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Sycamore poisoning in horses

Atypical myopathy or “Sycamore poisoning” can be fatal for horses. It is known as seasonal as it is thought that the helicopter seeds in autumn, and the saplings in spring, contain Hypoglycin-A that causes atypical myopathy in horses.


What is atypical myopathy?


Atypical myopathy is a highly fatal muscle disease, thought to be caused by caused by the ingestion of hypoglycin A, a toxin contained in seeds from the sycamore tree (Acer pseudoplatanus).


The disease is present in the UK and Northern Europe, and an equivalent condition in the USA, referred to as Seasonal Pasture Myopathy (SPM), has linked hypoglycin A toxins from the box elder tree (Acer negundo).


British vets have seen an alarming rise in new cases of atypical myopathy.


Young horses appear to be more susceptible, as are those being grazed on parched land.


Symptoms of sycamore poisoning

  • muscular stiffness
  • reluctance to walk
  • muscle tremors
  • sweating
  • depression
  • high heart rate
  • dark urine (reddish in colour).

Your horse may appear weak and may have difficulty standing, breathing difficulties, but may still want to eat. Call your vet as quickly as you can.



When to be aware of sycamore poisoning


Blue Cross vet Natasha Seely, of Bourton Vale Equine Clinic in Gloucestershire, said: “Horse owners need to be alert at all times but especially during the spring and autumn months.


“If they are worried that their horse may be showing any symptoms they must call their vet immediately.


“The signs range from depression, muscle weakness, recumbency, choke or colic-like symptoms to dark red urine.


“The sooner atypical myopathy is diagnosed the better the likely outcome.”



Top tips to prevent atypical myopathy in horses


The Blue Cross Education Team has worked with veterinary experts at Bourton Vale Equine Clinic to put together the following advice to help horse owners prevent atypical myopathy:


  • Feed forage, such as hay in parched fields, off of floor in haynets or feed racks
  • Do not over stock
  • Limit turnout.  Ideally stable horses over night
  • Section off areas around poisonous trees and collect and dispose of leaves safely away from horses
  • Remove young sapling plants
  • Be careful of streams running through paddocks as this is thought to be more prevalent in moist places
  • Be vigilant of the potential signs of this disease and act quickly if your horse becomes poorly.
  • Ensure you check your horse regularly at least twice daily
  • Check your vet insurance is up to date


— Page last updated 05/04/2022