Pistachio leaves potentially fatal to horses that ingest them

Horses and pistachio tree leaves don't mix—especially in the fall.

Leaves and seeds of Pistachia genus trees can be fatal to horses that ingest them. University of California equine veterinarians do not know the amount of pistachio leaves it takes to affect a horse's health, and are advising horse owners to restrict access to all species of Pistachia.

Rana Bozorgmanesh of the UC Davis Equine Medicine Service said veterinarians became aware of this issue in the fall of 2013, when two mares with jaundice and lethargy were brought to the facility. The owner told her that five other mares in the herd previously died after showing varying degrees of the same symptoms. All died within 48 hours of initial signs.

Bozorgmanesh, in a phone interview, said she suspected ingestion of toxic materials and visited the property to determine the source. Water, feed and vegetation were all sampled in the investigation. She said the horses likely had eaten some plant material that was causing disruption of blood cells' ability to deliver oxygen. This is usually caused by an oxidant toxin found in maple tree leaves, onions or other plants associated with oxidant damage, she said.

When none of those plants were found on the property, they looked at horses' access to recently downed pistachio trees, as the owner witnessed the horses eating leaves from the trees. Other horses on the property that did not have access to the downed pistachio trees had no signs of illness.

Toxins in the pistachio trees can cause a decrease in the oxygen-carrying capacity of blood cells, leading to jaundice, lethargy and kidney disease, Bozorgmanesh said. Acute signs can show up 24-48 hours after ingestion.

Studies by UC Davis toxicologists showed extracts of the seeds and leaves caused the breakdown of red blood cells in horse blood samples.

The horses had access to the pistachio trees throughout the year, but Bozorgmanesh said the problem stems from the wilted leaves. The felled trees in this case allowed for easy access and ingestion of large amounts of wilting leaves and seeds by the horses, compounding the effects of the toxins.

"We propose the problem lies with the felled and wilted leaves that allowed for easy access and ingestion of large quantities," she said.

"Try to avoid access as much as possible; don't turn out where trees are hanging over fences. Fall is a critical time, so watch wind-blown leaves," Bozorgmanesh said. She said she plans further research to identify the exact cause of pistachio tree toxicity, the toxic principles involved and the quantities required to cause clinical disease in horses.

This was not the first time pistachio leaves or seeds have caused animal losses. In 2012, dairy cows and heifers from three California dairies died as a result of eating feed containing pistachio shells. The materials were not toxic, but caused colon, abomasum and small intestine impactions. Deaths were due to intestinal rupture or ulcers that lead to peritonitis at sites of pistachio shell impaction.

The ration for the cows called for pistachio hulls, which can be used as part of the ration, but the load also contained at least 15 percent shells.

(Cecilia Parsons is a reporter in Ducor. She may be contacted at ceciliaparsons8@gmail.com.)