Equine atypical myopathy (EAM) is a serious potentially life threatening condition caused by eating Sycamore seeds or possibly leaves. Incidences tend to occur in autumn and in the spring following large autumnal outbreaks. Horses that develop EAM are usually kept on sparse pastures with an accumulation of dead leaves or wood present. There is often no supplementary feeding with hay or hard feed. Outbreaks frequently occur following a period of wet, windy or cold weather coinciding with when large numbers of seeds are falling. The amount of toxin within the seed is variable although the levels are thought to increase during the autumn months. It isn’t known how many seeds need to be eaten for a horse to become sick. It is likely that some horses are more susceptible than others, young horses and foals seem to be particularly susceptible. The disease results in muscle damage, affecting the muscles which enable a horse to stand, breathing muscles and the heart muscle. Due to the muscle damage, urine of affected horses becomes a dark red colour due to excretion of pigment from the affected muscles.

What are the signs?

Early signs of the disease include lethargy, dullness or mild weakness. These signs usually progress quickly to stiffness, muscle tremors, extreme weakness and increased periods of lying down. In severe cases the horse may be found lying down and unable to stand. Owners may be concerned their horse has colic. Some horses may be found standing rooted to the spot, with a low head carriage, vocalising (whinnying) and head nodding. If you suspect your horse has atypical myopathy phone the practice as a matter of urgency.


The physical examination and grazing history will often give a strong index of suspicion for the disease. A urine sample can be obtained and visually confirms ‘dark red urine’, laboratory testing can confirm the presence of muscle protein in the urine. Diagnosis can be confirmed by checking the blood to test and measure the muscle enzymes. Kidney parameters may also be elevated. If one horse is suspected to be showing signs of EAM the remaining field companions should be removed from the pasture, examined and the blood tested for early signs of the disease.


Horses treated for EAM require intensive 24/7 nursing care including intravenous fluid therapy to restore circulation and protect the kidneys from the damaging effects of the muscle protein.  EAM cases are often very painful and therefore require painkillers and supplementary vitamins and minerals may also be beneficial. Cases may get worse before they get better so if transport of the horse is possible early referral to a hospital is usually advisable. Those horses that do recover usually make a full recovery and return to work with no-long term effects of the disease.


The Sycamore seeds and to come extent the leaves are the only known source of the toxin although other sources may be elsewhere. Fence off any Sycamore trees or areas where seeds have fallen. Cases of EAM that arise in the spring are thought to be associated with ingestion of the seedlings therefore where possible remove seeds from the pasture. Supplementary feeding with hay or haylage will discourage horses from eating the seeds. Turning horses out for shorter periods of time where possible.

Due to the variability of the toxin levels in the seeds and to a lesser extent the leaves, cases of EAM have been seen in horses that may have grazed the same pasture previously with no signs of the disease. The introduction of a new herd mate may result in the new herd mate becoming affected despite other horses having shown no signs of the disease. Therefore it is always worth following the prevention steps above to avoid cases of EAM.

Phoebe Parker BVetmed MRCVS

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