High winds and wet weather put horses at risk of Atypical Myopathy

As autumn brings prolonged spells of wet and windy weather, with leaves falling from trees and sycamore seeds being blown far and wide, horse owners are urged to be vigilant for signs of Atypical Myopathy (AM).

Research has shown AM to be caused by the toxin hypoglycin A, contained in seeds and seedlings from some Acer trees, predominantly the sycamore. High winds often result in considerable contamination of pastures with sycamore seeds. The onset of the disease can be very rapid, with high mortality rates, and it can affect horses of all ages and types, but young horses may be more vulnerable than older horses.

Rossdales' Research Associate Victoria Colgate has written a detailed article for veterinary surgeons about Atypical Myopathy, which can also be downloaded at the end of this news story, but here are some of the key clinical signs of AM and ways to help prevent the disease.

What are the clinical signs of AM?

• muscular weakness and stiffness
• fine muscle tremors
• reluctance to move
• dark discoloured urine
• fatigue
• colic-like signs, including shivering, sweating and trembling
• depression
• high heart rate
• high respiratory rate and difficulty breathing

Prevention

Horses that develop AM are usually kept in sparse pastures with an accumulation of dead leaves, dead wood and trees in or around the pasture. They are often not fed any supplementary hay or feed. While the sycamore seeds may not be directly palatable, horses grazing on poor quality pasture may ingest considerable numbers of them.

Horse owners are advised to:

Restrict access to seeds by using temporary fencing.

• Ensure horses have access to good quality uncontaminated pasture.
• Move horses off pastures at times of risk.
• Provide supplementary feed in the field to minimise the risk of horses being tempted to ingest seeds.
• Avoid leaving wet hay on the ground where it will rot and potentially trap seeds.
• Discuss the risks and how to identify early clinical signs of AM with your vet.
• Be aware that a field without sycamore trees can still contain seeds spread by high winds or flood water.
• Do not prune seed laden trees as this can lead to massive pasture contamination and further increase the risk to horses.

The levels of the toxin vary from year to year and seed and tree samples can be tested to check for the presence of the toxin. Blood and urine samples can be analysed from horses to confirm the diagnosis of atypical myopathy.

If you suspect a case of Atypical Myopathy, or would like to discuss prevention, please contact your vet immediately.

Infographic by Victoria Colgate MA VetMB MSc MRCVS, © Rossdales Veterinary Surgeons 2023