Nutritional myodegeneration (NMD), more commonly known as white muscle disease, is a disorder that affects various animals, including horses. In equids, the condition primarily occurs in newborn foals, although adult horses can also develop it. [1][2]

The condition results in degeneration in the skeletal and cardiac muscle most often due to inadequate levels of selenium in the body. It also occurs less commonly in horses that are deficient in vitamin E. [1][2]

Selenium and vitamin E are important antioxidant nutrients for horses. Antioxidants are molecules that neutralize free radicals, which are unstable molecules that can damage cells.

Common signs of white muscle disease include weakness, poor coordination, and difficulty standing, eating, and nursing. [1] Due to a loss of muscle function, respiration can be impaired, and respiratory failure may occur in foals.

If left untreated, nutritional myodegeneration can be fatal. [1] But with early diagnosis and treatment, some horses make a full recovery from NMS.

You can help prevent white muscle disease by ensuring that your horse has adequate levels of selenium and vitamin E in their diet.

Nutritional Myodegeneration in Horses

NMD causes premature cell death of skeletal and cardiac muscles, resulting in the development of white scar tissue in muscles.

It is referred to as white muscle disease because affected muscles affected are paler in color, and can become completely white in advanced cases. [1][2]

NMD is a disease that most commonly presents in newborn foals, although it can also occur in adult horses. [1][2] 82% of nutrition-related muscle disease in horses occur in animals less than four years of age. [3]

White muscle disease can occur in foals that did not receive sufficient selenium from their mother during gestation. Broodmares that are deficient in selenium may not transfer enough of this nutrient to the placenta of the developing fetus. [1]

This disease may also be caused by a lack of vitamin E in the broodmare’s colostrum, which is the first milk produced by a mare after they deliver a foal.

Adult horses can develop the condition from chronic or prolonged low dietary intake of selenium. [1] Vitamin E deficiency can also contribute to NMD in adult horses but is believed to play less of a role than selenium deficiency. [1]

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Importance of Dietary Selenium

Selenium is an essential trace mineral that plays an important role in muscle function and development. It serves as an antioxidant, protecting cells from free radical damage.

A typical adult horse that weighs 500 kg (1100 lb) requires a minimum of 1 mg per day of selenium to avoid deficiency. According to the NRC, the optimal intake to support the immune system is between 2 to 3 mg daily. [9]

Horses that are heavily exercised, stressed or ill may also have higher needs for selenium to support recovery and immune function.

Antioxidant Status

Selenium is a component of glutathione peroxidase (GSH-Px), an antioxidant enzyme that neutralizes highly reactive chemicals in the body.

Antioxidants prevent damage from reactive oxygen species, which are natural compounds produced during the breakdown (metabolization) of fatty acids for energy.

If the horse does not have enough selenium in its body to support glutathione peroxidase activity, cell damage can accumulate at a faster rate and lead to muscle problems.

Vitamin E is another important antioxidant, preventing fatty acids peroxidation – a process by which free radicals damage fats in the body. [3]

Highly active muscles are usually most severely affected by nutritional myodegeneration. When muscles are worked, the production of reactive oxygen species is increased and if the horse does not have adequate antioxidant protections, these muscles will experience damage.

Muscles most often affected include: [3][7]

  • The diaphragm
  • Chest wall muscles
  • Gluteal muscles
  • The tongue
  • Masticatory (chewing) muscles
  • Muscular tissue of the heart

Selenium Content in Soil and Forage

NMD typically occurs in horses living in regions where the soil is deficient in selenium. Plants and grasses take up selenium from the groundwater and soil through their roots.

In areas with low selenium levels in the soil, the vegetation will also be deficient in this mineral. Selenium levels in feed crops should be above 0.1 mg/kg to prevent deficiency in grazing animals. [3]

Most forages in the coastal and Northern areas of North America have inadequate levels of selenium. Horses in these regions that are maintained exclusively on pasture and hay have a higher risk of deficiency. [8]

Selenium Map of Canada the USA

The prevalence of white muscle disease is expected to increase due to the depletion of this nutrient in soils. [1][2]

Acid rain can also affect selenium levels in forages. Soil exposed to acid rain releases less selenium, resulting in lower selenium levels in crops grown in that soil. [3]

Signs of White Muscle Disease in Horses

Common signs of white muscle disease in foals include rapid heartbeat and difficulty standing up. Severely affected foals experience tying up of their muscles or azoturia.

The first sign of NMD in adult horses is often difficulty eating because this condition affects the muscles used for chewing food. [6] The muscles involved in movement and cardiac function are also affected in adult horses. [2]

NMD can also contribute to the following clinical signs and progressive health complications: [4]

  • Sudden recumbency (laying down)
  • High respiratory rate (Tachypnea)
  • Shortness of breath (Dyspnea)
  • Inability to stand
  • Weakness
  • Inability to swallow (Dysphagia)
  • Jaw clenching (Trismus)
  • Muscle fasciculations
  • Trembling
  • Stiffness
  • Inadequate suckle reflex
  • Muscle breakdown

Nutritional myodegeneration is also associated with the following metabolic abnormalities, which may be detected in laboratory tests.

  • Increased potassium in the blood