Toxicology Brief: The dangers of yew ingestion
For millennia, people used yew alkaloids as both a method of suicide and a chemical weapon during hunting and warfare. Even sleeping beneath the shade of a yew bush was once considered dangerous.1
Yew also has a notorious reputation among livestock veterinarians in the Northern Hemisphere, and, within this context, Japanese yew (Taxus cuspidata), English yew (Taxus baccata), and Chinese yew (Taxus chinensis) are among the most toxic plants in North America.1 Chewing on Taxus species branches has caused death in dogs.1,2 And yew plants are potentially toxic to pet chinchillas and companion birds such as budgerigars and canaries, although macaws appear to be resistant.1,2
Identification and sourcesTaxus species' leaves are distinctive, making the plants relatively easy to identify. The simple, needlelike leaves are 1 to 2.5 cm long and less than 0.25 cm wide. They are alternately spirally arranged but twisted so they are two-ranked, linear-lanceolate, and decurrent (many lateral leaves with a central stem).1
Taxus cuspidata, T. baccata, and Taxus x media (T. baccata crossed with T. cuspidata) are common shelter, shade, and ornamental plants in the United States.1 Typically, they are planted as hedges or screens. In northern areas, T. cuspidata is preferred, probably because of its greater winter hardiness.1 Taxus baccata are long-lived; some English yews are more than 2,000 years old. Taxus canadensis (Canada yew, ground hemlock, American yew) is a native, cold-tolerant woodland shrub distributed from the Ohio River Valley to the far northeastern parts of Canada. Taxus floridana is a small tree whose distribution is limited to the Apalachicola River area of Florida. Taxus brevifolia (Pacific or western yew) is an understory tree in forests in the western United States. Pacific yew contains only minimal amounts of taxine alkaloids, the principal toxins associated with yew poisoning, and, thus, has a lower toxic potential than other Taxus species.
Toxic principles and toxicokinetics
While various potentially toxic chemicals are present in Taxus species, all parts of the plants except the aril (i.e. the fleshy covering of the seeds) contain cardiotoxic taxine alkaloids, the main compounds of toxicologic concern. The two important cardiotoxic alkaloids present are taxine A and taxine B.1,2 The cinnamate metabolites of both taxines are also cardiotoxic. Paclitaxel, which is of pharmacologic interest because of its antimitotic and anticancer effects, is also present in Taxus species and is potentially arrhythmogenic in some people; however, it is not the major toxic principle in this plant.
Taxines remain in the plant throughout the year, with the maximal plant taxine concentrations appearing during the winter.2 Dried yew plant material retains its toxicity for several months and remains a hazard to domestic animals.
The amount of plant material required to obtain a lethal dose is quite small: The LDmin in dogs is about 2.3 g of leaves/kg, or about 11.5 mg/kg of taxine alkaloids.2 So a dog could consume a potentially lethal dose while playing with Taxus species branches or sticks. Since cases have been recorded in which horses have collapsed within 15 minutes of consuming Taxus species, absorption of ingested taxine alkaloids in monogastric animals is rapid.1 One factor that may limit the ingestion of the leaves or bark is a volatile oil irritant in the plant.