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Atypical Myopathy

Atypical myopathy is a devastating disease that sadly, with no specific cure, is often fatal. However, as we are beginning to understand more about the cause of the disease, we are able to better advise and therefore prevent it.

The first labelled case was back in 1984, but it's only been in the last few years that researchers have been able to identify the exact cause: hypoglycin A, a toxin which is found (in the UK) in sycamore seeds as well as sycamore seedlings. After ingesting the toxin, hypoglycin A leads to a series of complicated biochemical pathways; disrupting energy metabolism in muscle fibres, causing a large amount of damage and majorly affecting their function. As the muscle fibres are destroyed, they release further substances that are toxic to the kidneys.

What are the signs of Atypical Myopathy?

Over the years, many clinical signs have been attributed to the disease. These are all resulting from muscle damage and loss of function. Sadly, as the respiratory and postural muscles are affected the most, horses become unable to stand and go into respiratory failure.

It has been recognised that the pattern of signs can vary between years depending on which muscles the toxin attacks, and it is thought that this is due to changes in toxin properties.

A diagnosis can be made based on the clinical signs, history of the horse grazing on pasture with access to sycamore seeds (Autumn) or seedlings (Spring) and can be further confirmed with laboratory findings. 

The most common signs seen are:

  • Weakness and depression;
  • Stiffness and trembling of the muscles;
  • Sweating;
  • Red or brown urine;
  • Recumbency;
  • Difficulty breathing;
  • Difficulty eating and choking.

What can we do to treat horses that are affected?

The toxin can take hold very quickly, so if your horse is kept near sycamore trees and they start displaying any of these signs, your vet should be called immediately. With no specific cure, treatment is based around supportive therapy; providing pain relief, fluids to help protect the kidneys, energy (via feed or intravenously) to try and restore the muscle metabolism and vitamins which help to act as anti-oxidants. Although this disease is difficult to treat and often fatal, with rapid and aggressive therapy mildly affected horses can recover.

How can we prevent Atypical Myopathy?

By preventing our horses from eating Sycamore seeds or seedlings…if only this was that easy!

The seeds are spread in the Autumn so frequently check for the seeds in your fields around this time of year; remember they can spread far from the parent tree and especially so after strong winds and poor weather. If seen, fence off areas with seeds, or do not turn your horse out on this pasture, provide extra forage to encourage them to only eat this and if possible collect and burn the seeds to reduce the chance of new seedlings come spring. In the spring, be vigilant and look out for seedlings growing. We cannot tell which individual Sycamore trees produce the toxin, and the level of toxin in a seed can vary between trees and from year to year, making it impossible to predict when they are dangerous. Young horses are more likely to be affected, however all horses can be, and some are much more susceptible than others. The only way to be safe is to prevent your horse from ingesting any seeds or seedlings.  

Is there further research going into Atypical Myopathy?

There is always more we can learn about a disease and in the case of Atypical Myopathy we still have plenty of questions to answer. For example, how does geographical location or climate affect the toxin production and properties of the toxin, and crucially can we find a cure? For these questions to be answered we still need to report cases of this disease and hopefully one day it won't be so devastating.

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