Yew
Horses are highly susceptible to yews (Taxus spp.), whose ingestion results in sudden death following incoordination, nervousness, difficulty breathing, diarrhea, and convulsions. Yews are evergreen shrubs or small trees with glossy, stiff, dark green linear leaves 1.2 to 2.7 inches (3 to 7 cm) long with pointed ends, closely spaced on the branches. Inconspicuous axillary male and female flowers are produced on separate plants, forming showy red to yellow fruits containing a single seed.
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Several species of yew grow naturally or are grown as ornamental plants in North America; perhaps appropriately Taxus species plants are used extensively for shrubbery around government buildings in Washington D.C. Yews generally prefer humid moist environments. Western yew (Taxus brevifolia) and Canada yew (Taxus canadensis) are two indigenous species. English yew (Taxus baccata) and Japanese yew (Taxus cuspidata) are commonly cultivated species in North America.
Yews contain the potent alkaloid taxine in all parts of the plant, green or dried, except the fleshy aril surrounding the seed. Horses are highly susceptible to the taxine's effect on the heart, frequently dying shortly after eating small quantities of the plant (an amount equal to 0.05% of their body weight of green leaves). Livestock are often poisoned when fed trimmings from cultivated yews.
Sudden incoordination, nervousness, difficulty in breathing, slow heart rate, diarrhea, convulsions, and death are characteristic of yew poisoning in all animals. Sudden death may be the only observed sign in horses. There is no effective treatment for acute yew poisoning. A diagnosis of yew poisoning is usually made from the history of sudden death, the absence of postmortem findings, the access to yew, and the finding of yew leaf fragments in the stomach.
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