12/19/2002 Entry: "Steer Clear of Poisonous Plants"

April 1998

Published by HorsePlay

Raymond Hyde, DVM

Charles Sr. was enjoying farm-sitting for his son, who was away on vacation with the grand kids. The elder Charles loved to work in the garden, so he decided to weed and trim his son's hedges. Seeing that the family's horses did not have much forage in the pasture, he threw the clippings and weeds over the fence to them. "Why let them go to waste?" he thought. The herd of 30 horses greedily consumed the unusual offerings.

A short while later, Charles noticed several of the horses acting restless and shaking; some were having difficulty breathing. Within minutes, a few were stumbling and even falling. Responding to his frantic call, I arrived shortly and was greeted by a ghastly sight: seven horses already dead and 10 more stricken. Upon examining the shrubbery, I spotted the trimmed Japanese yew--one of nature's deadliest plants.

Japanese and other varieties of yew are evergreen shrubs with small red olive-like berries. Fresh or dried, all parts are extremely toxic to all domestic animals (including people). A compound called taxine attacks the heart, causing abnormally slow rhythms and heart failure. Since there is no antidote to its poison, all I could do was give mineral oil and charcoal by nasogastric tube and atropine intravenously to bring the heart rate up and hope for the best, knowing only a handful of yew can kill a horse. A total of 11 horses were lost. Could this happen to you? Many plant poisonings of horses involve the unintentional feeding of ornamental plant clippings or fallen leaves or arise when horses escape their enclosures and dine on flowers and shrubbery. Plant poisonings are more likely to occur when more palatable grasses and forage are overgrazed or depleted by drought. Horses may then be forced to consume the more noxious plants if adequate hay is not provided. In some cases, horses actually have developed a taste for abnormal plants.

Below are descriptions of a few plants that are poisonous to horses.

What To Look For
Bracken fern is one poisonous plant often ingested when forage is scarce. Bracken grows in moist, sandy soils of open fields and woods. Not very palatable, horses normally won't eat it. Bracken's poison causes destruction of horses' stores of thiamin (vitamin B1)--an essential vitamin. As a horse's thiamin levels are depleted, incoordination develops, then the horse becomes reluctant to move; he loses his appetite, develops tremors and later severe spasms, his heart rate slows, he falls and can't rise. Convulsions and death sometimes result.

Early treatment of large intravenous doses of thiamin by your veterinarian often can save a horse with bracken poisoning.

Like bracken fern, horsetails and scouring rushes, sometimes found in hay, also contain thiaminase toxin. These plants grow up to three feet high and are found in damp places all across the United States. Unlike bracken fern, these plants don't cause appetite loss.

In early spring, before lush grass comes in, horses often consume sprouting weeds, some of which are toxic. Cockleburs are a prime example, causing liver disease and appetite loss.

Several kinds of trees also are poisonous to horses. Choke cherry, wild black cherry, and pin cherry are common--and toxic--wild trees. Their leaves and bark contain amygdalin, a material that converts to cyanic or prussic acid as leaves are crushed or wilt, which causes cyanide poisoning when ingested. (The same toxic principle occurs in Sudan grass, Johnson grass, sorghum, and some other plants if not harvested early enough). Cherry often causes poisonings when a tree or branch falls into a horse pasture; the branch wilts, and the horses eat the bark or leaves. Signs of toxicity often are seen within 20 minutes; they include apprehensive behavior, rapid and labored respiration, weakness, stumbling, spasms and convulsions, coma, and death.

Death can be sudden, so immediate treatment by your veterinarian is critical. He will probably inject a combination of sodium thiosulfate and sodium nitrate to rescue the hemoglobin from the cyanide. Some horses who are found dead following storms and are presumed to have been killed by lightning are sometimes actually victims of cherry poisoning.

Red maple trees are popular for landscaping. Their leaves--brilliantly red in the fall--are highly toxic to horses. The toxin causes rapid destruction of red blood cells, which causes anemia, kidney damage, and death. The urine of a horse poisoned by red maple leaves may appear red or coffee-like, due to the hemoglobin passing through the kidneys. Do not plant red maples near horses, and rake their leaves away from livestock.

Diagnosis, Treatment, And Prevention
Plants can contain multiple poisons, and concentrations depend on climatic and soil conditions. Symptoms of poisoning therefore can vary and be nonspecific. A good rule of thumb is that if several horses have a sudden onset of illness with no apparent cause, consider plant poisoning. Commonly associated signs include central nervous system agitation or depression, digestive upset, dermatologic problems, rapid loss of weight, accelerated or slowed heart rates, and a straining to defecate. Weakness, collapse, seizures, coma, and death can be seen in severe cases. The severity of the poisoning depends on the toxin concentration in the plant, the amount eaten, the time span over which it was eaten, and the sensitivity of the horse. Both acute (sudden and severe) and chronic (long-term) poisonings can occur. Treatment for plant poisonings depends on whether the culprit can be identified in the pasture or hay or--if the horse dies--in necropsy findings. This information could be used to treat remaining herd members and prevent re-exposure.

To avoid plant poisoning, become familiar with poisonous plants--identify and remove them whenever possible. Consult your extension agent for identification and control measures. Mow fields regularly to keep most weeds from growing. Check along fence rows after storms for downed branches and trees. Do not feed ornamental clippings, weeds, or flowers. If you suspect a plant poisoning, immediately call your veterinarian, who might be able to make a diagnosis, give an antidote and life saving, supportive care.