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When eaten, wilted leaves and seeds of certain pistachio trees are toxic to horses, according to researchers at the University of California, Davis.

In a recent case that involved the death of five mares in California, veterinarians and toxicologists identified three species of pistachio trees that had been felled and left in a pasture for horses to consume. These species included Pistacia atlantica, P. terebinthus, and P. chinesis, which should be distinguished from P. vera, the most common species of the Pistacia genus. P. vera bears the familiar pistachio nut, used in culinary pursuits, whereas the aforementioned species produce much smaller seeds with softer shells. The owner witnessed the deceased horses eating from the downed trees.

To determine if the trees were poisonous, toxicologists crafted a study* that revealed disintegration of red blood cells exposed to leaf and seed extracts in the laboratory setting. With this information, veterinarians deduced that horses likely die from acute hemolytic anemia following ingestion of pistachio leaves and seeds.

Horses with pistachio toxicity exhibit colic of varying severity, incoordination, pale mucous membranes, lethargy, and inappetence.

Pistachio trees thrive in desert conditions, so they are generally found in regions that have long stretches of hot weather, such as California and Arizona in the U.S.

This case of pistachio poisoning brings to light an oft-overlooked aspect of horse management: identification of all trees within or surrounding a horse habitat. Other trees are known to cause problems in horses, such as red maples, sycamore maples, and oaks.

“Without question, horse owners should take an interest in the plants and trees that their charges have access to,” said Catherine Whitehouse, M.S., a nutrition advisor for Kentucky Equine Research (KER). “Identifying and researching trees, shrubs, and plants can be difficult but, ultimately, it’s an important part of horse care. Horse owners wouldn’t dream of feeding their horses unidentified grains, so the same care should be taken with native plants.”

*Bozorgmanesh, R., K.G. Magdesian, D.M. Rhodes, K.A. Von Dollen, K.M. Walter, C.E. Moore, B. Puschner, L.W. Woods, K. Torrisi, and E.D. Voss. 2015. Hemolytic anemia in horses associated with ingestion of Pistacia leaves. Journal of Veterinary Internal Medicine. 29:1-410-423.

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