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Equine Atypical Myopathy

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Equine Atypical Myopathy

What is Equine Atypical Myopathy?

Equine Atypical Myopathy (EAM) is a severe condition affecting the muscles of horses and donkeys. EAM has been reported in various parts of the world since the 20th century and has a high mortality rate of 70-80%. The problem presents sporadically, although it is most common in autumn grazing horses when pastures are bare, close to a river or stream and where sycamore trees (Acer pseudoplatanus) are present. Outbreaks also tend to occur following a bout of wet or windy weather.

Unlike other myopathies that purely target the locomotory muscles, Atypical Myopathy also affects postural, cardiac and respiratory muscles, thus making the condition much more likely to result in death.

EAM is unrelated to exercise or the overuse of muscles, as it was once believed. It can present in any horse, irrespective of breed, sex or age, although it is more common in young horses of 3-4 years. Only if the condition is diagnosed in its earliest stages can it be treated, yet even with intensive care, there are no guarantees the horse will make a full recovery.

While it used to be rare, the UK has recently seen an increase in the number of cases of diagnosed EAM. On the 16th October, the Royal Veterinary College announced it had seen five cases of Atypical Myopathy in just 72 hours. With this in mind and autumn now upon us, it is vital we make ourselves aware of the condition and its early presenting symptoms.

What causes Equine Atypical Myopathy?

Much like Equine Grass Sickness (EGS) for which we are still yet to identify a definitive cause, Atypical Myopathy used to be a mystery to veterinarians and horse owners until last year, when Belgian scientists were able to link the fatal condition to the sycamore tree.

Today, it is believed that Hypoglycin A toxin, present in the seeds of sycamore trees (Box Elder trees in the US), is responsible for causing Atypical Myopathy. The condition occurs sporadically because Hypoglycin A isn’t present in all sycamore seeds or, in fact, all sycamore trees.

The levels of Hypoglycin A in seeds are highly variable so, while some sycamores will shed seeds that are practically free of the toxin, others will shed seeds containing lots of it. As there is just no knowing whether your trees are suspect or not, it is worth keeping your horses away from all sycamores as much as you can.

Even in fields with rich pasture, fence off sycamore trees from where your animals are grazing. Strong winds and rain can carry fallen seeds to neighbouring pastures so keep this in mind.

Hypoglycin A is accepted as the cause of EAM today, but further research still needs to be conducted to find out how disease occurrence varies seasonally and thus, how toxin levels must also change. Why is it that horses are more at risk during the months of spring and autumn? Probably because the seeds are shed and your horses have greater access to them on the ground.

If there is very little forage to be had by way of grass, your horse will content itself with rummaging through fallen leaves and seeds. If the grazing is poor then sycamore seeds are much more likely to get ingested. Also, if hay is left on the ground, your horse becomes more at risk. Keep forage in haynets attached to the fence and only leave bales out if you are sure there are no sycamores nearby.

What are the symptoms of Equine Atypical Myopathy?

Unfortunately for horses with Atypical Myopathy, clinical signs are severe, short-lived and almost always fatal. For owners whose horses have experienced EAM, seeing their rapid deterioration can be very distressing, especially since many people aren't familiar with the condition's presenting signs.

The earliest symptoms are often mistaken for colic as the affected horse may strain to urinate and/or have a distended bladder. The horse may also appear depressed and tucked up. Sudden onset stiffness and reluctance or inability to move will also be evident, leading eventually to recumbency and death. This usually happens within 12-72 hours. Another symptom to look out for is dark urine as this is a key indicator of Atypical Myopathy, presenting in roughly 95% of horses.

In order to distinguish EAM from similar diseases such as Equine Grass Sickness, Botulism, Acute Nutritional Myopathy and toxicosis caused by feed or plants, it is important to have your veterinarian examine your horse as soon as possible. All of these conditions are cause for emergency, so don't second guess your decision to contact your vet if your horse starts displaying symptoms.

If Atypical Myopathy is suspected and diagnosed early, intravenous fluids can be administered to reduce the clinical effects of the condition. IV fluids get to work by correcting electrolyte imbalances, restoring hydration, supporting renal function and ultimately restricting muscular destruction and associated complications. This autumn, be vigilant about where you graze your horse and if you suspect sycamore seed ingestion, contact your vet immediately.

Hopefully this article has been helpful and, should you have any further questions, feel free to contact me directly: [email protected]

Written by:


13th Nov 2014
Customer Since: June 2014
From: Somerset, United Kingdom

Brilliant article, I didn't realise about the sycamore seeds being so dangerous to horses. There seem to be so many seeds this Autumn. Will keep my horse off the ground, where they are. Thankyou will pass on to other Horsey friends.

13th Nov 2014

I know how sad this is because my section A 18 month old colt died from this last week people need to be educated the more people know the better

13th Nov 2014

Hi there
My Spanish mare aged 7 at the time contracted an atypical myopathy, when out riding her one day. I got off her and walked home when I noticed she was struggling to walk and started sweating profusely. She collapsed as I took her saddle off, I called the vet and he came out within minutes and gave her a injection to relax her muscles. She was then taken to lip hook where she was put on a drip for a week after having lots of tests. I nearly lost her but the vets were amazing, I was told that her muscles were so severely damaged that I would probably not ever be able to ride her again, but I was just happy she was still alive. She's 10 now and doing amazingly well, I had bloods taken from her every year to see how her enzymes were doing and at long last after 3 years of rest I have been able to start riding her again. She truly is a miracle and we are both enjoying getting back in the saddle. I also just want to mention that there was no sycamore trees in her field.

14th Nov 2014

Great article to share around as so many people are unaware of this condition. My 5 year old warmblood died because of this 2 weeks ago today and my sister's 23 year old the day before. People need to be vigilant

14th Nov 2014

My mare died in September with something similar, as she had very dark urine & a distended bladder. She was put down 12 hours after the onset of the colic.

23rd Apr 2015

Our stallion died last year by AM.
Here is the Facebook Group link from members of AM

24th Apr 2015

Can anyone tell me if Field Maple, which I believe is of the Sycamore family, causes the same problems?

24th Apr 2015
Customer Since: March 2009
From: South Yorkshire, United Kingdom

my 23 year old suddenly collapsed without warning,was diagnosed as EAM. my vet introduced a stomach tube and administered pain killers and copious fluids via the gastric tube, ( we all expected him to give up and die , the horse, not the vet! , but he rallied round and survived the night. My vet returned the next day and repeated the procedures at which point `Blue` continued to improve and now appears fit and well. He has overwintered with no problem and is fat to the point of being overweight ( he is greedy ! ) Having lived here and grazed my horses on my land for 43 years surrounded by Sycamore trees this is the first incident of poisoning to happen.

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