Wednesday, October 9, 2013

Red Maple Toxicity (RMT) – A Fall Danger for Horses

The drought in Missouri has caused an early leaf change in our trees this year at the research center.  As I got into my truck this morning, there were fallen leaves from the huge silver maple that sits next to my driveway stuck in my windshield wipers and it got me to thinking about the dangers dead maple leaves can pose to horses and I decided to write a blog post about it.

Red maple tree just beginning to show its’ fall colors – Photo by K. Williamson

Red maple trees (Acer rubrum) are absolutely gorgeous in the fall when their leaves turn bright flame red, which is one reason why they are among the most popular ornamental trees in the United States.  However, red maples can pose a serious health risk to horses.  The dried or wilted leaves of the red maple contain an unknown oxidant toxin or combination of toxins which cause damage to red blood cells leading to a condition known as acute hemolytic anemia and/or increased formation or abnormal accumulation of methemoglobin (an abnormal form of hemoglobin that is not capable of carrying oxygen).  Horses are more susceptible and commonly affected by RMT than other species.  Red maple toxicity may occur throughout the growing season but cases are more common during the late summer and fall.  The seasonal increase in cases of RMT appears to be due to 2 main factors:  First, the toxic principle(s) in the leaves appear to increase later in the growing season and are especially abundant in the fall.  Second, horses have more access to the toxic leaves during the fall months when the trees are shedding their leaves which may fall or be blown into pastures.  Consumption of as little as 1 gram of red maples leaves per kilogram body weight can result in fatality.  

Here is a list of possible clinical signs of red maple toxicity.

  • sudden death within 12-18 hours of exposure or:
  • lethargy/depression
  • increased heart rate and respiratory rate
  • abdominal pain (colic)
  • going off feed
  • signs of liver damage (icterus-orange/yellow discoloration of the white around the eyes (sclera) and the gums)
  • renal damage (brown urine)
  • brown discoloration of mucous membranes and/or cyanosis
  • laminitis

Horse owners that suspect their horse may have ingested red maple leaves should call their veterinarian immediately.  There is no antidote for red maple toxicity, and treatment is directed toward decreasing the amount of toxin absorbed, mitigating the organ damage caused by the remnants of the destroyed red blood cells and supportive care. Unfortunately, the prognosis for horses with RMT is guarded to poor, even with aggressive treatment and the reported mortality rate is 60-65%.  So it is extremely important for horse owners to take steps to decrease their horses’ risk of exposure to wilted or dead red maple leaves. 

Here are a few prevention measures to consider:

  • If possible, remove red maple trees in or near pastures and make sure none are subsequently planted nearby. 
  • Never throw red maple tree trimmings into pastures. 
  • Remove horses from pastures in the fall when leaves from nearby red maples may be blown in.
  • If it is impossible or impractical to move horses away from red maple trees, be sure they have access to plenty of high quality forage or hay which may make them less likely to browse on fallen leaves. 
  • Check pastures carefully after a storm for fallen branches and remove any that you suspect may be red maple.

SIDE NOTE:   Eating live green leaves directly from the tree has not been reported to cause toxicity in horses.  The leaves are toxic only when wilted or dead.  


  1. Thank you, Dr. Williamson, for this very helpful posting. As a horse owner, I didn't know this about Red Maples and I appreciate your info so much. If my calculations are correct - it means that eating 2 pounds of wilted or dead red maple leaves could kill my 1,000 pound horse. I'm guessing that she'd need to eat that within a short time period, days rather than over several weeks or months- is that correct? Or do the toxins accumulate? Either way, I'm going to check around my property and see if I can identify this particular tree here in the NW Arkansas Ozarks. Some of our trees are turning red right now in late October though most seem to turn yellow or orange. Thank you again!

    1. That is a great question. The toxic dose referred to in the article is what is reported for a short term exposure. However, due to the way that the toxin alters and destroys red blood cells, longer term but lower magnitude exposure may lead to clinical disease. So it is best to take steps reduce the risk of any exposure to the leaves.