Published June, 2010

Red maples (Acer rubrum) are beautiful trees that grow extensively throughout eastern North America. However, horse owners should be aware these trees can pose a serious health risk to horses. Ingestion of wilted or dried maples leaves can cause damage to red blood cells and potentially death in horses (and possibly camelids such as alpacas and llamas).

The red maple tree is a tall hardwood with green leaves that have three large "fingers" or points and five prominent veins in the leaves. The leaves turn a brilliant crimson and sometimes yellow in the fall.

Poisoning typically occurs in late summer or fall or after storms cause tree limbs to fall within reach of horses. Other maple species can be toxic to horses, but most documented cases involve red maple exposure. The toxin responsible has not been identified but is apparently only found in dried or wilted maple leaves, not in fresh growing leaves.

Clinical signs in poisoned horses include depression, poor appetite, yellow or brown color to gums and membranes, dark red or brown urine, colic, and fast heart and respiratory rates. Abortion, sudden death, and kidney failure might also occur. Signs typically present within 12-48 hours of ingestion of leaves, and death can occur within days. If ingestion of dried or wilted maples leaves is witnessed, treatment (i.e., intravenous fluid therapy, blood transfusion, administration of activated charcoal) should be instituted and is effective in many cases. The mortality rate can be high and is dependent on the dosage ingested, among other factors. As little as 1.5 grams of wilted or dried leaves per kilogram of body weight can be fatal (roughly 1 pound of dried leaves in an adult horse). Post-mortem diagnosis usually is made by finding large amounts of maple leaves in the stomach and intestinal contents.

Prevention is key: Do not plant maples trees near horse pastures or barns, and remove any maples that are already present; do not incorporate maple leaves into bales of hay; and remove fallen maple leaves and downed branches from pastures after storms.

Cynthia Gaskill, DVM, PhD, clinical veterinary toxicologist at the University of Kentucky Livestock Disease Diagnostic Center, provided this information.