Yew Toxicology in Domestic and Wild Species
History and General Information
- Yew is known as the “tree of death,” dedicated to the gods of death
- Yew leaf extracts were frequently used for murder and suicide
- Caesar writes about Catuvolcus, one of the kings of Eburones, who chose death from Taxus baccata rather than be taken prisoner
- Yew is now used as an ornamental shrub and frequently used for Christmas wreaths
Species and Distribution – Taxus sp. most common varieties and their distribution
Taxus baccata- European Yew
Taxus cuspidata- Japanese Yew – most common ornamental shrub in US and Canada
- Distribution- Widespread due to cultivation in landscape architecture and design
- Japanese and European Yew are both imported species that have become well established in US as ornamental shrubs
- Both grow as shrubs and never grow larger than 20 feet
- Thought to be the two most toxic species
Taxus brevifolia- Western Yew
- Distribution- Western US and Canada, ranging from California to Montana and to Alaska
- Grows as an evergreen tree with drooping branches ranging in height from 15 to 75 feet
Taxus cannadensis- American Yew or Ground hemlock
- Distribution- mid-western and northeast US, ranging from Kentucky to Minnesota and to Maine
- Grows as spreading shrub, ranging in height from 3 to 5 feet
- Browsing by white-tailed deer is thought to have led to selection for spreading growth over the arboreal form (escape browsing below snow pack)
Yew Anatomy
- Darkest green of all evergreen shrubs
- Leaves are dark green dorsally and pale green to yellow-green ventrally (pictured left) with a prominent mid-rib
- Alternate, stiff, flat to needle-like leaves
- .5 to 1 inches long
- Fruit is bright red, ovoid, fleshing cupped berry (aril)
- Aril surrounds a single small brown seed
- Chemical composition – hundreds of distinct molecules have been isolated from Yew sp. (mostly flaveniods and toxins)
- Hydrocyanic (HCN) esters
- Ephedrine
- Taxol
- Oil of Yew
- Taxines- Taxine A, B, C
- Others – taxicatin, taxicin I and II, taxiphyllin, taxiresinol, iso-taxiresinol, taxusin, taxinine A, B, E, H, J, K, and L, anhydrotaxininol
- Oil of Yew
- Intestinal irritant responsible for colic and diarrhea symptoms of yew poisoning
- Found in sap of yew
- Taxines (generic pictured left)
- Non-irritating, diternepoid alkaloid
- Taxine A and B are most the most common alkaloids (taxine B being the most abundant)
- Responsible for Yew deaths
- Found in all parts of the plant except aril
Yew Toxicology
- Yew are toxic to all animals to varying degrees
- White-tail deer and certain seed eating birds are much less susceptible
- Yew are toxic all year round
- Yew are more toxic later in the year due to a build up of toxins
- Most cases of yew poisoning are seen later in the year because of the scarcity of food and the enhance plant toxicity
- Yew is always poisonous
- Fresh and dried yew are both toxic
- Yew eaten directly from plant is as toxic as yew clippings
- Health of plant does not seem to significantly reduce toxicity (green yew is as toxic and brown yew)
Yew Poisoning & Treatment
Two Syndromes associated with Yew Poisoning (both caused primarily by taxine A and B)
- Acute Syndrome
- Symptoms- Death.
- Animals are frequently found dead next to yew bushes
- Death usually follows 1 to 3 hours after ingestion
- Onset of acute syndrome is rapid – Animal will appear normal, then unexpectedly gasp a few times and die
- Cause of death is cardiac arrhythmia
- Taxine acts as a cardio-depressant
- Taxine inhibits sodium and calcium currents, blocking myocardial conduction
- Heart suddenly stops in diastole
- Subacute Syndrome
- Symptoms – Ataxia, diarrhea, hypotension, colic, hypothermia, coma, seizures, weakness, respiratory failure, bradycardia and sudden death
- Animals (usually cattle) die within 24 to 48 hours after ingestion
- Survival without treatment is possible but occurs infrequently
Severity of yew poisoning depends on:
- Health status of animal- Sick animals seem to be more susceptible to acute syndrome
- Age of animals- Young animals are more prone to acute syndrome
- Amount of yew consumed- The more yew that is eaten, the more severe the poisoning is
- Type of animal poisoned- Monogastrics are more susceptible to acute syndrome
- English yew is lethal to ruminants at around 0.5% of the animal’s body weight
- English yew is lethal to monogastrics at around 0.1% of the animal’s body weight
Diagnosis – Finding fragments of yew leaves and twigs in the mouth, stomach and intestines
- In cases where no yew detritus is found in GI tract diagnosis of yew poisoning may be determined by GC/MS of stomach or rumen
Necropsy
- No pathogenic lesions
- Exception – In cases of animals dying subacutely, there is a mild inflammation of the upper intestinal tract
- Inflammation is due to the action of the irritant oil of yew
- In grams per pound BW
- Horse0.9
- Ox 4.5
- Sheep4.5
- Goat 5.5
- Pig 1.4
- Total Fatal Doses (in grams)
- Horse100-200
- Ox~500
- Pig 75
- Dog 30
- Fowl30
Typical case of yew poisoning
- “Three cows from a herd of 14 crossed a cattle guard into a driveway lined with yews and consumed some of the branches. Two of the cows died suddenly and the third died a few hours later after showing signs of nervousness, trembling and ataxia. No gross lesions were seen at necropsy, but large quantities of yew leaves were present in the ruminal ingesta. The bushes were later identified as T. cuspidata.”
----Veterinary Medicine – Small Animal Clinician, Sept. 1984.
Treatment of yew poisoning
- No treatment for acute syndrome
- Aggressive decontamination of stomach using activated charcoal and a cathartic (MgSO4)
- Rumenotomy and removal of rumen contents
- Administration of atropine sulfate to counteract bradycardia
- Problem – atropine slows gastrointestinal peristalsis and prolongs the elimination of the ingested toxic plant
- Therefore, treatment with atropine but must be done judiciously
Deer resistance to yew poisoning
- Anecdotal evidence for white-tail deer resistance to yew poisoning
- Newspaper articles about people in Cayuga heights complaining about local deer eating their ornamental yew bushes
- Occasional references to white-tail resistance in yew toxicology articles
- Unrecorded experiment at Vet School about a white-tail deer being fed yew ad libidum without any detrimental effects
- Not all deer are resistant
- Dutch article indicating that fallow deer are susceptible to yew toxicosis
Bibliography
For this project I have compiled over 30 articles concerning every aspect of yew biology and toxicology. These are the most salient articles in regard to this presentation.
Hare, W.R. (1998). Bovine Yew (Taxus spp.) Poisoning. Large Animal Practice January/February: p. 24-28
Lang, D. G., Smith, R. A., and Miller, R. E. (1997). Detecting Taxus Poisoning Using GC/MS. Veterinary and Human Toxicology 39 (5): p.314
International Yew Resources Conference: Yew (Taxus) Conservation Biology and Interactions. 12-13 March, 1993. Berkeley, California, USA.
Kingsbury, J. M.: Poison Plants of the United States and Canada. Prentice Hall, Englewood Cliffs, NJ. 1964; p. 121-123
Clarke, E.G.C and Clarke M.L.: Garner’s Veterinary Toxicology. 3rd ed. Williams and Wilkins Co., Baltimore, MD. 1967; p.399-401
Kerr, L.A., Edward W.C. (1981). Japanese yew: a toxic ornamental shrub. Veterinary Medicine – Small Animal Clinician September: p.1339-1340
[History and General Information]
[Species and Distribution of Yew]
[Yew Anatomy]
[Yew Toxicology]
[Yew Poisoning & Treatment]
[Deer Resistance to Yew Poisoning]
[Bibliography]