Equine Motor Neuron Disease (EMND) is a disease that causes progressive damage to nerve cells in the brain and spinal cord of adult horses.

The disease damages cells that are involved with muscle movement, rendering them dysfunctional and leading to muscle wasting.

Early signs of EMND typically include weight loss and muscle atrophy. As the disease progresses, other signs may become evident such as muscle trembling, an abnormal stance, excessive recumbency (lying down), a raised tail, and a low head and neck carriage.

The primary risk factor for EMND is vitamin E deficiency. [1] Other factors may also contribute to the disease but more research is needed to understand these factors.

When horses are supplemented with sufficient amounts of vitamin E, clinical symptoms improve in some cases of EMND. [6] However, some horses continue to deteriorate despite receiving high doses of vitamin E.

The only treatment available for horses affected by EMND is to increase vitamin E intake in the diet by providing access to green pasture or a supplemental source.

Equine Motor Neuron Disease

First identified in 1990 in the Northeastern United States [1], EMND is a neurodegenerative illness that affects nerve cells (neurons) located in the spinal column and brain stem of adult horses.

This condition affects the neck (cervico-thoracic) and back (lumbar-sacral) areas of the spinal cord. In particular, EMND affects a part of the spinal cord known as the ventral horn where upper motor neurons interact with lower motor neurons. [1]

EMND causes muscle wasting and weakness due to the deterioration of lower motor neurons of the somatic nervous system. These neurons send neural signals from the spinal cord to muscle, glands, organs and other tissues.

The condition causes degenerative changes in the nerve fibre (axon) – the part of the nerve cell that carries nerve impulses away from the cell body. EMND also results in damage to the lipid-rich material (myelin) that insulates the nerve cell and enables nerve signals to travel efficiently. [6]

The degeneration of motor neurons in horses with EMND is similar to what occurs in humans with amyotrophic lateral sclerosis (ALS or Lou Gehrig’s disease). [1]

Prevalence

EMND typically affects horses between 15 months and 25 years old. Both genders are equally affected. Breeds including Standardbreds, Quarter horses, and Thoroughbreds may be more likely to develop the condition for unknown reasons. [1]

EMND has been reported in countries around the world. In the US, cases are more prevalent in the Northeast. Based on referrals to Cornell Unversity, the prevalence was higher in the early 1990s compared to recent years. [2]

Increased awareness of the importance of vitamin E and increased fortification of feeds and supplements with this nutrient being may be responsible for a decline in the number of cases. [1]

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Clinical Signs

Horses that are clinically affected by EMND have both dead motor neurons and alive but dysfunctional motor neurons.[6]

A small proportion of clinically unaffected horses may have some deterioration of their motor neurons due to EMND, but don’t exhibit signs or symptoms of the condition. [6]

The clinical signs of EMND typically become apparent when affected horses have lost approximately 30% of their motor neurons. [6]

Common clinical signs of Equine Motor Neuron Disease include:

  • Progressive weakness
  • Symmetrical muscle wasting and atrophy
  • Weight loss
  • Abnormal stance with the hind legs positioned underneath the abdomen
  • Muscle twitching and trembling
  • Excessive time spent laying down (recumbency)
  • Shifting the weight in the hind legs
  • Short-strided gait
  • Low head and neck carriage
  • Elevated position of the tail head
  • Discoloration on the interior surface of the eye
  • Sweating
  • Hyperalert behavior

Some horse owners report that weight loss occurs before muscle trembling. [6] In some cases, trembling and constant weight shifting may not occur but muscle wasting is evident. [6]

Causes of EMND

Researchers do not yet have a complete understanding of what causes EMND. However, a dietary deficiency in vitamin E is believed to be the predominant factor that contributes to the condition. [1]

EMND occur