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Selenium Toxicosis in the Horse

Selenium is an essential element in the diet of horses. However, certain plants, environmental conditions, and overzealous administration can result in toxicity. Depending on the dose and length of exposure three different sets of symptoms can result. Clinical signs seen depend on the amount ingested. Large doses, approximately 2.5 mg / lb of horse, results in peracute to acute disease. Ingestion of feed stuffs containing greater than 10,000 ppm can result in poisoning at this level. In peracute cases death occurs rapidly due to shock and respiratory failure. In acute cases depression and colic may be the first sign noticed followed by weakness, frequent urination, difficult breathing, shock, and death. Lower doses, 0.05 to 0.5 mg / lb of horse (5 to 50 ppm minimum dry weight in forage) in the diet for more than 30 days can result in chronic poisoning. The clinical sins are loss of mane and tail hair, rings and cracks in the hoof wall, and secondary laminitis. Also seen in this chronic condition is ill thrift, anemia, and stiffness. Severe cases may result in sloughing of the hoof.

Additional information

  • Selenium is an essential trace element but, unlike most nutrients, it has a low-toxicity threshold. Care is required to achieve the right balance between the correct amount and over-supplementation. The authorities also take a prudent view and list only certain forms of selenium for use in feeds and supplements.
  • Selenium is best known as an antioxidant, but it has other roles including iodine metabolism, repair of DNA and in the immune system (although this may be linked to its antioxidant properties).
  • As an integral part of the enzyme glutathione peroxidase, selenium's antioxidant role is within the cells of the body to prevent the formation of free radicals. It is often mentioned with vitamin E because both nutrients work within the cells to prevent free radical damage. A deficiency of one increases the requirement for the other, and vice versa.
  • Horses typically require 0.1mg/kg selenium in the total diet when inactive, increasing to 0.3mg/kg in exercising horses and broodmares.
  • A classic sign of selenium deficiency is white muscle disease in foals and young stock. The mare transfers selenium to the foetus during pregnancy and passes more to the foal through her milk. She must receive plenty of selenium (via a dietary supply above 0.05 mg/kg) throughout her gestation to set the foal up for early life.
  • At the other end of the spectrum, the first signs of toxicity are poor hoof quality and pain around the coronet band, although the classic signs are the sloughing off of hooves and loss of mane and tail hair.

    Selenium specifics
  • Compound feeds are generally supplemented with selenium at levels of 0.2-0.3mg/kg feed. The selenium source is sodium selenite. The EU has recently banned selenium yeast
  • Supplements, particularly those supplying antioxidant or immune system support, can typically supply 1mg selenium per dose
  • A healthy adult horse eating 10-12kg of compound feed and forage per day will consume about 2mg of selenium per day, before supplement addition
  • The toxic threshold for selenium is as low as five times the requirement. Double-dosing — giving supplements and feed that both contain selenium — may approach toxic levels
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