WHAT IS ATYPICAL MYOPATHY?

Atypical Myopathy (AM) is a serious and potentially life-threatening disease caused by horses ingesting sycamore seeds, seedlings, or leaves that contain a toxin called hypoglycin A. Once ingested, this toxin causes severe muscle damage, affecting the horse’s ability to move, breathe, and maintain normal bodily functions.

HOW MUCH DOES A HORSE NEED TO INGEST TO BE AFFECTED?

Unfortunately, there is no simple answer.

Not all sycamore trees contain the same levels of toxin, and concentrations can vary significantly between trees and from year to year. As a result, there is no known “safe” amount of sycamore material that a horse can consume.

Fortunately, sycamore seeds and seedlings have a bitter taste, and horses generally avoid eating them when sufficient grazing or forage is available. However, young horses can be more inquisitive and may be at greater risk of consuming them.

If you are concerned about sycamore contamination in your paddocks, samples of seeds, seedlings, and leaves can be tested for hypoglycin A. The Royal Veterinary College offers testing services for horse owners wishing to assess the risk on their land.

WHAT ARE THE CLINICAL SIGNS?

Signs of atypical myopathy can develop suddenly and may include:

Depression and lethargy
Weakness
Muscle tremors
Stiffness or reluctance to move
Rapid of irregular heart rate
Dark red or brown urine
Difficulty breathing
Colic-like symptoms

If you notice any of these signs, particularly during periods when sycamore seeds or seedlings are present, veterinary attention should be sought immediately.

HOW IS ATYPICAL MYOPATHY DIAGNOSED?

Diagnosis is based on a combination of:

A history of access to sycamore trees, seeds, seedlings, or leaves
Compatible clinical signs
Blood tests showing evidence of muscle damage

Additional blood and urine tests can be performed to detect the presence of the toxin and help confirm the diagnosis.

HOW IS IT TREATED?

Unfortunately, there is currently no specific cure for atypical myopathy. Treatment focuses on intensive supportive care and nursing.

This may include:

Intravenous fluid therapy
Pain relief
Nutritional and vitamin supplementation to help limit ongoing muscle damage
Careful monitoring and supportive nursing

Early recognition and aggressive treatment are crucial. The chances of survival depend largely on the extent of muscle damage that has already occurred by the time treatment begins.

For severely affected horses, the prognosis remains poor, with survival rates reported between 10% and 25%.

HOW CAN I PREVENT ATYPICAL MYOPATHY?

Prevention focuses on reducing exposure to sycamore seeds, leaves, and seedlings.

Practical steps include:

Fencing off areas around sycamore trees
Regularly clearing paddocks of seeds and seedlings
Providing supplementary hay when grass is sparse
Removing sycamore trees from grazing areas where possible
Monitoring fields closely during autumn and spring, when the risk is highest

Remember!
Sycamore seeds, often known as “helicopters”, can travel surprisingly long distances. They may be found up to 200 yards from the parent tree, so neighbouring trees can still pose a risk even if there are no sycamores within your paddock.

EARLY ACTION SAVES LIVES

Atypical myopathy is a veterinary emergency. Prompt recognition of the signs and immediate treatment provide the best chance of survival. If you suspect your horse may have ingested sycamore material or is showing any of the signs listed above, contact us without delay.

Atypical Myopathy is a very serious and often fatal disease that horses can get when out at pasture from consuming sycamore seeds.

What is Atypical Myopathy?

An acute, severe rhabdomyolysis (a severe form of azoturia / tying up) that occurs independently of exercise. The disease causes extreme muscle cramping and often irreparable muscle damage.

Muscle enzymes that are released when the muscles are damaged, cause multiple organ failure.

What is the cause?

A toxin called Hypoglycin-A found in sycamore seeds. Ingestion of these seeds at pasture causes Atypical Myopathy. Outbreaks may occur with multiple horses affected in one geographical area within a few weeks of each other.

What are the clinical signs?

The onset of Atypical Myopathy is acute. Early presentation of the disease may be confused with colic or laminitis. Horses may be found dead, but more frequently they start by showing signs of weakness, stiffness, falling over, lying down more than usual, sweating, apparent sedation or depression, fine muscle tremors, dark brown urine that can look like blood and respiratory distress.

Progression to recumbency is often rapid and once at this stage, the prognosis is hopeless. Mortality is seen in 70% – 90% of cases within 3-5 days.

Who is at risk?

The condition predominantly affects young animals, but all horses are at risk. No sex or breed predisposition has been described, but to date, no cases of the disease have been reported in donkeys. Underweight body condition is associated with increased risk.

Overgrazed pastures and poor quality grazing gives a greater risk of disease.

What can I do to prevent Atypical Myopathy?

Clear fields and water troughs of sycamore seeds, leaves and saplings. If this is not possible, do not allow horses to graze affected paddocks or fence off affected areas. Do not over graze paddocks and avoid overgrowth of weeds on pasture. Provide hay if the fields are low on grass and where possible consider stabling horses over night.

What to do if you think your horse may have Atypical Myopathy

Ring your vet immediately. Try and move the horse to a stable with deep bedding in case they want to lie down. If you notice your horse urinating try and catch a urine sample with a clean jar or mug so that we can test it.