The plant is exceptionally toxic, with one mouthful able to kill a horse or cow within 5 minutes. Toxicity is compounded by the apparent palatability of yew. Many animals are poisoned accidently when yew trimmings are thrown into the pasture or when yew is planted as an ornamental within browsing reach. Infrequent reports of dogs chewing the leaves resulted in gastroenteritis, seizures, and aggressive behavior. The toxin is taxine, a mixture of alkaloids, that slow down cardiac conduction.
First aid is usually impractical, since the animals die so quickly.
Prevent other animals from being exposed and use caution around animals showing clinical signs to prevent human injury. If animals are still alive, contact a veterinarian. Cardiac drug therapy may be attempted, but success is unlikely. Yew is toxic even when dry, therefore hay with yew in it is never safe in any amount. Never allow yew plants or trimmings within reach of horses, cattle, sheep, goats, pigs, birds, or any other animal likely to eat plants.