Atypical myopathy: what you need to know right now to keep your horses safe

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    This article has been edited and approved by Karen Coumbe MRCVS, H&H’s veterinary advisor since 1991.
  • Atypical myopathy in horses (also known as seasonal pasture myopathy, sycamore myopathy or sycamore poisoning) is an often fatal illness, usually found in grazing equines — mostly in the autumn and spring. This has been more widely recognised in recent years as more information about the disease has become available.

    The condition weakens the muscles of the horse’s body, including the heart, and can present with sudden stiffness, muscle tremors, collapse and colic-like signs, accompanied by a low temperature. Often dark urine is seen because the damaged muscle cells release the pigment myoglobin, which can damage the kidneys.

    Atypical myopathy has a fatality rate of around 70% so should always be considered a veterinary emergency if it is suspected.

    Atypical myopathy is not infectious and can affect horses of all ages and types, although young horses may be more vulnerable. Some horses appear to be more susceptible than others, which may be due to genetic differences.

    Atypical myopathy: Causes | Risks | Signs | Diagnosis | Treatment | Prognosis | Prevention

    What causes atypical myopathy in horses?

    Studies have established that atypical myopathy is caused by ingestion of the hypoglycin A toxin from the seeds, seedlings and possibly flowers of certain trees of the genus Acer. The most common of this type is the sycamore tree, but different trees contain variable amounts of the toxin. It is important to note that the toxin is not present in every seed, seedling or tree.

    The Royal Veterinary College provides a hypoglcyin A test which can be used on both plant samples and equine serum to identify the level of the toxin.

    Hypoglycin A prevents energy being produced within the muscle cells.

    Much research has been done by the University of Liege and involved horses from Belgium, Germany and the Netherlands. High concentrations of hypoglycin A were found in all the sick horses. Botanists visited the pastures that had been grazed and the sycamore was found to be nearby in every case.

    In the United States research has linked seasonal pasture myopathy (the US equivalent) to toxins from the box elder tree. Both trees produce seeds containing the agent hypoglycin A.

    Sycamore seeds are linked to atypical myopathy in horses, which can be fatal

    Sycamore seeds have a downward V shape with “wings” to help them travel on the wind.

    The “helicopter” wing-shaped seeds are thought to the primary source of the toxin, but sycamore seedlings have also been found to contain the toxin, being a particular concern in the spring. It is recommended that seedlings are mowed and the cuttings collected and removed from the pasture afterwards as the toxic hypoglcyin A remains in the plant material after it has been cut.

    Acer pseudoplatanus, a sycamore seedling, which is linked to the often fatal condition of atypical myopathy in horses

    Sycamore seedlings are also a cause for concern and should be removed from pastures where horses graze.

    When are horses most at risk?

    Outbreaks of the fatal condition tend to be seasonal, with cases typically occurring in the autumn and spring. Victims are usually kept in sparse pastures, where seeds or seedlings are on the ground and are eaten when there is not much other grazing available. Windy autumn weather is particularly risky as it can deposit large numbers of sycamore seeds on to grazing land, which are then eaten. 

    What are the signs of atypical myopathy in horses?

    The onset of the disease can be extremely rapid, with some horses being found collapsed or dead in their fields. Signs include:

    • Stiffness
    • Weakness
    • Reluctance to move
    • Lethargy
    • Muscle tremors
    • Muscle pain
    • Sweating
    • Difficulty breathing
    • Shivering
    • Dark red or brown urine
    • Fast, irregular heart beat
    • Colic signs
    • Unable to stand
    • Sudden collapse and death

    How is it diagnosed?

    Your vets will be able to make a likely diagnosis based on the clinical signs as well as the horse’s environment, along with lab tests to confirm the levels of toxicity.

    What treatment is available?

    Horses diagnosed early will need treatment with intravenous fluids, pain relief and intensive care at an equine hospital, but once the signs are present it is already serious. Affected animals often deteriorate for the first 24 to 48 hours so should be moved to an equine hospital as soon as possible after the diagnosis is made as the period the horse is able to remain standing while in transit is going to be limited.

    There is a compromise in that the horses do not want to be moved far, however effective intensive care is not really possible in the field, where they may be ingesting more toxin.

    Supportive therapy including vitamins, minerals and antioxidants may be helpful.

    A 2018 study showed that activated charcoal may bind the toxin, so your vet may consider stomach tubing with this following removal of any cases from pasture.

    Atypical myopathy in horses: what is the prognosis?

    With some 70% of cases proving fatal, the prognosis for horses with atypical myopathy is cautious at best. However, if a horse can survive the first few days of treatment, they can go on to make a full recovery over a period of some months with no long-term effects of the disease.

    How can owners reduce the risk?

    • Fence off areas where sycamore seeds and/or leaves have fallen
    • Regularly inspect fields to ensure seeds have not blown in from nearby sycamore trees
    • Mow areas where seedlings are growing and collect the cuttings
    • Supply extra forage (hay or haylage), especially where pasture is poor
    • Reduce stock density, so there is enough good grazing for every horse
    • Turn out horses for short periods (ideally less than 6hrs.)
    • Pick up and remove sycamore seeds, if possible
    • Test for the presence of hypoglcyin A in your pastures
    • If concerned, contact your vet immediately

    If a case occurs and your horse is in the same field, ask your vet for advice and consider moving your horse.

    Further reading:
    Answers to the Frequently Asked Questions Regarding Horse Feeding and Management Practices to Reduce the Risk of Atypical Myopathy – February 2020

    Equine atypical myopathy in the UK: Epidemiological characteristics of cases reported from 2011 to 2015 and factors associated with survivalApril 2017

    The Story of Equine Atypical Myopathy: A Review from the Beginning to a Possible End – November 2012

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