Poison hemlock toxicity

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Poison Hemlock Toxicity

Poison Hemlock Poisoning

Poison hemlock (Conium maculatum) is an annual or biennial herb which is native to Europe and western Asia. It has been introduced to America and Oceania as an ornamental plant. C. maculatum can grow up to 9 feet in height and resembles a carrot plant. It has large lacy leaves and a usually hollow stem which has purple spots. It produces small white or greenish-white flowers which present as flat clusters like an umbrella which blossom in the spring. Small fruits appear in the summer. When the leaves, stem or flowers are crushed, they produce an offensive "mousy" odor.

Toxic components
C. maculatum contains eight piperidinic alkaloids, however y-coniceine and coniine are generally the most abundant and the predominant cause of acute and chronic toxicity. Coniine is a pyridine derivative similar in structure and function to nicotine. The toxicity of the plant varies depending on the stage of growth, environmental conditions (rain, temperature, cloud cover), and soil. Although all parts of the plant are toxic, the most toxic parts are full formed fruit that is still green, ripe fruits, and stems.

Signs of poisoning develop 1 to 2 hours after ingestion. The toxic dose in horses is 15.5 mg/kg. Horses don't typically graze hemlock unless food is scarce or it is contaminated in hay or haystuffs.


Muscle tremors
Frequent urination
Sudden death


  • History
  • Clinical signs
  • Physical exam
  • Necropsy



Supportive therapy


  • 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.
  • Quarantine new animals in a separate paddock the first 10 days to 2 weeks after arrival. Weed seeds can be passed through an animal's digestive tract.

Scientific Research

General Overviews

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Risk Factors

  • Letting horses graze in weedy pastures that contain poison hemlock.