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Atypical Myopathy Joint Position Statement

 29/01/2018    Last Modified: 31/07/2023

Arboricultural Association and British Horse Society joint policy statement on

Felling of Sycamore trees in relation to Atypical Myopathy: A statement for arborists and horse owners


This 2022 document is a revision of the previous joint document issued by the two organisations in 2017. Following recent research and improved understanding, this document is intended to bring equestrian and arboricultural managers and practitioners up to date and to offer advice to enable a reasonable and pragmatic approach in dealing with the risks.

Atypical Myopathy (AM), also referred to as Seasonal Pasture Myopathy, is a disease associated with horses eating sycamore seeds and seedlings1. This sycamore material has been found, in varying concentrations, to contain a substance called Hypoglycin-A (HGA). When eaten, HGA is converted into a toxin within the horse’s body. The toxin has a rapid negative effect and can reduce or even stop energy production in the horse’s muscle fibres. The onset of AM is rapid, and horses can quickly deteriorate within 6-12 hours. Prognosis for the horse is often poor if veterinary treatment is delayed; mortality rates are around 56-61% in the UK2.

The seeds are windblown from trees in October onwards, with the seedlings appearing from March, however some vegetative material from sycamores may be found all year round. HGA levels can differ between individual sycamore trees, possibly due to the time of year or due to different climatic conditions or even genetic provenance; the reason why and risk factors have not yet been confirmed, however, this variation is crucial in the management decision process.

It is widely accepted that trees provide us with many benefits and have a significant role to play in our wellbeing, recreation and place making. Trees also provide benefits to horses, including shade, shelter and screening as well as contributing to wider habitats and ecosystems. In some locations, such as poor land or sites suffering from exposure, sycamore trees can be of huge significance in establishing and maintaining tree cover. All these benefits are in addition to trees’ unseen contributions such as purifying the air, trapping pollutants and storm water attenuation.

The British Horse Society advise horse owners to minimise the exposure of their horses to sycamore seeds and seedlings to help prevent AM.

Although much is understood about the effects of HGA, there are many unknowns and variables - not only the variability of HGA levels in different samples of sycamore material but also the range of circumstances when horses may choose to eat them in preference to other vegetation in the paddock.


Clinical signs include:

  • Muscle stiffness / tremors
  • Sweating
  • High heart rate
  • Depressed with their head hung low
  • Brown or dark red urine
  • Weakness, struggling or reluctance to walk and/or have difficulty standing
  • Breathing difficulties

Decreasing the risk

There are steps that horse owners can take to help decrease the risk of horses consuming sycamore seeds and seedlings, which includes:

  • Where horses have poor grazing, ensure supplementary forage/feed is provided
  • Ensure the pasture is not over-stocked
  • Maintain good pasture management to prevent weeds taking over grass growth
  • If moving horses is not an option, fence off areas outside the sycamore tree canopy and be vigilant for seeds that may have been blown outside of this area
  • Where possible, consider stabling the horses overnight to prevent over-grazing of pasture
  • Rake up and dispose of the seeds on a regular basis throughout autumn, and pull up seedlings in the spring
  • Where horses are exposed to sycamore material, have it tested for HGA levels before making any further management decisions.

Testing service

A testing service is available from the Comparative Neuromuscular Diseases Laboratory at the Royal Veterinary College (RVC) which tests sycamore material for HGA.

The test assesses the HGA levels in the sycamore materials. The toxicity levels that the horse may be exposed to is linked to how much toxin the seeds or seedlings contain, but also to the amount of plant material the horse eats, which is difficult to control/measure.

Horse owners need to be aware that the felling of sycamore trees in the immediate vicinity of horse pastures will not guarantee the full prevention of AM as cases have occurred where horses have consumed sycamore seeds that have been blown into, or carried downstream into, their paddocks. It is possible for sycamore seeds to disperse hundreds of metres1. It should also be noted that if you fell a mature sycamore, it is highly likely many more seedlings will grow up in that space from previously dispersed seeds.

Tree owners and those working in arboriculture should be aware that felling sycamore trees due to concerns about AM is not an appropriate first course of action. It is strongly advised that the HGA test should be undertaken first with management decisions based on the results. Therefore, arborists should advise horse owners that the test is available and not facilitate the unnecessary removal of sycamore trees.

If, after sensible consideration, tree owners or arborists are convinced that felling is required, we strongly recommend you consider if the trees are protected by law.

Details of professionals working in arboriculture and answers to other tree related questions can be found from The Arboricultural Association (www.trees.org.uk).

Further information on sycamore seed and seedling identification, and Atypical Myopathy is available here.

If horse owners are concerned about their horse’s health, they should contact their vet immediately.


1. Votion D-M et al. (2020) Frequently Asked Questions Regarding Horse Feeding and Management Practices to Reduce the Risk of Atypical Myopathy. Animals 10, 365.

2. Dunkel B et al. (2018) Atypical myopathy in the South-East of England: Clinicopathological data and outcome in hospitalised horses. Equine Veterinary Education 32, 2, pg. 90-95.

Statement revised: May 2022

Arboricultural Association, Atypical Myopathy, horses, Royal Veterinary College, sycamore, The British Horse Society