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Sycamore Seed Poisoning

Equine atypical myopathy, also known as atypical myogloninuria or seasonal pasture myopathy (SPM), a highly fatal muscle disease, is a condition which can affect grazing horses in the spring or autumn caused by consumption of sycamore tree seeds.

Each year in Ireland there have been from 6-12 cases diagnosed with perhaps many more occurring, but not identified.  A typical story is of a horse demonstrating initially stiffness and a reluctance to move.  The muscles suddenly become weak to the point the horse can no longer remain standing.  Then, as quickly as clinical signs set in, the horse dies.   Just 48 hours earlier the horse grazed happily at pasture—an overgrazed field full of seed heads and dead leaves.

Equines that develop Atypical Myoglobinuria are usually kept on sparse pastures with an accumulation of dead leaves, dead wood, and trees in or around the pastures. Unlike other muscle disorders, the disease’s clinical signs are not associated with exercise and include:

  • Reluctance to move
  • Muscle weakness
  • Stiffness and fine muscle tremors
  • Increased periods of recumbency (unable to rise after lying down)
  • Tachycardia (irregular and overly rapid heartbeat)
  • Myoglobinuria (red-brown coloured urine)
  • Occasionally choke (oesophageal obstruction)

These clinical signs progress quickly with rapid respiratory rate within 48 hours and difficulty breathing and death within 72 hours in at least 75% of cases.  The cause of death is a very specific metabolic block in the muscle’s ability to burn fat for fuel.

Horses at SPM farms are less likely to receive supplemental hay or concentrate while grazed in proximity to Sycamore trees (Acer pseudoplatanus). A paper published in the March 2014 edition of the Equine Veterinary Journal by Votion et al1 concluded that Atypical myopathy in Europe is highly associated with the toxic metabolite of hypoglycin A (MCPA-carnitine) – HGA- present in the seeds of Acer pseudoplatanus.

Protecting your equines

Some equines do not develop ATYPICAL MYOPATHY after years of living on affected pastures. Young animals and those new to an affected pasture appear to be at great risk if:-

  • Pasture is overgrazed in autumn and early spring
  • Turnout time is greater than 12 hours per day
  • No supplemental forage is provided on pasture

Removal of sycamore trees from affected pastures can be one approach, although, this might not always be feasible. In cases where the trees can’t be removed, it is recommended decreasing turnout time on affected pastures from October through mid-December and in the early spring. Remember that even paddocks free of sycamore trees may still be at risk from seeds being blown on to the pasture.

Other important preventive measures could include providing additional forage, preventing over grazing of pastures through rotational grazing, and limiting turnout to less than six hours per day during autumn and early spring3. Remember that feeding supplemented forage from the ground or close to sycamore trees can increase the risk of ingesting seeds. 

Also, if mowing such pastures then collect and dispose of the cut grass and seedlings. Pastures contaminated with sycamore material should not be used to produce hay, haylage or silage as research has found that seeds and seedlings present in bales after six to eight months storage still contained appreciable amounts of HGA2.

HGA is a water-soluble toxin that may pass from plants to water through direct contact. For this reason, pastures with a natural water source, such as a river or stream, should not be used during the high-risk seasons and an alternative water supply should be provided3.

The condition has progressed from a sporadic condition to more frequent large outbreaks throughout Europe. Climate change may have increased sycamore tree seed production, seed dispersal by weather or toxin concentrations in seeds.  It is also possible that increasing awareness of the disease has resulted in increasing reporting rather than an increased incidence.



  1. Votion, D.-M. et al (2014). Identification of methylenecyclopropyl acetic acid in serum of European horses with atypical myopathy Equine Veterinary Journal
  2. González-Medina S et al. (2019). Atypical myopathy-associated hypoglycin A toxin remains in sycamore seedlings despite mowing, herbicidal spraying or storage in hay and silage. Equine Veterinary Journal
  3. Votion, D.-M. et al (2020). Frequently asked questions regarding horse feeding and management practices to reduce the risk of atypical myopathy. Animals 10