Equine Motor Neuron Disease: What We Know

There's something not right with your horse. He's sweating, his muscles are twitching, and he can't seem to stand still. He just looks uncomfortable. You call your veterinarian and suggest it could be colic, but at the 2012 Western Veterinary Conference, held Feb. 19-23 in Las Vegas, Nev., one researcher suggested another ailment to consider: equine motor neuron disease, or EMND.

Elizabeth G. Davis, DVM, PhD, Dipl. ACVIM, associate professor and equine section head at the Kansas State University College of Veterinary Medicine, explained that while the first cases of EMND were reported in 1990 by Divers and colleagues, researchers and veterinarians are still working to better understand both the etiology and the disease itself. She gave an overview of what we, as an industry, know about EMND and what we're still working to figure out.

EMND Basics

Davis began by describing the EMND-affected horse and relaying what is known about potential disease causes. Affected horses, she said, appear thin due to a loss in muscle mass. These horses often stand in a "camped under" stance, hold their heads in a relatively horizontal position, and have an elevated tail carriage.

According to Davis, clinical signs of EMND include:

  • General weakness;
  • Anxiousness;
  • Muscle fasciculation (twitching);
  • Recumbency;
  • Tachycardia (elevated heart rate); and
  • Sweating.

Affected horses also have an urge to constantly shift their weight and continue moving, she added. Although researchers have not yet identified one specific etiology, they have demonstrated that EMND is associated with vitamin E deficiency. However, other mineral deficits or intoxications are likely involved because some horses with EMND exhibit vitamin E levels within the normal reference ranges.

Researchers have also observed that Quarter Horses and Thoroughbreds seem to be overrepresented in some patient populations and gender does not appear to be a factor in the disease, she added. The breed association is most likely a result of management practices rather than a true breed predisposition.

"Horses with EMND have frequently been housed at the same location for at least 18 months," Davis noted. "There is minimal or no pasture for the great majority of cases." Additionally, she noted, the majority of affected horses consume grass hay and concentrate grain.

Diagnosis

Davis relayed that EMND diagnosis is based on clinical signs along with histology and serologic tests. Specifically, she noted, pathologists examine a muscle biopsy from the tailhead to look for characteristic abnormalities typically associated with EMND.

"The characteristic lesions within muscle of the tailhead are a result of damage of very specific motor nerves," she explained. "The site of biopsy sample from the tailhead is very specific in order to obtain a sample of muscle tissue that has lost the normal innervation from health nerves; instead it has been innervated by these damaged nerves, which is referred to as an axonopathic effect."

In serologic tests a low plasma vitamin E level is often, but not always, found in EMND horses, Davis said. Additionally, she said, researchers and veterinarians consistently identify a mild to moderate elevation of the muscle enzymes creatine kinase and aspartate aminotransferase in EMND horses.

"The muscle enzymes are slightly elevated because of low-grade chronic muscle damage due to the loss of proper (normal) innervation from healthy nerves," Davis explained.

Management and Prognosis

Once a horse is diagnosed with EMND, owners should implement management changes carefully but aggressively: Provide horses with moderate to severe EMND with a deeply bedded stall to encourage rest, provide green forage, and administer about 10,000 IU of natural vitamin E daily via a feed supplement, relayed Davis.

For horses with mild to moderate EMND, feed green forage, administer 10,000 IU vitamin E, and if possible, offer pasture access, Davis said.

She stressed that when choosing a vitamin E supplement, avoid synthetic products. Natural vitamin E products are "well absorbed" by horses and are able to enter the central nervous system (CNS) at therapeutic levels, she explained, while synthetic products aren't well absorbed and typically don't reach therapeutic levels in the CNS. In most cases Davis recommends EMND horses be kept on vitamin E as long as they remain in an at-risk environment (see sidebar at left).

The prognosis for EMND varies and depends on the severity of clinical signs, Davis explained.

"Approximately 40% of horses with EMND continue to deteriorate and are euthanized within four weeks of the onset of clinical signs," she said. "Approximately 40% have marked improvement in clinical signs within four to six weeks after either relocation to another stable and/or administration of antioxidants, and approximately 20% remain permanently and noticeably atrophied."

Take-Home Message

Although EMND is not yet fully understood, it's a treatable--and often time preventable--disease. As with many equine ailments, early detection and treatment tends to yield more positive results, so consulting a veterinarian at the first sign of a problem is advisable.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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