One of the most important hazards for horses in the autumn is atypical myopathy, a muscle disease with high mortality. It is associated with chronic ingestion of seeds or seedlings of some commonly grown Acer species including sycamore (Acer pseudoplatanus) and ashleaf maple (box elder, Acer negundo). Sycamore is the most common source of hypoglycin A poisoning in Europe and the box elder is the most common source in North America.
Seeds of these plants contain hypoglycin A which prevents the oxidation of fatty acids and therefore the production of energy in mitochondria. Type 1 muscle fibres (found in cardiac, respiratory and postural muscles), which are more dependent on fatty acid oxidation for energy needs, are affected.
Most cases of poisoning occur in the autumn from ingestion of the seeds or in the late spring from ingestion of seedlings. It has been calculated that approximately 20 g of samaras (winged fruits, see image), 50 seedlings, 150 g of inflorescences or 2 litres of water that has been in contact with seedlings would contain the maximum tolerated daily dose of hypoglycin A for a horse. It is also worth noting that seedlings still contain hypoglycin A after mowing or treatment with herbicide. Recently, atypical myopathy from ingestion of hypoglycin A has also reported in deer.
Resources for veterinary surgeons and owners are available on the Royal Veterinary College website.