’s system and maintaining hydration to prevent further damage. Nursing and palliative care are crucial for recovery and early diagnosis is critical. Even so, mortality levels are around 50% with affected ponies appearing to have a worse prognosis than horses, perhaps due to the relative size of their muscles.

What can you do to protect your horses from Atypical Myopathy? Learn to identify the sycamore tree and its seeds, and fence them off to keep horses away. Clear up seeds regularly as part of your pasture management programme and provide supplementary forage so that your horses are less likely to eat any seeds remaining in the field.

If you need further advice or help from the practice regarding this or any other matters relating to keeping your horse in good health, call us on 080 8168 5580.