Selenium, in conjunction with vitamin E, is necessary for proper functioning of the immune system and to protect the integrity of cell membranes. However, there is a delicate balance between too little selenium and too much.

"Because selenium can be toxic if fed in too great a quantity, many people are unwilling to feed it to their horses," says Dr. R. D. Scoggins, retired equine Extension veterinarian at the University of Illinois College of Veterinary Medicine at Urbana. "But in the proper
quantities, it is very safe and effective.

Selenium is a mineral found in the soil in many areas of the country. Horses normally ingest selenium while foraging. In other areas–the Midwest for example–selenium is deficient in the soil, and without proper supplementation horses can have significant problems.

"Clinically, selenium deficient horses will often ‘tie-up,’ a degenerative condition of the muscles also known as rhabdomyolysis," says Dr. Scoggins. "It can affect the heart muscle, the muscles of respiration, as well as the large muscles of the back and limbs. It can also cause a decrease in the efficiency of the immune system, leading to opportunistic infections."

A horse that has rhabdomyolosis will have severe muscle cramps resulting in sweating, stiffness, and increased pulse. The breakdown of muscle cells can result in coffee-colored urine. "Do not walk a horse that is tied up," stresses Dr. Scoggins.

While selenium deficiency is not related to the time of year or the horse’s sex, it is especially a problem in growing horses. Detection of selenium deficiency can be done with a simple blood test that can be run inexpensively at many veterinary diagnostic laboratories.

Dr. Scoggins does not recommend routine testing for selenium deficiency or toxicity unless it is suspected based on clinical signs.

"Many different sources of selenium are available for supplementation," says Dr. Scoggins. "The most important factor is selecting a source of the proper strength and selenium content that is available for digestion. Only one consistent source should be used. A free-choice salt/mineral mix containing selenium is the safest form of supplementation."

Acute selenium deficiency can be treated with an injectable product, which is commercially available and quite safe. This medication is used to increase selenium levels immediately, and oral supplementation is used thereafter.

"The supplementation rate for selenium is generally 1 mg per horse per day," says Dr. Scoggins. "Supplementation can go as high as 2 to 3 mg per day without any ill effects. Doses of 5 mg per day can lead to problems with selenium toxicity."

Dr. Scoggins emphasizes that there is no one proper supplementation method. It is important to discuss this problem with your local equine veterinarian to determine the soil availability of selenium in your area and to decide what methods are right for you and your

Contributed By: Joseph Hahn (Information Specialist)
University of Illinois / College of Veterinary Medicine

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