Red Alert: What should horse owners know about falling leaves?

Autumn's fine fall foliage holds a hidden dangers for horses
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Autumn's fine fall foliage holds a hidden dangers for horses
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Autumn leaves: do love them or hate them? They're beautiful to look at and fun to play in, but you know that, sooner or later, you are going to spend hours raking them. You also know they are destined to clog your barn’s gutters and drains. You also know before it even happens that every time the barn door opens, a swoop of dry leaves will blow in on your just-swept aisle.

Where's that leaf blower?

And why can’t you see those fall perennials you planted? They’re covered with leaves.

But more importantly, does your horse love or hate them?

Once the novelty of stuffing leaves into a Halloween dummy has worn off, you may be left with acres of leaves to bag up and compost. Maybe just this morning you looked up at the big oak tree, knowing it will be the last one to drop its leaves. No sense raking until they are all down on the ground…

On another level, many horse owners are unaware that autumn leaves, especially red maple leaves, can present a serious health threat--even a deadly one--to their horses. So let's take a look at where the danger lies, and what you can do to keep your horse as safe as possible.

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What's the problem?

Red maple leaves, and possibly some other types of maple leaves, can be fatal if they are eaten by horses. The risk is generally thought to be highest in wilted red maple (acer rubrum L) leaves, which cause a severe toxic reaction in some horses.

What’s the risk?

What trees are in or near your horse's pasture? You may have red maple trees within reach of your horse and not know it.

Red maples are a second-class maple tree; the wood is soft and not the best for furniture making. It is quick to seed and form a new generation wooded area. Some people even consider the red to be the maple equivalent of a weed, compared to the noble (and valuable) sugar maple used for the maple syrup industry in the Northeast and Canada. Red maples are also called swamp maples, since they are able to survive in wetlands.

But when fall comes, the weedy red maple redeems itself by flaming into high color on every horizon.

Many horse owners have had their horses in the same pasture for years without any red maple problems, and not all horses are equally affected by eating the leaves, although some horses may have more of an appetite for them than others.

Red maple trees don’t have to be in your pasture, or even very near it. The wind blows leaves into and out of a pasture. Sometimes, a red maple can be cut down in a pasture, and branches are left behind. These leaves wilt and become a snack for the horses. Or, a branch can be blown off a large tree in or near the pasture, and fall within reach of the horses.

One risk that is not often mentioned is that dried fall leaves, especially on the edges of a field, can end up being baled into hay. This usually occurs during the season’s last cutting, or a cutting after a strong windstorm. In this case, the toxic effect could be seen months later, after the ground is carpeted in snow, or leaves are no longer on a horse owner's mind.

Credit: US Geological Survey Distribution of the red maple tree in the United States and Canada.

Credit: US Geological Survey Distribution of the red maple tree in the United States and Canada.

What happens to the horse?

“Toxins (gallic acid and tannins) within the red maple leaves cause oxidative damage to the hemoglobin component of the horse’s red blood cells,” explains Dr. Alisha Gruntman of the Cummings Veterinary Medical Center at Tufts Hospital for Large Animals’ Internal Medicine Service in a public service announcement (1), “leading to decreased ability to carry oxygen and eventual red cell destruction, resulting in a potentially severe anemia.”

Red maple toxin works quickly, once in a horse's system. The horse may become seriously ill within 18 hours of ingesting leaves.

Dr. Gruntman describes the possible complications as:

  1. Anemia: A below-normal number of red blood cells in circulation.
  2. Methemoglobinemia: An alteration of the hemoglobin within the red blood cell that renders it unable to transport oxygen.
  3. Intravascular hemolysis: A rapid breakdown of red blood cells within the blood vessels.
  4. Sudden Death

What are the outward signs of red maple toxicity?

  • Lethargy
  • Lack of appetite
  • Pale to yellow gums
  • Increased respiratory rate and heart rate
  • Dark brown or reddish urine
  • Progressive weakness

While those signs sound straightforward, if horrific, the diagnosis of red maple toxicity can be a difficult one. The symptoms may not be obvious, and they may mimic other medical conditions. Most horse owners won't be aware that their horses have eaten the leaves, or that they have been chewing on the also-toxic bark of a red maple tree.

In a retrospective study published in 2006 in the Journal of Veterinary Internal Medicine, thirty-two horses with red maple toxicosis were identified, 19 of which died. The authors described the breakdown by the numbers: "Twenty-nine horses presented with anemia and 24 had clinicopathologic evidence of systemic inflammation. Renal insufficiency was identified in 12/30 (41%) horses. Laminitis (9/28) and colic (13/30) also were identified in horses with red maple toxicosis, but development of these 2 conditions did not have a negative effect on short-term survival." (2)

Tufts' Dr. Gruntman advises bringing a sample of leaves from a sick horse's pasture to the attention of the veterinarian. Horse and owner are in for an intensive care situation. “Treatment consists of maintaining hydration and preventing kidney damage with intravenous fluids. Blood transfusions are often performed due to severe anemia,” says Dr. Gruntman. “The prognosis is good if the toxicosis is caught before the anemia is severe and (if) the animal responds well (to) the initial treatment.”

How can red maple poisoning be prevented?

Land management is the first step in preventing red maple poisoning, but it sounds a bit like the “closing the barn door after the horse is loose” analogy. Fall isn’t always the best time to cut down a tree, and many horse owners like having trees in a pasture to create a more natural environment, attract birds, and give horses some natural shade.

Sometimes, a new horse owner creates a pasture without realizing that a stand of red maples is inside the fence. Young weedy trees just blend in with other "brush".

Step One in prevention is to learn not just the red maple locations, but to identify all the trees within reach of your horses or along your fence line. Ask a professional, perhaps from your County Extension Office, to help you with this task. You can physically label the trees or mark the tree on an accurate map and take photos. A drone may help here. 

Oak trees, or their acorns, may be troublesome to horses as well, so look for acorns on the ground in October each year.

Whenever there is a storm, check your pasture's far reaches and make sure that no branches are down within reach of hungry horses. Consider fencing off any trees that are toxic to horses, even if they have never caused a problem before.

Whenever you buy hay, inspect it carefully. Chances are, there would be no leaf problems in hay sold for horses, but if you are at a show or trail ride, your horse may be eating hay from an unknown source. Always inspect hay for leaves and weeds before you toss a flake to your horse.

If you have a feeder or hay rack in your pasture, clean it out often in the fall. Leaves will collect in the bottom and it will be the hungriest horses that devour even the leaves at the bottom if they are left there.

Learn the leaves in your area. Remember “leaflets three” for poison ivy? It works for maple leaves, too. Red maples have three distinct lobes when they are immature, and they grow out into a more substantial shape--like the one on the Canadian flag. Other maples have five lobes. The edges of the red maple look they have been trimmed with a serrated kitchen knife.

Many horse owners who think they have seen it all, are aghast when faced with red maple toxicity. It's not something we often hear about, and yet we know it happens. Sometimes, horses become very ill without a known cause, and ingesting leaves is suspected, but can never be verified.

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(1) "Seeing Red: Reminder About Red Maple Toxicity." News at Cummings School of Veterinary Medicine at Tufts. Cummings Veterinary Medical Center at Tufts, 28 Mar. 2016. Web. 24 Oct. 2016. .

(2) Alward, Ashley, et al. "Red maple (Acer rubrum) leaf toxicosis in horses: a retrospective study of 32 cases." Journal of veterinary internal medicine 20.5 (2006): 1197-1201. (Note: this is an Open Access article, and may be read without a subscription.)