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Identifying poisoning signs, sequelae


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.
Taxus spp. (yew, Japanese yew, American yew, English yew, Western yew — Photo 3)

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 essential.

Vascular toxins

  • 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 or shock.

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 stress-resistant.
  • Ergot (Claviceps purpurea) is a fungus that infects the seed heads of many grasses, producing a sclerotium containing the ergopeptides, ergotamine and ergocryptine.
  • 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.


Source: DVM360 MAGAZINE,
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