Red Maple Leaf Toxicosis

Your favorite mount, Trigger, has been "a little off" lately. He wasn't as enthusiastic as usual at breakfast, and he appears a little sluggish in his stall. A closer inspection reveals that Trigger has an elevated heart rate (Vital Signs: The Link to Your Horses Health), brown mucous membranes and is becoming progressively weaker. Concerned, you call your veterinarian who insists that you bring Trigger in at once.Your favorite mount, Trigger, has been "a little off" lately. He wasn't as enthusiastic as usual at breakfast, and he appears a little sluggish in his stall. A closer inspection reveals that Trigger has an elevated heart rate (Vital Signs: The Link to Your Horses Health), brown mucous membranes and is becoming progressively weaker. Concerned, you call your veterinarian who insists that you bring Trigger in at once.

Story originally posted by: Michael Lowder, DVM, MS University of Georgia

Your favorite mount, Trigger, has been "a little off" lately. He wasn’t as enthusiastic as usual at breakfast, and he appears a little sluggish in his stall. A closer inspection reveals that Trigger has an elevated heart rate (Vital Signs: The Link to Your Horses Health Part 1), brown mucous membranes and is becoming progressively weaker. Concerned, you call your veterinarian who insists that you bring Trigger in at once.

Your veterinarian begins the examination and asks a few imperative questions that makes you stop and think. What types of trees are in the pasture? None. What types of trees ares accumulate on the close to the pasture? You remember that last spring your spouse planted red maple trees along the driveway leading up to the house. With autumn in the air, the leaves are falling and blowing into the pasture. It appears that Trigger has acquired a taste for the leaves.

Red maple (Acer rubrum) toxicosis is most commonly seen in late summer and autumn (fall) when their leaveground (but toxicosis can occur at any time of the year when horses come in contact with dried leaves). Red maple trees are one of the most common ornamental trees in eastern North America ranging as far north as Newfoundland to west Texas southward to the Mississippi River Valley. It is only the dead leaves that are toxic when consumed by horses. Leaves eaten off the tree (fresh leaves) are not toxic to the horse.

Just how much does your horse have to eat? It has been shown that ingestion of 1.5 grams of dried red maple leaves per kilogram of body weight can induce clinical signs and cause death in horses (that’s 1.5 lbs of leaves for a 1000 lb horse). Mortality is approximately 60% to 65% in experimental and naturally occurring cases. Most horses show clinical signs within 48 hours of ingesting a toxic dose, and death can occur as fast as 3-6 days in some cases. The dried leaves retain their toxic components for about 30 days, and overnight freezing does not decrease their toxicity.

What does the leaf look like? The leaves are about 1.5 to 6 inches in length and have three to five irregular double-serrated or toothed lobes with shallow sinuses, a light green top and a silvery white underside. The red maple leaves range in color from red, orange, or yellow in the fall (Figure 1).

Clinical signs most commonly seen in affected horses include, but are not limited to: lethargy, anorexia, abdominal pain, brown mucous membranes, ataxia, colic, reddish-brown urine, an increased heart rate, fever, difficultly in breathing, abortion, weakness, and dehydration. Horses may show some or all of the clinical signs and to various degrees depending upon the level of toxicity. At the start of the disease, the most common clinical sign of red maple leaf toxicosis that the owner notices is the passage of the reddish-brown urine. Horses that eat a larger amount of leaves over a shorter period of time may have a more acute onset with a rapid progression of clinical signs than those horses that consume just a few leaves over an extended period of time.

Your veterinarian will need to obtain a blood sample to confirm red maple leaf toxicosis and to determine the degree of damage. They will examine the blood to assess the degree of anemia (a low red blood cell count) and the formation of Heinz bodies, which are deformed proteins on the surface of a diseased red blood cell. The initial diagnosis is usually made from the history and clinical signs of the affected horse.

There are a number of diseases that your veterinarian will have to differentiate from red maple leaf toxicosis. They include immune mediated diseases, equine infectious anemia (EIA), Potomac horse fever, piroplasmosis, wild onion poisoning, hepatic failure, snake bite, etc. Most of these diseases can be ruled out via laboratory testing and physical examination.

Although most horses with red maple leaf toxicosis will not survive despite aggressive treatment and supportive care, the return of mucous membranes and urine to a normal color may be a good prognostic indicator. Prevention is the cure. Do not plant any red maple leaf trees in or in close proximity to your pastures. Remove or isolate trees that are easily accessible to livestock on your property. If you suspect a case of toxicosis on your farm, notify your veterinarian at once. Although they may provide beautiful fall foliage, the old adage may ring true, "pretty is as pretty does."