Sycamore Toxicity / Atypical Myopathy -2020 update

A fatal muscle disease affecting horses in the UK has been linked to sycamore seed and seedlings ingestion.

Atypical Myopathy (AM) causes damage and destruction to muscles involved in breathing, the heart and maintaining posture. Horses show a variety of clinical signs including trembling, pain, lethargy, passing red/brown urine and eventual recumbency. Sometimes it can be confused with colic – it is extremely painful.

Risk factors for the disease have been identified as poor grazing with lack of supplementary forage, more than one horse can be affected in any location and disease is more likely in autumn.

Ways to decrease the risk of an outbreak include removal of seeds where possible, fencing off areas where sycamore seeds have fallen to the ground, supply extra forage if grazing is poor, reduce stocking densities and decrease time out at grazing.

The use of herbicide such as Graze-on should be discussed with the agricultural company manufacturing it. Please keep reading for update.

You do not have to have sycamores either on your own land, or close by, for your horse to have access to sycamore seeds. The little helicopters travel quite a distance so please be vigilant on your grazing! Concerned your horse has eaten Sycamore seeds, flowers, leaves or seedlings, please phone the clinic 01728 685123.

Spring time

It is vitally important in the spring time to check your grazing and ensure that if you find any of the two-leaf seedlings (shown above) they are either sprayed or fenced off. Please do not ignore them!

Two-leaf seedling stage.

Autumn time

Be vigilant for the “helicopter” seeds falling on your grazing. If you find any please remove them or fence off the area.

‘Helicopter’ seed.

How common is it?

At the last count on 16th December 2016, 181 clinical cases compatible with the diagnosis of atypical myopathy have been communicated to Liege University and to the RESPE in 2016. These cases were recorded in Belgium (38 cases) and France (120 cases) Great-Britain (3 cases), Switzerland (6 cases), Germany (13 cases), and The Netherlands (1 case). 2015 was much worse in the UK for this condition whilst 2016 was not so bad because the conditions were less favourable for the sycamore seeds to be distributed.

We have had three cases at Deben Valley Equine over the last few years, two of these are dead.

Flowering sycamore maple is toxic.

Testing for Atypical Myopathy

There is now testing available at the Royal Veterinary College (RVC) and more information on their website or via Liege University.

Please see the link below for information on testing (plants & horses).

RVC Diagnostics – Atypical Myopathy

Update on Atypical Myopathy / Sycamore poisoning – July 2019

More research papers have been published recently regarding Atypical Myopathy, caused by ingestion of the toxin hypoglycin A (HGA) found in Sycamore seedlings, seeds and even leaves.  As little as 20g of toxic sycamore, which equates to JUST 50 SEEDLINGS is enough to kill a horse. The inflorescences (flowers) of sycamore also can contain HGA (150g of flowers would be toxic).

There are more than 25 species of Acer (Sycamore being just one), not all 25 species are toxic.  Originally, we considered that only the 2-leaf stage of seedlings was toxic, but research has shown that as the seedling grows, HGA is still present in 1 year saplings!  Worryingly this toxin has also been detected from in-contact rainwater, so how toxic could the surrounding grass become?

Spraying with herbicides designed to kill broad-leaved weeds including Sycamore seedlings will indeed kill the seedling but the toxin remains within the dead seedling for more than 6 months; hay or haylage cut containing Sycamore seedlings and seeds shows HGA is still present in the bits of sycamore for beyond 6 months.

Fencing off areas of seeds or seedlings can be an effective method of control; whilst physically collecting and removing seeds and seedlings will also lower the toxin from the grazing.  Mowing the pasture will only remove the toxin if all of the seeds/seedlings are picked up afterwards; the same applies after herbicide use.  This is very concerning for anyone with horses near possible toxic Acer / Sycamore trees.  Not all sycamore trees contain the same amount of toxin…

The Royal Veterinary College (RVC) Laboratory offer you a testing service for seeds, seedlings and leaves so you can send samples and find out if the trees at your location are toxic. The RVC Lab has also developed a blood test to help vets confirm the diagnosis in suspected cases, but treatment must start immediately.  Even with intensive care only 25% of horses with AM survive. There is ongoing research into the genetics of horses which have suffered AM as not all horses may be susceptible.  More info on

Last updated July 2019

Identifying toxic maple trees.


Our European counterparts have many cases in France this year so we therefore advise:

– To prohibit horses access to pastures containing sycamore maple samaras (helicopters!)

– Fence off the pasture to avoid areas where samaras are present in large numbers

– Or, at least, limit the grazing time to a few hours per day (the majority of Atypical Myopathy cases grazed > 6 hours / day). If possible, feed the animals before putting them on the meadow; thin, hungry horses are more at risk…..

– Do not place food (hay or other) on the ground.