Selenium, one of the most interesting of all nutrients, was known for its toxicity before being identified as an essential nutrient. Selenium is important in immune response and as an anticarcinogen. The power of its antioxidant activity has been widely acclaimed. While selenium is not a panacea, there is evidence that selenium can have enormous health benefits for animals and humans.

Selenium was first identified as a toxin in the 1930s. Alkali disease of horses and cattle was shown to be caused by selenium. Signs in the horse included hair loss from the mane and tail, sloughing of hooves, joint erosion, and lameness. Blind staggers characterized by ataxia, blindness, head pressing, and respiratory failure were also thought to be caused by selenium. The mechanism of selenium toxicity is still not clear, but the blocking of sulfur uptake and use within the cells is a likely prospect. The tolerable level has been shown to have somewhat of a wide range, possibly related to differences in availability among sources and other interfering factors in the environment.

Selenosis (alkali disease) has been reported in horses grazing on soils known to be high in selenium. In some cases related to alfalfa hay, it was hypothesized that the alfalfa extracted selenium from deeper subsoils after drought-stimulated extensive tap root development. Selenosis has also been reported occasionally because of overzealous use of selenium supplements and water with a high selenium content.

Tying-up in horses was associated with selenium deficiency in the past, but selenium does not seem to be a major factor in cases of tying-up today, probably because of the widespread use of commercial feeds containing selenium. Reduced reproductive performance and sudden death in horses have also been attributed to selenium deficiency.

Selenium is necessary for the development of the acquired immune system. However, not all classes of antibodies are affected to the same extent by selenium deficiency, and differences in animal species, age, sex, and antigens affect the degree to which antibody production responds to selenium supplementation. The selenium requirement for optimal immune response in the horse is not known. However, in other species, selenium supplementation at a supranutritional level may not be needed to improve immune response. A level of 0.1 ppm selenium can be adequate to prevent white muscle disease and maintain glutathione peroxidase levels in horses, but many factors can influence selenium utilization.