Neuroaxonal Dystrophy in Quarter Horses: Case Series

Neuroaxonal dystrophy, or NAD, is a neurologic disease that can affect horses as well as humans, sheep, cats, and dogs. The condition is not yet fully understood, although researchers believe there might be a genetic component. At the 2010 American Association of Equine Practitioners convention, held Dec. 4-8 in Baltimore, Md., one presenter discussed findings in a series of cases in Quarter Horses at a single cutting horse breeding farm.

"Neuroaxonal dystrophy is a degenerative disease of selected neurons and their axonal processes in the nervous system," explained Carrie Finno, DVM, Dipl. ACVIM, a graduate student at the University of California, Davis (UC Davis). Clinical signs of the disease include:

  • Symmetric ataxia (incoordination) that can appear much like wobbler syndrome,
  • A base-wide stance at rest,
  • Abnormal circling,
  • Dull mentation,
  • Often laterally odd foot placement (inappropriate proprioception, or the horse not knowing where his feet are),
  • Toe stabbing when walking up inclines,
  • Weakness behind when going downhill, and
  • Trouble walking over curbs (such as those in parking areas/on road edges).

Mild cases can present with performance issues, said corresponding author John Madigan, DVM, MS, Dipl. ACVIM, a professor of veterinary medicine at UC Davis.

"This disease is clinically indistinguishable from equine degenerative myeloencephalopathy," Finno noted. "The lesions (in the nervous system) are the same, just distributed differently. The disease should be referred to as NAD/EDM to encompass the full extent of the lesions."

The disease is not restricted to Quarter Horses, she added; she mentioned another study in which half of one farm's Lusitano foal crop was affected with NAD/EDM.

The case series Finno described involved a cutting horse farm with 148 Quarter Horses from two months to 34 years of age (but mostly yearlings), 59% of which had ataxia of Grade 1 or greater on a five-point ataxia scale.

"These cases showed symmetric ataxia of all four limbs, with the pelvic (hind) limbs more severely affected than the thoracic (front) limbs," she reported. "They also often had an inconsistent menace response (not always flinching from quick hand movement towards their eyes), dull mentation almost as if they were sedated, and we could induce this sleepiness by raising their heads for a short time."

Also, many cases had low levels of vitamin E in their bloodstreams; Finno suggested that affected horses might have an underlying susceptibility to developing this disease if they don't receive enough vitamin E.

"Why a cluster (of cases) in cutting horses?" commented Madigan. "Could it be that horses with the genetic mutation for NAD/EDM who get adequate vitamin E have an ability to be tolerant of extreme limb position relative to their body (away from the midline)? Are the exceptional movements of the cutting horse related to this trait and disease simply occurs when vitamin E is low? The future will tell. For the meantime, feed plenty of vitamin E beginning in early pregnancy and the first few years of life of at-risk horses."

All three index (initial) cases from that farm that were euthanized showed characteristic NAD lesions in their nervous systems, strongly suggesting that the same process was at work in other affected horses. Lesions can be subtle and easy to miss when tissues are examined, Finno commented. Additionally, Finno is working to determine the underlying genetic basis for the disease.   

"This disease is probably an autosomal dominant trait," she went on. "After genetic counseling (and breeding selection modification), 10% of this farm's 2009 foals were affected (compared to the 59% found in 2007).

"Neuroaxonal dystrophy/equine degenerative myeloencephalopathy is an important rule-out for any breed of horse with proprioceptive deficits," she concluded.

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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