MAPLE LEAF TOXICITY
Maple trees are very common in the Northeastern United States. Albeit the maple leaf is beautiful, the wilted leaves can be fatal to horses, ponies, donkeys, mules, and even zebras. The Red Maple (Acer rubrum; also called the Swamp Maple) is known to be toxic to horses, it has been suggested by researchers that other maple trees can be just as poisonous. Included in the list of toxic maples are the Sugar and silver maples and their hybrids. Horses are most likely to come into contact with the wilted maple leaf after summer storms drop the branches full of leaves into paddocks and pastures where curious horses have unlimited access to them. In the fall, fallen maple leaves are generally less palatable to horses, but they also pose a serious threat when they are consumed. The green leaves are less of a danger but still may contain some level of toxins. The bark and twigs of maple trees may also be toxic if consumed by horses.
Horses may eat an occasional maple leaf without any symptoms of toxicity. An adult horse would have to consume 1-2 lbs of dried or wilted maple leaves to be affected by the toxin. Ponies can consume as little as a half-of-a-pound of wilted leaves and succumb to the toxicity. Lethality is based on amount of leaves consumed by the horse in a rapid amount of time. Death may occur within hours to days.
MODE OF ACTION
(Gallic Acid and Pyrogallol)
Plant toxins damage the hemoglobin the horses's red blood cells, so the red blood cell (RBC) can no longer carry oxygen; thereby starving the body of oxygen. The affected RBC can rupture and become clogged in the kidney as a waste product. The liver and spleen will attempt to remove the trashed cells from the bloodstream faster than they can be replaced by the bone marrow. Consequently, the horse will become Anemic. Because the body is now starved of oxygen, tissues and organs will begin to shut-down and fail.
Gallic Acid is present in some level in silver maple, sugar maple and Norway maple leaves. The extracts derived from the silver and sugar maples do less damage to the equine red cells than do the red maple extracts but nonetheless, the changes are significant enough to be potentially harmful to a horse. The Norway maple extracts are far less toxic and a horse could not consume enough to cause serious harm. Gallic Acid is then converted into Pyrogallol by the microbes found in the lower portion of the digestive tract of the horse. Since the digestive tract in a horse differs from a cow/sheep/goat/deer, this theory may explain why Maple leaves are toxic to horses only. Most cases of Maple Leaf Toxicity is reported from June through October in the eastern USA.
Green, fresh leaves are less toxic because the leaf is 80% water. Therefore, the toxins are diluted out verses a dry and wilted leaf where the toxins are concentrated.
As the horse's tissues become starved of oxygen and waste products begin to build in the kidneys, the horse will become lethargic. His appetite will become poor and he may even show signs of colic. The gums will become pale yellow and progress to dark and muddy color. Due to anemia, the heart rate will be elevated and the respirations (breaths) will become faster. Often laminitis (founder) may occur. Ultimately, the urine will become dark red to black colored. Many times, urine discoloration is the first symptom that the owner may notice.
If you see your horse eating wilted maple leaves, deny your horse access to the leaves by removing him from the source of the leaves.
Call the veterinarian immediately!
Do not wait for symptoms to occur because the damage caused to your horse by the maple leaves may be irreversible by the time the symptoms appear. Immediate treatment is your best course of action.
Treatment by your veterinarian usually will include IV fluids, mineral oil and other supportive treatments. Your horse may require a blood transfusion and may receive large doses of Vitamin C (antioxidant). Vitamin C has to be given very early in the treatment modality for it to be effective. Once signs of maple toxicity appear, a horse's odds of survival drop significantly, even with hospitalization.
Avoid access to Maple Trees by your horses. This may involve fencing off the trees from the pastures or even cutting down the trees. If you have nearby maple trees, immediately rake and remove storm damaged leaves and branches from the pasture before they become dried and wilted.
Safe trees to plant near horse pastures include: Ash, Fir, Birch, Hickory, Hackberry and Magnolia
Trees to AVOID: Maple, Oak, boxelder, walnut and chinaberry
Inspect hay that may have been contaminated with maple tree leaves. Eliminate access to contaminated hay by your horse.
(Further reading on this subject may be found in Equus Magazine. Issue 457 October 2015)