"Locoweed" is a group of plants with over 300 species that grow in the U.S. Not all of them are toxic. However, given their dire effect on horses, one should assume that any species of locoweed is toxic until proven otherwise. The most lethal species grow in the American West and Southwest.

These plants grow to about 12" in height and have flowers that are purple blue, yellow or white. All parts of the poisonous varieties of this plant contain the toxin swainsonine, which interrupts the metabolism of certain sugars in cells. This causes the buildup of these sugars in the cells, causing dysfunction. The cells typically affected are within the brain.

Signs of locoweed poisoning include incoordination, apparent blindness and confusion, weight loss, depression and poor appetite, among many others.

A horse must eat a large quantity of these plants over days to weeks to show signs of poisoning. Once horses show signs of poisoning, the prognosis is poor for return to function. Once affected, horses are also more susceptible to poisoning in the future.

In the Western U.S. "locoism" is most common in the late spring in dry years. These plants have a long taproot and may be the only green plant on pasture.

Unlike many other toxic plants, some horses develop a taste for locoweed and must be removed from pastures containing this plant.

There is no treatment for toxicity once it takes place, other than removing access to the plants. Usually, severely affected horses never fully recover.