The danger posed when horses eat wilted or dried leaves of the red maple tree (Acer rubrum) has long been recognized. Toxins in the leaves damage the hemoglobin in the horse’s red blood cells so they can no longer carry oxygen—a condition called “Heinz body anemia.” Damaged red blood cells may rupture, clogging the kidneys with waste products. If the horse loses red cells faster than they can be replaced, his tissues will starve for oxygen, and vital organs may begin to fail within hours or days. Once signs of toxicosis develop, the horse’s odds for survival are poor: Red maple poisoning is fatal in 60 to 70 percent of cases.
Fortunately, red maple toxicosis is fairly rare. Grabbing the occasional mouthful of green leaves off of a live tree probably does little harm, if any—an average adult horse has to consume one to two pounds of dried or wilted red maple leaves to experience ill effects. (As little as half a pound of wilted leaves may be fatal to a small pony or miniature donkey.) But most horses who have access to good pasture and/or hay are unlikely to eat too many leaves. In addition, the amount of toxin in the leaves can vary dramatically from year to year in the same tree—even if a horse ate leaves from a particular tree without harm in the past, eating them again may pose a grave risk.
The threat of red maple toxicosis is not to be taken lightly. And now is the time to be especially vigilant: Most cases occur in late summer and early fall—when storms cause branches to drop and blow down green leaves, and when autumn leaves change color and fall into pastures. If you notice that your horse has consumed downed leaves from any maple tree (leaves from other maple species, including the sugar maple and silver maple, may also contain toxins), his survival depends on the fastest possible veterinary care. Here’s what to do:
CALL YOUR VETERINARIAN IMMEDIATELY IF:
• You discover—or even suspect—that your horse has eaten dried or wilted maple leaves. Apart from actually witnessing him, evidence might include a fallen branch in the pasture that has been stripped of leaves or accumulations of fallen leaves that have been disturbed. Do not wait for signs of illness to appear.
• You find your horse colicky and/or lethargic, with elevated heart and respiratory rates and darkened gums. He may also pass urine that is red or dark brown.
AS YOU WAIT FOR THE VETERINARIAN:
• Keep the horse as still as possible. If he must be walked in to the barn, go slowly and allow him to rest if he has difficulty. Agitation or unnecessary movements that tax the limited supplies of oxygen in his blood may increase the strain on his internal organs.
• If your horse urinates, catch some in a clean bucket. The color will give your veterinarian important clues about his condition.
• Allow the horse to eat hay or drink fresh water, if he will.
• Remove all fallen maple leaves still within reach of other horses. If necessary, remove horses from the pasture until the fallen branches and debris can be cleared.
Cutting down all maple trees in and around your property isn’t practical or necessary. But you can take steps to protect your horses:
• Inspect turnout areas after summer storms and promptly remove fallen branches or leaves.
• Hire an arborist to inspect mature maple trees and have weaker branches pruned out. Also remove lower branches within reach of horses.
• Rake or blow to remove autumn leaves from pastures.
• Monitor horses in dry lots. Those confined with limited forage may be more inclined to eat leaves that fall into their paddocks.
• Avoid maples when choosing new trees to plant in or around pastures and turnout areas.
Maple Tree ID
Red maple trees (Acer rubrum) are widespread throughout the eastern United States. The leaves are three- to five-lobed and green, with bright red stems and silver-white undersides. In spring, the tree produces bright red buds, and in fall the leaves are typically fiery scarlet, although different varieties may produce bright yellow or orange foliage. Other maple species, especially sugar maple (Acer saccharum) and silver maple (Acer saccharinum), may also pose a threat.
Cultivars and hybrids of all these species may be toxic. Trees whose leaves remain red throughout the summer, such as the Japanese red maple (Acer palmatum) and the “Crimson King” cultivar of the Norway maple (Acer platanoides), are not believed to be toxic.
This article first appeared in EQUUS issue #468, September 2016.