Yew (Taxus spp.) are a common ornamental tree or shrub found worldwide, often used as hedges. In its tree form, Taxus spp are densely branching evergreens with a massive trunk, reaching up to 20 m tall. The leaves are dark green, linear and up to 3 cm long, with a pointed tip, and appear to spread in two rows on either side of the shoot.

All parts of Taxus spp. are very poisonous and contain a complex mixture of alkaloids. Taxines A and B are two of the major chemicals found in the mixture, which is absorbed rapidly and interferes with calcium channels in cardiac myocytes, resulting in cardiac arrest and death within 30 minutes of ingestion. Other toxins that are also found in Taxus spp.are ephedrine, a cyanogenic glycoside (taxiphyllin) and a volatile oil.

Consumption of as little as 0.1% body weight in Taxus spp. leaves is lethal to horses. Symptoms are rarely observed since horses die quickly following ingestion. Most cases have resulted from horses gaining access to yard/hedge clippings. Taxus spp. is one of the plants where the poison is not destroyed when the plant dies. Thus, branches removed from a yew by high winds or pruning will retain their poison.

Taxus spp have caused many poisonings in livestock since ancient times. Plants of Taxaceae, or yew family are known to be palatable to horses. Risk of toxicity increases during the winter months, as the palatability and concentration of toxins increases in the leaves.