Monday, 21 September 2015

Atypical Myopathy

Just hearing those words is enough to scare me.  A few years ago I had never heard of it but for me and many other horse owners it is now a horrifying thought.  There have been awful stories of owners arriving at their field to find their horse dead.  So I have tried to find out as much as I can.  Now is the time to be especially vigilant!

What is it?

Atypical Myopathy is also known as sycamore poisoning is a disease which damages the muscles.  In addition to the Skeletal Muscle this also includes the Smooth and Cardiac Muscle (see my Muscle blog).  It is very often fatal.  Mostly seen in the autumn in horses or ponies kept on bare pasture it is also occasionally seen in the spring.  

Atypical Myopathy is seen in the UK and Europe, the American equivalent is Seasonal Pasture Myopathy.


  • Muscle soreness 
  • Stiffness 
  • Muscle tremors 
  • Weakness 
  • Lethargy 
  • Fast or laboured breathing 
  • Reluctance to work 
  • Red or brown urine 
  • Choke 
  • Whinnying 
  • Head tossing or low head carriage 
  • Fast or irregular heartbeat 
  • Sudden death
If your horse exhibits any of these signs DON'T WAIT call the VET IMMEDIATELY.    


Research is still in it's early stages but Atypical Myopathy is thought to be caused by a toxin which prevents energy being produced in muscle cells.  This toxin is found in Sycamore seeds (sometimes called helicopters) and also to some extent Sycamore (Acer pseudoplatanus) leaves.  These seeds usually fall in October and there is evidence that more cases are seen in this month.  It also seems that bad weather is a trigger!?

The level of toxin is variable and some horses seem to be more susceptible than others. There is research suggesting that young adult horses (ie: 3 years) are more likely to succumb but also evidence that older horses are more susceptible.  As with many other things it is likely that some horses are more likely to be affected than others and perhaps some build up a tolerance to the toxin. 

In the USA the Box Elder tree (Acer negundo) has been found to contain the toxin.  Current research suggests that other Acer's do also contain the toxin  but that the Field Maple (Acer campestre) does not.

Box Elder


This can be done by the vet, using clinical examinations and information about the horse's grazing. Muscle damage can be confirmed by measuring enzymes in the blood (these are released from damaged muscle cells).   Urine colour is one of the most reliable signs as the dark colour is unusual in any other diseases.  It is caused by the release of a muscle pigment from the damaged muscle cells. 


There is treatment, if a horse or pony is found early enough with mild signs of the disease there is a 50:50 chance of survival.  Treatment needs to start immediately and will involve fluids, pain killers, anaesthetics, vitamins and minerals and perhaps anti-inflammatory drugs.  However, there is no specific treatment.   If possible it is better for the horse to be treated in an equine hospital but it is not always possible for them to be moved.  Recovery is slow and euthanasia is often considered the kindest option.


As Sycamore seeds and leaves are a source of the toxin it seems sensible to prevent horses and ponies from eating these. Unfortunately, as these are spread in the wind some will inevitably blow into fields from the surrounding area.
  • Move horses and ponies away from fields with Sycamore trees. 
  • If this is not possible put a fence up to keep the horses/ponies away. 
  • Consider picking up the seeds and leaves too. 
  • Check your fields regularly. 
  • Turn horses and ponies out for shorter periods. 
  • Provide extra forage, eg: hay if the pasture is bare. 
  • As with other Poisonous Plants (see my blog) horses and ponies are more likely to find and eat the seeds and leaves if they are hungry so ensure there is grass and/or hay to eat.  This will prevent them foraging - however, autumn is also a dangerous time for Laminitis (see my blog).

Don't take any chances, keep your horses and ponies safe from this awful disease.

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Until next time!

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