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03 October

What is sycamore poisoning?

author image Edeline Bourrier Edeline Bourrier
What is sycamore poisoning? image

Atypical Myopathy (AM) or sycamore poisoning is a form of non-exertional rhabdomyolysis (NRE). It is a rare but potentially fatal pathology that can affect horses. It doesn’t seem to affect other members of the Equidea family, however, the reasons behind that are yet to be asserted. 

Whilst the first recorded cases in Europe were decades ago, it was only in 2014 that it was linked to the seed and saplings of the Acer pseudoplatanus (sycamore maple). The condition is caused by the ingestion of sycamore tree ‘helicopter’ seeds or saplings. It can have devastating effects if not diagnosed and treated on time. According to the Royal Veterinary College, Atypical Myopathy is fatal for around three-quarters of affected horses [1]. Within 24-72 hours of ingesting a significant amount of the toxin, the condition tends to be fatal if urgent action hasn't been taken.

As autumn sets in, the risk of atypical myopathy in horses increases. Let’s investigate the causes, symptoms, treatment, and steps you can take to prevent it.

Good to know: 

Not all sycamores carry the toxin and it may vary year on year. The Royal Veterinary College offers a plant testing service. You can find all the information on their website. The form is available here.

1. What are the causes of atypical myopathy?

Atypical Myopathy is primarily caused when horses eat seeds, saplings, leaves, or bark from trees belonging to the Acer family, commonly known as sycamore trees, hence why it is also known as sycamore poisoning. The seeds, especially those that have fallen onto pastures and are mixed with grass, can be a significant source of danger.

Good to know: 

In Europe, Atypical myopathy is linked to Acer pseudoplatanus, whilst in the USA it is linked to another variety of the sycamore species, Acer negundo.

The toxin responsible for AM is mainly hypoglycin A, which the tree produces as a defense mechanism to protect seeds and the tree against herbivores and illnesses.

When the toxin enters the horse’s body, the toxin affects the metabolism of carbohydrates, leading to the energy available in muscle cells to plummet. This can result in symptoms ranging from muscle damage to multiple organ failure.

Acer pseudoplatanus - Leaves and seeds

2. What are the symptoms of sycamore poisoning?

Early diagnosis is crucial in lowering the risks of fatality. A variety of symptoms can be observed in horses who have ingested sycamore seeds.

  • Muscle weakness: Myopathy literally means muscle disease. Unsurprisingly, hoses affected by the condition usually display signs of muscle weakness with difficulty moving or standing.
  • Dark urine: Reddish-brown urine is caused by the kidneys processing the myoglobin (muscle protein) released into the bloodstream by the muscles.
  • Rapid breathing and heart rate: Increased respiratory and heart rates are the usual signs of stress, pain, and discomfort in horses.
  • Lethargy: Every muscle in your horse’s body can become affected, including those allowing them to drink, eat, breathe, pump oxygen-rich blood throughout the body, etc. leading to intense fatigue and unwillingness to move, drink, or eat.
  • Muscle twitching: In some cases, muscle tremors can appear.

A lot of these symptoms can be associated with other conditions such as Monday morning disease, anemia, colic: fatigue, rapid breathing, rolling, lying down, refusing to eat or drink, etc. Environmental factors are to be taken into consideration to be able to reach the right diagnoses rapidly.

Good to know: 

As the toxin levels in the body take over the horse or pony will still want to eat but may suffer choke as the swallow reflex muscles weaken.  Wet hay is one of the safest things to offer.

3. What are the treatments for atypical myopathy?

There are no specific treatments for atypical myopathy. Most of the care is focused on treating the symptoms and the complications associated with the condition. Therefore, prompt diagnosis and treatment are crucial in avoiding extensive and fatal muscle damage.

  • Fluid therapy: As horses affected can become reluctant to drink and eat, intravenous fluid therapy is key to maintaining hydration and electrolyte balance. In addition, this can help prevent kidney damage as they process the myoglobin released by the muscles.
  • Nutritional support: As horses may have difficulty eating, supplying them with intravenous or nasogastric tube feeding ensures the supply of essential nutrients.
  • Pain management: As the muscles deteriorate they become painful. Anti-inflammatories are often given to help keep horses more comfortable.
  • Respiratory support: Respiratory muscles and the heart can be weakened and respiratory support can help ensure oxygen-rich blood carries on flowing through the body and vital organs. Once they become recumbent their respiratory system tends to fail, trying to keep the horse upright can help.
  • Monitoring: Monitoring vital signs (heart and respiratory rates, body temperature, blood oxygen saturation etc.) is crucial as well as ensuring that the horse is kept in a quiet and safe environment to minimise muscle and organ damage.  
  • Recovery: The period of recovery will vary widely according to the extent of the damage sustained, the age of the horses, and their fitness level. Most horses who make it past the 72-hour mark tend to recover. Rehab often involves physical therapy to help restore muscle strength and function.  

4. How can I prevent my horse from suffering from atypical myopathy? 

As we’ve seen above, the prognosis for horses suffering from atypical myopathy is dire and diagnosis can be complicated. As often, prevention is better than cure which in this case lies in managing your horses’ environment, grazing, and forage.

  • Pasture management: If possible, avoid giving access to zones where sycamore seeds could contaminate the grazing by fencing the area off. Keep in mind that whilst most of the seeds will fall in the autumn if your area is affected by a drought, they may fall earlier in the season, or after a period of high wind, seeds may travel quite far. Inspect your fields regularly and collect any seeds you may find. Letting your horse out grazing for short periods of time can help limit the risk of ingesting large quantities of the toxin.
  • Supplement grazing: Horses will naturally avoid eating sycamore seed, however, when grass is scarce and short, they may accidentally ingest some. Providing them with good quality hay and feed when grazing is limited can help avoid this. When feeding additional forage, it is best to keep it off the ground to limit the risk of contamination.
  • Water: Ensure a contaminant-free water source is available at all times. Be mindful that seeds can travel in waterways and the toxins can contaminate streams several fields away from the affected tree.

Good to  know: 

If you’re planting hedges and trees to provide your horses with shade and shelter, choose the species wisely. Avoid varieties from the sycamore family and oaks or walnut trees as they all present a toxicity risk.  

Atypical Myopathy is a serious and potentially fatal condition in horses. In order to reduce the risks, a proactive approach centered around pasture management and awareness is crucial to reduce the risk to your horses.

If you suspect your horse may be affected by sycamore poisoning, consult with your vet immediately to ensure the correct diagnosis and treatment plan.


[1] Atypical Myopathy Fact File,

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