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Key takeaways

Hemlock poisoning is a potentially life-threatening condition in horses caused by the ingestion of a toxic dose of the Conium maculatum plant, also known as hemlock or poison hemlock. 

  • Hemlock contains compounds called pyridine alkaloids which can impact the nervous system causing a number of symptoms, including nervousness, dilated pupils, tremors, paralysis, and difficulty breathing
  • Diagnosis depends on a history of exposure or identification of the plants in the stomach contents
  • There is no available treatment for hemlock toxicosis
  • Prognosis depends on the amount of toxin ingested; if acute poisoning does not progress to paralysis, chances of recovery are good
  • If respiratory system paralysis occurs, prognosis is extremely poor 
  • Due to the absence of a specific treatment, prevention is of the utmost importance
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A closer look: Hemlock Poisoning in Horses

Hemlock is native to Europe and North Africa, but it is a hardy plant capable of growing in many different climates and conditions with populations found worldwide.

Symptoms of hemlock toxicosis vary depending on the amount of plant ingested and the amount of toxin contained within the ingested plants. Typically, hemlock toxicosis progresses rapidly, with many horses dying within a few hours of ingestion. In severe cases, horses are often found dead with few other symptoms.

Horses ingesting water hemlock specifically commonly show seizures as their main symptom.

No antidote or treatment options exist, and as such, prevention is the only way to ensure the animal's safety.

Risk factors

Hemlock toxicosis is uncommon in horses. Hemlock toxicosis is a life-threatening condition, as ingestion of hemlock plants can cause rapid death due to respiratory failure. Horses showing symptoms such as difficulty breathing or paralysis require emergency veterinary attention.

Hemlock plants are commonly found along roadways, along streams, and at the edges of fields. Horses eat hemlock even if there is other forage available, which differs from most other plant toxicoses.

Possible causes

The ingestion of a toxic dose of the Conium maculatum plant causes hemlock toxicosis.

The Conium maculatum plant contains pyridine alkaloids, a potent class of neurotoxin that affects the nervous system. In high doses, the alkaloids cause paralysis of the horse's muscles, including those that control the respiratory system.

A similar plant, water hemlock (Cicuta spp.), has similar effects when ingested by horses.

Main symptoms

Testing and diagnosis

Diagnosis of hemlock toxicosis is based on symptoms, the presence of toxic plants in the stomach, and history of exposure.

Steps to Recovery

There is no specific treatment or antidote for hemlock poisoning. In cases of mild toxicosis, treatment focuses on supportive care. Cases identified soon after ingestion may benefit from decontamination of the stomach to prevent further absorption of the toxin.

If the toxicosis progresses to respiratory muscle paralysis, the condition is usually fatal due to respiratory failure. Mechanical ventilation may be used in some cases to maintain breathing while the muscles recover. Other horses in the pasture must be given alternative feed immediately to decrease the risk of toxicosis in the rest of the herd.

In severe cases, the prognosis is extremely poor. Many horses die within five to 10 hours of developing symptoms.

In mild cases of poisoning, prognosis is better as long as the respiratory system is not affected by the toxin and ingestion is promptly discontinued. Recovery time ranges from several hours to one to two days, depending on the amount of toxin consumed.


Hemlock toxicosis is not contagious.

As there is no antidote or treatment for hemlock poisoning, prevention is paramount. Strategies include:

  • Ensuring hay and feed does not contain hemlock plants
  • Providing access to an abundant amount of alternative forage
  • Removal of hemlock plants from pastures

Removal of hemlock plants from pastures is the best way to prevent hemlock toxicosis; however, attention is needed as some herbicides can also be toxic to animals. Herbicide-treated hemlock plants may also be more palatable to horses, so careful examination to ensure all plants are dead is recommended before turning horses out onto a treated pasture.

Is Hemlock Poisoning in Horses common?

Hemlock toxicosis is uncommon in horses.

Typical Treatment

  • None
  • Supportive care for mild cases

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