Horse owners warned of pistachio family’s deadly autumn drop

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Beautiful but deadly: Pistacia terebintha flowers.
Beautiful but deadly: Pistacia terebintha flowers. © greffer.net; CC BY 2.5

Horses should be kept away from Pistacia orchards as autumn nears, as the leaves and seeds of this genus – from the same family as pistachio nuts – can be fatal to horses.

Five mares from a large herd in California died from hemolytic anemia in the autumn of 2013 after ingesting the genus, and veterinarians at UC Davis treated two surviving mares after two days of lethargy and icterus (jaundice).

All of the horses who died had varying degrees of colic, ataxia, pigmenturia, pale and icteric mucous membranes, lethargy and inappetance. All died within 48 hours of the initial signs.

Dr Rana Bozorgmanesh
Dr Rana Bozorgmanesh

While UC Davis’ Equine Medicine Service worked to save the two mares, one of the veterinarians on the case, resident Dr Rana Bozorgmanesh, started researching the cause of the deaths. She and the service team, along with members of the toxicology department, discovered that the sick horses had access to a planted Pistacia orchard (containing the species P. atlantica, P. terebinthus, and P. chinensis) following the fall harvest. (The most common species of the Pistaciagenus is P. vera, or the pistachio nut.)

There were no recent changes in herd management or housing, except for the felling of the Pistacia orchard shortly before the first horse developed clinical signs; the owner had witnessed the horses eating from trees that had been cut down. Other horses on the property that were not allowed access to the orchard did not exhibit any signs of illness.

A site visit Dr. Bozorgmanesh, along with veterinary students and staff, was performed to inspect the property for possible toxin exposure. Sampling of the water, hay, trees, and vegetation was conducted to investigate potential intoxication as the cause of hemolytic anemia, usually associated with an oxidant toxin such as maple trees, onions, and other plants. None of these were found on the property.

The owner was instructed to move the herd away from access to Pistacia trees and to administer activated charcoal to horses seen eating Pistacia leaves or seeds. Upon moving the mares to a smaller area of the property with no access to Pistacia trees, there were no additional illnesses or deaths.

Pistacia trees are not native to North America, but are found in California, as well as several other states ranging from the Southwest to the Southeast of the United States, where fertile land produces much of the world’s marketable Pistacia products. The California outbreak arose in the fall, the same seasonality as reported with red maple leaf toxicosis, when the leaves are wilted and falling off the trees. A similar incident with Pistacia occurred in Arizona, also at a time when leaves were falling off the tree.

Pistacia chinensis showing its autumn colors. © USDA Agriculture Research Service
Pistacia chinensis showing its autumn colors. © USDA Agriculture Research Service

“While the horses had access to the trees throughout the remainder of the year, we propose the problem lies with the felled and wilted leaves,” said Bozorgmanesh, who ultimately became the lead author on this groundbreaking clinical research.

“The felled trees in the California outbreak would have allowed for easy access and ingestion of large quantities of wilting leaves and seeds by the horses, thus accentuating these effects.”

In vitro studies by UC Davis toxicologists showed extracts of the seeds and leaves induced lysis of horse red blood cells in the laboratory setting. Further research is required to identify the exact pathophysiology of Pistacia tree toxicosis, the toxic principles involved, and the quantities required to cause clinical disease in horses.

“Until that time, it is clear that horses must be isolated from these trees to prevent acute hemolytic anemia and death,” Bozorgmanesh added.

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