Yew (Taxus baccata L.). As stated above, the Yew is the only British conifer that is likely to prove seriously poisonous to live stock. It has unfortunately caused the death of many horses and cattle, while asses, mules, deer, pigs, rabbits, and pheasants have also been poisoned. Many cases of fatal human poisoning have also occurred. For example, the poisoning of a horse, and of cattle, horses, and deer were mentioned respectively in 1885 and 1893 (Jour. Roy. Agric. Soc.). “Through eating the poisonous leaves of a Yew tree, four cross-bred cattle were found dead in a shrubbery at Elmswell, near Driffield” (Daily Mail, 13th Dec., 1913). Many cases of poisoning and death of animals due to eating the foliage direct from the trees have been recorded in the veterinary journals—e.g. the death of 15 cattle in The Veterinary News, 7th Dec., 1912. Tegetmeier recorded the death of pheasants due to Yew leaves and seeds (The Field, 11th Nov., 1893). On the other hand many cases have been recorded in which fatal results have not followed the ingestion of the leaves, and it appears that the lower branches of Yew trees in parks are constantly cropped by cattle without ill effects. (Trans. Chem. Soc., 1902; Jour. Bd. Agric., 1903).

The bark, leaves, and seeds are all poisonous, the leaves usually being the parts eaten. Henslow states that of 32 cases of Yew poisoning 9 were due to the berries and the remainder to the leaves, while 20 of the 32 cases were fatal. In 1879 R. Modlen published a circumstantial account of children poisoned at Oxford by the fruits, and demonstrated that the arillus or scarlet mucilaginous cup surrounding the ripe seed is not poisonous, but that the seeds are so. Apart from idiosyncrasy of appetite stock are perhaps more likely to eat the dark green foliage of the Yew in winter or during scarcity of green fodder than at other times. The old leaves or shoots are the most poisonous parts.

By experiments with autumn and winter leaves Cornevin found that the following quantities would be necessary to kill various animals:—

Horse0·2lb.per 100 lb. live weight.
Ass and Mule0·16lb.„ „
Cow and Sheep1·0lb.„ „
Goat1·2lb.„ „
Pig0·3lb.„ „
Rabbit2·0lb.„ „

Other authorities, however, quote much lower quantities as necessary to kill the horse and ox, and Pott says that in the case of horses 150 to 180 grammes (0·33 to 0·4 lb.) is sufficient to cause death in one-quarter to one-half an hour, without previous symptoms. Eaten by an animal on a full stomach a small quantity of Yew may cause little or no dangerous results.

Toxic Principle. The researches of Marmé pointed to the alkaloid Taxine (C37H52NO10) as the toxic substance. Later investigations of Thorpe and Stubbs confirmed this view, and it is generally agreed that this substance is the toxic principle, though doubts as to its being the only poison have been expressed. Taxine has a bitter taste, is a heart depressant, and may cause death from suffocation. The yew also contains a considerable quantity of Formic Acid, and the acrid, irritant volatile Oil of Yew.

Thorpe and Stubbs (Trans. Chem. Soc., 1902) found in the fresh leaves of the common Yew from 0·1 to 0·18 per cent. of Taxine. In an investigation of the Irish Yew (T. baccata var. fastigiata) Moss found Taxine present as follows:—

Leaves from female tree, fresh0·596per cent.
Leaves from female tree, fresh0·623„
Seeds from fruit of same tree0·079„
Another female tree—leaves0·323„
Another female tree—seeds0·082„

In the leaves of the male common Yew Moss found 0·082 per cent. of Taxine, or less than one-half the quantity found by Thorpe and Stubbs.

Symptoms. The Yew is irritant and narcotic, and the poison is not cumulative, but on the other hand rapidly effective, so that animals may die apparently suddenly, no previous symptoms having been observed.