Horses and Sycamore Seeds – Atypical Myopathy

I read a few sad cases in the press last year of horses dying following ingestion of Sycamore seeds, which led to an understandable run of enquiries from concerned clients. With a life threatening condition such as this, you need to know and understand the causes, symptoms and means of prevention, to keep the risk to your horse as low as possible.

Your first port of call should be your local Equine Veterinary service, and Ricky Farr from Farr & Pursey explains further;

What is Atypical Myopathy and what causes it?

Atypical myopathy is a condition seen within horses which affects the muscles as a result of ingestion of a plant toxin called ‘Hypoglycin A’. Strangely the association of Hypoglycin A, Atypical Myopathy and horses was almost by chance. A study into the condition of ‘Jamaican Vomiting Sickness’ in human athletes found that the ingestion of the Ackee fruit resulted in degeneration of muscle fibres found within the respiratory system. Further research found the fruit contained Hypoglycin A and there were distinct similarities found within the muscle fibres of horses suffering from Atypical Myopathy.

We now know that sufficient levels of the toxin can result in severe degeneration of muscle tissue, which not only affects the function of the muscle (particularly those associated with breathing), but also can result in the by-products of muscle breakdown causing kidney failure.

All horses can be affected with even documented cases in donkeys; however some studies have suggested younger animals (< 3 yrs) be more vulnerable.

So where do we find Hypoglycin A? Thankfully the UK is not renowned for abundant crops of Ackee fruit; however we do have a reasonable amount of Acer pseudoplatanus – aka the Sycamore, which is also known to contain the toxin. In the United States other trees within the Acer family have also been implicated in the development of the condition, for example Acer negundo, aka the Box Elder Tree. At present in the UK it is only the Sycamore which has been associated with Atypical Mypopathy.

What are the symptoms?

Clinical signs associated with Atypical Myopathy can vary immensely and can include any of the following:

  • Postural abnormalities and severe gait changes.
  • Reluctance to move
  • Sweating
  • Muscular weakness
  • Muscle tremors
  • Stiffness
  • Behavioural changes – depression, confusion, sedation
  • Dark urine
  • High heart and respiratory rates
  • Difficulty breathing
  • Distressingly in some cases sudden death

The challenge with Atypical Myopathy is to differentiate the condition from more common diseases as the clinical signs are also consistent with conditions such as laminitis and colic.

What should you do if you suspect your horse is affected?

If any horse owner suspects a case of Atypical Myopathy they should call their local vet immediately.

Incidence of the condition is low, but over the past few years there have been increasing numbers. We do know that the disease is particularly prevalent during the autumn months; however some cases have been documented in the spring and summer. Unfortunately by the time genuine clinical signs of Atypical Myopathy become apparent the mortality rate is very high – up to 90%.

From personal experience I can also say that the onset and development of the condition is extremely quick and many patients are lost with 72hrs. There is no specific treatment for Atypical Myopathy and the most horses undergo extensive supportive therapy to try and minimise kidney damage – therefore if you ever suspect a case, rapid referral to your vet is imperative.

Is it just Sycamore they should be concerned about? What about other species of Acer trees – such as Field maple, so common in hedgerows?

At present there has been no definitive association with any other UK based species of plant, but that is not to say there won’t be. Research into the condition is still undergoing – one of the best sources of up to date research is at: which is run in conjunction with the Animal Health Trust.

Does it depend on the maturity of the trees?

At present we only have an association with the Sycamore tree in general, however research into clinical cases has found that factors including dry conditions, compacted soil, and wind strength may affect the number of seeds produced. Furthermore, seed dispersal and concentration of different substances within fruits and seeds has also been shown to have an effect. It could be speculated that a mature tree is likely to produce more seeds, therefore pose a potential increased risk to a horse population – however this is not definitive.

Is it just the seeds of the trees causing the problem, or is it the leaves as well?

At present we only believe the seeds provide a potential risk.

What practical steps can you take to prevent the problem in the first place?

Here are a few practical tips I try to give horse owners:

  • Avoidance of pasture with a high proportion of Sycamore trees.
  • Avoidance of areas where Sycamore seeds seem to be dispersed.
  • Manual removal of excessive seeds / leaves
  • Supplementary feeding for horses away from Sycamore trees
  • Adequate clean drinking water from a mains source
  • Avoidance of pasture where there has been an association with the condition – especially in the autumn months
  • Maintenance of good body condition – poor body condition scores have been correlated with increased risk
  • Regular vaccination – unvaccinated horses have been associated as a risk group
  • Regular worming – horses not associated with a worming program have also been isolated as a risk group
  • Young horses avoiding pastures where cases have been reported & or grazing around Sycamore trees – 3 yrs. or less has been shown to be a risk group.

And so back to the forester for questions on tree management …

Can we or should we cut Sycamore trees down around their paddocks?

With regards the trees, there is no immediate suggestion they need to be removed to manage the problem, but obviously if there are practical reasons to do so, then consider the following: –

1. Do you need a felling licence? Not all situations will demand this, but the removal of a group of mature Sycamore may require a licence. Phone before you fell!
2. Are your trees covered by a Tree Preservation Order or are they located in a Conservation Area? You will need to advise or gain consent from your Local Authority.
3. And finally, if you’re removing the tree, have you considered a suitable replacement species?

Species to avoid planting due to their natural toxicity would include: –

  • Sycamore
  • Box Elder Tree
  • Alder
  • Cherry species
  • Laburnum
  • Oak (acorns)
  • Horse Chestnut
  • Yew
  • Red Maple

And whilst we’re on the subject, hedgerow species which can be toxic to horses include the following: –

  • Box
  • Cherry Laurel
  • Rhododendron
  • Yew
  • Spindle

In conclusion, if the Sycamore trees are a manageable and desirable feature, and offering shade and cover to your horses, your best option may be to take practical steps in managing grazing during seed fall, without resorting to felling. If the risk cannot be managed, then better to have the trees removed, and consider planting some sensible alternatives.

Our Bloggers

Colin Finlay has been managing and creating new woodlands with Nicholsons since 2010.  With a background in biodiversity, and the more mundane sounding grants & regulations with the Forestry Commission, his natural incline is to make woodland management approachable and understandable.  The enjoyment comes in supporting landowners; not just in terms of day to day management, but also making meaningful improvements to their woodlands through conservation and diversification projects.

Ricky Farr is the co-founder and one of the principle partners at Farr and Pursey Equine Veterinary Service Ltd. Based in the little village of Albury in Hertfordshire. This sole Equine Practice prides itself on providing a completely ambulatory service for all horse owners; whilst being devoted to supplying a transparent veterinary service, with preventative medicine at its core.

Horses and Sycamore Seeds - Atypical Myopathy