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Atypical Myopathy

September 2019

The latest research and tests on this continued threat posed to horses

Atypical Myopathy

Atypical Myopathy (AM), also referred to as Seasonal Pasture Myopathy (SPM), often results in a rapid deterioration of the horse’s health with reports indicating around a 70% mortality rate1 ,2. Numerous research studies have found that AM is highly associated with the ingestion of sycamore seeds and seedlings which have been found, in varying concentrations, to contain a substance called Hypoglycin-A3. This substance poses a huge problem for horses, as it is converted into a toxin within the horse’s body4. The toxin has a rapid negative effect on the normal functioning of the horse’s cells, as it deprives the horse’s muscle fibres of the energy it needs to work.

Vigilance Required

The BHS urges all horse owners to be vigilant for sycamore seedlings during the spring and seeds during the autumn. Be aware that the sycamore seed, more commonly known as ‘helicopter seeds’ can land between 30-80 metres from its parent tree, but in strong winds can travel several kilometres5. As a result, seedlings can appear to grow in areas which aren’t surrounded by sycamore trees.


Sycamore seedling

Clinical Signs

The postural (muscles responsible for keeping the horse standing) and respiratory (breathing) muscles are most commonly affected and, therefore, the clinical signs shown by the horse most often reflect muscular problems, signs associated with pain and breathing difficulties. The onset of AM is rapid and horses can quickly deteriorate within 6-12 hours4. As the toxin’s effects progress, the horse’s heart muscle can also be affected.

A recent study6 compared the respiratory capacity of 15 horses diagnosed with AM against 10 healthy horses. The study concluded that the horses with AM showed up to a 49% decrease in their respiratory capacity compared to the healthy horses. This highlights the severe internal changes in the horse and how this contributes to the high mortality rate associated with the disease.

Sycamore Trees

The sycamore tree belongs to the Acer family group. In the UK, the most common Acer family trees are the field maple, sycamore maple and the Norway maple. Numerous studies continue to investigate the HGA levels in all of these trees, but so far have found that the field and Norway maple do not contain this substance7,8 and, therefore, is currently related to sycamore trees1, 9-11. For many horse owners in the UK, the sycamore tree is a common and attractive feature in paddocks used to graze horses. The fact that sycamore trees now pose a risk to horses may come as a surprise to many horse owners whose animals have been happily grazing alongside them for years with no ill effects. This anomaly poses an important question, to which there are yet no answers, as to ‘why are there variable amounts of HGA in different sycamore trees’ plant material?’ This question is particularly pertinent when nearly 100 horses died in the UK in 2014 due to AM12. Suggestions have included that HGA levels may vary due to the time of year or due to different climatic conditions; however, these risk factors have not yet been confirmed, but research is ongoing.

New Test Available

A test is available from the Comparative Neuromuscular Diseases Laboratory at the Royal Veterinary College  for the testing of seeds, seedlings and leaves for HGA. Further details on the test are available from the Royal Veterinary College website.

Multiple sycamore seeds

Sycamore seeds & leaves 
 Sycamore seeds Sycamore leaves and seeds

Removal of Sycamore Seedlings

AM cases in the spring are less common than those in the autumn but compared with sycamore seeds, the seedlings have the highest HGA levels13 so still pose a risk to grazing horses. Removal of the seedlings is not easy and very recent research has shown HGA levels remain in seedlings when they are mowed or sprayed with herbicides (dimethylamine-based and picolinic acid-based) up to two weeks after intervention. In fact, when mowed, the HGA content temporarily significantly increased in the youngest seedlings within the first two days of cutting13.

An important consideration is that HGA can have different effects in different species; for example, in rats the intoxication causes mainly liver damage and intestinal problems rather than causing the same clinical signs observed in horses. Therefore, we are currently not able to recommend the use of livestock to help graze sycamore material.

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 6-8 months storage still contained appreciable amounts of HGA13.

The BHS appreciates that the clearance of sycamore material poses a huge task and headache to horse owners. The current recommendations by lead researchers is mowing followed by the collection and removal of the cut grass and sycamore seedlings to avoid HGA toxicity in horses13 and to decrease the risk of colic from the cut grass.

Further Information

Further advice on AM is available on the BHS website. Alternatively, you can contact the Welfare Team on 02476 840517* or If you have any concerns for the health of your horse, contact your veterinary surgeon immediately.

*Calls may be recorded for monitoring purposes

Case Study

Joseph was a healthy horse first thing in the morning but by the afternoon he was in an equine hospital being treated for Atypical Myopathy. Find out more about Joseph’s story here


1. Votion DM et al. (2014) Identification of methylenecyclopropyl acetic acid in serum of European horses with atypical myopathy. Equine Veterinary Journal March 46(2) pg. 146-9.

2.Richard Piercy. (2019). EVJ In Conversation Podcast No. 37- atypical myopathy. [Accessed online 09.09.2019].

3. Draper A. (2016) Should we cut down all sycamores? BEVA Congress Handbook of Presentations 7-10 Sep pg. 142-3.

4. Royal Veterinary College. (2017) Plant sample testing for Atypical Myopathy – owner information sheet. [Accessed 01.03.17].

5. Squirrell J. (2015) Sycamore, Acer pseudoplatanus. [Accessed 01.03.17].

6. Lemieux H et al. (2016) Mitochondrial function is altered in horse atypical myopathy. Mitochondrion Sep 30 pg. 35-41.

7. Westermann CM et al. (2016) Hypoglycin A concentrations in maple tree species in the Netherlands and the occurrence of atypical myopathy in horses. Journal of Veterinary Internal Medicine May/June 30 (3) pg. 880-4.

8. Grὂndahl GB et al. (2015) Detection of the toxin Hypoglycin A in pastured horses and in the European sycamore maple tree (Acer Pseudoplatanus) during two outbreaks of atypical myopathy in Sweden. Equine Veterinary Journal. 47 Supplement 48:22.

9. Zuraw A et al. (2016) Equine atypical myopathy caused by hypoglycin A intoxication associated with ingestion of sycamore maple tree seeds. Equine Veterinary Journal July 48 (4) pg. 418-21.

10. Baise E et al. (2016) Samaras and seedlings of Acer pseudoplatanus are potential sources of hypoglycin A intoxication in atypical myopathy without necessarily inducing clinical signs. Equine Veterinary Journal 48 (4) pg. 414-17.

11. Zuraw A et al. (2016) Equine atypical myopathy caused by hypoglycin A intoxication associated with ingestion of sycamore maple tree seeds. Equine Veterinary Journal 48 (4) pg. 418-21.

12. Université de Liège (2014) Alert of the 28th of October 2014 – update 19/12/2014[Accessed 02.03.17].

13. 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 0 pg.1-4.

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