Water hemlock toxicity

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Water Hemlock Toxicity

Water Hemlock Poisoning

Water hemlock (Cicuta virosa) is one of the most violently toxic poisonous plants known. Consuming as little as 0.5% of their body weight is lethal to an adult horse. Over 20 different species of Cicuta have been identified and found worldwide. Most species are found in North America, with nine common in the U.S. It is most toxic in the spring, during the early stages of its growth, when it is also more difficult to identify. The toxicity of plants is also highly variable on its location, environmental conditions ( moisture, temperature), and time of day.

Water Hemlock Toxic components

Water hemlock toxic horses
Cicuta contains cicutoxin, which is a long-chain, highly unsaturated alcohol. It acts very quickly, and early clinical signs of poisoning occur 10 to 15 minutes after ingestion. The toxin affects the central nervous system, with initial signs starting out as excessive salivation, nervousness, stumbling and incoordination, and frothing; quickly leading to intermittent periods of tremors, convulsive seizures, and muscular weakness. The horse usually will die during a period of convulsions, which are extreme and violent.


Dilated pupils
Excessive salivation
Laboured breathing
Teeth grinding
Muscle fasciculations
Muscular weakness


  • History
  • Clinical signs
  • Physical exam
  • Laboratory testing



Supportive care


  • Make yourself aware of the weeds and plant species that can be invasive in pastures and/or poisonous to horses.
  • Take periodic walks around pastures to check for the presence of potentially poisonous plants
  • Check that hay does not contain dried up poisonous plants
  • If you borrow or hire farm machinery ensure it is clean prior to arriving on your property, the same goes for lending of your own equipment.
  • Control water hemlock from growing in pastures. Phenoxy herbicides such as 2,4 D is effective at controlling them if applied appropriately.

Scientific Research

General Overviews

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Risk Factors

  • Pasture is adjacent to or provides horse access to streams, creeks, lakes, rivers, ditches, marshes, and/or swampy areas.