Atypical myopathy (AM) is a muscular disease that affects horses and is caused by Hypoglycin A (HGA), a toxin found in sycamore seeds and seedlings. It is a life-threatening disease with mortality rates of around 75% and the onset can be rapid. Horses can deteriorate quickly when clinical signs become visible, so it is extremely important to call your vet immediately if you are worried about your horse. Not all horses, however, are affected by the toxin, so you may see individual cases of AM despite a whole herd being exposed to sycamore seeds or seedlings.  

Clinical Signs of AM

Once ingested, HGA causes the muscles to breakdown, which leads to symptoms such as muscle tremors, high respiratory rate and dyspnoea (difficulty breathing), muscle weakness and stiffness, and recumbency. Myoglobinuria is another common sign that causes the urine to turn dark red or brown due to the products of the muscle breakdown being released into the blood and filtered out into the urine. Horses appear depressed with their heads hung low and are often reluctant to move. They may also have colic-like signs such as sweating, trembling, and shivering, as well as a high heart rate. It is critical to be able to identify these signs of atypical myopathy and to call your vet immediately. 

When does AM occur?

Outbreaks of atypical myopathy are most prevalent during the autumn and spring. Outbreaks are more likely to occur after high winds and wet weather. Seed fall is heaviest in the autumn, and although horses often avoid other plants such as sycamores, it is common for those on restricted or poor grazing to eat anything in their pastures when hungry. Similarly, during the spring, when the seedlings begin to grow, this can be enticing to horses. It is important to provide plenty of supplementary forage which will help to prevent them from having to look for extra food.  

How to prevent AM

The most effective method of prevention is to remove the seeds from your pastures during the autumn before they germinate into seedlings the following spring. Seeds can be carried long distances by the wind, so be vigilant even if you don’t have sycamore trees directly in your fields. It is a good idea to fence around sycamore trees where seed fall will be most dense as this will help to reduce the likelihood of them being inadvertently ingested . You can also pull up and remove the seedlings in spring to prevent them from continuing to grow. 

Where possible, remove your horses from contaminated paddocks and always provide supplementary feed to minimise the risk of horses being tempted to eat the seeds. It can be difficult to prevent this disease entirely if you have sycamore trees in or near your fields, but by being vigilant, you can reduce the risk of your horse getting atypical myopathy.

If you are concerned about your horse, recognising the clinical signs of AM and seeking immediate veterinary attention are vital.