We advise donkey owners to be aware of Sycamore-related atypical myopathy in donkeys and take steps to reduce the risk to their donkeys and learn to recognise the disease symptoms.

Atypical myopathy (AM) is a frequently fatal metabolic disease affecting the muscles. Cases have been reported in horses and donkeys. The disease has been reported in the UK, Northern Europe and North America. In Europe it is thought to be caused by a toxin in the 'helicopter' like seeds from the sycamore (Acer pseudoplatanus) whereas in the USA, seeds from the box elder (Acer negundo) have been implicated.

Disease development

Autumn 2014 saw a sharp increase in cases in the UK, due to extreme seasonal changes in weather. The disease is relatively new and aspects of it are still not fully understood. A link was made between ingestion of seeds containing the toxin hypoglycin A and development of AM in 2013. The condition has a high mortality rate; initially approximately 70% of affected horses died or were euthanased, but more recently this rate has dropped to around 50% as cases are more quickly identified and therefore treatment begun earlier. The time between onset of clinical signs and death can be as little as 12 hours or as long as 10 days. Horses that have made it through the first five days are more likely to survive.

Symptoms in horses include:

  • Depression
  • Muscle weakness/tremors
  • Inability to stand
  • Loss of appetite
  • Increased heart and respiration rate
  • Choke
  • Depression/lethargy
  • Sweating
  • Colic
  • Extremely low head carriage
  • Fast or irregular heart beat
  • Dark red/brown urine
  • Dark red mucous membranes (eg gums).

As the number of donkeys affected has been very low, it is not clear whether donkeys would display the same clinical signs as horses.


High winds can carry the seeds far from the tree, as can streams and run-off water. Very wet conditions may also release the toxin from the seeds into the surrounding land. Donkey owners need to be particularly vigilant in the autumn as seeds can even be found in pastures not necessarily containing sycamore trees. Extreme weather at other times of year may also be of significance. Cases have been seen to rise in the spring as seedlings start to germinate.

Tips for donkey owners

  • Ensure adequate food sources are available
  • Fence off around sycamore trees but be aware that seeds may travel further than expected
  • Inspect fields daily and remove any seeds/leaves
  • Ensure water sources are kept clean and fence off any ponds or streams that may have been contaminated
  • Remove young tree saplings
  • Young donkeys are thought to be more susceptible
  • Check your donkeys twice daily and contact a vet immediately if any abnormal signs are seen
  • Ensure all people caring for your donkeys are aware of the dangers and symptoms
  • If one of your donkeys shows signs of AM remove all others from grazing immediately
  • Check your insurance policy is up to date.

Pasture management

Poor pasture management, high stocking densities and poor provision of safe food sources are often major risk factors in the development of many kinds of poisoning in horses, donkeys and mules. Hungry animals will explore alternative food sources and eat plants that they would normally reject. Once eaten, animals often get a taste for many plants and seeds that we know are toxic, such as bitter acorns. Prior to winter it is important to ensure a steady supply of straw and hay/haylage is available to avoid your donkeys looking elsewhere to satisfy their appetites.

We would not recommend removing your donkeys from pasture altogether, but be especially careful choosing grazing for 'at risk' animals such as the very young or old, and those who are ill or convalescing as they will be more susceptible to and less able to cope with AM.

Sycamore has traditionally been used as a source of shelter/browse for horses and donkeys, and there are undoubtedly many situations where equids and sycamores get along just fine. Cut sycamore logs (minus any leaves/seeds) are still safe to be given as environmental enrichment.

The major risk factor is thought to be an abundance of seeds/leaves on the ground together with a lack of suitable forage. The field maple (Acer campestre),  which is a common plant among British hedgerows, has been identified as not producing the deadly toxin.

Poisonous plants

As part of pasture management, we would recommend donkey owners familiarise themselves with safe trees and shrubs for donkeys and avoid poisonous plants and trees.

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