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The top five trees poisonous to large animals are the RED MAPLE, OAK, BOX ELDER, CHOKECHERRY and BLACK WALNUT. Careful attention must be paid to animals pastured close to these trees, and every effort must be made to prevent access. Pastures should be examined, especially after storms, and fallen limbs, branches and leaves should be removed. The black walnut tree itself is not toxic, but shavings made from it are and should not be used as horse bedding.



Red Maple Tree

Red Maple (Acer rubrum)


Red maple trees are commonly found in the northern and eastern parts of the United States. Dry or wilted leaves from these trees are poisonous for up to four weeks after falling from the tree. Horses ingesting 1 to 3 lb of dry or wilted leaves/1000 pounds of body weight may show clinical signs within 24 hours. Common signs include anorexia, pale mucous membranes, red to brown urine, an increased respiratory rate and recumbency. Hemoglobin, the cause of the discolored urine, is toxic to the kidneys and can cause acute renal damage with little or no urine production.

The toxin in red maples is unknown but is suspected to be a gallotannin present within the wilted and dried leaves that is metabolized to pyrogallol by intestinal bacteria. This toxin, once  absorbed,  causes reactive oxidation of red blood cells (RBCs), RBC lysis, methemoglobinemia and the formation of Heinz bodies. The decrease in the number (acute hemolytic anemia) and oxygen-carrying capacity (methemoglobinemia) of circulating RBCs causes a severe lack of oxygen delivery and poor perfusion. Horses can die from the secondary effects of poor perfusion as well as from acute renal failure.


Oak Tree

Oak (Quercus species)


Oak trees, well distributed throughout the eastern half of the United States, are poisonous to all large animal species. Tasty green buds in the spring and green or sprouted acorns in the fall are sources of the toxin. The tree’s toxicity does not decrease with drying or freezing. Poisoning is associated with the ingestion of large amounts of buds or young leaves and acorns over a 2-3-day period.

The toxins are tannins that bind and precipitate proteins. The toxins primarily affect the gastrointestinal (GI) tract of horses causing anorexia, colic, bloody diarrhea and less commonly kidney damage.


Box Elder Tree

Box Elder (Acer negundo)


Box elder trees are widespread throughout pastures in North America. The seeds pose a serious and often fatal threat to horses. The toxic dose is not yet well-defined, but ingestion of as few as 165 seeds could cause toxicosis.

Box elder seeds contain the toxin hypoglycin A, which has been associated with the onset of seasonal pasture myopathy, a syndrome affecting horses in the fall. Significant necrosis of the respiratory and postural muscles causes horses to be weak and reluctant to move. Recumbent horses may not be able to stand without assistance. Urine is often dark brown to red. The respiratory rate is rapid, and breathing is difficult by 48 hours after ingestion. 75% of affected horses die within 72 hours. Those horses not as severely affected show signs of stiffness, lethargy, ataxia and muscle weakness within three days of ingestion.


Chokecherry Tree

Chokecherry (Prunus virginiana)


Chokecherry trees, along with other Prunus species (wild cherry, apricots, peaches and cherry laurel), are often found adjacent to pastures as windbreaks or ornamental trees.

The toxins, cyanide glycosides, are found in the leaves and seeds of the plants but not in the fresh fruit. Ruminants are more sensitive because of the rapid break down and absorption of cyanide by the rumen.


Black Walnut Tree

Black Walnut (Juglans nigra)


Black walnut trees are commonly found in the eastern half of the United States. Due to the rich appearance of black walnut wood, it is commonly used in carpentry. Shavings generated from black walnut wood are poisonous to horses when utilized as stall bedding material. Clinical signs of laminitis often occur within 24 hours of exposure.

The toxin within the black walnut shavings is unknown but was originally thought to be juglone. Aged or old wood shavings are both toxic, but shavings exposed to air for more than a month are considered to be less harmful.


Pet Poison Helpline

Pet Poison Helpline, an animal poison control center based out of Minneapolis, MN, is available 24/7 for pet owners and veterinary professionals who require assistance treating a potentially poisoned pet. The staff provides treatment advice for poisoning cases of all species, including dogs, cats, birds, small mammals, large animals and exotic species. As the most cost-effective option for animal poison control care, Pet Poison Helpline’s fee of $39 per incident includes follow-up consultation for the duration of the poison case.

Pet Poison Helpline is available in North America by calling 800-213-6680. Additional information can be foundHERE.


If you have any questions regarding Toxicosis in Horses please call our office at (678) 867-2577. We look forward to serving you!
THE ATLANTA EQUINE CLINIC: 1665 Ward Road, Hoschton, Georgia 30548 - ph. 678-867-2577

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