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.