Taxus spp. (yew, Japanese yew, American yew, English yew, Western yew — Photo 3)
Photo 3: Taxus spp. (yew, Japanese yew, American yew, English yew, Western yew) are evergreen ornamental shrubs that have
glossy, rigid, green, linear leaves. The fruits are red to yellow and they contain one seed. They grow best in moist, humid
areas and most poisoning occurs when livestock are allowed access to clippings.
Yew is an introduced ornamental that most commonly poisons animals when clippings are thrown into paddocks. Yew toxicity is
attributed to taxine alkaloids but they also contain traces of cyanogenic glycosides, lignins, flavonoids and volatile oils.
The taxines and taxols alter myocardial calcium and electrolyte balance, resulting in cardiac arrest (negative ionotroph).
All portions of the plant, green or dry, are toxic. Doses as low as 0.1 percent to 0.5 percent of body weight are toxic (just
over 100 mg could be toxic for a 450-kg horse).
Clinical signs are related to toxin disruption of cardiac function. They include colic, sweating, incoordination, shallow
and difficult breathing, muscle tremors, recumbence, diarrhea, bloody diarrhea and death from cardiac arrest. Most poisonings
result in sudden death, with minimal gross or microscopic lesions. Some animals may have non-specific lesions of myocardial
hemorrhage, pulmonary congestion and edema. Diagnosis most often is made by documenting exposure and identifying plant parts
in gastrointestinal contents.
As most cases are fatal, prevention of ingestion of trimmed shrubs is paramount. If possible, treatments should include evacuation
of the GI tract and minimization of absorption with activated charcoal and cathartics. As poisoned animals are likely to have
compromised cardiovascular function, precautions are needed to avoid stress and minimize animal exertion.
Persea americana (avocado)
Avocado leaves, fruit, bark and seeds contain persin and an unidentified cardiac toxin. Persin produces a non-infectious mastitis
and agalactia. Signs of poisoning in lactating mares are mastitis with occasional gastritis and colic. Other signs related
to cardiotoxin-induced heart failure include dsypnea, coughing and increased respiratory and heart rates. Other less-common
changes include cardiac arrhythmias, edema of the neck and ventral abdomen, cyanosis, anorexia, general weakness and recumbency.
Post-mortem findings include microscopic lesions in the heart, with generalized edema and congestion of many organs.
As damage may be fatal and many of the cardiac changes are irreversible, preventing a horse's access to avocado groves is
Zigadenus spp. (death camas)
Death camas contains a steroidal alkaloid called zygacine. Vasoactive zygacine causes arteriole dilation and venous constriction.
This floods vascular beds (hypotensive effect), resulting in reduced venous return and, if severe, cardiovascular collapse
Though poisoning in horses is rare, the signs of poisoning are likely to be similar to those seen in sheep, including depression,
staggering, profuse salivation, papillary constriction, decreased respiration and heart rates, weakness, dsypnea, prostration
and death. Post-mortem lesions are those of hypotensive shock. As most animals are found dead, few treatments have been reported.
Ergot and fescue
Photo 4: Festuca elatior or F. arundinace (tall fescue) is among the most common cool-season pasture grasses in North America
and many other countries with temperate climates. Nearly all tall-fescue pastures planted before 1980 are infected with toxin-producing
Neotyphodium coenophialum, a microscopic fungus. Endophyte-infected fescue has a growth advantage, because it is more drought-and
- Ergot (Claviceps purpurea) is a fungus that infects the seed heads of many grasses, producing a sclerotium containing the ergopeptides, ergotamine
- Tall fescue (festuca elatior or F. arundinacea — Photo 4) often is infected with the endophyte [Neotyphodium (Acremonium) coenophialum]. The endophyte produces ergot alkaloids such as ergovaline.