Red Maple Toxicosis

By: Molly Kaplan, DVM

Now that the fall is upon us, it is that time of year that owners should be aware of Red Maple leaf toxicosis. The Red Maple (Acer rubrumRed Red Maple is a deciduous tree found in the northeast of the United States and is a common tree found along pastures.  The disease is most common in the late summer and fall months when storms cause limbs to break and the leaves to wilt.  It is only the wilted Red Maple leaves and bark that can cause Red Maple toxicosis, the fresh leaves do not appear to cause any clinical signs and neither do other trees within the maple tree family.

The toxin within the wilted maple leaves is unknown at this time, but it has oxidative properties that damage the horse’s red blood cells leading to a hemolytic anemia. Essentially, the toxin causes the red blood cells to break and leads to a very low red blood cell count (anemia).

Clinical signs that owners first notice is that their horse appears depressed and lethargic. They may also notice yellow-brown mucous membranes (jaundice), an increased respiratory rate and heart rate, and discolored reddish-brown urine.  The lethargy and increased respiratory rate is a direct result of anemia and is the body’s way to adapt to the decreased oxygen carrying capacity.  The discolored mucous membranes and urine is associated with the breaking down of the horse’s red blood cells, which release hemoglobin.  A complicating factor to this disease is that the hemoglobin is toxic to the kidneys that filter the blood, so it can lead to acute renal failure.   After the horse has eaten the wilted leaves it usually takes 1-2 days before you start to see clinical signs.

Unfortunately, there is no specific identifying toxin that can be isolated, so diagnosis tends to be based on a combination of clinical signs and bloodwork. A blood smear will show a Heinz body anemia indicative of an oxidative toxin and bloodwork tends to reveal elevated kidney and liver values.

Treatment includes trying to bind the toxin within the gastrointestinal tract with activated charcoal and mineral oil. The rest of the treatments are focused on addressing the hemolytic anemia.   Corticosteroids are used to inhibit and suppress the hemolytic aspect of the toxin and to prevent the further lysing of red blood cells.  IV fluids are instituted to diurese or flush the hemoglobin circulating in the blood stream.  And blood transfusions and oxygen therapy may be performed, depending on how low the horse’s red blood cell count is.

The key to preventing this disease is to correctly identify the Red Maple tree. It’s recommended to remove any Red Maple trees that grow in pastures or fence off pasture that has access to Red Maple trees.  If this is not possible, owners should be diligent about removing any fallen branches or wilted leaves after a storm.  If you suspect your horse has eaten wilted Red Maple leaves, contact your veterinarian immediately since your horse’s chances improve the sooner treatment is received.

To identify a Red Maple tree you should examine the leaves which have a number of identifying characteristics. The leaf is palmate (like the palm of your hand) with 3-5 lobes.   The leaf margin has a jagged or serrated edge and the underside of the leaf is silver in color.  If you have  difficulty in identifying or differentiating which trees are Red Maples on your farm please feel free to contact Dr. Kaplan who has taken Toxic Plants and Dendrology (study of trees) classes in school.Red MapleRed Maple Back