Selenium from the soil is absorbed by plants, and when livestock eat those plants, Se toxicity may develop. Selenium toxicity was first reported in farm animals in China in the 13th century, but the first documented report in the United States occurred during the 1850s in South Dakota. Acute selenium toxicity is often called blind staggers, although the affected livestock are not always blind nor staggering. Recent research has shown that blind staggers is sometimes associated with high sulfur intake and is not always due to Se toxicity. Typical symptoms of blind staggers include head pressing, perspiration, blindness, abdominal pain, colic, diarrhea, increased heart and respiration rates, and lethargy.
Death can occur quickly.
Seleniferous soils are widespread in the Rocky Mountain and Great Plains regions of the western United States. Although these soils exist throughout Colorado, most Colorado soils are not seleniferous. The primary source of Se is weathering of rocks that contain Se, especially shales. Excessive irrigation can lead to dissolution of these rocks and speed up weathering. In addition, mining increases the potential for Se toxicity through exposure of seleniferous rocks to air, which leads to the solubilization of the Se.