If your horse lives in the pasture or often goes out to the paddock, you have probably already heard about the atypical myopathy also known as the Seasonal Pasture Myopathy. Let's take a closer look at it to better understand it and prevent it.

What is atypical myopathy?

Atypical myopathy is often a lethal disease which corresponds to a significant degeneration of the muscles, including those involved in breathing, posture as well as the heart muscle.

This muscle degeneration is the result of intoxication from Sycamore trees (Acer pseudoplatanus) and Boxelder trees (Acer negundo). The intoxication comes from the seeds and seedlings, which is why atypical myopathy is a seasonal disease. In spring, it is usually the seedlings that cause the poisoning, while in autumn it is the samaras, which contain the seeds, that fall to the ground.

Atypical myopathy can affect all equines and is generally more common in horses that are grazed but there are cases in horses that spend only a few hours at grass.

Studies of atypical myopathy have not shown a predisposition to the sex of the horse, nor to physical condition. However, factors such as age and the duration of grazing influence the risk of intoxication. Thus, as for many diseases, young horses (less than 5 years old) and older horses (over 20 years old) are more affected.

Atypical myopathy is not a contagious disease between horses or between horses and humans. However, as it is related to the ingestion of seeds, seedlings, samaras in one or more pastures, horses in the meadow or in very close meadows can become intoxicated at the same time.

Atypical myopathy is a disease that is mainly found in Europe with about 75% of the cases in Belgium, France and Germany. For some years now, a disease with similar symptoms has been present in the United States.

The causes of atypical myopathy

While the Sycamore was responsible for almost all cases of atypical myopathy in Europe a few years ago, there are more and more cases of Boxelder tree poisoning, particularly in the south of France. Indeed, it is an invasive species that is developing more and more in Europe.

The Sycamore is a tree found in the forests of Europe that can reach 30 metres in height. Its leaves are generally dark green with 5 lobes. The special feature of the seeds is that they are contained in samaras, which resemble two "wings" that allow the seeds to be spread by the wind. This is why there are cases of atypical myopathy, even in meadows that do not have Sycamores.

A lot of research has been done and it is now known that Hypoglycin A is responsible for the pathology. This toxin is found in the seeds and seedlings of Sycamore and Boxelder trees.

Horses are usually poisoned by eating the seeds and seedlings. In fact, once ingested, Hypoglycin A will be transformed into another toxin which will lead to the disruption of the metabolism muscle cell dynsfunction.

As mentioned earlier, cases of atypical myopathy are related to seasons. In autumn, the samaras detach from the tree and fall to the ground. If they fall and there is wind, their "wings" will scatter them and carry them away from the tree. In spring, the seeds that have been dispersed, thanks to the samaras, will germinate. They then become seedlings which are just as toxic to horses.

Symptoms of atypical myopathy

Atypical myopathy is an acute disease with symptoms usually appearing suddenly. In most cases, the horse is found lying down when it was in good shape the day before.

It is important to spot the symptoms as early as possible, as atypical myopathy is fatal in 75% of cases within 48 to 72 hours of the onset of symptoms. The earlier it is caught the better the chances are for the horse to recover.

In more than half of all cases, horses do not develop a fever and usually continue to eat. Symptoms therefore include:

  • difficulty or inability to stand
  • muscle stiffness
  • depression
  • tremors
  • excessive sweating
  • Very dark urine (coffee colour), due to the presence of myoglobin released by the muscle cells when they are destroyed
  • Congestion of the mucous membranes (darker than normal); red gums and eyes
  • Increased heart rate (over 45 beats per minute)

Other symptoms exist, but occur only rarely:

  • Difficulty breathing
  • Hypothermia (below 37°C) or hyperthermia (above 38°C)
  • Difficulty swallowing
  • Exacerbated appetite
  • Colic

 Studies show that horses that are able to stand are more likely to recover.

Diagnosing Atypical Myopathy

If you suspect your horse has atypical myopathy, call your vet urgently.

While waiting for your vet to arrive, there are a few things that you may do to increase your horse's chances of survival and facilitate a rapid diagnosis.

Firstly, if your horse is able to, try to move him to a place that is sheltered from the rain and wind and as comfortable as possible (with thick bedding) and that will allow you to monitor him easily and to do any treatment. Once the horse is settled, try to limit its movements which will aggravate the destruction of the muscles.

Then you can check for specific elements:

- The temperature: if your horse is hypothermic don't hesitate to cover him with a blanket to warm him up.

- Sweating: if your horse is sweating, don't hesitate to rub him with straw.

- Feed intake: see if your horse is hungry and is able to swallow properly.

  • If so, you can give him a little hay or grass and pellets in small quantities
  • If swallowing is difficult, you can give him water with sugar

Finally, you can go to your horse's pasture to see if you can find any seeds, seedlings or samaras.

To make a diagnosis, your veterinarian will have two solutions: urine observations and blood tests.

For the urine, it will be a question of colour. If your horse urinates before the vet arrives, do not hesitate to collect it. If the horse does not urinate on its own, your veterinarian can do a urinary catheterization.

The disadvantage of the blood test is that results usually take longer to – although they are more precise and allows a reliable diagnosis.


At present, there is no antidote for the toxin.

The veterinarian will treat the symptoms.

Most of the time he will give infusions of vitamins and antioxidants (Vitamin B12, Vitamin E and selenium) which will support muscle function and energy metabolism and will provide water to support the kidneys. The infusion will also dilute the toxins present in the blood and encourage their elimination in the urine.

It may also help the horse to fight pain with analgesics and anti-inflammatories.

Horses with atypical myopathy may develop urinary retention in the bladder. It is important to try and maintain urine elimination as this is the outlet for the toxin.

Generally, horses that recover do not have any after-effects.

Prevention of atypical myopathy

As you will have understood, the prevention of atypical myopathy consists mainly of ensuring the absence of seeds, seedlings and samaras of sycamore and boxelder trees.

However, this is sometimes difficult to do, especially in the autumn when the samaras can be displaced for more than 100 metres from the tree and thus end up in a pasture.

However, there are some tips to try and prevent poisoning during the risky seasons:

  • Regularly monitor pastures and carry out grazing checks for seeds and seedlings.
  • Avoid feeding horses on the ground - Offer mineral supplementation to horses on pasture (salt stone and adapted CMV)
  • Limit grazing time in high-risk seasons (if possible) and/or bring horses in on windy days in autumn.
  • Regularly clean troughs where samaras can have landed.

We also recommend you to learn to identify the different trees as well as their leaves, samaras and seedlings.... Not all maples are poisonous to horses.

In addition, if a case of atypical myopathy occurs in a pasture (or in its immediate vicinity), it is important to remove all horses from the pasture. These horses should be removed from the pasture and must be monitored very closely in the following days.

In conclusion, the best protection for your horse against atypical myopathy is prevention. The possibility of ingesting seedlings or samaras should be kept to a minimum. If you suspect your horse has been poisoned, call your veterinarian.

If you have any questions about atypical myopathy, please feel free to contact us on social media.

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