Atypical myopathy is a disease that primarily affects muscles. It is also known as autumn pasture associated myopathy and is caused by the ingestion of a toxin found within sycamore seeds and saplings (see photos).

The disease is not very common in the UK, but cases have been reported. It occurs mainly in autumn when seeds are on the grass and ingested by grazing horses.

The toxin held within the seed interferes with the energy metabolism within muscle fibres. The muscles most affected are the group that maintain normal posture (ie standing) as well as in the heart and chest.

There is no cure and horses are managed with supportive care.

Veterinary advice should be sought if any of the signs below are observed:


Stiff muscles

Difficulty moving


Breathing difficulties

Dark coloured urine


A blood sample will reveal very high levels of the muscle enzyme, creatine kinase. The disease can progress rapidly within 24 hours and the prognosis is very poor if the horse becomes recumbent.

Any horse grazing in the same field as one diagnosed with an atypical myopathy should be moved immediately and closely monitored for symptoms.

Treatment involves high levels of fluid therapy to flush the toxin out through the kidneys as well as pain relief and muscle relaxants. Additional vitamin supplementation and antioxidants may also be beneficial.

These cases often require intensive nursing care and are best managed in a hospital.

To prevent the disease occurring, pastures should be inspected for contamination with sycamore seeds and horses moved if possible. If horses are unable to be moved, additional hay or haylage should be provided to prevent horses grazing close to ground level and ingesting high quantities of seeds.

Be mindful of pasture contamination after very wet and windy weather as seeds are blown off the trees and call your veterinary surgeon if you have any concerns.