AM is a severe and often fatal muscle disorder caused by the ingestion of sycamore seeds, leaves or seedlings. It is fatal for around three quarters of the affected horses. It was first reported in the 1940s, but it seems like there has been a significant increase in recent years.

Young sycamore seeds and leaf


What causes AM?

AM in horses is caused by the ingestion of sycamore seeds that are often found in pastures in autumn and winter and their germinating seedlings in spring.

Recently, looking into similar diseases in human medicine lead to the discovery of the hypoglycin A and its toxic metabolite as the cause of the degeneration of the muscle cells; this toxin is present to a various degree in trees from the maple family, and their toxicity can be affected by a number of environmental factors such as climatic and soil conditions. There is currently research ongoing to establish exactly which factors are involved in deterring the toxicity from the ingestion of sycamore, such as quantity, genetic factors, diet and previous exposure. For reasons yet to be understood, even fields that had horses grazing for many years with no signs of AM are not to be considered safe in presence of sycamore trees.

Are all horses susceptible to AM?

AM has been reported in horses of all ages and breeds; however, some horses seem to be more susceptible than others, perhaps due to genetic differences. There is also some evidence suggesting that younger horses are more severely affected.

What are the clinical signs of AM?

The onset of AM is usually rapid, and the major clinical signs are the result of the severe damage to postural and respiratory muscles. Early signs are general weakness, reluctance to walk, stand and breath; often colic signs are present, despite the horse showing appetite. As it progresses, the rapid breakdown of the muscle cells can cause acute kidney failure and heart problems.

How is AM diagnosed?

Clinical signs, history, environmental factors and laboratory findings are often sufficient for a presumptive diagnosis of AM; recently, specific tests for the hypoglycin A toxins have been made available to confirm AM diagnosis in horses.

What is the treatment for AM?

Horses affected by AM require immediate intensive care aimed at reducing the risk of kidney failure; this is preferably carried out in a specialist equine hospital for intensive 24h care.  At present there is no specific treatment for AM and affected horses are treated symptomatically.

What is the prognosis?

Survival rate is around 30-40% and is highly dependent on fast diagnosis and treatment.

If horses survive following the first few days of treatment, they usually go on to a full recovery, although this can take several months.

What are the risk factors?

Unlike muscle disease azoturia (“tying up” or exertional rhabdomyolysis) AM occurs independent of exercise; it affects horses kept at pasture, and it has been reported mainly in the autumn with some cases in spring, winter and summer. Wind strength is thought to be a crucial factor for dispersal of the seeds.

Can AM be prevented?

Recently  the Royal Veterinary College developed a reliable test to identify and quantify the presence of hypoglycin A toxins from leaves, seeds and seedlings of sycamore trees; owners are encouraged to test the safety of their paddocks by submitting the samples directly to the RVC.

Owners whose fields are close to Sycamore trees should consider providing supplementary forage and picking up/hoovering sycamore seeds off the pasture; precautions also include reducing the turn out during seasons at risk.


If you are concerned about your horse and wish to discuss this further with us, do not hesitate to contact us at the practice.

For more information about atypical myopathy toxin tests and tree sample testing, visit the RVC website.

RVC Atypical Myopathy Testing


About the author : Serena Rubechini

Serena Rubechini
Serena has a keen interest in general equine medicine and often deals with medical problems in horses and ponies.

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