Locoweeds are perennial herbaceous plants composed of Astragalus
spp. and Oxytropis
spp. known for poisoning livestock.
These plants contain swainsonine, a toxic indolizidine alkaloid which causes a neurological condition known as 'locoism' in horses. Swainsonine is produced by fungal endophytes that contaminate locoweeds.
Locoweeds are a concern for horses living in the rangelands of the western United States, Asia, and South America. Consumption of locoweeds causes horses to display behavioral disorders, gait abnormalities, and neurological symptoms such as ataxia, difficulty standing, standing abnormally, and leg paresis.
Locoweeds are low or small bushy plants with white, purple, yellow or orange pealike flowers. Leaflets are arranged opposite one other along the stem, with a single leaf extending from the tip of the stem. Seedpods are often inflated.
All parts of the plants are toxic, resulting in locoweed toxicity
if ingested. It remains toxic during all stages of growth, and whether it is fresh or dried. Locoweeds can also accumulate potentially toxic selenium concentrations. Symptoms are usually seen after consumption of large amounts for 2 to 9 weeks. Signs last for several weeks. Young animals are most susceptible. Occasionally horses can recover however it is generally thrifty and will eat locoweed to the exclusion of other forage if returned to infested pasture.