Selenium Deficiency in Horses

Happy New Year! This is part of Owl Equestrian's new series, Health Spotlight, featuring a number of topics related to equine health. 

A number of significant health impacts are associated with selenium deficiencies in the equine diet, including severe muscle disorders like white muscle disease, myodegeneration, and acute exertional rhabdomyolysis (tying-up), the latter of which may not actually be a sign of deficiency at all. Hypothyroidism has also been associated with selenium deficiency, and a link has recently been found between low blood selenium levels and the stereotypic behaviour of crib-biting.

White Muscle Disease in Foals

A degenerative disease typically classified as a developmental disease and most commonly observed in foals of selenium deficient mares, acute or subacute white muscle disease can also affect adult horses, often in the form of masseter (cheek muscle) myopathy. Horses affected by this have difficulty opening their mouths and therefore, difficulty eating, which can lead to complications from anorexia, or death from starvation.

In foals,  the age group most commonly affected by white muscle disease, observable signs of severe deficiency include recumbency, increased heart rate, arrhythmias, rapid or slow and laboured breathing, stiffness and muscular pain in the limbs, back and neck, discoloured urine, and in extreme cases, sudden death. At the subacute stage, foals may exhibit severe weakness, a stilted gait or inability to stand, muscular twitches, lethargy, and difficulty suckling and swallowing. Common complications include aspiration pneumonia and starvation.

A retrospective study of approximately 270 horses tested for selenium levels at the Cornell University Equine and Farm Animal Hospital between 1996-2011 indicates that of the horses tested, 65% in the less than 30 day old age group had low selenium concentrations, with decreasing incidences of selenium deficiency in older horses, 43% in the 30 day to 2 year-old age group, and 37% in those older than 2. The higher incidence of selenium deficiency in foals less than 30 days old was directly associated with nutritional myopathy and low blood serum selenium in dams.

In some instances, when the mare is selenium deficient during pregnancy, the foal will be born with white muscle disease, additionally, said mares are at an increased risk for “early embryonic death, retained placentas, cystic ovaries, and weak or silent heat periods”. Minimum daily requirements of selenium for foals, pregnant mares, and lactating mares, all with an assumed body weight of 500kg, are presented below.

Minimum Daily Requirements of Selenium

FoalsPregnant MaresLactating Mares
1 month .16 mg
2 months .23 mg
3 months .28 mg
6 months .43 mg
12 months .64 mg
1 mg1.25 mg

(National Research Council, 2007)

Myodegeneration in Adult Horses

At the 2016 American College of Veterinary Internal Medicine Forum, Dr. Andrew Allen presented a 2014 case in which 13 horses on a farm had died. All of the horses had exhibited similar clinical signs. Two of these horses had been examined post-mortem and were found to have muscle cell death of the head and skeletal muscles (myonecrosis), fluid accumulation beneath the skin (subcutaneous edema), fluid accumulation in the chest cavity (pleural effusion), and ulcerated and inflamed tongues. The remaining herd of 12 horses was subsequently examined and all were found to have low body condition scores. Of the 12, 3 horses had elevated heart rates and one had edema in the abdomen. Blood was drawn from 4 of the horses, those that exhibited the most severe clinical signs, and all of them had low blood selenium levels and elevated creatine kinase activity and aspartate aminotransferase levels, indicative of muscle damage/degeneration. The entire herd was prescribed selenium injections and supplementation. When tested after several months, even the most severely affected horses had attained normal blood selenium level

In 2015, Dr. Allen investigated a second farm where there was a similar incidence of “herd-wide myodegeneration secondary to selenium deficiency”.  During this second investigation, the owners indicated they had learned about the toxicity of selenium and chose not to supplement their horses’ diet. Both of these cases underscore the role of selenium in maintaining muscle integrity in adult horses.

Acute Exertional Rhabdomyolysis

Commonly known as ‘tying-up’, exertional rhabdomyolysis can cause muscle necrosis and in severe cases, kidney failure. Veterinary attention is necessary within 24 to 48 hours of mild cases, and immediately in acute cases. In suspected acute cases, amongst other tests, blood serum levels will be tested for creatine kinase 4 to 6 hours after the episode, and aspartate aminotransferase 18 to 24 hours after elevated levels of which are indicative of muscle damage. It has been suggested that there is a relationship between selenium and vitamin E deficiencies and these elevated levels, however, horses with acute exertional rhabdomyolysis typically have adequate selenium and vitamin E serum levels, and increased supplementation has not proven to be beneficial.


There has been speculation that because two selenoproteins, iodothyronine deiodinase, or tri-iodothyronine (T3), and thyroxin (T4) are integral components of thyroid hormone activation, that a horse deficient in selenium will also develop hypothyroidism. A 2010 study by Muirhead et al. found this to be untrue. In a study of 201 horses on Prince Edward Island, blood serum was measured to determine levels of selenium, vitamin E, T3, and T4. In instances where an animal is in a hypothyroid state, both T3 and T4 concentrations will fall below the range of what is considered normal. The majority of the horses in the study had low or deficient selenium concentrations, however, T3 serum concentrations fell within the normal range for all horses, while T4 was more varied, leading the researchers to conclude that there is not a significant correlation between selenium deficiency and hypothyroidism in horses.


A recent study by Omidi et al. found a direct correlation between low blood serum concentrations of selenium and crib-biting in horses. Blood samples were drawn from 10 horses with an established history of crib-biting during basal conditions and during or immediately following cribbing episodes. The samples were compared to blood samples of a control group of non-crib-biting horses.

Low levels of blood serum selenium were observed in the crib-biting horses both at rest and during cribbing episodes, with the lowest levels of concentration recorded when the behaviour was performed, suggesting that low levels of blood serum selenium may have a role in crib-biting in horses, possibly as a result of oxidative stress. To give more credibility to their study, Omidi et al. reference several recent studies that have linked low Selenium levels to stereotypic behaviours in humans with psychiatric conditions, like schizophrenia.

To learn more about the role selenium plays in the equine diet, visit Health Spotlight: The Role of Selenium in the Equine Diet


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Cavanagh, K. and S. Ternan. (2014). From the Horses’ Mouth: Nutrition, Feeds & Feeding. Fonthill, ON: Matrix Multimedia

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Muirhead, T.L., Wichtel, J.J., Stryhn, H. and J.T. McClure. (2010). “The selenium and vitamin E status of horses in Prince Edward Island.” The Canadian Veterinary Journal, 51(9), pp. 979-985. Retrieved from  

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National Research Council. (2007). Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press.

Omidi, A., Jafari, R., Nazifi, S. and M.O. Parker. (2018). “Potential role for selenium in the pathophysiology of crib-biting behavior in horses.” Journal of Veterinary Behavior, 23, pp. 10-1

Patton, K.M. (2008). “Hiding in Plain Sight: Selenium deficiency masks itself as different diseases.” The American Quarter Horse Journal May 2008. Retrieved from

Streeter, R.M., Divers, T.J., Mittel, L., Korn, A.E., and  J.J. Wakshlag. (2012). “Selenium deficiency associations with gender, breed, serum vitamin E and creatine kinase, clinical signs and diagnoses in horses of different age groups: A retrospective examination 1996–2011.” Equine Veterinary Journal, 44, pp. 31-35

Thunes, C. (2011). “Supplementing Selenium.” The Horse Jan 11, 2011. Retrieved from

Valberg, S.J. (undated). “Exertional Myopathies in Horses.” Merck Manual: Veterinary Manual. Retrieved from