Yew Poisoning in Horses and Ruminants

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Table of Contents

1. Introduction
2. Clinical Signs
3. Identifying Yew Evergreens
4. Take-Home Messages
5. References

Introduction

The genus Taxus consists of three commonly grown ornamental shrubs: English yew; Canada yew, a native shrub; and Japanese yew. The needles and seeds of all yews are highly poisonous to horses and other livestock (Figure 1). However, the red fleshy seed covering is not poisonous. Wild deer, moose and elk browse on yews as winter food and are not affected by the yew toxin.

Yew species contain a number of toxic alkaloids, the most toxic of which are taxine A and B. They are collectively referred to as 'taxine.' Taxine inhibits normal sodium and calcium exchange across myocardial (heart muscle) cells. This depresses the electrical conduction across the heart and results in arrhythmias (abnormal heart rhythm). Horses are commonly poisoned when they wander into areas around the house or barn where they eat yews that are often used for landscaping. Livestock are accidentally poisoned when yew trimmings, or wreaths made of yew, are thrown either onto manure piles or over the fence where they are easily accessible to horses or cattle.

Consumption of as little as 0.05% of body weight (0.5 lbs.) of either fresh or dried yew is all that is needed to kill a 1000-lb horse.

Clinical Signs

Sudden death, often within two hours of ingestion, is the most common observation with yew poisoning. Other causes of acute death include: poison hemlock, botulism, ionaphore toxicity (monensin - Rumensin, lasalocid - Bovatec) and red maple leaf poisoning. Prior to death, muscle trembling, incoordination, nervousness, difficulty breathing, slow heart rate, vomiting, diarrhea and convulsions may be observed. No post-mortem signs are specific to yew poisoning, unless partially digested leaves are found in the stomach.

Identifying Yew Evergreens

Yews grow as shrubs or medium-sized trees. The native species include: the Canada yew (Taxus canadensis), a native shrub in eastern Canada; and the Western yew (Taxus brevifolia), a medium tree up to 20 metres high found on the west coast. Various yews are common as landscape ornamentals and include: the English yew (Taxus baccata), a tree up to 25 metres high, and Japanese yew (Taxus cuspidata), a small tree. All are poisonous to horses.

Yews are evergreens, with soft, flat, abruptly-pointed, needle-like leaves that are 15 to 30 mm long. The upper surface is dark green with a lighter green underside. Leaves are arranged spirally or in a flat plane. Most cultivated varieties are compact and the plants retain a dense character with age.

Young twigs are green, becoming greenish-brown or reddish-brown with age. They are flexible. The bark is thin and scaly and varies from dark reddish brown to purplish brown.

Yew seeds are hard, dark brown/blue, nut-like in shape and about 8 mm in diameter. Seeds are set inside an attractive, red, fleshy fruit (aril) that can be found on the underside of recent twigs (Figure 2).

Yews range from shrubs up to medium sized trees 25 metres high. They can live to several hundred years of age. With age, the trunk can become twisted and fluted with an irregular shaped canopy, often with multiple tops, especially in the absence of pruning.

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Take-Home Messages

Yews are planted for many landscape purposes. They can withstand trimming and shaping and are easily transplanted (Figure 3). They can tolerate urban pollution but not road salt. Yew trees and shrubs should not be planted near horse pastures or areas of the property where horses or other livestock can nibble while pastured, ridden or exercised, e.g. around the farm house. Yews growing wild should be removed from horse pastures or wooded pastures.

Evergreen branch clippings of yew are sometimes used to make Christmas ornaments, such as wreaths and hanging decorations.

Wreaths of yew should never be hung on barn doors or on fence posts where horses or ponies can reach them (Figure 4).

Yew ornaments or hedge clippings of yew should never be discarded onto manure or compost piles where they are accessible to horses and/or other livestock.

References

Dirr MA. Manual of Woody Landscape Plants: Their Identification, Ornamental Characteristics, Culture, Propagation and Uses, 5th ed. Champaign, IL: Stipes Pulbishing, 1998.

Farrar JL. Trees in Canada. Markham, Ontario: Fitzhenry & Whiteside Limited, 1995. ISBN 1-55041-199-3

Knight AP, Walter RG. A Guide to Plant Poisoning of Animals in North America. Jackson, Wyoming: Teton New Media, 2001.

Related Links

Further information on plant poisonings can be obtained from the Canadian Poisonous Plants Information System
Munro Derek B. Canadian Poisonous Plants Information System

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