23 October 2023

Atypical myopathy: what it is and how to lower the risk to your horse this autumn

Atypical myopathy is a potentially fatal disease caused by a toxin found in the seeds and seedlings of the common European sycamore, from the Sycamore tree. Because it is potentially deadly for horses, it is sensible to do what you can to protect your horse from suffering with atypical myopathy, which is prevalent in spring and autumn.

If ingested, the toxin found in sycamore seeds and seedlings can make its way to the horse’s muscles, causing the muscle cells to become inactive and even die. Horses that eat the seedlings or seeds can quickly become affected by the toxin, and the disease is fatal in around 70% of affected horses. Atypical myopathy is not contagious and is usually seen in autumn when the sycamore seeds are on the ground, and in spring when the seedlings are growing.

It is important for all horse owners to be aware of the signs of atypical myopathy in horses, which are:

  • Lethargy
  • Depression
  • A reluctance to move
  • Muscle tremors (shivering)
  • Sweating
  • Difficulty breathing
  • Red/brown (dark) urine, due to the products of muscle breakdown being released in the urine.

As soon as you notice these symptoms, call your vet immediately and describe what you’ve seen. Your vet will examine your horse and may take blood and urine samples.

How to lower the risk of atypical myopathy

Pictured is a sycamore seedling on the ground, which can cause potentially fatal atypical myopathy in horses if eaten

Pictured: a sycamore seedling on the ground, a common sight in the autumn

Ensuring that horses don’t eat sycamore seeds or seedlings is key to preventing the disease occurring in your horse. Even if your horse has lived in pasture with sycamore trees before, it is important to be vigilant and take precautions with each tree.

Preventative measures include:

  • Being able to recognise and identify a sycamore tree, as well as its seedlings.
  • Fencing off areas around sycamore trees.
  • Moving your horse to other pastures during high-risk periods, in spring and autumn.
  • Remembering that sycamore seeds can be carried in the wind and travel some distance, so even if there aren’t any sycamore trees in your horse’s field, but there are sycamores in the vicinity, you will still need to regularly inspect the field and remove any that have blown in.
  • Providing extra forage for your horse so that they aren’t tempted to eat any sycamore seeds they find.
  • Clearing up sycamore seeds when they have fallen from the tree, and uprooting any seedlings.

The British Equine Veterinary Assocation also warned farmers about the grave danger of cutting hay from pastures contaminated with sycamore seedlings and saplings earlier this year.

Prognosis for a sick horse

To have a chance of recovery from atypical myopathy, an affected horse will need intensive care at an equine hospital. It can take several days to make a diagnosis from a blood sample and time is of the essence, so treatment may be started before the test results come back.

Unfortunately, there is no antidote, so treatment consists of supportive care, such as fluid therapy, which helps to protect the kidneys from damage, and pain relief. The first 24-48 hours are the most critical. If the horse survives the first few days, it can still take several months of treatment to make a full recovery.

However, if the horse does recover, they can often go on to live a normal life and return to full time work without any long-term side effects. The key is identifying sycamore trees in and around your horse’s field, and doing all you can to prevent them accidentally eating seeds in autumn and spring. The golden message is: atypical myopathy is better prevented rather than cured.

Your Horse

by Your Horse

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