Equine atypical myopathy 

Increasing numbers of horses are being diagnosed with highly fatal atypical myopathy this Autumn.

Equine atypical myopathy (EAM) or atypical myoglobinuria has been a recognised disease in horses and ponies for many years but only in 2013 was the cause of this deadly disease finally identified.

What is EAM and what are the clinical signs

Equine atypical myopathy causes the breakdown of muscles; both skeletal (the ones used for body movement) breathing, and heart muscles. Outbreaks tend to occur in the autumn, and in the spring following autumn outbreaks. This leads to a rapid onset weakness, stiffness, reluctance to walk, muscle trembling, depression, pain and collapse as well as myoglobinuria (dark coloured urine). Clinical signs can be mistaken for colic or laminitis particularly in the early stages.

The cause

The cause of EAM in horses is a compound called hypoglycin A which is found in the seeds from two species of elder tree; the sycamore and box elder. The concentration in seeds varies from tree to tree and from seed to seed and it is as yet unclear what contributes to this difference and the research is ongoing.



Once clinical signs are evident mortality rates are high and have been reported to be from 75% to almost 90% even with intensive veterinary care. A collapsed horse carries a worse prognosis.


The discovery of the cause of EAM has been a huge advancement and means that owners can take specific measures to prevent the exposure of horses to sycamore seeds. It is imperative that pastures and the surrounding fields are inspected for maple (sycamore and box elder) trees; don’t forget that the seeds (frequently called helicopters) can travel a long way in windy conditions, so check pastures regularly even if they aren't bounded by maple trees! Fencing off areas where seeds have fallen or alternatively hovering/picking them up is important unless the horses can be moved to alternative clear pasture. If there is a plentiful supply of forage horses are much less likely to want to eat seeds in the first places. Reducing stocking density (number of horses per field) and providing adequate amounts of supplementary hay/haylage/hard feed is essential especially where the pasture has become poor.

Where EAM is suspected all horses should be removed from the pasture and a blood sample taken to identify any subclinical (early stage) cases, which can then be monitored or treated as necessary.



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