Atypical myopathy (AM) is a frequently fatal muscle disease of horses caused by chronic ingestion of Acer pseudoplatanus (sycamore). Mortality in horses with atypical myopathy caused by sycamore toxicosis is reported to be as high as 57%; in Europe, most cases occur in the autumn, but there is also risk of poisoning in late spring.

An article published earlier this year discusses the frequently asked questions regarding equine husbandry, to reduce the risk of atypical myopathy. The paper covers 5 main aspects:

  1. There are approximately 561 maple species. As a result, there is a demand to distinguish the potential toxicity of other maple species. Testing for the incriminating toxin has been done in many Acer species, with multiple testing positive for the toxin including Acer palmatum (Japanese maple), Acer japonicum (downy Japanese-maple or fullmoon maple), Acer macrophyllum (bigleaf maple), Acer spicatum (mountain maple), Acer saccharinurn (silver maple) and Acer saccharum (sugar maple).
  2. To reduce the risk of AM at pasture level, avoid humid pastures, permanent pasturing; spreading of manure for pasture with sycamores in the vicinity and avoid sycamore maple trees around pasture.
  3. To reduce the risk of AM at horse level, reduce pasturing time according to weather conditions and to less than six hours a day during risk periods for horses on risk pasture; provide supplementary feeds including toxin-free forage; water from the distribution network; vitamins and a salt block.
  4. All pastures with a sycamore tree in the vicinity are at risk.
  5. The seasonality of AM – 94% of cases occur within two 3 month periods, starting in October and March.