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Farm Management
First Posted: Oct 7, 2008
Jan 21, 2020

Poisoning in Horses: Most Common Toxic Substances

For information on Toxic Plants to Horses and Weeds Toxic to Horses please follow these provided links. Below is an article of the Most Common Substances Toxic to Horses. It may be extremely helpful in preventing poisoning in your horse.

  1. Poison Hemlock
  2. Tansy Ragwort (below)
  3. Field Horsetail
  4. Buttercups
  5. Yew (below)
  6. Oleander
  7. Bracken Fern
  8. St. John's-Wort
  9. Nightshade
  10. Hoary alyssum
  11. Death Camas, Deadly Zigadene, Hog Potato and Mystery-grass
  12. Wild parsnip
  13. Bracken Fern


There are a number of different kinds of Yew: American, English, Japanese, and Western yew are ornamental evergreen hedge-type plants that grow red berries in the fall. They are commonly used in landscaping as hedges or as ornamental plantings across much of North America. Yew trees are extremely toxic to horses and all grazing animals. Yew poisoning is the commonest form of animal poisoning. All parts (dead or living) are poisonous, especially the leaves. Yew contains an alkaloid that depresses the action of the heart. Just a handful of yew can kill a horse. Symptoms if a moderate amount of yew has been ingested are mild to severe digestive upsets, respiratory distress and cardiac failure. A vet should be called immediately. Even after the plant is dead it still remails toxic.

Oleander Poisoning, Nerium oleander, in Horses

Image: Wikipedia/Alvesgasar
Oleander Nerium

Oleander is a decorative perennial evergreen shrub. It produces white, pink, or red flowers in spring and summer and is common in the southern United States. It only grows in climates where temperatures remain above freezing. Danger to horses are potent cardiac glycosides in the that plant affect the heart's ion balance, causing irregular heart activity that can ultimately result in cardiac failure and death. Relatively small quantities (0.005% of the horse's body weight, or 0.05 pounds for a 1,000-pound horse) are considered lethal. Imagine as little as 1 ounce of oleander leaves can cause death in horses. Ingestion might also cause colic. "...Horses that consume a lethal dose of oleander leaves are often found dead 8 to 10 hours later, and symptoms of poisoning rarely last more than 24 hours before death occurs. Clinical symptoms include colic, diarrhea, labored breathing, muscle tremors, ataxia, and the inability to stand. Furthermore, an irregular and weak pulse, due to the decreased cardiac output, will lead to cold extremities, and convulsions prior to death are not uncommon. If you suspect that your horse may be suffering from these symptoms of oleander poisoning, it is extremely important to contact your veterinarian immediately. While there is no specific treatment for counteracting the effects of the toxic principles, animals that have not consumed a lethal dose may be treated with a guarded prognosis for recovery over the next several days. ..." Oleander Poisoning of Horses If clippings are thrown into pastures or onto trails where horses are ridden the horse is at risk.


What is an ionophore? Scientifically they are "a lipid-soluble molecule usually synthesized by microorganisms to transport ions across the lipid bilayer of the cell membrane. The two broad classifications of ionophores are: (1) Chemical compounds (mobile ion carriers) that bind to a particular ion, shielding its charge from the surrounding environment, and thus facilitating its crossing of the hydrophobic interior of the lipid membrane and (2) Channel formers that introduce a hydrophilic pore into the membrane, allowing ions to pass through while avoiding contact with the membrane's hydrophobic interior. ..." For purposes of this article they are antibiotic feed additives which are used as growth promoters in poultry and cattle. One such additive is monesin. Monensin is a polyether antibiotic isolated from Streptomyces cinnamonensis. It is widely used in ruminant animal feeds although horses do not chew their cud. Horse feeds can become contaminated with ionophores. Horses are more sensitive than other livestock to ionophores. Nerves and muscles are really affected in horses and can do real damage to the heart muscle. Symptoms in horses are loss of appetite, rapid heart rate, sweating, colic, a weakness in the legs, thrashing, inability to get up and death. Prognosis is poor.

Blister beetles

Image: Muhammad Mahdi Karim/GNU Free Documentation License/Wikipedia
Image: Muhammad Mahdi Karin

Cantharidin is toxic to horses. Blister beetles harbor this toxin. This toxin is spread to horses when the horse eats alfalfa that has blister beetles in it. Blister beetles swarm alfalfa fields and feed on the tops of alfalfa plants. Sometimes during harvesting of alfalfa, (aka crimping process--when hay stems are broken to hasten drying) adult blister beetles are killed in the processing of the alfalfa, baled into the hay contaminating the crops with their body parts and fluids. When the horses eat this contaminated feed they become ill. This agent, cantharidin, is a toxic chemical and blistering agent. It causes blisters in the horse's mouth and esophagus. It also can cause ulceration of the stomach, intestines and bladder. The cantharidin can circulate through the horses's blood and cause damage to the kidneys and damage the cells of the heart. Clinical signs appear within hours of ingestion. There is gastrointestinal distress, strained and frequent urination and lesions around the mouth. If a horse ingests a lethal quantity (believed to be 0.5-1 mg of cantharidin per kilogram of body weight), it can die within 72 hours.

Rodenticides and Pesticides

Rats, mice, gophers, birds, snails, slugs, ants, and other pests are often poisoned with rodenticides or pesticides. These products come in different forms such as pellets, granular, or powder. The result of inappropriate pesticide use or handling by humans. If these pesticides are inappropriately used by humans, other animals can also be poisoned and die. Among these animals are horses. Many of these poisons are grain based and attract horses. They are also sweet smelling and sweet flavored. It does not take much ingestion of this bait to kill a horse or a foal. Symptoms may include: fever, intense thirst, increased rate of breathing, colic, uncontrollable muscle twitches, pinpoint pupils, convulsions, inability to breathe, unconsciousness. Most of the bait contains an anticoagulant such as rat poison. It causes bleeding and hemorrhaging. The animal bleeds to death internally.


When using herbicides in horse pastures it is really important to keep the horses off the pasture for the specific amount of time indicated. Follow the directions carefully. Using herbicides can change the chemistry of the vegetation in the field that can to both attractive and toxic to horses. Two of the most used herbicides are glyphosate and phenoxy . Clinical symptoms are diarrhea and colic.

Decaying Organic Matter

Botulism is the biggest danger of a horse who ingests decaying organic matter. Bad hay and silage that has been stored improperly can become contaminated. Decomposition of rodents, birds, and other carcasses in baled hay are often blamed. Large round bales of hay put a horse at greater risk for botulism. These toxins can also enter the horse's body through wounds. These botulism causing toxins are produced by Clostridium botulinum. Horses are highly susceptible to C. botulinum toxins. Horses are highly susceptible to C. botulinum toxins, which attack the nerves that communicate with muscles, leading to general weakness that progresses to paralysis. Clinical signs of botulism might include: inability to swallow food, inability to drink water, saliva drips from lips (drooling), grain falls out of mouth, loss of coordination (ataxia), muscle tremors, depression, protruding tongue, dilated pupils, constipation, colic, shortness of breath, spasms and/or seizures, inability to stand, nasal discharge and death.

An antitoxin can be given but it is very expensive. Botulism may be prevented through vaccination with BotVax B.

Fumonisin (moldy corn)

Fumonsins do well in moldy corn and can cause poisoning. Sometimes this poisoning is referred to as Blind staggers (technically known as equine leucoencephalomalacia) occasionally occurs in horses, mules, or donkeys foraging corn left standing in the field after harvest or fed grain or screenings heavily infected with F. moniliforme. The toxins fumonisin B1 and B2 are produced only by certain strains of F. moniliforme. Lameness and/or staggering and seizures can occur if a horse eats more than 5 parts per million. Fumonisin toxicity is usually fatal. Central nervous system disease and death can occur a day or so after the horse has ingested the toxin. It is extremely important to have clean corn. It has also been suggested to feed only limited amounts of corn to horses.

This mycotoxin (fungal toxin) can infect corn prior to harvest or during storage. Hot, dry conditions followed by high humidity are associated with increased fumonisin concentrations in growing corn, usually in the Midwest and South. Danger to horses that eat corn containing toxic fumonisin levels develop moldy corn poisoning, or equine leukoencephalomalacia (ELEM), a rapidly progressing, often fatal neurologic disease such as head-pressing, circling, muscle tremors, weakness, or strange, sometimes violent behavior. For those that survive, the prognosis is not good. Veterinarians typically recommend euthanasia. It attacks the white matter in the brain.

Acer rubrum (Red Maple, also known as Swamp, Water or Soft Maple)

Image: Famartin /Wikipedia Creative Commons License
Acer rubrum (Red Maple, also known as Swamp, Water or Soft Maple)

"...The leaves of red maple, especially when dead or wilted, are extremely toxic to horses. The toxin is unknown, but believed to be an oxidant because it damages red blood cells, causing acute oxidative hemolysis that inhibits the transport of oxygen. This not only decreases oxygen delivery to all tissues, but also leads to the production of methemoglobin, which can further damage the kidneys. The ingestion of 700 grams (1.5 pounds) of leaves is considered toxic and 1.4 kilograms (3 pounds) is lethal. Symptoms occur within one or two days after ingestion and can include depression, lethargy, increased rate and depth of breathing, increased heart rate, jaundice, dark brown urine, colic, laminitis, coma, and death. Treatment is limited and can include the use of methylene blue or mineral oil and activated carbon in order to stop further absorption of the toxin into the stomach, as well as blood transfusions, fluid support, diuretics, and anti-oxidants such as Vitamin C. About 50% to 75% of affected horses die or are euthanized as a result. ..." Acer rubrum (Red Maple, also known as Swamp, Water or Soft Maple)

Researchers now believe that gallic acid may be the toxic agent. There is an increase in gallic acid in the leaves in summer. "The leaves in combination with certain bacteria produce a strong oxidant that damages horses' red blood cells, hindering their ability to carry oxygen or destroying them completely. ..." Wilted leaves may remain toxic for more than 30 days. It is also thought that the bark may also be toxic. It should also be noted that sugar and silver maple might also be toxic to horses.

Tansy Ragwort

Image: Claus Ableiter/ Creative Commons Attribution-Share Alike 4.0 International/Wikipedia
Image: Claus Ableiter/ Creative Commons Attribution-Share Alike 4.0 International/ Jacobaea vulgaris/Tansy Ragwort

Jacobaea vulgaris, syn. Senecio jacobaea is a very common wild flower in the family Asteraceae that is native to northern Eurasia, usually in dry, open places, and has also been widely distributed as a weed elsewhere. Common names include ragwort, common ragwort, tansy ragwort, benweed, St. James-wort, ragweed, stinking nanny/ninny/willy, staggerwort, dog standard, cankerwort, stammerwort, mare's fart and cushag. In the western US it is generally known as tansy ragwort, or tansy, though its resemblance to the true tansy is superficial.

"...Ragwort contains many different alkaloids, making it poisonous to certain animals. Alkaloids which have been found in the plant confirmed by the WHO report EHC 80 are -- jacobine, jaconine, jacozine, otosenine, retrorsine, seneciphylline, senecionine, and senkirkine. Other alkaloids claimed to be present but from an undeclared source are acetylerucifoline, (Z)-erucifoline, (E)-erucifoline, 21-hydroxyintegerrimine, integerrimine, jacoline, riddelline, senecivernine, spartioidine, and usaramine.

Ragwort is of concern to people who keep horses and cattle. In areas of the world where ragwort is a native plant, such as Britain and continental Europe, documented cases of proven poisoning are rare. Horses do not normally eat fresh ragwort due to its bitter taste. It loses this taste when dried and can become a danger in hay. The result, if sufficient quantity is consumed, can be irreversible cirrhosis of the liver. Signs that a horse has been poisoned include yellow mucus membranes, depression, and lack of coordination. Animals may also resort to the consumption of ragwort when there is shortage of food. Sheep and goats suffer the same process of liver destruction but at a reduced rate to horses and pigs.

The danger of Ragwort is that the toxin can have a cumulative effect. The alkaloid does not actually accumulate in the liver but a breakdown product can damage DNA and progressively kills cells. About 3-7% of the body weight is sometimes claimed as deadly for horses, but an example in the scientific literature exists of a horse surviving being fed over 20% of its body weight. The effect of low doses is lessened by the destruction of the original alkaloids by the action of bacteria in the digestive tract before they reach the bloodstream. There is no known antidote or cure to poisoning, but examples are known from the scientific literature of horses making a full recovery once consumption has been stopped. ..."

Clinical signs of poisoning may take many months to develop. Among these signs are head-pressing, circling, loss of appetite, weight loss, irreversible and chronic liver damage and other odd behavior.

Poisoning in Horses: Common Toxic Substances

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