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.