Wilted or dried red maple leaves contain a toxin that causes damage to a horse's red blood cells, causing rupture of the cells (hemolytic anemia), and an inability of the red blood cell to carry oxygen. It is a life threatening condition. A large percentage of affected horses die from this condition.

Toxicity is a problem primarily when trees or branches have fallen on the ground, especially in the autumn months when the leaves fall. Just a pound or two of wilted or dried leaves can be fatal. The toxicity is probably lessened or lost by the following spring. Other maple species seem to have similar toxins but possibly at a lower concentration and so are not thought to be as dangerous.

Diagnosis is often presumed based on common signs of serious illness including loss of appetite, rapid breathing, pale or reddish gums and high heart rate in a horse that has had access to red maple leaves.

Treatment includes blood transfusion and the use of medications that prevent damage to red blood cells.