The red maple (Acer rubrum), also known as the swamp maple is common in eastern North America. The toxicity of red maple leaves has been recognized in veterinary medicine since the early 1980s.  

Signs

• Lethargy
• Fever
• High heart rate
• High respiratory rate
• Brown or Yellow mucous membranes  
• Brown / Red urine
• Colic
• Sudden Death

Causes

Ingestion of the wilted leaves and bark leads to severe toxicosis in horses, other equids and alpacas. It is the wilted red leaves only that carry the toxins, not green or fresh leaves. This condition most commonly occurs late in the summer following a storm where limbs have been blown down. The toxic dose is 1.5g/kg, which is roughly 1.5lbs of leaves for a 1000lbs horse. The toxin in the leaves, a tannin, causes oxidative damage to the surface of red blood cells causing them to lyse (explode) and alters the way hemoglobin binds to red blood cells. The profound anemia cause by the destruction of the red blood cells impairs oxygen delivery throughout the body.

Diagnosis

Diagnosis is based on historical evidence of exposure to wilted maple leaves, demonstration of anemia, hemolysis (destruction of RBCs), discolored urine and microscopic examination of a blood smear.

Treatment

Treatment of this toxicosis is intensive and costly, requiring hospitalization in an ICU facility, intranasal oxygen supplementation and blood transfusions, among other supportive care.  If you know a horse has ingested maple leaves, contact your veterinarian immediately. Lavaging of the stomach with water may remove some of the offending leaves and treatment with mineral oil may prevent absorption of the toxin and speed passage of the leaves through the intestinal tract. Though the best way to prevent red maple leaf toxicity is to not house horses on a property containing these trees.

Prognosis

Prognosis in these cases is grave and few cases survive despite aggressive medical treatment. The mortality rate is reported between 59% to 100% in the scientific literature. Because treatment is often futile, prevention is key! As it is most commonly the wilted, fallen leaves that lead to the toxicity, and leaves or branches that may fall or be blown into pastures pose a threat, pastures should be checked thoroughly after storms that could blow debris from these trees into the field.