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Key takeaways

Selenium (Se) is a micronutrient mineral that, in small doses, plays an essential role in a healthy diet in horses. 

  • Selenium poisoning is a potentially life-threatening condition that occurs when an excessive amount of selenium is ingested
  • As little as 5 mg per day over the course of a month can cause toxicosis
  • Symptoms of acute Se poisoning include lethargy, abdominal pain, and death
  • Symptoms of chronic Se intoxication include hoof cracking, lameness, and hair loss
  • There is no antidote for selenium intoxication, and treatment of chronic toxicosis involves dietary modifications and pain control
  • Prognosis is generally poor; in cases of acute intoxication, most animals die within hours or days from ingestion
  • Horses with chronic toxicosis are often euthanized due to a poor prognosis and prolonged recovery
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A closer look: Selenium Poisoning in Horses

Selenium is a mineral micronutrient that, in small doses, is an essential to a healthy diet. When ingestion exceeds safe levels, it is absorbed by the digestive tract, enters the bloodstream and leads to poisoning. Selenium toxicosis can be either acute or chronic.

Acute selenium toxicosis occurs as the result of ingestion of a single high dose. As little as one gram of selenium can cause acute toxicosis in an adult horse, which in most cases leads to rapid death.

Most horses suffering from acute selenium toxicosis die from respiratory failure.

Chronic selenium toxicosis is caused by ingesting small doses of selenium over a prolonged period of time.

Horses may become so lame that they cannot move to eat or drink, resulting in starvation.

Risk factors

Selenium toxicosis is uncommon in horses. While selenium deficiency is more common than selenium toxicosis, the latter is a potentially life-threatening condition.

Selenium toxicosis is more common in regions where plants store high Se levels, or there are high levels of selenium in the soil. Plants growing in alkaline soils have the highest Se concentrations during periods of drought.

Horses living in South Dakota, Montana, Nebraska, Kansas, Utah, New Mexico, Colorado, and Wyoming are at a higher risk of Se intoxication. Soil analysis can identify the risk of selenium toxicosis in a specific area.

Most high selenium-absorbent plants are unpalatable to horses, and animals with proper access to sufficient forage are less likely to suffer from selenium intoxication.

Possible causes

The ingestion of excessive levels of selenium causes selenium toxicosis.

Common causes of excessive ingestion include:

  • Accidental excess selenium supplementation
  • Ingestion of high selenium forages or hay
  • Drinking from high selenium concentration water sources (> 0.05 ppm)

Main symptoms

Testing and diagnosis

Horses presenting symptoms consistent with selenium toxicosis usually undergo the following diagnostics:

  • Physical examination
  • Blood test: to assess the concentration of selenium in the plasma
  • X-rays of the hooves
  • Diet analysis

Steps to Recovery

Once diagnosed, treatment options include:

  • Elimination of high selenium plants or feed from the diet
  • Dietary modification: in the case of chronic toxicosis, treatment involves a high-quality elevated protein, low-selenium diet
  • Pain medication to control hoof pain

There are no effective treatments for acute selenium toxicosis.

Prognosis of acute selenium toxicosis ranges from poor to extremely poor, as supportive care for cardiac collapse is rarely successful.

The prognosis of chronic selenium toxicosis is poor and depends on the extent of hoof damage and level of pain experienced. Recovery can take up to 18 months.


Selenium intoxication is not contagious. Prevention is the best way of safeguarding the animal's health, as there is no specific treatment for selenium toxicosis.

Prevention strategies include:

  • Providing an appropriately balanced diet with adequate levels of selenium
  • Eliminating selenium accumulating plants from pasture
  • Providing adequate protein levels in the diet
  • Consulting a veterinarian or equine nutritionist before adding selenium-containing grain or feed to a dietary program
  • Testing soil concentrations of selenium

Is Selenium Poisoning in Horses common?

Se intoxication is uncommon in horses. It is more common in areas where there is a high concentration of Se in the soil.

Typical Treatment

  • Diary modification
  • Elimination of exposure
  • Hoof care


Karyn Bischoff , DVM, DABVT - Writing for Merck Veterinary Manual
Nettie Liburt, MS, PhD, PAS - Writing for The Horse
University of Kentucky College of Agriculture, Food and Environment - Writing for The Horse
Chad Mendell - Writing for The Horse
Erica Larson - Writing for The Horse
Peter Aitken BVetMed MRCVS; Graham Munroe BVSc PhD DESM CertEO DipECVS FRCVS; Wilson Rumbeiha BVM PhD DipABT DipABVT; Rachael Conwell BVetMed DipECEIM CertEM(IntMed) MRCVS - Writing for Vetlexicon

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