Locoweed Poisoning in Horses: Neurologic Signs and Grazing Risk

Locoweed poisoning, sometimes called locoism, is a toxic plant exposure that affects the nervous system and other body systems in horses. It develops after a horse eats certain Astragalus or Oxytropis species that contain swainsonine, a toxin that disrupts normal cell function and can lead to an acquired storage disease. Horses are considered particularly sensitive to this toxin.

This problem is most often linked to grazing in the western United States, where locoweed grows on rangeland and dry pastures. Some horses avoid the plant, but others develop a taste for it and may keep seeking it out. Risk rises when forage is sparse, pastures are overgrazed, or toxic plants are mixed into hay.

The course is usually chronic rather than sudden. That means signs often build over days to weeks instead of appearing all at once. Early changes can be subtle, like dullness, weight loss, or mild incoordination. With continued exposure, horses may develop more obvious neurologic problems, trouble eating and drinking, reproductive losses, and a higher risk of injury.

Locoweed poisoning is caused by eating toxic species of Astragalus and Oxytropis that contain swainsonine. This toxin interferes with the enzyme alpha-mannosidase, which leads to abnormal material building up inside cells. In horses, the nervous system is especially vulnerable, which is why gait changes and behavior changes are so common.

Exposure usually happens on pasture or open range, but it can also happen through contaminated hay. Horses may be more likely to eat locoweed when desirable forage is limited, after drought, during early spring growth, or in overgrazed fields. Some horses appear to keep returning to the plant once they start eating it, which can make management harder for pet parents.

Not every plant called "locoweed" is equally toxic, and plant identification can be tricky. That is one reason your vet may ask for photos, pasture samples, or a fresh plant specimen. In some regions, locoweed may also be confused with other toxic range plants, so the grazing history matters as much as the physical exam.

Your vet usually diagnoses locoweed poisoning by combining the history of exposure, the horse's neurologic and physical exam findings, and evidence that locoweed is present in the pasture, range, or hay. There is no single perfect test for every case. Because many other conditions can cause ataxia, weight loss, or behavior changes, your vet may also work through a list of look-alike problems.

Testing often starts with basic bloodwork to check overall health and rule out other causes of weakness or poor condition. In some cases, specialized testing for swainsonine or changes linked to alpha-mannosidase inhibition may be discussed, but timing matters because the toxin does not stay detectable for long after exposure stops. Plant identification from the field or hay source can be very helpful.

If signs are severe or the diagnosis is uncertain, your vet may recommend a more complete neurologic workup, toxicology consultation, or referral. Diagnosis can also become clearer over time if the horse improves after strict removal from the source. Even then, recovery may be incomplete in horses with more advanced nerve damage.

Prevention starts with pasture awareness. Walk grazing areas regularly, especially in the western U.S., and learn what locoweed looks like in your region. If you are unsure, ask your vet, local extension service, or an experienced range specialist for help with identification. Horses should not be turned out on pastures where toxic locoweed is abundant, especially when other forage is limited.

Good grazing management matters. Avoid overgrazing, provide enough safe forage, and be cautious during seasons when locoweed is one of the first green plants available. Check hay for weeds and buy from reliable sources when possible. If one horse has been eating locoweed, consider evaluating the whole group because herd mates may also be at risk.

If locoweed is established on your property, talk with your vet and local land or weed professionals about a realistic control plan. That may include restricting access, improving desirable forage, and using region-appropriate weed management. Prevention is especially important because there is no specific antidote, and some horses do not fully recover once neurologic injury has developed.