The Fall season brings cooler weather and beautiful colored foliage. Leaves start to fall and the grass starts to die back. Although we may see this as a time to get off the mower and get on our horses to enjoy the beautiful scenery, it can be a very dangerous time for horses if the wrong things end up in there pasture.

Red Maple leaves and acorns from Oak trees pose very serious threats to horses. Both are highly toxic and can cause serious harm and death if ingested. Horses often eat Red Maple leaves or acorns if there is not enough grass left in the pasture and they are not supplied with supplemental forage such as hay or hay cubes. Some horses accidentally taste acorns that have fallen into a field and then develop a taste for them and seek them out.

The native Red Maple (Acer rubrum), also called swamp or soft maple, is a potent killer of horses and ponies. Red Maple is a tree native to the eastern half of North America.

Red Maple leaf toxicity


The toxic ingredient in Red Maple leaves is believed to be gallic acid. Gallic acid causes methemoglobinemia and is found in the leaves of Red Maple, Sugar Maple and Silver Maple trees. Ingestion of wilted or partially dried Red Maple leaves from fallen or pruned branches causes lysis of the red blood cells with the subsequent development of a hemolytic anemia, which can be deadly. The problem can occur from June to October. Ingestion of dried or wilted, but not fresh, maple leaves is associated with the toxicosis. Although dried leaves may remain toxic for 4 weeks, they are not generally believed to retain toxicity the following spring. Older wilted leaves, e.g., those collected after September 15, cause faster poisoning than wilted leaves of early summer growth. This indicates that the amount of toxin increases in leaves during the summer. Red cell damage has been reproduced in horses ingesting 1.5 to 3 pounds of dried leaves per 1,000 pounds of body weight. Ingestion of fresh leaves does not appear to cause disease.

Clinical Syndrome

Horses often die within 18-24 hr of ingestion of wilted leaves. Horses that remain alive for 18-24 hr after ingestion of wilted leaves will be severely depressed and cyanotic and produce dark red or brown urine. The mucous membranes are blue to brown from poor oxygenation. They suffer intravascular and extravascular hemolysis (red blood cell breakdown). The percentage of red blood cells circulating in the blood - packed cell volume (PCV) - can drop as low as 8%-10% and the hemoglobin (Hb) concentration can be as low as 50 g/L. The normal PCV and Hb concentrations in horse blood are 28%-44% and 112-169 g/L respectively. Death is due to a severe lack of oxygen delivery to vital cells from hemolysis of red blood cells, anemia and the oxidation of hemoglobin to methemoglobin, which is incapable of transporting oxygen. The clinical signs observed in horses that eat Red Maple leaves include: colic, fever, followed by laminitis and disseminated intravascular coagulation.

Blood changes of horses with Red Maple leaf toxicity include anemia, hemoglobinemia, Heinz body formation, increased AST, SDH, plasma protein, and bilirubin.


Early treatment is aimed at preventing absorption of the toxin if the syndrome is recognized quickly, especially if the owner saw the horse eat leaves. Activated charcoal or mineral oil can be used to slow absorption. Activated charcoal will also bind some of the toxin. Once clinical signs have occurred, treatment is symptomatic and aimed at maintaining a viable PCV and oxygen level. IV fluids are used to help flush the products of red blood cell breakdown out of the kidneys and to prevent dehydration. Blood transfusions may be necessary if the PCV drops below 15%. Nasal oxygen supplementation can also be used to keep oxygen levels within normal limits. The condition also causes colic like symptoms and the pain can be carefully treated with non-steroidal anti-inflammatories or opiates as needed.


Prognosis is guarded to poor for horses who consume large amounts of wilted Red Maple leaves.

Identifying Red Maples

The leaves of Red Maples are palmate (like the palm of your hand), 5-15 cm long and about as wide, with 3 to 5 lobes. The two sides of the center lobe are almost parallel to the mid-vein (5). Between the lobes, the leaf edge or leaf margin is serrated or jagged, while the leaf margin of Sugar Maple and Norway Maple is smooth with no serrations. The underside of the Red Maple leaf is silver grey and the keys are red. Red Maple can hybridize with silver maple, creating crosses of intermediate forms that should also be avoided near horse pastures. Silver Maple is a soft maple with heavily indented lobes compared to Red Maple or Sugar Maple. In northern parts of Ontario, Mountain Maple with its small, heavily serrated 3 to 5 lobed palmate leaves could be confused with Red Maple. However, it only grows to 3-5 m or as a shrub. Red Maple trees can grow up to 25 m high. Researchers have identified the presence of gallic acid in Silver and Sugar Maple as well as Red Maple. Although no reports citing either of them as a cause in poisoning have been published, there have been anecdotal reports of possible poisonings.


Remove any limbs or leaves from Red Maple trees that have fallen into a pasture, looking after each rain or wind storm in particular. Trim trees near pastures so that horses are not able to reach the branches to eat leaves.

Oak Leaf and Acorn Toxicity

Poisoning can occur in Spring when young oak leaves are eaten, but it mostly occurs due to ingestion of the acorns in the Autumn. This is due to the tannic and gallic acids in the acorn, which can cause severe damage to the gastrointestinal system, liver, and kidneys.

Clinical Signs

If horses are eating acorns, the husks can often be seen in their droppings. Many horses are unaffected but clinical signs to look out for include depression, loss of appetite, mouth ulcers, abdominal pain (colic), constipation followed by diarrhea which may contain blood, blood in the urine, leg edema, weakness and incoordination.


There is no specific antidote for acorn toxicity and treatment of these cases involves intravenous fluids to prevent dehydration and correct electrolyte abnormalities. If horses develop kidney failure, intravenous fluids can help to increase blood flow to the kidney and furosemide can be used to increase urination. Mineral oil or activated charcoal can be given orally to help remove the toxin from the gastrointestinal tract as soon as possible. The prognosis for horses with acorn poisoning is guarded, so it is much better to prevent the problem than treat it.


The only way to prevent acorn poisoning is to prevent your horses from having any access to the oak trees and the acorns that fall from them. This will undoubtedly involve fencing off the trees and the area of ground where the acorns fall or alternatively picking up fallen acorns daily and remove low branches.

Individual animals have different levels of tolerance. Therefore it is not possible to say how many can be eaten in a given period of time without causing symptoms, however small amounts do not usually cause problems. Acorns can become addictive; some horses will actively search for them once they have acquired the taste.

Registered 2013 by Equestrian Collections
Author: Sallie S. Hyman,  VMD, DACVIM, CVA

Information in this article is for educational purposes only and is not a substitute for evaluation by an equine professional. In particular, all horse owners should seek advice and treatment from a licensed veterinarian, such as TEVA, for their horses' medical care.

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