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Water Hemlock

Poisonous to: Cats, Dogs, Horses, Cows

Level of toxicity: Generally moderate to severe/life-threatening

Common signs to watch for:

  • Drooling
  • Dilated pupils
  • Weakness
  • Agitation
  • Nervousness
  • Twitching
  • Seizures
  • Cardiac abnormalities
  • Difficult breathing
  • Death from respiratory paralysis

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Water hemlock, which is also called Cowbane, poison parsnip, or poison parsley, is often referred to as the most violently toxic plant in the United States. Water hemlock is considered a natural wildflower and prefers wet areas, such as irrigation ditches, marshes, damp areas in pastures, and riverbanks. Water hemlock contains the toxins cicutoxin and cicutol, which affect the neurons in the brain and central nervous system. All parts of the plant are poisonous, with the roots containing the highest concentration of the toxin. It is highly poisonous to horses, and only takes about 0.2 to 2 lbs hemlock root per 1,000 lbs of a horse’s body weight to cause death. Clinical signs include drooling, dilated pupils, weakness, agitation, nervousness, twitching, seizures, cardiac abnormalities, difficult breathing, and death from respiratory paralysis.

Content written by: Dr. Lynn Hovda, DVM, RPH, MS, DACVIM, Pet Poison Helpline

Poison type: Plants

Scientific name: Cicuta maculata

Alternate names: Cowbane, Poison parsley, poison parsnip