Conditions or ailments that are the cause of a problem that you see - your observation.

Your vet may diagnose

Poison Hemlock or Water Hemlock Toxicity

Synonyms: Poison Hemlock, or Water Hemlock, Cowbane, Poison Parsnip


These are two very different plants that I decided to place under one record because of their roughly similar appearance and name.

Poison hemlock (Conium maculatum) is a multi-stemmed, 3-6 foot tall weed with fern-like, toothed leaves. It is found throughout North America in low lying moist areas along field borders, fences and streams. The stem has purple spots which are most obvious at the base. The plant has an obvious smell, described as similar to a dead mouse, or parsnips. Regardless, horses generally find poison hemlock unpalatable.

The roots, stems, seeds and leaves of Poison Hemlock contain a potent nerve toxin called conine, which affects the brain, spinal cord and peripheral nerves. It requires 4-5 lbs of plant material to cause severe toxicity. Signs of toxicity are colic, trembling, incoordination, diarrhea, dilated pupils, and death. Horses avoid the plant unless they are very hungry. Thus it is a rare cause of poisoning.

Water hemlock (Cicuta spp.) is a perennial plant that grows throughout the United States as well. It grows over 8 feet tall and has small green or white flowers arranged in the shape of an umbrella. Water hemlock contains the toxin cicutoxin and causes violent seizures and convulsions and rapid death within a very short period after ingestion. As little as 8 oz of this plant can kill a full-sized horse. The toxin is concentrated in the roots.

Signs of toxicity include depression, rapid breathing, frothing at the mouth, dilated pupils and muscle twitching, lying down, seizures and death. Those signs usually start 15 minutes to 4 hours after ingestion. Signs are usually progressive and death follows in a few hours.

Diagnosis requires evidence that the plants has been consumed, along with compatible clinical signs. The toxins might be retrieved from stomach or intestinal contents.

my vet's role



Other conditions or ailments that might also need to be ruled out by a vet.

Very Common
Less Common
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The prognosis for poisoning from these plants is generally grave, although it depends on the amount of plant ingested. Horses that ingest a small amount of poison hemlock and are promptly prohibited from eating more generally recover. Water hemlock consumption usually results in rapid death.

my role


I might observe

You might make these observations when a horse has this condition.

Very Common
Less Common
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Questions To Ask Your Vet:
  • How can I minimize risk to my horses of consuming these toxic plants?

Ensure that pastured horses are not allowed to overgraze their pastures, leaving toxic weeds as the only accessible green plants. If horses must be kept on overgrazed pasture, they must be supplied with adequate to excess palatable hay so they are not tempted to try toxic weeds.

Be extremely careful when on the trail to avoid tying horses near these plants or any plant you cannot identify. Remove these plants from horse enclosures whenever possible.

Related References:

Higgins AJ, Snyder JR eds. The Equine Manual. 2nd Ed. Edinburgh: Elsevier Saunders 2006.

Author: Doug Thal DVM Dipl. ABVP