Taxus species: Yew

Family

Taxaceae

Common Names

Yew

North American indigenous species of yew include:

Taxus brevifolia: Western or Pacific yew

T. canadensis: Canada yew, American yew, ground hemlock

T. floridana: Florida yew

Introduced species of yew include: T. baccata: English yew T. chinensis: Chinese yew T. cuspidate: Japanese yew

Numerous hybrids have been developed from indigenous and introduced species.

Plant Description

A genus of 5 - 10 species of evergreen shrubs or trees which are native to most northern temperate zones and extending into Central America and the Philippines. Taxus species are branching shrubs or trees ranging from 2 - 25 m in height. The bark is scaly and reddish brown. Leaves are dark green above, paler underneath, arranged spirally. Pollen cones are produced on the male plants while the female plants produce fleshy, red to orange, single seeded, cup-shaped arils (Fig. 383 and Fig. 384).

Figure 383. Taxus baccata.

Figure 384. Taxus baccata berries.

Toxic Principle and Mechanism of Action

Yews contain a variety of toxins including cyanogenic glycosides, taxane derivitives, ephedrine, irritant oil, and molting hormone[1,2] Only the nitrogenous taxane alkaloids known as taxines are associated with the cardiotoxic effects of the Taxus species. Taxine B is more toxic than taxine A, and the proportions of the taxines varies depending upon the species of Taxus [3,4]. Taxine B causes decreased cardiac contractility and marked slowing of atrial and ventricular rates. This effect is mediated through taxine B’s inhibition of sodium, potassium, and calcium channels in cardiac muscle [5] All parts of the plant are toxic except for the ripe red-orange aril. The seed itself is toxic. The plant also appears to be more toxic in winter [6]. Taxus canadensis has the lowest concentrations of taxines and probably has little potential for toxicity. All domestic animals including birds are susceptible to poisoning by taxine B [7-11]. Deer and moose appear to be far less susceptible to poisoning, apparently because their rumen microflora are able to detoxify taxines [12].

Taxol has antineoplastic properties, and has been used as a chemotherapeutic agent in treating certain types of cancer. Its effects are attributed to its inhibition of cell mitosis through the promotion of microtubule formation and stability [12].

Taxol is found in T. brevifolia but is also present in other species. It has also been synthesized [1,14].

Risk Assessment

Yews are one of the most toxic plants that are commonly planted for their dense green evergreen foliage that is ideal for hedges and pruning into ornamental shapes. In many instances, yew poisoning of animals occurs when animals are accidentally fed yew leaves after the plants are pruned. At other times horses or livestock gain access to the yew in gardens. As little as 200 g of yew is lethal to a 500 kg horse.14 Humans are susceptible to yew poisoning with approximately 100 g of the leaves being lethal [13]. An ounce of the leaves is lethal to dogs. No species of Taxus should be planted in or around animal enclosures, and it is important not to dispose of yew trimmings into animal pens.

Clinical Signs

Horses and cattle that have eaten yew are often found dead with no other signs being noticed prior to death [16,17]. Animals may die suddenly several days after they have consumed yew. Weakness, ataxia, muscle tremors, nervousness, bradycardia, jugular distension, and difficulty in breathing may all be present in an animal affected by yew poisoning [6,7]. Dogs eating yew leaves vomit, develop muscle tremors, seizures, panting, dilated pupils, and marked increase in heart rate [9]. Stressing the animal can precipitate sudden collapse and death. There are generally no gross post mortem lesions.

Cases of human poisoning are rare and result in vomiting, diarrhea, delirium, and convulsions [18]. Death occurs as a result of bradycardia and complete heart standstill.

Treatment must be initiated shortly after the yew is consumed to ensure success. Activated charcoal orally, and a rumenotomy to remove recently eaten yew from the rumen may be life saving. Gastric lavage may be attempted to try and remove yew leaves from the stomach. Atropine may be helpful in counteracting the bradycardia if administered early.

Intravenous lidocaine hydrochloride has been used in a case of human poisoning to control ventricular fibrillation. Seizuring dogs may need to be given pentobarbital or given diazepam to control seizures [9,10].

A diagnosis of yew poisoning in animals is generally made by finding evidence of the plant in the stomach or rumen of the animal, absence of other pathology, and the detection of the taxine alkaloids in the stomach contents using liquid chromatography and mass spectrometry [4].