Essential trace element component of glutathione peroxidase – one of the most important antioxidants in the body which detoxifies peroxides (R.O.S.). Selenium is also involved in the metabolism of Thyroid hormone as a component of a selenoenzyme, and, in selenoproteins it plays a role in sperm formation and female fertility. Protection from damage by R.O.S. in both skeletal and heart muscle tissues require adequate selenium. Hair and hoof horn quality are in part selenium related and the proper functioning of Vitamin E also seems to require selenium. Deficient in soils in several areas of Australia, selenium is commonly included in manufactured feeds and supplements.
Selenium deficiency has traditionally been associated with Nutritional Myopathy or White Muscle Disease found in foals. Affected foals are weak, have difficulty standing, suckling, and swallowing and may have heart failure. Post -mortem reveals pale white discoloration of areas of skeletal and heart muscle tissue. The condition is usually associated with low selenium status of the mare during pregnancy and low glutathione peroxidase level in the foal coupled with elevated muscle enzymes found on blood tests. Symptoms mimic those of Vitamin E deficiency as the functions of both are interdependent.
Selenium is essential in very small amounts but can be toxic if overdosed. Both acute and chronic forms of toxicity are recognized.
Acute – “Blind Staggers” manifests as apparent blindness, sweating, colic, diarrhoea, kidney damage, lethargy and elevated heart and respiratory rate. The LD50 (the dose rate which would be expected to be lethal for 50% of animals) for selenium in the form of sodium selenite is only 3.3 mg/kg bodyweight, or 1.65 grams for a 500kg horse, a tiny amount.
Chronic – Alkali Disease shows as cracking of the hooves especially at the coronary band, sometimes with separation and sloughing of the hoof, as well as hair loss, particularly of the mane and tail. This is believed to be caused by the substitution of Selenium for Sulphur in the keratin of hoof and hair which weakens it’s normal structure or possibly by damaging the keratin forming cells.
Toxicity can occur from ingesting selenium concentrating plants in pasture but is most commonly the result of accidental over supplementation or use of vitamin E/Selenium injections. Recently organic chelates of selenium and selenium yeast have become popular in supplements and premixes but to date there is little evidence that they are more bio-available or effective.