It turns out that a mysterious and generally fatal muscle illness affecting horses and other equines (donkeys, mules, etc.), atypical myopathy, often called seasonal myopathy in North America, is caused by the ingestion of maple seeds, seedlings and leaves containing hypoglycin A (HGA). Symptoms include depressed or sedated behavior, high heart rate, muscle stiffness and reluctance to walk, sweating and trembling and red to dark brown urine. According to different sources, 70 to 90% of affected horses affected die of the disease. To complicate things further, atypical myopathy can be confused with colic in its early stages.
However, what is becoming clearer as further studies are carried out is that not just any maple species is involved.
In Europe, the culprit is the sycamore maple (Acer pseudoplatanus), unfortunately the most common species on that continent. It appears to be the most toxic species overall as well, with dangerous levels HGA present in leaves, seedlings and seeds. However, levels do vary widely from tree to tree, with some containing little HGA. Poisoning is therefore unpredictable.
In North America, the tree to be concerned about is the box elder or Manitoba maple (A. negundo). Only its seeds appear to have toxic levels of HGA. Since it’s one of the rare maples that are dioecious (male and female flowers are produced on separate plants), only female trees produce seeds and are a cause for concern. Note too that equines will not normally eat box elder seeds unless other feed is insufficient or unavailable.
A Dutch study has shown that two other common European maples—Norway maple (A. platanoides) and field maple (A. campestre)—are harmless to horses.
HGA has also been discovered in the seeds of other maples, including Japanese maple (Acer palmatum), silver maple (Acer saccharinum), mountain maple (Acer spicatum) and sugar maple (Acer saccharum), but at levels too low to cause poisoning. Certainly, though, further studies are required to determine which of the some 75 other maple species are toxic and which are safe.
Owners of horses or other equines should be made aware of this problem. Sycamore maples and female box elders should not be planted in or around pastures where horses graze and it may be wise to remove any that already grow there. This is only a partial solution, of course, as seeds can blow in from quite a distance.
Horses should not be allowed to graze in pastures when seeds are present (fall through early spring). Seedlings can be killed by low mowing shortly after they germinate. Then rake off the mowed seedlings or wait until they decompose before allowing horses back into the pasture.
Hypoglycin A is also toxic to people, but few human poisonings related to maples occur given since we rarely eat maple seeds. It is, however, also found in other plants. Jamaican vomiting sickness, a human version of hypoglycin poisoning, is notably caused by eating unripe fruit of the ackee tree (Blighia sapida), which contains both hypoglycin A and B.
Thanks to Belgian reader Cécile Vause, who sadly lost several horses to atypical myopathy, for suggesting the idea for this article.
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Thank you for this article, which is more detailed than any I’ve found to date about the various maples and their relative toxicity to horses. I had always heard “all maples are toxic to equines.” I have what I believe is a silver maple in a pasture was grazed by horses for 20 years by a previous owner, but I stopped putting my horses on it after not knowing definitively whether it was safe or not. If it is indeed a silver maple, looks like I can start using that pasture again. Or go ahead and take down the tree just to be safe. Thanks again.
One would think that horses would know what they should not eat. We boarded horses where I lived in college, which was not easy for a horticulturist. Horse were sometimes tied too close to my back yard where they reached over to eat what they wanted, no matter how important it was to me, or how rare.