Atypical myopathy (AM, atypical myoglobinuria or seasonal pasture myopathy) is a muscle disease that hit the headlines in 2014 due to a peak in cases. The disease has been documented since 1985 but we have seen a gradual increase in numbers over the last 6 years. It has come under a lot of scrutiny recently due to a lack of knowledge regarding its exact cause and its highly fatal nature.
It appears that atypical myopathy is caused by a toxin called hypoglycin A that is contained in sycamore seeds (‘helicopter seeds’). Although horses do not usually eat these seeds by choice, if there is scarce grazing or a lot of seeds on the pasture they may be eaten accidentally. There is usually a peak of cases in the autumn when the seeds fall. There may also be a peak in the spring if there has been an especially high number of cases during the previous autumn.