Maple trees are commonly used for landscaping in many areas of North America and especially in the northeastern states. Some species such as the sugar maple are economically valuable for the production of maple syrup, and wood from the trees is used for the manufacture of furniture and musical instruments. However, some maple species have a sinister side—horses and ponies as well as donkeys, mules and zebras eating the fallen, wilted or dried leaves can be fatally poisoned.

Although the dangers of poisoning from one species, the red maple (Acer rubrum, also called swamp maple or soft maple), are well known, research suggests that other trees of the species, including the sugar and silver maples and their hybrids, may also pose a threat. In fact, cases of maple poisoning have been identified in horses that consumed wilted leaves from the sugar maple (Acer saccharum).

Horses are most likely to encounter wilted leaves after summer storms bring down branches or blow leaves into pastures and paddocks. In the autumn, fallen maple leaves are generally less palatable to horses, but they also pose a serious threat when they are consumed. Fresh, green leaves of any maple species are less dangerous but may still contain some level of toxins. The bark and twigs of maple trees may also be toxic if consumed by horses.

“Red maple toxicity is not common, simply because most people feed their horses well and pay attention to what their horses are eating,” says Anthony Knight, BVSc, MS, DACVIM, a large animal veterinarian, plant toxicologist and professor emeritus at Colorado State University. “A horse may eat a few maple leaves on occasion, but an adult horse would need to eat one to two pounds of the dried or wilted maple leaves to be affected by the toxin. It is the dose that makes the poison.”

Still, to keep your horse safe, it’s a good idea to be able to identify the maple species—sugar and silver as well as red maples—because they are common in or around pastures and may be encountered on the trail.

The nature of the threat

The danger posed by wilted or dried red maple leaves has long been recog-nized: Horses who consume them may sicken or die within hours or days. Tox-ins in the plant damage the hemoglobin in the horse’s red blood cells, so they can no longer carry oxygen. Affected red cells may rupture, clogging the kidneys with waste products, and the liver and spleen will remove damaged cells from the bloodstream faster than they can be replaced by the bone marrow, which results in severe anemia.

Starved of oxygen, other tissues and organs begin to fail. Outward signs of red maple poisoning include lethargy, poor appetite, colic, pale yellowish gums that progress to dark muddy brown, increased heart rate, faster respiration and distinctive dark red to black-colored urine.

The speed and severity of red maple toxicosis depends on how many wilted leaves have been eaten relative to the horse’s body weight. As little as half a pound of wilted leaves can kill a small pony or miniature donkey; a pound or two can be a fatal dose for an average adult horse.

“If you see your horse eating fallen red maple leaves, first remove the horse from the source of the maple leaves, and call your veterinarian immediately,” says Knight. Do not wait for signs to appear—your horse’s best chance of survival depends on the earliest possible intervention.

There is no specific antidote to red maple toxicosis, but supportive treatment including intravenous fluids and possibly blood transfusions may help a horse survive long enough for the toxins to clear his system. “Some studies have shown that large amounts of vitamin C [a potent antioxidant] can counter the oxidative effect of the toxins,” says Knight, “but that treatment has to be given very early to be effective.”

Once signs appear, a horse’s odds of survival drop significantly, even with hospitalization. One 2006 study from North Carolina State University reviewed the cases of 32 horses with red maple poisoning admitted to referral hospitals in the Southeast. Of those cases, 29 horses were anemic, 24 had serious systemic inflammation, 12 had kidney dysfunction, nine had laminitis and 13 had colic—19 (59 percent) died.

The exact mechanism of poisoning has not been fully documented. Studies from Cornell University have uncovered significant clues. Compounds extracted from red, sugar, silver and Norway maple leaves incubated with equine blood samples caused oxidation, hemolysis (the breakdown of red blood cells) or other types of damage to the cells. Several chemicals were identified as toxic to the equine red blood cells, most notably gallic acid. “While gallic acid was a major player, most likely it is not the only player,” says Jeanelle Boyer, PhD, who conducted the study. “Quite likely there is a synergistic effect in combination with other chemicals.”

In the next phase of the research, Boyer analyzed extracts from silver maple, sugar maple and Norway maple leaves and found that all contained some level of gallic acid. The extracts derived from the silver and sugar maples did less damage to the equine red cells than did the red maple extracts, but the changes were significant enough to be potentially harmful to a horse. The extracts drawn from the Norway maple were far less toxic. In fact, Boyer speculated that horses would most likely not be able to ingest enough Norway maple leaves to cause themselves any serious harm.

In a separate study, Cornell researchers Karyn Bischoff and Karan Agrawal investigated the effects of gallic acid and tannins from red maple leaves that were incubated with samples of digestive fluids drawn from the equine ileum, the lowest portion of the small intestine. Their findings revealed another piece of the puzzle: The equine digestive tract contains microbes that can turn gallic acid into an even more damaging substance called pyrogallol.

Maple toxicosis is unique to horses and other equids—it does not occur in sheep, cattle or other farm animals. The reason, says Knight, is that “horses do not have a digestive system like that of ruminants [such as cattle, sheep and goats] that can break down gallic acid into harmless components.”

Diluting the danger

The idea that even green maple leaves contain a toxin deadly to horses may be unsettling but, says Knight, they are likely to pose a significant threat only if they wilt or dry out: “This is because green maple leaves contain about 80 percent water. A horse would have to eat a huge amount to get a toxic dose, but when the water evaporates out as the leaves wilt and dry, the gallic acid in the leaves becomes more concentrated.” When leaves drop or break off and begin to dry out they can become toxic in as little as a day, and they can retain their toxicity for weeks, if not months. Branches and bark of the red maple tree also contain toxins, but most horses are not likely to eat enough of them to cause trouble.

In general, the occasional nibble of fresh green leaves or twigs still on a red maple tree probably will do little harm to a horse, but dropped, wilted or dried leaves are dangerous. However, the caveat is that the concentrations of toxins in different parts of a tree may vary. The amount of gallic acid may vary with growing conditions, time of year and stage of leaf maturity. The majority of reported cases occur between June and October—which suggests that growing leaves become more toxic later in the season. The same may be true for sugar and silver maples.

Red, silver and sugar maples are among 13 species native to the United States, and there are more than 100 introduced species, hybrids and cultivars (selectively bred variations, analogous to breeds of animals) that are popular with landscapers. So far, only a few have been tested for toxicity—but it’s wise to assume that all maples may be potentially poisonous until proven otherwise. 

Preventing maple poisoning

Eradicating all red and sugar maple trees that grow in or around horse properties is not practical or even advisable. But it is wise to consider the danger when choosing new trees. “When selecting trees to plant in or around horse pastures,” says Knight, “choose such trees as ash, fir, birch, hickory, hackberry, magnolia, etc. Avoid maples, oaks, boxelder, walnut and chinaberry because of their potential toxicity.”

If there are existing maple trees on the property, take steps to reduce the risk for horses.

1. Promptly remove fallen branches and leaves from turnout areas. Horses are most likely to encounter wilted red maple after a storm has dropped a tree branch or blown large numbers of green leaves into a pasture. Make it a habit to inspect turnout areas after any storms, especially one that brought high winds, and do not return horses to the area until any fallen leaves and branches have been cleared.

If you have many mature trees, consider having an arborist inspect them. He may be able to identify weaker branches, which can be pruned out safely before they fall. An arborist may also identify unhealthy trees, which can be either treated or removed before they develop significant damage.

2. Rake and remove autumn leaves from pastures. As maple leaves change color and dry out each fall, their toxicity levels become high. If red or other dangerous maples are positioned where they consistently shed their leaves into turnout areas, rake and remove the leaves to keep them away from grazing horses. If maple leaf fall is especially heavy in some pastures, it may be wisest to turn horses out in different areas for the season.

3. Fence off large maple trees in pastures. Gallic acid is present in bark and branches as well as the leaves, but few horses are likely to eat enough woody material to be affected. However, if the horse is a cribber or habitually chews on trees, it is a good idea to fence off the maples. The lower branches of maple trees should be regularly pruned to keep them out of reach of horses.

4. Monitor horses in dry lots. Horses who are on good pasture or have free-choice access to hay are less likely to eat enough maple leaves to do them harm. Those restricted to dry lots with limited forage may be more inclined to reach over the fence for nearby branches or eat what falls into their area. A hungry horse is more likely to eat dried maple leaves.

5. Inspect hay. If your hay is grown in an area where maple trees are common, you may occasionally find fallen leaves incorporated in the bales. (This may be more likely to happen in fall cuttings.) Remove the leaves before offering the hay to horses.

6. Remove maple trees when necessary. If there are a combination of circumstances that could place horses at risk, such as a horse who cribs living in a pasture with old red maple trees, it might be best to remove the trees. Double-check the identity of the trees before removing them by showing the leaves to an extension agent or horticulturalist at a local nursery. Make sure all debris, including stray twigs and leaves, are removed from the field before returning horses to the area. Better yet, have the trees cut while they are leafless, to simplify cleanup and reduce the risk of leaves being left in the field.

With these simple precautions, the chances of red maple poisoning can be greatly reduced or eliminated.

This article first appeared in EQUUS issue #457, October 2015.