The College of Veterinary Medicine at Washington State University (WSU) recently reported cases of two horses with severe selenium deficiency, which, for both horses, resulted in death.

Selenium, an essential trace element, is naturally found in plants, and horses can eat the selenium they need by grazing. However, soils in some areas of the United States, particularly in the Northwest, are often deficient in selenium, and plants growing in deficient soil are themselves selenium deficient.

Selenium plays an essential role in maintaining a healthy body, including the immune system. Horses with selenium deficiency may show severe muscle cramping, sweating, rapid pulse, brown urine and stiffness, according to Dr. Macarena Sanz, WSU resident veterinarian in equine medicine. According to Dr. Sanz, a horse with severe selenium deficiency can appear in a few different ways. He may be reluctant to walk and sore to the touch. Or he may appear normal but refuses to chew and swallow.

Selenium-deficient horses can experience compromised immune systems or nutritional muscular dystrophy where the muscles break down. “When the heart is affected, the prognosis is very poor,” she said. Additionally, foals with selenium deficiency can be either weak at birth or normal at first, then unable to stand or nurse.

The reason foals may be affected is because the broodmare mare transfers selenium to the foal during pregnancy and then again through milk when the foal nurses, according to Ruth Bishop’s “Understanding Selenium” article in June 2005 Horse & Hound magazine.

Bishop explains that horses usually need just 0.1 mg/kg selenium daily when inactive, then 0.3 mg/kg when in an exercise program or working as a broodmare.

If selenium deficiency is suspected or of concern, horse owners should consult their veterinarians for confirmation by blood test before attempting to supplement on their own because many feeds, mineral blocks and supplements already contain selenium, and as one can see in the paragraph above, the daily selenium intake recommended is a very small amount.

The Ohio State University Bulletin on horse nutrition notes that while selenium is essential for horses for muscle and immune system functions, the mineral is a concern because of  “its narrow range between the requirement and toxic levels.” Too much selenium can be toxic to horses, causing mane and tail hair loss, lameness, hoof problems (including loss of the hoof wall) and, in the most severe cases, blindness and death. A veterinarian can determine if there is a selenium deficiency, and, if so, how best to create a safe and effective supplement program.