Equine Atypical Myopathy

Equine atypical myopathy is a life-threatening disease that interferes with the uptake and use of glucose in the muscles. It is extremely serious with a mortality rate of around 75%, though this has been noted to be as high as 97% in some study populations (Votion et al., 2020). For this reason, early veterinary care and intervention is essential for a horse to have a chance of survival.

What causes atypical myopathy in horses?

Atypical myopathy is caused by the ingestion of the compounds hypoglycin A and methylenecyclopropylglycine (MCPG), which can primarily be found in the seeds, leaves and seedlings of sycamore trees (Acer pseudoplatanus). Hypoglycin A and MCPG are broken down in the horse’s body into toxic products which are then responsible for causing the symptoms of the disease. These toxins work by disrupting energy production from the breakdown of fatty acids in the body. This can lead to a reduction in the energy available for the horse’s muscles, particularly the heart muscle as well as those involved in maintaining posture and controlling breathing. If muscle fibres are starved of energy then they can start to degrade and release a molecule called myoglobin into the blood. This is what can lead to the characteristic red or dark urine commonly seen in horses with atypical myopathy.

It is important to note that it is not just sycamore trees that are the problem, as the leaves, seeds and seedlings of box elder trees (Acer negundo) have also been found to contain hypoglycin A. This type of tree has previously been associated with atypical myopathy in North America (where it is more commonly referred to as seasonal pasture myopathy), but more recently has been linked to cases in Europe (Jahn et al., 2024). Although box elder trees aren’t native to the UK, they can commonly be found in gardens and parks, so it’s particularly important to look out for these if your horse’s grazing is close to residential areas.

Is the risk of atypical myopathy increased at certain times of the year?

Atypical myopathy is most commonly seen in the autumn and into early winter when large numbers of sycamore or box elder seeds and leaves have fallen from the trees and may accumulate in fields where horses are grazing. Climatic conditions that increase seed fall such as storms and high winds should also be considered as higher risk periods. Spring is another high risk time as lots of sycamore or box elder seedlings may be emerging after the winter months. It is important to note though that hypoglycin A is not thought to be destroyed by freezing conditions in the winter, and high levels have also been identified in box elder seedlings even into the summer months, so care should really be taken at all times of the year.

What are the symptoms of atypical myopathy in horses?

The toxins produced through the metabolism of hypoglycin A and MCPG affect the horse’s body by slowing or preventing energy from being produced within the muscle cells, including the heart. Symptoms therefore often include general weakness, and the horse may even struggle to stand and walk. It is often the case that symptoms will come on very suddenly, but some horses are much more susceptible than others; for example some horses may have a high concentration of hypoglycin A in their blood but show few or no clinical signs.

The symptoms of atypical myopathy in horses include:

Muscle tremors
Laboured breathing
Stiffness
Depression and lethargy
Sweating
Struggling to walk or stand
Low-hanging head
Fast or irregular heart rate
Recumbency
Dark red or brown urine
Colic-like symptoms – although horses usually still have an appetite in contrast to those with gastrointestinal related colic

How is atypical myopathy in horses treated?

There is no specific cure for atypical myopathy, and so as with any illness, prevention is key. A vet should be called immediately if a horse is suspected to be suffering from the disease, as early recognition and treatment is essential to maximise their chance of survival. Your vet will discuss appropriate treatment options for your horse but this may include pain relief, supplementary vitamins and minerals, and fluid therapy to manage dehydration. Usually this will involve a hospital stay and intensive care for several days.

Actions to help reduce the risk of atypical myopathy:

Check your fields and neighbouring fields, woodland and hedgerows for sycamore and box elder trees – seeds may spread up to several hundred metres in windy conditions or in flood water, so you need to be aware of what is growing in proximity to your fields and not just on the immediate boundaries.
Remove sycamore and box elder trees, seeds, leaves and seedlings where possible.
If you cannot remove the sycamore or box elder trees or seedlings, ensure any affected areas are well fenced-off.
Using plant ID mobile apps or search engine AI technology can be helpful to identify tree species near to the horse’s grazing. These can also be used to distinguish other types of plants that may be poisonous to horses. A useful downloadable guide on identifying sycamore leaves, flowers and seeds can be found here (Renaud et al., 2019).
Supplementary feeding in the field (such as hay, haylage or a bucket of a suitable forage replacer feed) can help to minimise the risk of horses being tempted to ingest seeds. Any supplementary feed or forage should be placed well away from any sycamore or box elder trees to reduce the risk of falling leaves or seeds causing contamination.
Stocking density should be kept low to ensure there is plenty of grazing for all horses in the field.
Avoid spreading manure or harrowing grazing pastures near to sycamore or box elder trees as this has also been found to increase the risk of atypical myopathy (Votion et al., 2009) – droppings should instead be manually removed.
A common misconception is that mowing pastures or the use of herbicides on pastures with sycamore seedlings present is sufficient to reduce the content of hypoglycin A they contain. However, research has found that this is not the case, with mowing potentially even increasing the levels (Gonzalez-Medina et al., 2019) – these methods should therefore not be considered as useful preventative measures for atypical myopathy.
Contaminated pasture should not be used for producing hay or haylage, as sycamore seeds or seedlings that have inadvertently ended up dried in bales of hay have also been found to contain detectable levels of hypoglycin A and can remain toxic even after extended periods of storage (Gonzalez-Medina et al., 2019).
When a case is suspected, the horse’s field mates should be removed from the pasture immediately. A recent study that compared the blood test results from horses with atypical myopathy and their apparently healthy field mates found that 97% of the field mates also had hypoglycin A in their blood (Renaud et al., 2024). This study also found that there had already been changes in the breakdown of fatty acids in these horses, even though they weren’t showing any outward symptoms of atypical myopathy. This demonstrates why the management of other horses on the same grazing as a suspected case is so important in trying to reduce the impact of the disease.
It is now also possible to send off samples of seeds, leaves and seedlings for testing to find out if they contain hypoglycin A. The Royal Veterinary College offers this service to horse owners to help assess the risk of plant material located near to where their horses are grazing.

Remember that all pastures with sycamore or box elder trees in the vicinity may present a risk, even if no cases of atypical myopathy have ever occurred from those fields in the past. It has been suggested that around 80% of cases of atypical myopathy were horses grazing in pastures where there had been no previous problems (Votion et al., 2020). This could be because climatic or soil conditions may affect hypoglycin A contents at different times, and levels can also vary considerably between trees. Additionally, some horses may be more careful about what they are consuming than others, so there is no guarantee that any new horses to the field will avoid the leaves, seeds or seedlings of affected trees. Individual horses may also have different sensitivities to the toxin, as subclinical atypical myopathy has been identified whereby fatty acid breakdown is affected but no symptoms are shown (Renaud et al., 2024). Environmental conditions that lead to poorer quality grazing may also increase the likelihood of seed, seedling or leaf consumption. More recent research has also indicated that hypoglycin A content in sycamore seeds within the same bunch from the same tree can vary by up to around 60% between seeds, which highlights the importance of taking a representative sample for testing (González-Medina et al., 2025).

Feeding advice for horses with suspected atypical myopathy:

Avoid fasting – continue to offer plenty of forage.
Horses with atypical myopathy may develop hyperlipaemia which means there is an abnormally high level of fat in the blood – for this reason it may be preferable to avoid high fat diets in horses suffering from atypical myopathy.
Dengie Hi-Fi Senior can often be a good choice, as the blend of precision dried grasses and alfalfa supplies a very digestible source of fibre without any added oil.
Avoid feeding excessive levels of cereal-based feeds, as high intakes of starch may present further problems for digestive health.
B vitamins and antioxidants such as vitamin E have been shown to be beneficial to help support muscle function – ensure a balanced diet is provided in the form of a balancer, broad-spectrum vitamin and mineral supplement, or recommended quantities of a nutritionally balanced feed.
Providing a source of good quality protein within the diet is also recommended – for example, alfalfa can work well for this purpose as it contains around 12-16% protein, including good levels of essential amino acids such as lysine.
Alfalfa is also rich in many naturally occurring vitamins and minerals, and the precision drying techniques used at Dengie help to lock in the natural nutrients. When fed alongside a balancer, it’s also a great way to provide slow-release energy from fibre to help maintain weight and condition as an alternative to feeding high levels of fats or cereals.
Colic and choke have been associated with equine atypical myopathy, so soaked fibre feeds such as Dengie Alfa-Beet or Alfalfa Pellets may be beneficial in these situations.
Feeds should be provided little and often rather than in large meals.
It is advisable to contact a nutritionist for further advice and information on a suitable feeding plan.

Where can I get further information?

Further information on atypical myopathy can be found using the following resources:

References

González‐Medina et al., (2019). Atypical myopathy‐associated hypoglycin A toxin remains in sycamore seedlings despite mowing, herbicidal spraying or storage in hay and silage. Equine Veterinary Journal, 51(5), pp.701-704.
González‐Medina et al., (2025) Assessment of tree-associated atypical myopathy risk factors in Acer pseudoplatanus (sycamore) seeds and leaves. Equine Veterinary Journal
Jahn et al., (2024) Horse with myopathy caused by consumption of box elder tree seedlings in the Czech Republic. Equine Veterinary Education.
Renaud et al., (2019) Identification of the Maple Tree Responsible for Atypical Myopathy. Available online: http://hdl.handle.net/2268/242221
Renaud et al., (2024) Large-scale study of blood markers in equine atypical myopathy reveals subclinical poisoning and advances in diagnostic and prognostic criteria. Environmental Toxicology and Pharmacology, 110, p.104515.
Votion et al., (2009) Atypical myopathy in grazing horses: a first exploratory data analysis. The Veterinary Journal, 180(1), pp.77-87.
Votion et al., (2020) Answers to the frequently asked questions regarding horse feeding and management practices to reduce the risk of atypical myopathy. Animals, 10(2), p.365.

Sign up to the Dengie newsletter

Quick Links