Toxic Principle

An unidentified toxin with oxidant properties is present in the wilted or dried leaves of red maples. Only the red maple (Acer rubrum) and possibly closely related hybrids are known to be toxic. The toxin in red maples oxidizes hemoglobin with the formation of Heinz bodies, methemoglobinemia and subsequent hemolytic anemia. Poisoning is especially likely in the fall or following a storm when leaves of fallen branches become accessible to horses. The fresh green leaves apparently are not toxic, but once dried they may remain toxic for up to 30 days. The bark from red maple trees is also toxic. Fatal poisoning of ponies fed 3.0 kg of dried red maple leaves occurred in 1 to 5 days. Doses of half this amount will induce formation of Heinz bodies. Other maple species including the sugar maple (Acer saccharum)have been suspected of causing poisoning in horses.

Description

Trees attaining heights of 70-100ft. Leaves are distinctly 3, or occasionally 5 lobed, simple, opposite, and toothed, with red petiole, mat green top side and light (silvery) underside. Leaves turn bright red in the fall. The buds are rounded and not pointed. Dense clusters of red flowers appear before leaves, the male and female flowers being on separate trees. Fruits are red in color and have 2 wings 2-2.5 cm long. The bark of young trees is smooth and gray in color, developing scaly plates with age.

Treatment

Horses should be denied access to red maple leaves. Blood transfusions may be given as necessary. Administration of intravenous fluids are of benefit in preventing dehydration and loss of kidney function. Methylene blue advocated for the treatment of erythrocyte oxidant damage should be used with extreme caution in horses, and it is contraindicated if Heinz bodies are already formed as methylene blue induces Heinz body formation. The dosage of methylene blue should not exceed 8 mg/kg body weight and should be administered slowly intravenously as a 1% solution. High doses of vitamin C given intravenously has been shown to be beneficial in preventing the oxidative damage from the red maple toxiin.

Cardiovascular system

Acute hemolytic anemia. Cyanosis and brown discoloration of the blood due to methemoglobin.

Respiratory System

Increased respiratory rate.

Nervous System

Depression.

Renal System

Dark red-brown urine due to hemoglobinurea and methemoglobinurea.

Reproductive System

Pregnant mares may abort without showing signs of hemolytic anemia.

Hepatic System

Icterus (jaundice)

Diagnosis

Hemolytic anemia, with Heinz body formation, with evidence that horse has had access to wilted or dried red maple leaves

Special Notes

The prognosis is always guarded to poor for horses with red maple poisoning because of the rapid development of intravascular hemolysis, coagulopathy, hemoglobin nephropathy and vascular thrombosis. Red maple trees should not be planted in or around horse enclosures. Regular removal of the dried maple leaves in the Fall to prevent their accumulation in horse enclosures is recommended.