By Amanda House, DVM ,DACVIM, University of Florida College of Veterinary Medicine

Selenium is a trace mineral that is essential for cellular function in the body. Fortunately, large doses of selenium causing acute toxicity and death are uncommon in the horse. In fact, more often the opposite situation is problematic. Many areas of the United States produce selenium deficient forage including parts of the Pacific Northwest, the Great Lakes, and down the Eastern Seaboard into Florida. The clinical syndrome that results from selenium (and Vitamin E) deficiency is called white muscle disease. White muscle disease is a degenerative disease that affects skeletal and cardiac muscle in foals and other farm animals. Young, fast growing animals nursing from dams fed a diet low in selenium and vitamin E are commonly affected. The primary signs in young animals with white muscle disease are recumbency, fast heart rates, failure to suckle, difficulty swallowing, and discolored (red to brown tinged) urine. Laboratory tests are available to diagnose selenium deficiency.  

Selenium toxicity is more often a chronic condition. Certain “indicator” plants may reveal high levels of soil based selenium (such as locoweed), and are common in areas such as Colorado and New Mexico. As previously mentioned, Florida is typically considered selenium deficient or adequate soil, depending on the region. The chronic signs of selenium toxicity are characterized by hair loss of the mane and tail, cracking of the hooves, and often signs of lameness, excess salivation, and respiratory failure. Severe overdose of selenium can lead to death. In these severe cases, the signs of overdose may include a staggering gait, blindness, labored breathing, respiratory failure, collapse, and muscle tremors. Selenium status in horses can be measured using serum, plasma, or whole blood selenium levels. If you are concerned about selenium levels, consult your local veterinarian for additional information on testing.  

The FDA has set a daily recommended level of selenium for an "average" horse at a total of 3 mg per day. Many different types of feeds and supplements contain selenium. Take the time to read the labels and calculate how much, if any, selenium is contributing to your horse's diet. Know what part of the country your hay comes from and test it on a regular basis. Consult often with your veterinarian or nutritionist when making changes to your horse's diet.

Reviewed by original author in 2016.