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Revered for their refulgent fall foliage and ease of cultivation, red maples stand out in both urban and pastoral settings, both in splendor and bounty. However beautiful in the fall they may be, when leaves drop from the tree and begin to dry, they become deadly to horses, no matter the season.

Though usually referred to as red maples (Acer rubrum), these trees are known also as scarlet maples, swamp maples, soft maples, and Caroline maples. They thrive throughout the entire eastern half of the United States and Canada in a variety of habitats, including both wet and dry soil.

Problems arise in horses one to two days after eating wilted red maple leaves. In spring and summer, this usually occurs when horses have access to felled limbs left in grazing areas after storms or pruning. Parenthetically, fresh, green leaves, like those plucked from a healthy tree, pose little to no risk to horses, though consumption should not be encouraged. Once wilted, however, leaves remain poisonous for as long as 30 days. Ingestion of 1.5 lb (0.75 kg) of leaves is toxic, and ingestion of 3 lb (1.5 kg) of leaves can be fatal in mature horses, though less may cause problems in smaller equids. The bark of red maple trees is also toxic.

While the identity of the specific toxin in red maple leaves is unknown, the toxin causes oxidative damage to red blood cells that results in serious illness or death. Clinical signs include lethargy, pale mucous membranes, elevated respiratory and heart rates, and red-brown discoloration of the urine. A diagnosis of red maple poisoning can generally be made when horses develop an acute hemolytic anemia with Heinz body formation, and when environmental evidence reveals they had access to the leaves. Heinz bodies are indicative of oxidative injury to red blood cells. Death occurs due to profound oxygen deprivation of tissues secondary to severe anemia and methemoglobinemia, a blood disorder in which too little blood is ferried to cells.

Because there is no reliable treatment for horses with red maple poisoning, prognosis is generally guarded to poor. Supportive care comes in the form of intravenous fluid therapy and blood transfusions. Horses with the best care sometimes develop complications during recovery, including acute renal failure, laminitis, and colic.

Prevention of red maple poisoning is best accomplished by astute management. Here’s a place to start:

  • Remove all red maples that horses can contact. Even horses with access to abundant forage will often gnaw on tree bark, exhibiting natural investigative behavior. If sufficient tree bark is actually consumed, it can be as fatal as wilted leaves.
  • Carefully manage trees that stand outside of paddocks or pastures but are situated nearby. Promptly remove all fallen limbs and branches from turnout areas, taking care to rake up and discard leaves that drop off the limbs during cleanup.
  • Provide horses with plenty of good-quality, palatable forage so they are less likely to consume red maple leaves. It is important to offer sufficient forage when pastures are not productive, such as during winter or drought. Horses on forage-only diets should be fed a well-formulated ration balancer or vitamin and mineral supplement.
  • Any leaves in hay should be carefully inspected. If red maple leaves are discovered, the hay should be discarded or given to other livestock, which seem to be unaffected by the toxin in wilted red maple leaves.

Alward, A., C.A. Corriher, M.H. Barton, D.C. Sellon, A.T. Bilkslager, and S.L. Jones. 2006. Red maple (Acer rubrum) leaf toxicosis in horses: A retrospective study of 32 cases. Journal of Veterinary Internal Medicine 20:1197-1201.

Burrows, G.E., and R.J. Tyrl. 2013. Toxic plants of North America. Wiley-Blackwell, Ames, IA, p. 1110-1113.

Knight, A.P., and R.G. Walter. 2001. A guide to plant poisoning of animals in North America. Teton NewMedia, Jackson, WY. p. 190-191.

Knight, A.P. 2017. Poisonous plants. In: B.M. Waldridge, editor. Nutritional management of equine diseases and special cases. John Wiley & Sons. Ames, IA. p. 162-163.

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