Skip to: main navigation | main content | sitemap | accessibility page

Telephone: 01749 830666
24 hour emergency pager: 07623 984669

Atypical Myopathy

There has recently been a number of cases of atypical myopathy (AM) diagnosed in the UK and subsequently a lot of information in the press and online.

As a result of several people asking questions, we feel our clients would probably appreciate a summary of the latest research and information regarding this disease.

There is still a lot about this disease which is not fully understood and much research is ongoing. Outbreaks of AM largely occur in autumn and it has been strongly linked to a toxin (called hypoglycin A) within the leaves and seeds of the maple family. Unfortunately for us in the UK, sycamore trees fall into this family. We do get other members of the maple family in this country as well, such as Box Elder but they are not native. Sycamore trees are obviously very common but it is very difficult to predict the risk to your horse even if they are grazing in a field surrounded by Sycamores. This is because the amount of toxin in the leaves and seeds varies between trees, therefore the chance of your horse getting AM depends on the amount they digest as well as the concentration of toxin that in the leaves/seeds they have digested.

Risk factors for AM

  • Strong winds often precede outbreaks as more leaves/seeds are blown onto the pasture
  • Affected horses need to be grazing in order to pick up AM
  • Most common in autumn


AM is highly fatal but about a quarter of cases survive, therefore the sooner the vet is called the faster AM can be diagnosed and treated which will improve the chances of survival. AM affects multiple organ systems, so can cause varying symptoms and makes them truly intensive care patients. The more mild symptoms are, the better the prognosis. 

  • Weakness
  • Pain
  • Stiffness
  • Tremors
  • Recumbence
  • Respiratory difficulty
  • Dark coloured urine


Reduced exposure to sycamore seeds/leaves will reduce the risk this can be done by either removing the leaves from the pasture or restricting grazing to areas surrounding the trees.

Remember there are numerous examples of horses co-existing with maple species and having no problems but increased awareness will hopefully lead to better prevention and horses receiving treatment sooner.  

Browse by category

Subscribe to our e-newsletter to stay up to date with our latest news, events, and upcoming special offers. Newsletters are sent out quarterly but on occasions we may vary from this frequency so you don’t miss out on any important news or offers.

  • This field is for validation purposes and should be left unchanged.