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Red Maple Disease

Red Maple/Swamp Maple/Acer rubrum is a very common tree in North America. Leaves of this tree are potentially poisonous to your horse from June-October.  The most dangerous time is from mid-September on, because the toxin is most concentrated. The toxin is thought to be gallic acid. It causes destruction (hemolysis) of red blood cells. Green, wilted leaves and partially dried leaves are the problem. Fresh, green leaves on the tree and dried fall-time leaves appear to be safe. Wind storms in the fall and well-meaning people giving horses cut down branches to play with cause the most dangerous leaves to litter paddocks. It is thought that as little as 1/3 of a pound (.7kg) is enough to poison a 1000 lb horse. It is dose and size dependent, so smaller horses and ponies need only ingest a very small amount.

Effected horses can die within 18-24 hours. The most obvious signs of poisoning are:  severe depression and muddy brown gums. The treatment is a blood transfusion and medical management. There is not a drug to counteract the poison. It is a matter of staying ahead of the rate of red blood cell destruction. Treatment is not always successful. 

The good news is that many of our horses have lived for many years with beautiful yellow and red leaves littering their pasture every fall. In most cases horses opt to eat grass and hay over leaves. This of course, has lulled us all into complacency. As previously stated, wind storms and well meaning people providing access to wilted leaves are the big problem. Please be aware of this and either rake up carefully or do not turn out. 

Identifying the trees is the first challenge. There are many species of maple in New England. See the above picture for a sample of leaves in my very own yard. You may also go to for a more in depth description. The jagged edge of each lobe is one of the major distinguishing features. 

One more time….it is the wilted and partially wilted leaves that cause the problem.