What Are Horse Toxins?

Oleander

• Oleander is an evergreen shrub that has long leaves and white, pink or red flowers. All parts of the plant contain oleandrin and neriin, toxins that can disrupt a horse̵7;s natural heartbeat. Oleander leaves remain toxic when dried; ingesting approximately 30 to 40 leaves can be damaging, even deadly, to a horse.̷2; Signs of toxicity, which appear a few hours after ingestion, may include difficulty breathing, tremors, colic and irregular heart rate. A horse can survive if treated early with activated charcoal to block continued absorption of toxins and medication to regulate the heart rate.

Red Maple Tree

• The red maple is a medium-size tree with green leaves in the spring and summer, and bright red leaves in autumn. Eating fresh red maple leaves may not be harmful to horses, but wilted leaves are extremely toxic. Horses are most likely to find wilted leaves after a storm or in the autumn when fallen leaves blow into pastures and grazing areas. If a horse ingests wilted red maple leaves, the wilted leaves' toxins can cause red blood cells to break down, resulting in damaged organs and blood that cannot carry oxygen. Symptoms include depression, loss of appetite, dark urine, pale gums, dehydration, and increased respiration and heart rate. Treatment is typically intravenous administration of large amounts of fluids; some horses also need blood transfusions.

Yew

• The yew shrub is evergreen and woody with bright red or yellow berries. Yew poisoning tends to occur in horses after pruning when brush trimmings are discarded in areas where horses have access to them. All parts of the yew plant, except for the soft part of its fruit, contain an alkaloid called taxine that can cause cardiac and respiratory collapse. One mouthful can be deadly to a horse within minutes, so avoidance is essential. Yew-poisoned horses found alive will be shaky and colicky, and will have a slowed heart rate and difficulty breathing. There is no known treatment for yew poisoning, and death is the usual outcome.

Water Hemlock

• Water hemlock is among the most toxic plants in the United States. Most of the water hemlock's toxin is found in the weed's root, but the entire plant contains a cicutoxin alkaloid that acts on the horse̵7;s central nervous system. The toxins affect neurons within the horse̵7;s brain and lead to excessive salivation, pupil dilation and nervousness that quickly causes breathing difficulty, heart and muscle degeneration, and convulsions. Signs of poisoning are apparent about an hour after ingestion, and death from respiratory paralysis follows within a few hours. Receiving care before the onset of convulsions may ameliorate the seizures̵7; effects, but horses that survive usually have permanent damage to their heart and muscles.

Botulism

• Botulism can occur when horses ingest toxins produced by Clostridium botulinum bacteria found in spores on vegetables and other foods, and in soil. The spores are not toxic until they end up in a protein-rich environment, such as decaying vegetable matter, that is low in acidity, is moist and has little or no oxygen. This results in the severing of communication between nerves and muscles and may lead to deadly neuromuscular damage. The chance of a horse surviving depends on how promptly a veterinarian starts treatment, but botulism in horses is usually fatal. The most effective way to prevent botulism is by vaccinating horses with the equine botulism vaccine that is effective against the form of botulism common in horses.

Tomatoes

• Fruits such as tomatoes are dangerous for horses. The tomato plant is part of the toxic plant family that includes potatoes, deadly nightshade and horse nettle. The green, leafy parts of tomato plants contain atropine, which slows digestion and causes potentially deadly colic in horses. If the horse has muscle tremors, dilated pupils and slowed intestinal movement, a veterinarian can treat her with physostigmine. If you find your horse shortly after the horse has consumed contaminated plants, administer activated charcoal, which acts as an absorbent. It may or may not be effective.

Fruit Trees

• Cherry, plum, apple and peach trees belong to the same plant species and all are toxic to horses. The seeds, bark, leaves and stems of these fruit trees -- especially the highly poisonous wild black cherry tree, produce cyanide, a toxin that when ingested deprives horses of oxygen and leads to respiratory failure. Toxicity is most potent when leaves are wilted or dying, because this is when the most cyanide is released. Wilted leaves and stems store sugar, and the sweetness tempts horses to eat them. Symptoms of cyanide poisoning include dark red mucous membranes, gasping, drooling, increased respiration, weakened pulse, convulsions and coma. If your horse displays any of these symptoms, contact a veterinarian immediately in case an antidote can be administered. Unfortunately, horses usually succumb to the deadly toxin.