10 Most Poisonous Plants for Horses

Water hemlock (Cicuta spp.)
Also known as: Spotted water hemlock
ID: A perennial weed with erect hairless stems that can grow to six feet from clusters of fleshy roots. The stems are hollow and branching, thicker at the base. Leaves are elongated and toothed, and the small white flowers form flat, umbrella-shaped clusters at the ends of branches.
Range: Water hemlock grows throughout the contiguous United States and is most likely to be found in marshy areas of meadows and along streams and irrigation ditches.
The danger: Water hemlock is considered one of the most toxic plants in the United States. All parts of the plant contain a cicutoxin alkaloid that affects the central nervous system, but the toxin is most concentrated in the root. Because cattle are more likely to pull up and consume the root, that species is considered most at risk of poisoning, but horses have also been known to browse the plant; less than a pound of the leaves and stems can be fatal. The toxin levels in the leaves and stems diminish as the plant ages during the growing season, and additional amounts of toxin are lost when the plant is dried, but water hemlock is never considered safe for consumption. Most animals will avoid the plant.
Signs: The toxins affect neurons primarily within the brain, causing various signs, including excessive salivation, dilated pupils and nervousness, progressing rapidly to difficult breathing, degeneration of the heart and skeletal muscles, seizures and convulsions; death usually results from respiratory paralysis. Signs of poisoning appear within an hour of ingestion, and death typically follows within two to three hours.
What to do: Supportive care initiated before the convulsions begin can offset the worst effects of the seizures, but horses who survive are likely to have experienced permanent damage to the heart and skeletal muscles.

Yellow star thistle/Russian knapweed (Centauria spp.)
Also known as: Barnaby's thistle
ID:Yellow star thistle is an annual weed that branches out from a single base stem to form a spherical plant up to three feet tall; its round yellow flowers are surrounded by stiff spines 1/2 to 3/4 of an inch long. Russian knapweed spreads via a creeping root system; its erect, stiff stems grow two to three feet high and are covered with gray hairs, and its thistlelike flowers range from purple to white; Russian knapweed has no spines or prickles.
Range: Both plants appear throughout the Western United States, approximately from Missouri to California, and from Mexico northward, almost to Canada. They appear as weeds along roadsides, in cultivated fields and pastures.
The danger: Both plants contain a toxic agent that has a neurological effect on the brain that inhibits the nerves and control chewing. The poisoning is chronic in nature; to receive a toxic dose, horses must consume 50 to 200 percent of their body weight over 30 to 90 days.
Signs: Affected horses may appear to have tense or clenched facial muscles, and they are unable to bite or chew their food effectively. Weight loss is also common.
What to do: There is no treatment, and any neural damage is permanent. Euthanasia is recommended if the horse is too debilitated to eat.

Yew (Taxus spp.)
ID: A woody evergreen shrub with closely spaced, flat, needlelike leaves a half-inch to one inch long. Berries are bright red or yellow, soft and juicy with a hole in the end, where the dark seed is visible.
Range: Western yew and American yew are native to the West Coast and to the Eastern and central United States, respectively, but these two species along with the Japanese and English yews are commonly planted as ornamentals nationwide.
The danger: All parts of the yew plant, except for the fleshy portion of the berries, contain taxine, an alkaloid that causes respiratory and cardiac collapse. The leaves remain toxic even after dried. A single mouthful can be deadly to a horse within minutes.
Signs: Sudden death is the most typical sign of yew ingestion. Animals found alive may be trembling and colicky, with difficulty breathing and a slowed heart rate.
What to do: There is no treatment for yew poisoning. Avoidance is critical; most yew poisonings occur when trimmings are thrown into a pasture after a pruning.

EQUUS thanks Anthony Knight, BVSc, MRCVS, and Jill Richardson, DVM, for their assistance in the preparation of this article. For more information, visit Knight's website, Guide to Poisonous Plants.

This article originally appeared in the June 2004 issue of EQUUS magazine.