Each fall, the trees change to beautiful and brilliant colors and the leaves begin to fall from the trees. For some species, this can be a dangerous and potentially fatal time of the year. For horses, the most deadly tree is the red maple (acer rubrum).
Ingestion of fresh, dried or wilted leaves and bark can result in serious and frequently fatal toxicity. So far, toxicities have occurred primarily in the northeastern United States, though a few reports of toxicity have occurred in Georgia. Toxicity is primarily associated with access to leaves from trees growing in the pasture or from trimmed branches thrown into the pasture or field. To date, only horses seem to be affected.
Even though red maple trees pose a significant threat to horses, the toxic principle involved has eluded scientists. Even the amount of red maple leaves that need to be ingested to cause toxicity is not thoroughly understood. In a recent report, it was found that ingestion of 0.3 percent of body weight resulted in toxicity. For example, a 1000-pound horse would only need to ingest 3 pounds of leaves for toxic signs to develop.
The primary toxic signs associated with red maple toxicity are anemia (below normal red blood cell count), 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) or sudden death. These generally occur within 18 hours of ingestion.
What to Watch For
The diagnosis of red maple toxicity can be difficult unless ingestion of the offending leaves or bark is known. Red maple toxicity is similar to piroplasmosis (a blood parasite also known as babesia), various immune diseases, phenothiazine drug exposure (type of sedative), onion exposure (grazing in onion field) and liver failure.
Expect your veterinarian to recommend blood tests and urinalysis. Blood tests may reveal anemia, increased bilirubin (pigment) in the blood, increased hemoglobin and methemoglobin. Urinalyis 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 will 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.
Home Care and Prevention
There is no home care for red maple toxicity. If you suspect that your horse has ingested red maple leaves or bark, consult a veterinarian immediately.
Preventing exposure and access to red maple trees is the only way to prevent red maple toxicity.