UC Davis vets warn horse owners about the dangers of pistacia trees

Pistacia seeds and leaves, which includes the pistachio, can be fatal if ingested by horses

UC Davis veterinarians are sending a call-out to equine owners to keep their horses away from Pistacia orchards. According to UC Davis, the leaves and seeds can cause hemolytic anemia, which can be fatal if ingested.

UC Davis veterinarians learned about this in the fall of 2013, when five mares died suddenly. Two of the mares were brought to UC Davis veterinary hospital, and were found to have degrees of colic, ataxia, pigmenturia, pale and icteric mucous membranes, lethargy and inappetance. All died after 48 hours.

According to UC Davis:

“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. atlanticaP. terebinthus, and P. chinensis) following the fall harvest. (The most common species of the Pistacia genus is, of course, 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 by 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, or other plants associated with oxidant damage or hemolysis in horses. 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 observed to ingest 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.”

“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,” Bozorgmanesh said. “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.”

UC Davis toxicologists did tests and revealed that extracts of the seeds and leaves induced lysis of horse red blood cells in the laboratory setting. More research is needed 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 said.

Since her discovery,  Bozorgmanesh became the lead author of a study called “Hemolytic Anemia in Horses Associated with Ingestion of Pistacia Leaves.”

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