Atypical Myopathy

Atypical Myopathy was first recognised back in 1984 but has largely come to prominence over the last few years with outbreaks in the UK and Europe. It occurs in individuals or groups of horses at pasture and is likely caused by ingestion of seeds known as ‘helicopters’ (and possibly to a lesser extent leaves) of the sycamore tree (Acer Pseudoplatanus) that contain a specific toxin. This toxin has been identified as hypoglycin A. The amount of toxin within seeds is variable and it is not understood why some seeds have more toxin than others nor is it understood how many seeds have to be eaten for a horse to become sick. It is likely that some horses will be more susceptible than others with younger horses appearing particularly susceptible, particularly those in poor bodily condition, on relatively poor quality pasture. As older horses are less likely to become affected it may be that they develop some tolerance to the toxin.

Incidences tend to occur in the autumn and in the spring following large autumnal outbreaks and are often following a sudden adverse change in weather conditions, such as a frost or heavy rain. 

It can cause a variety of clinical signs which can present as dullness or lethargy or as a sudden onset of muscle stiffness or weakness that can progress rapidly to recumbency. Other reported signs also include reluctance to work, choke, whinnying, head tossing or an abnormally low head carriage. It can affect the diaphragm resulting in difficult or laboured breathing and some cases may show some colic like symptoms such as paddling or stretching the limbs. Due to the breakdown of the muscle urine often appears dark red and brown. If it affects the heart muscle it can result in a fast or irregular heart beat and in the worst case some horses may present as a sudden death. The mortality rates vary from 40-100% and vary from year to year.


Diagnosis is based on clinical signs alongside an increase in serum muscle enzymes (AST/CK) and the presence of red/brown urine.


Prompt diagnosis and treatment is essential if horses are to have any chance of survival. There is no specific treatment only supportive care which involves hospitalisation, intensive intravenous fluid therapy and nursing. Fluids are required to provide cardiovascular support as horses can become very dehydrated and to support the kidneys as the product of muscle breakdown (myoglobin) can affect their function and can cause renal failure. If there is concern over the kidney function then they may also be given a diuretic to help the kidneys maintain a good urine output. This condition can be extremely painful and so a variety of powerful painkillers and anti-inflammatories may be required. When horses become recumbent then they require frequent turning to encourage them to stand and to prevent sores. If they are inappetant then they are provided with an alternative source of nutrition usually by stomach tube in order to provide their energy requirements. Supplementary vitamins and minerals have also been shown to be useful in some cases.

Prognostic factors for survival are normal mucosae, no respiratory distress signs, a standing position most of the time, no temperature, and normal abdominal transit.
Factors unfavourable to a recovery include recumbency, abundant sedation, anorexia, tachycardia (a high heart rate), tachypnoea (high respiratory rate), respiratory difficulties and severe acid-base disturbances. Euthanasia may have to be considered when horses appear to have reached the stage where they are no longer likely to respond to supportive treatment.

In horses that do recover, recovery is initially slow, but most go on to make a complete recovery and return to work with no long-term effects of the disease.


Given the high mortality rate prevention is better than cure. If sycamore seeds are present in your fields then the following is advised

- Avoid letting horses graze pasture that are contaminated with the sycamore seeds. 
- Move horses out of the field to as distant a point as is practical, or stable the horses during the risk period. 
- If you are unable to remove horses from pastures then fence off areas where the seeds and leaves have fallen and offer supplementary hay but do not leave it on the ground to get wet and feed extra concentrate.
- Rake up or hoover and remove the seeds, dead leaves and saplings where possible.
- Reducing the stocking density can help to ensure there is good grazing for every horse.
- If you are suspicious that your horse may have ingested some of these seeds then call your vet out to check the muscle enzyme levels (AST/CK) to identify subclinical and pre-clinical cases.

If you suspect that your horse is showing any signs of atypical myopathy then contact your vet IMMEDIATELY. If you have any other concerns then please do not hesitate to contact your vet practice for further advice.

Images of Sycamore trees and seeds.