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The brilliantly colored fallen leaves of autumn bring a potentially fatal danger to horses. The leaves and bark of the red maple tree are highly toxic to them and care should be taken to prevent ingestion.
No one knows for sure why the leaves of the red maple (Acer rubrum) are poisonous to horses, but they are dangerous, whether they are fresh, wilted or dried. How much a horse needs to eat to be sickened is not thoroughly understood either. A recent report found that ingestion of 0.3 percent of body weight resulted in toxicity. For example, a 1000 pound horse would need to ingest only 3 pounds of leaves for toxic signs to develop. In another case, a pony suffered toxicity after consuming a kilogram of leaves.
Toxicities have occurred primarily in the northeastern United States, with a few reports in Georgia. Typically horses come across the leaves as they fall into a pasture or branches are trimmed.
The red maple is a medium sized tree with bark that is smooth and gray when young and dark and broken when older. Its leaves have three to five lobes. They are green during the growing season and turn red in the fall. The underside of the leaves is white.
What to Watch For
These generally occur within 18 hours of ingestion:
Red maple toxicity can be difficult for a veterinarian to diagnosis unless it is clear that a horse has consumed the leaves or bark. The signs of red maple toxicity are similar to those shown by other conditions including piroplasmosis (a blood parasite also called babesia), various immune diseases, overexposure to the dewormer phenothiazine sulfoxide (PTZ), liver failure and onion exposure (grazing in an onion field), which can cause anemia and red cell breakdown if ingested in sufficient quantity.
Expect your veterinarian to recommend blood tests and urinalysis. Blood tests may reveal anemia, increased bilirubin in the blood, increased hemoglobin and methemoglobin. Urinalysis may reveal excess hemoglobin. Since the toxic principle is not known, there is no specific test for red maple toxicity.
Treatment of red maple toxicity is based on the severity of the signs. Most horses require hospitalization with intravenous fluids. Oxygen is required for those horses severely affected with methemoglobinemia. Blood transfusion may be necessary in cases of severe anemia. Ascorbic acid can help in treatment. Recently, the use of a blood substitute (Oxyglobin®) saved a pony with red maple toxicity.
Unfortunately, despite aggressive treatment, most horses poisoned by red maple leaves do not survive, unless they are caught very early, and that is usually not possible. The most important tip-off is the recent availability of red maple leaves or bark to the horse. Preventing exposure and access to red maple trees is the only way to prevent red maple toxicity.