sycamore tree

Atypical Myopathy (also known as Seasonal Pasture Myopathy)



A relatively new and thankfully RARE but important disease of horses.

The condition is seen with increasing frequency during autumn months.

Early recognition of signs is vital as the condition is potentially fatal but reducing the risk of development of this disease is possible by some management changes.

The condition is caused by a pasture based toxin (Hypogyclin A) in the ‘helicopter’ seeds of the sycamore tree.

Atypical Myopathy affects younger horses (especially <3years), those that are thin and not in regular work and those kept on sparse pastures without supplementary feeding.

This year’s warm summer and early autumn has seen an abundance of seeds within sycamore trees. The recent winds and rain has seen a large number of seeds falling from trees in a short period of time, posing a risk to horses grazing on fields containing sycamore trees and surrounding fields.


The disease causes severe muscle damage, so horses can present as if ‘tying-up’, and results in

  • Severe generalised weakness and stiffness, leading to muscle tremor and potentially collapse.

  • Decreased appetite and colic like signs.

  • Dark coloured urine.


Firstly, if your horse is unwell, removed from pasture and obtain veterinary advice, otherwise identify whether your horses are at risk….

ACT NOW- check your horses field and identify whether sycamores surround the fields.

If your horse grazes on pasture surrounded by sycamore trees or if large number of seeds blow into your field you should:

  • Consider fencing off areas around trees

  • Remove fallen sycamore seeds
    provide supplementary hay especially if pasture quality is poor (horses won’t usually eat the seeds if grass is good or provided with extra forage source)

  • Reduce stocking density within pastures

  • Stable horses and turn out for short periods of time

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