Equine Toxicity Series: Poisonous Plants to Horses

Dec 19, 2022

Written by Lauren M. Kiningham, Jennie Ivey, PhD, and Assistant Professor Lew Strickland
The United States is home to numerous plants that are poisonous to horses. Some are rare, but most are common weeds and trees. The toxicity of the plant generally depends on soil, climate, and life stage of the plant, as well as the horse’s age, weight, and tolerance. Normally, a horse must ingest a large amount of the plant before toxicity occurs, while others require only a slight nibble or repeated grazing to be deadly. Most poisonous plants have low palatability, and horses tend to avoid them. However, when they are hungry, horses will eat anything they can access. The most common cause of ingestion is hunger when a horse is on an overgrazed pasture or its nutrition is not balanced. In addition to pasture plants, toxins can also be found in hay, contaminated grain, ornamentals, and clippings.
Red Maple (Acer rubrum)
The red maple tree is a medium-sized tree that can be identified from its bright red-stemmed leaves containing shallow notches. Red maple leaves turn a distinct red in the fall. Red maple toxicosis occurs in horses when wilted or dry leaves are consumed. Fresh leaves and red autumn leaves are less toxic, but care should still be taken to avoid any consumption. Toxicity occurs in a 1,000-pound horse when approximately 1.5 pounds of leaves are consumed. Three pounds of leaves can be a lethal amount. Dried and wilted red maple leaves damage hemoglobin in red blood cells, inhibiting their ability to transport oxygen. Clinical signs usually occur one to two days after ingestion and include general symptoms such as weakness, depression, lack of appetite, and increased respiratory and heart rates. Yellow/brown mucous membranes and dark red urine are also classic signs of red maple toxicity.
To prevent red maple toxicity, minimize access to red maple trees. It is important to check pastures after storms or high winds and remove any leaves that have fallen prematurely. Also, provide adequate forage so that horses are not tempted to eat the leaves. Death occurs in 75% of cases, so it is critical that appropriate measures be taken to minimize exposure to red maple leaves.
Pyrolyzadine Alkaloids (PA) Toxicity
Numerous species of plants in the United States cause Pyrlyzadine Alkaloid (PA) toxicity. Two common genera are Crotalaria and Senecio. Crotalaria are hardy annuals with yellow flowers. Senecio are multistemmed annuals or perennials with small, daisy-like yellow flowers. They include ragworts and groundsels. Every part of the plant of these two genera is toxic to horses whether fresh or dry. Consumption of these plants causes liver damage. Cell growth is slowed, and cells are unable to divide. The damage is cumulative and irreversible. This poison is extremely dangerous because clinical signs can take weeks or months after ingestion to appear. Weight loss, anorexia, and weakness commonly occur with PA toxicity, as well as incoordination, jaundice, and photosensitization. In some cases, a coma can occur and lead to death. Death can also be sudden if a large amount of toxin is ingested.
PA toxicity can be diagnosed if there is a history of consumption and availability to the horse. Increased liver enzymes indicate PA toxicity, and a liver biopsy can confirm the diagnosis. Horses showing signs of PA toxicity rarely recover. A low-protein diet that has less nitrogen is recommended as it lessens the liver workload. It is important to always provide adequate forage to discourage horses from grazing on weeds. Also, keeping pastures mowed appropriately will decrease weeds, resulting in decreased exposure.
Oleander (Nerium oleander)
Oleander is an evergreen shrub with red, white, or pink clusters of flowers. All parts of the oleander plant are toxic, including dried leaves. Oleander contains a cardiotoxic compound, oleandrin, that disrupts the normal heartbeat, leading to cardiac arrest and death. Approximately 30-40 leaves can be lethal to a horse, and death usually occurs within 24 hours after symptoms appear. Colic, diarrhea, labored breathing, weakness and muscle tremors, and an inability to stand are clinical signs of oleander poisoning. An irregular or weak pulse is another classic sign.
A veterinarian should be contacted immediately after symptoms appear or ingestion is observed. If a lethal amount has not been consumed, horses can be treated with a guarded prognosis for recovery. To prevent oleander poisoning, pastures should be kept clean of oleander leaves, and all plants should be removed from the property.
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