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Seeing Red: Reminder About Red Maple Toxicity

The bright colors and fallen leaves of autumn are beautiful to look at, which makes for great riding excursions. But they also bring a potentially fatal danger to horses. The leaves and bark of the Red maple tree are highly toxic to them, so it is important to take note of what the tree and its leaves look like and to prevent ingestion.

(Photo above: Can you tell which is the Red maple leaf?)


What you need to know about Red maple:

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 distinct lobes (sometimes with two additional, smaller lobes at the bottom of the leaf), and their most distinct feature is their serrated edges. Their lobe depths vary, from very shallow to a medium depth. They are green during spring and summer and turn red in the fall. The underside of the leaf is white or silvery.

Note: Toxicities can occur in fresh, wilted, or dried leaves, though they are most toxic when wilted.

“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 Hospital for Large Animals’ Internal Medicine Service, “leading to decreased ability to carry oxygen and eventual red cell destruction, resulting in a potentially severe anemia.”

Red maples are green in summer and a brilliant red in fall, and are most distinctive by their serrated edges.

Red maples are green in summer and a brilliant red in fall, and are most distinctive by their serrated edges.


The following may occur within 18 hours of ingestion:

Anemia: A below normal amount of red blood cells in circulation.

Methemoglobinemia: An alteration of the hemoglobin within the red blood cell that renders it unable to transport oxygen.

Intravascular hemolysis: A rapid breakdown of red blood cells within the blood vessels.

Sudden Death


What to watch for:

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



Red maple toxicity can be difficult to diagnose unless it is clear that a horse has consumed the leaves or bark of a Red maple tree. “Diagnosis of RMT is based on ruling out other causes of anemia,” explains Dr. Gruntman. “Bringing or reporting any dry or wilted leaves that the horse(s) might have had access to can aid in the diagnosis.”


“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 the animal responds well the initial treatment.”


Horses typically come across the leaves as they fall into their pasture or when branches are trimmed. So, the best way to prevent Red maple toxicity is to keep its leaves and branches away from paddocks and pastures. Red maples are common in the Northeast, so while it may be difficult to avoid them entirely, it is easy to trim them and make sure that no branches are hanging near your horse’s paddock, pastures, or stall.

Dr. Gruntman notes that other forms of maple leaves can also cause disease if consumed in large quantities, so it is always important to know what your horse has access to and is consuming.