Atypical Myopathy is a highly fatal muscle disease associated with the ingestion of sycamore tree seeds occurring predominantly in the autumn and occasionally in the spring.
The disease is known to be caused by toxin hypoglycin A which is contained in the seeds from the sycamore tree (Acer pseudoplatanus) or box elder trees (Acer negunda). The toxin affects muscles causing clinical signs such as trembling, weakness, staggering, respiratory distress, recumbency and can lead to death. There is no antidote available and treatment is limited to symptomatic treatment.
Sycamores produce a helicopter blade like seed in the summer (the samara). These contain the toxin and are shed in the autumn, rotating to the ground where accidental ingestion can occur. It is usually when animals are on sparse pasture with lots of leaf litter on the ground and not much supplementary feed. It is not known how much needs to be ingested before signs are seen.
As not a lot can be done once a horse is affected then prevention is best. If you have these trees on your property, it is best to avoid grazing your horses in these paddocks over the autumn and possibly early spring. Making sure they have enough supplementary feed if there is not a lot of feed around. Ultimately removing these trees may be the best option in the long term.