Veterinarians at the University of California, Davis (UC Davis), are warning owners to keep their horses away from Pistacia orchards this autumn, as the leaves and seeds of this genus can cause hemolytic anemia (when the body's immune system attacks and kills its own red blood cells) and be fatal if ingested.

In the fall of 2013, following the acute deaths of five mares from a large herd, two surviving mares were brought to the UC Davis veterinary hospital after two days of lethargy and icterus (jaundice). The deceased horses had varying degrees of colic, ataxia (incoordination), pigmenturia (urine discoloration), pale and icteric mucous membranes (a yellowish tinge to their gums and sclera, the white around the eye), lethargy, and inappetance. All died within 48 hours of initial signs.

While UC Davis’ Equine Medicine Service worked to save the two mares, one of the veterinarians on the case, resident Rana Bozorgmanesh, BVSc, started researching the cause of the other 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 Pistacia genus 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.

Bozorgmanesh, along with veterinary students and staff, visited the site to inspect the property for possible toxin exposure. The team collected samples of the water, hay, trees, and vegetation 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