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Autumn Watch – Sycamore poisoning & atypical myopathy

September 20, 2021

Atypical myopathy, known as sycamore poisoning, is a frequently fatal disease of horses in the UK caused by eating sycamore seeds, and possibly the leaves. The disease is most common in the autumn, typically around October, and often occurs as an outbreak when large numbers of seeds fall onto the pasture following bad weather. These helicopter shaped seeds fall onto the pasture and contain a toxin known as hypoglycin A (HGA) that causes severe muscle damage, in particular to the muscles that enable the horse to stand, to breathe and the heart muscle. The amount of toxin within the seeds varies, and some horses appear to be more susceptible to the disease and become sick after eating only a small number of seeds. Individual grazing habits and the condition of the pasture are also likely to determine why some horses become sick and others don’t. Even with intense veterinary treatment and hospitalisation, the survival rate is around 50% for affected horses.


What are the signs?

• Muscle soreness and stiffness
• Muscle tremors
• Reluctance to move
• Fast and laboured breathing
• Lethargy and reluctance to exercise
• Red/ brown urine
• Choke
• Colic symptoms
• Head tossing and low head carriage
• Sudden death


How is the disease confirmed and treated?

Recognising the signs early give the best chance of survival. If your horse is showing any of the symptoms above with sycamore seeds on the grazing, we often assume the diagnosis and begin treatment immediately. In particular, if your horse is passing red/ brown urine, as this is caused by a pigment released from the muscle breaking down into the urine. A more definitive diagnosis is made by taking a blood sample to check for muscle breakdown. Unfortunately there is no anti-toxin, but some medications can be used to stop the absorption of the
toxin from the intestines.

The best chances of survival are to transport the affected horse to an equine hospital for intensive care within the first 24-48 hours of symptoms. If the horse survives the first few days of treatment their chances of recovery are good, although this may take several months.



Check for sycamore trees around your fields and avoid letting horses graze pasture with over hanging sycamore trees in the autumn. The seeds can spread up to 3x the height of the tree, but this distance may increase in bad weather.
If horses have to remain on pasture with sycamore trees, fence off areas where the seeds and leaves fall, and clear them from the grass as often as possible. Ensuring there is adequate grazing, and if not supplementing with hay or haylage placed well away from the trees, will encourage horses not to eat the seeds and to graze away from the trees. Reducing stocking density on the pasture may also help ensure there is enough grass.


For more information on sycamore poisoning and atypical myopathy, or if you are concerned your horse is at risk, please contact the practice to speak to one of our equine vets for further advice.


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