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Red Maple Leaves: Poisonous to Horses

Charlotte Means, DVM

Red Alert

Photo Credit: Thinkstock

Much of the beauty of autumn on the East Coast of the United States can be attributed to the majestic red maple tree (Acer rubrum). In the spring, these trees bloom with brilliant red flowers, and in the summer, the red maple – also known as scarlet maple, swamp maple, water maple, soft maple and Carolina maple – provides lots of cooling shade. As a result, it’s often a staple in horse pastures.

In fact, horses enjoy more than just the shade of the red maple – they also like to eat the tree’s leaves. Unfortunately, red maple leaves have been linked to a condition known as acute hemolytic anemia with methemglobinemia and/or Heinz body formation in some horses, which can result in death. The toxin that causes the disease is not yet known. Oddly, some horses can live in pastures containing red maple trees for years without adverse reactions. Other times, only some of the horses in a pasture are affected. In the worst cases, horses have died from ingesting only small amounts of the tree leaves. The disease is usually more common in the fall when wilted and dried leaves are easily accessible to the animals, but it can occur at any time of year that maple leaves are present.

Pinpointing the Disease
Hemolytic anemia is a medical condition that causes lysis, or the destruction of red blood cells. Methemoblobinemia is a condition where the hemoglobin, the part of a red blood cell that contains iron and carries oxygen, is changed so that oxygen cannot be carried to tissues and the blood becomes brown in color. Heinz bodies, which are microscopic structures associated with hemolytic anemia, are actually injured hemoglobin released by damaged red blood cells. Symptoms of acute hemolytic anemia include depression, anorexia (refusal to eat) and dehydration. In addition, the horse will be jaundiced (a condition that turns the whites of the eyes yellow), his heart will beat faster, and his respiration will increase due to the lack of oxygen in his bloodstream. Hemoglobinuria (reddish-brown urine) might also occur because of the lysis of the red blood cells. Death by red maple ingestion can occur within a few days of the onset of the disease.

Before treatment begins, a veterinarian will have to rule out diseases with similar symptoms, such as Equine Infectious Anemia (EIA) and babesia infections. Overdoses of phenothiazine sedatives such as acepromazine or plants such as onions can also cause similar clinical signs. Initial treatment for the disease must be very aggressive in order for the horse to survive. Activated charcoal is effective, if the ingestion is recent, but blood transfusions are sometimes required. The horse will also need intravenous fluids to prevent shock and lessen dehydration – high volumes of fluids also help prevent kidney damage caused by the hemoglobin. Vitamin C is often given to help oxidize the blood and resolve the methemoglobinemia.

Avoiding Contamination
While treatment is effective, prevention is always the best cure for disease by red maple leaves. If maple trees can be removed from the area where the horses live, the risk of poisoning is eliminated. However, many of these trees have been established for years and removal is not possible. In these cases, good nutrition is imperative, especially in the fall and winter when underfed horses might look for supplements to their diet. Measures should be taken to prevent boredom – balls and other toys can decrease the desire to eat the maple leaves. If possible, branches should be trimmed to above the horses’ reach and limbs that fall after storms should be removed immediately. Leaves that fall or blow into pastures should be raked daily. When landscaping new pastures, stables and arenas, select other species of shade trees to avoid possible contamination.

Although red maples are the only maple species that has caused confirmed disease, some research-ers believe that other species could also be to blame. To be safe, restrict your horse’s access to all species of maple.

Dr. Means is a veterinary toxicologist at the ASPCA’s Animal Poison Control Center in Urbana, Illinois.

© 2002 ASPCA
ASPCA Animal Watch – Winter 2002

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