Poisonous plants and common toxicoses in horses

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Poisonous plants and common toxicoses in horses - horse grazing in pasture

There are vaccinations that prevent communicable diseases in horses, but there is little you can do to prevent poisoning in horses except for environmental management. It’s important as a horse owner to be able to identify and remove toxic plants, fungi, mold, and minerals from your horse’s environment. Read on to learn answers to questions about poisoning every horse owner should know, including:

  • What plants are poisonous for my horse?
  • What will happen if my horse eats mold?
  • Which fungi are toxic to horses?
  • How can I identify mineral or heavy metal poisoning in my horse?
  • How to keep your horse safe from poisoning?

Toxic plants and contaminants can be found in pastures, paddocks, hay bales, and horse feed. By learning the signs and symptoms of toxicoses relevant to your geographic area, you can prevent your horse from getting sick or recognize when they need treatment right away.

What plants are toxic to horses?

There are many plants throughout the United States and Canada that are toxic to horses. Some toxic plants are local to certain regions of North America, while others are more widespread. “It’s crucial for horse owners to know and be able to recognize the toxic plants found in their area, so prevention and treatment of toxicoses can happen early,” explains Dr. Jo Myers. Note that the total number of possible toxins that affect horses is extensive. This guide covers some of the most common found in North America. Always check with your local equine vet to identify what other risks of poisoning are associated with your specific area and horse’s lifestyle.

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Toxic plants found widespread in North America

There are many plants found throughout the majority of the United States and Canada that are toxic to horses. Some of these toxic plants include:

Oak trees

There are sixty species of oak trees found throughout North America. The buds, twigs, leaves, and acorns from all of these oak tree species are potentially harmful to horses if eaten. Symptoms of oak toxicity include bloody urine, colic, and bloody diarrhea. Oak toxicosis in horses is rare, but it can be fatal if the quantity of oak plant material consumed is high enough. Treatment for oak toxicity primarily involves supportive care.

Pyrrolizidine alkaloid toxicosis

Pyrrolizidine alkaloids%20%5B1%5D.) are a type of organic compound found in a large number of plants around North America. Symptoms of pyrrolizidine alkaloid toxicosis include yellow gums, sloughing of the skin, and diarrhea. Once symptoms of liver failure appear, the prognosis is generally very poor. Treatment for pyrrolizidine alkaloid toxicosis is supportive care for the resulting symptoms and reducing exposure to sunlight.

Cardiotoxic plants

Cardiotoxins occur naturally in some plant species, and damage the heart muscle of affected horses after ingestion. While there are many species of cardiotoxic plants, oleander is the most common to cause toxicosis in horses. Symptoms of cardiotoxic oleander poisoning include abdominal pain, difficulty breathing, seizures, collapse, and sudden death. If caught early, oleander toxicosis can be treated with decontamination of the GI tract and the use of activated charcoal to limit the absorption of the toxin. Once symptoms appear, treatment is supportive care. The prognosis for the ingestion of cardiotoxic plants depends on the quantity, type, and how early treatment was given, but is generally guarded or extremely poor.


Nightshade is a family of plants that are all toxic to horses, including jimsonweed, potato plants, and deadly nightshade. There are more than seventy types of nightshade. Nightshade poisoning in horses is relatively rare due to a horse’s general distaste for the plant, but it can cause serious symptoms. Symptoms of nightshade toxicosis include abdominal pain, diarrhea, and muscle tremors. Treatment for nightshade ingestion includes neostigmine and supportive care. However, the prognosis is generally guarded to poor depending on the amount ingested and how quickly treatment is initiated.

Black locust trees

The black locust tree is found throughout the United States and Canada. The seeds, leaves, and inner bark of the tree are all toxic to horses. Symptoms of black locust poisoning include abdominal pain, diarrhea, blood in the feces, and severe lameness. Treatment for the poisoning is supportive and based on the symptoms, but horses may die within a few days of ingestion if treatment is not given immediately.


Hemlock contains a compound that damages the nervous system, resulting in symptoms such as uncoordinated movement, muscle tremors, paralysis, difficulty breathing, and collapse. The prognosis and treatment for hemlock poisoning in horses depend on the amount ingested and the resulting symptoms. If the acute poisoning does not progress to paralysis, the prognosis is usually good. If paralysis occurs, especially paralysis of the respiratory system, the prognosis is very poor and horses often require mechanical ventilation. Hemlock plants are commonly found along roads, waterways, and the edge of pastures.


Cyanide is a compound produced by many species of plants in different circumstances, but is most commonly produced  when part of the plant is damaged. This can be due to cutting or grazing on the plant. Fortunately, cyanide toxicity is less common in horses than in ruminants, like cows and sheep. However, horses can still be affected by ingesting cyanide, especially if the leaves of the plant are dried or wilted. Symptoms of cyanide poisoning in horses include difficulty breathing, uncoordinated movement, seizures, and red gums. The antidote for cyanide consists of sodium nitrite and sodium thiosulfate, but these are not always effective.

Blue-green algae

Blue-green algae is not a plant, but an algae bloom containing toxic bacteria that occurs in stagnant water. These bacteria can produce two different types of toxins that can affect the liver or the nervous system. Symptoms of blue-green algae toxicosis include colic, muscle tremors, diarrhea, yellow gums, and seizures. Treatment involves gastrointestinal decontamination and supportive care of resulting symptoms. Many horses die from blue-green algae toxicosis, and many who survive develop long-term liver damage.

Alsike clover

Alsike clover thrives in northern latitudes and high elevations but can be found all over North America. The plant can cause two forms of toxicosis in horses: liver failure and dew poisoning (photosensitivity). Symptoms include signs of liver failure, such as jaundice and abdominal pain, or signs of dew poisoning, such as sensitivity to sunlight, and skin swelling or sloughing. The photosensitivity form of alsike clover poisoning generally has a good prognosis, while the liver failure form varies from guarded to poor based on the extent of liver damage.

Toxic plants found in Canada and Northern USA

While some toxic plants are found in all areas of North America, others thrive in the cooler areas in northern American states and Canada.

Hoary alyssum

Hoary alyssum is a short-lived weed that is endemic to states in the Northern U.S. and throughout Canada, though it can sometimes be found in the Southwest region of the U.S. Ingestion of hoary alyssum in horses typically occurs when it becomes mixed with their hay or other grazing material. When more than thirty percent of ingested hay is composed of hoary alyssum, it becomes toxic. Some horses never show symptoms, while others seem to be more sensitive. Symptoms include swelling of the lower limbs, lameness, and unwillingness to move. Treatment involves supportive care, anti-inflammatories, and icing the hooves.

Toxic plants found in Western USA and Canada

The unique weather conditions of abundant rain and mild winters in the western parts of the U.S. and Canada allow specific plants to thrive where they cannot in other areas. Many of these plants are toxic to horses.

Yellow star thistle

Yellow star thistle is found primarily in the Western regions of the United States and Canada but can grow nearly anywhere in North America. Yellow star thistle poisoning is often referred to as “chewing disease” due to causing difficulty in eating and swallowing in horses. In addition, the toxic plant can cause lethargy, weakness, head pressing, circling, and wandering. Yellow star toxicosis is cumulative, so once a horse ingests between twenty and two hundred percent of its body weight, its prognosis is very poor.

Toxic plants found in Eastern USA and Canada

The East Coast of North America experiences vastly different weather conditions than the West Coast and the Midwest. Hot summers and freezing winters result in specialized plants in these areas.

Red maple trees

Red maple trees are found primarily in the Midwest and East Coast of North America. Eating the dry or wilted leaves of the red maple tree leads to toxicosis in horses. Symptoms of red maple poisoning include jaundice, colic, bloody or brown urine, and fever. Treatment is supportive, providing IV fluid therapy, blood transfusions, and activated charcoal to reduce absorption of the toxin. Red maple poisoning has a poor prognosis but is rare in horses.

Toxic plants found in Southern USA

The Southern U.S. also has its share of toxic plants that are dangerous for horses. Lantana is one of the most common.

Lantana (yellow or red sage)

Lantana, also known as yellow or red sage, is a toxic plant that grows in the Southern states of the U.S. The plant contains toxins that affect the liver, leading to liver damage and failure when ingested, as well as photosensitization. Symptoms of lantana toxicosis include jaundice, swelling or sloughing of the skin, and excessive urination. Treatment for lantana poisoning is focused on supportive care, reduced sunlight exposure, and treatment of skin wounds. Depending on the amount that was ingested and the severity of the symptoms, lantana poisoning has a guarded to poor prognosis in horses.

What molds are toxic to horses?

There are many toxic molds that commonly grow in plants that make them toxic, even if the plant itself is not. Molds can also invade and affect grains, feed, and forage material

Tall fescue grass

Tall fescue grass grows in pastures all over North America and is not itself toxic. However, the grass is often infected with a toxic fungus that affects breeding and pregnant mares. Ingestion of contaminated fescue grass causes prolonged gestation, decreased milk production, abortion, stillbirth, and difficulty delivering. Foals born to mothers infected with fescue toxicosis are at risk of developing infections if the mare does not produce colostrum, the first milk that contains antibodies to boost the foal’s immune system. Prognosis is guarded for foals born to mares consuming contaminated fescue.

Moldy sweet clover

Sweet clover is a well liked and popular foraging material for horses. However, when sweet clover goes bad, the resulting mold is toxic. This mold produces a powerful anticoagulant that can cause fatal hemorrhages. Symptoms of moldy sweet clover poisoning include bruising, red spotted gums, nosebleeds, stiffness, and lameness. Treatment for sweet clover toxicosis includes blood transfusions, and synthetic vitamin K1 to assist with clotting.

What fungi are toxic to horses?

Similarly to molds, fungi can invade and affect horse grains, feed, and forage. Fungi can also appear quickly in pastures in the right climate conditions.


Slaframine is a fungal toxin that typically contaminates the red clover plant, alfalfa, and legumes. Slaframine toxicosis is not life-threatening, but some symptoms can mimic more dangerous conditions. Symptoms include excessive salivation, excessive urination, excessive tear production, diarrhea, and mild abdominal pain. Symptoms of slaframine toxicosis will generally clear on their own once the contaminated feed is removed.


Aflatoxins are produced by a fungus that grows on feed crops, such as corn, peanuts, and soybeans. Once ingested, aflatoxicosis causes liver damage that can be life-threatening. Symptoms include excessive bleeding, muscle tremors, uncoordinated movement, and yellow gums or skin. Treatment options are limited other than removing the affected feed and providing supportive care. Prognosis depends on the severity of the damage and symptoms, and how quickly treatment was given after ingestion.

Moldy sweet corn

Despite its name, moldy sweet corn toxicity is the result of a fungal toxin called fumonisin that grows on sweet corn. Fumonisin can cause two syndromes: equine leukoencephalomalacia (ELEM) or liver damage. The two syndromes can occur at the same time or separately. Moldy sweet corn toxicosis is life-threatening with a grave prognosis once symptoms begin. Symptoms include yellow gums, uncoordinated movement, blindness, circling, recumbency, and sudden death.

Ryegrass staggers

Ryegrass is often contaminated with fungal or bacterial toxins, causing ryegrass staggers. Symptoms of ryegrass staggers include muscle tremors, incoordination, and recumbency. In addition, the perennial variety of ryegrass can cause heat stress and reproductive issues in pregnant mares. Ryegrass staggers is uncommon but is considered a life-threatening medical emergency. Early supportive care is essential, and the majority of severely affected horses die despite supportive care.

What other sources of poisoning can be dangerous for horses?

Though horses need essential minerals and other micronutrients in their diet, too much can be toxic. Mineral and heavy metal poisonings can be life-threatening and difficult to remove from a horse’s system once they have been ingested. Always consult an equine vet before administering supplements to your horse’s diet.

Selenium poisoning

Selenium is a micronutrient that, in small doses, aids in muscle development and growth in horses. However, as little as 5 mg a day over the course of a month can cause selenium poisoning. Acute symptoms include lethargy, abdominal pain, and death. Symptoms of chronic selenium poisoning include hoof cracking, lameness, and hair loss. There is no antidote for selenium poisoning, and treatment includes dietary change and pain management. The prognosis is poor for both acute and chronic selenium poisoning.

Nitrate and nitrite poisoning

Nitrate and nitrite toxicosis cause a condition in horses called brown blood disease. The disease is rare and potentially life-threatening. Nitrate and nitrite poisoning are typically caused by an excessive amount of nitrate-containing forage material, contaminated water, or the presence of some fertilizers. Nitrate/nitrate poisoning causes a reduction in oxygen transport in the body, preventing organs and tissues from receiving the oxygen they need to survive. Inadequate oxygen results in weakness, tremors, recumbency, blue gums, and sudden death. Treatment options for brown blood disease include IV administration of methylene blue salts, removal of nitrate sources, and supportive care. Symptoms of brown blood disease usually occur quickly, often resulting in death before treatment can be given.

Heavy metal poisoning

Heavy metal poisoning occurs when horses ingest toxic levels of heavy metals, such as lead, arsenic, and mercury. It is a life-threatening condition with symptoms such as colic, seizures, uncoordinated movement, and recumbency. In severe cases, heavy metal toxicosis can result in organ failure and death. The prognosis depends on the severity of the toxicosis and what type of metal has been ingested. Treatment includes removal of the source, decontamination of the stomach, and chelation therapy.

Oxalate poisoning

Oxalates bind to calcium in the gastrointestinal tract of horses, preventing them from absorbing calcium essential for the health of their skeletal system. Ingestion of oxalate-containing plants is generally not an issue unless it represents the majority of a horse’s diet. Oxalate poisoning can occur acutely or chronically. Symptoms can appear immediately or two to eight months after grazing in oxalate-heavy pastures. They include changes in the head shape, lameness, bone fractures, and increased urination.

Iron poisoning

Horses can be exposed to iron in high doses through supplements, drinking water, grass, or hay. Chronic iron overload causes symptoms, such as jaundice, rough hair coat, weight loss, and head pressing. Treatment generally involves supportive care based on symptoms, but iron poisoning can cause widespread iron deposits in major organs around the body and can be very difficult to remove.

How do I keep my horse safe from poisoning?

For poisonings and toxicoses, prevention is key. Learn to identify and remove toxic and poisonous plants from your horse’s environment, and test feed and water quality to identify potential sources of mold or excess minerals before feeding. Never add supplements to your horse’s diet without consulting a veterinarian, as this can lead to acute poisoning by overdosing on minerals in the supplements. Finally, be sure to contact a horse vet if your equine is showing symptoms of toxicosis of any kind. The sooner a veterinarian can diagnose and treat your horse, the better. You can schedule a virtual vet appointment if you have any questions about toxicities in your area.

FAQ - Poisonous plants and common toxicosis in horses

What is the most poisonous plant to horses?

Many highly poisonous plants, molds, and fungi are dangerous for horses. The risks of different plants vary on geographic location across North America, so it is important to know what the most common toxic plants are in your area. Some of the most common toxic plants for horses include Johnson grass, red maple trees, and oleander.

How do horses get poisoned?

While there are many ways a horse can ingest something poisonous, the most common way is by grazing on toxic plants. It is important to maintain pastures and grazing areas to ensure your horse does not have access to toxic plants, and to acquire hay from a reliable supplier.

What plants cause colic in horses?

Colic refers to persistent abdominal pain in horses. It is a symptom and not a disease or diagnosis itself. When toxic plants are ingested, they are digested in the stomach, often causing abdominal pain or colic. Nearly every plant that is poisonous to horses can cause colic, but others, such as oak trees or oleander, are more common than others.