Horses can safely eat many fruits and vegetables, but in the case of some innocent-seeming produce, the ingestion of even a few pieces can cause a serious toxic reaction. One such fruit is the avocado, a large berry that is found in some southern states as well as in Mexico and other Central American countries.

The fleshy dark-green fruits grow on trees and are cultivated commercially in areas with tropical climates. When the trees grow where animals are pastured, or when grazing is allowed in an avocado orchard, horses may eat the fallen fruit and become seriously ill. Some affected horses die, even with supportive veterinary care.

Persin, a substance derived from fatty acids contained in avocados, is believed to be the toxin responsible for health problems in horses and other animals that eat the fruit. In horses that ingest avocados, the toxin can cause colic, irregular heartbeat, respiratory distress, neurologic dysfunction, edema, and other signs of illness.

Horse owners should not feed avocados to their horses, and should not allow the animals to graze in areas where they might have access to avocados. As well as the fruit, avocado tree leaves and bark are said to be harmful, and horses turned out on poor pasture might be tempted to eat these parts of the tree if they don’t have access to other forage.