Nutrition, Genetics, and Equine Neuroaxonal Dystrophy
NAD/EDM has been identified in numerous breeds including Appaloosas, Arabians, Haflingers, Lusitanos, Mongolian horses, Morgans, Norwegian Fjords, Paints, Paso Finos, Pony of the Americas, Quarter Horses, Standardbreds, and more.
Photo: Stephanie L. Church, Editor-in-Chief
Editor's note: This article is part of TheHorse.com's ongoing coverage of topics presented at the 2012 American College of Veterinary Internal Medicine Forum, held May 30 - June 2 in New Orleans, La.
Researchers are still working to fully understand equine neuroaxonal dystrophy (NAD), a relatively common neurologic disorder that is considered the underlying cause of equine degenerative myeloencephalopathy (EDM). At a recent veterinary conference, one NAD researcher updated attendees about recently identified links between NAD, nutrition, and genetics.
Carrie J. Finno, DVM, Dipl. ACVIM, a graduate student at the University of California, Davis, reviewed disease basics and discussed new findings at the 2012 American College of Veterinary Internal Medicine Forum, held May 30-June 2 in New Orleans, La.
Back to Basics
Finno first reviewed NAD/EDM, a disease that generally develops in horses around six to 12 months of age. EDM is considered the more severe variant of NAD. "NAD is clinically indistinguishable from EDM," she said. "The only difference between these disorders is the location of axonal degeneration within the central nervous system."
Finno said that NAD/EDM were considered the second most common causes of equine spinal cord cases admitted to Cornell University in 1978, and ranked second in causes of spinal ataxia (incoordination) at the University of Montreal from 1985 to 1988.
NAD/EDM has been identified in numerous breeds including Appaloosas, Arabians, Haflingers, Lusitanos, Mongolian horses, Morgans, Norwegian Fjords, Paints, Paso Finos, Pony of the Americas, Quarter Horses, Standardbreds, Tennessee Walking Horses, Thoroughbreds, Welsh ponies, and grade horses.
Clinical signs of NAD include:
- Symmetric ataxia (incoordination);
- A base-wide stance at rest;
- Abnormal circling;
- Possible dull mentation, though not always noted;
- Odd foot placement, often laterally (inappropriate proprioception, or the horse not knowing where his feet are);
- Toe stabbing when walking up inclines;
- Weakness behind when going downhill; and
- Trouble negotiating obstacles (including walking over cavalettis, stepping up or down a curb, or loading into a step-up, for example).
Additionally, she said, NAD/EDM lesions can be subtle, so cases often go undiagnosed at postmortem examination, currently the only definitive diagnostic tool veterinarians have.
NAD and Genetics
"Although clinical signs of NAD/EDM are similar across breeds, it is not known whether all breeds share the same etiologic basis," Finno said. That said, she discussed a study yielding strong evidence for the disease having a genetic basis with either a polygenic or an autosomal dominant with variable expressivity mode of inheritance in Morgan horses.
"One study found that foals from dams that had an NAD/EDM-affected foal (previously) were 25 times more likely to develop NAD-EDM than foals from other dams," she said.
Further, she noted, "Candidate gene discovered by a genome-wide association analysis using single nucleotide polymorphism markers are being evaluated for NAD/EDM in the Quarter Horse." Should a genetic marker be identified, Finno said, it will help breeders decrease the disease's prevalence in the general population (through selective breeding). It would also allow at-risk foals to start consuming nutritional support to prevent or delay the onset of severe clinical signs.
NAD and Nutrition
The aforementioned nutritional support mainly consists of vitamin E administration. Finno discussed several studies that have shown the vitamin can be beneficial when administered to at-risk foals. Findings included:
- When provided in the first year of life, vitamin E supplementation appears to lower NAD/EDM incidence and severity in susceptible foals; and
- In one herd of horses vitamin E supplementation for pregnant mares and foals appeared to decrease the number of severely affected cases; however, the overall incidence was unchanged.
Finno stressed that for vitamin E supplementation to be effective, it must begin prior to signs of disease: "Empirical treatment with vitamin E once clinical signs are apparent is often attempted but is ineffective, and (horses treated in this manner) exhibit lifelong, stable neurologic deficits that prevent use for any performance activity."
With that in mind, Finno relayed, "In equine families where previous NAD/EDM cases have been diagnosed, supplementation above the baseline 2007 NRC (National Research Council) requirement (1,000 IU for a 500 kg horse) is advised to pregnant broodmares and foals during the first two years of life."
When supplementing with vitamin E it is important to measure vitamin E concentrations before starting supplementation, and determine if the vitamin E supplement is natural or synthetic. Finno issued the following recommendations for affected or susceptible horses:
|Case||Amount of Synthetic Vit. E Fed||
Amount of Natural Vit. E Fed
(1.5 to 2 times the activity level of synthetic)
|Healthy Horse (1,000 pounds)||1,000 IU||500-660 IU|
|NAD/EDM Affected Pregnant Broodmare (first two trimesters)||5,000 IU||2,500-3,300 IU|
|NAD/EDM Affected Pregnant Broodmare (last trimester)||10,000 IU||5,000-6,600 IU|
|NAD/EDM Affected Foals (in the first year of life)*||
500 IU (100 pound foal)
1,500-1,900 IU (400 pound weanling)
250-330 IU (100 pound foal)
750-12,60 IU (400 pound weanling)
|NAD/EDM Affected Foals (after the first year of life||1,350-2,250 IU (660 to 1,000 pounds)||675-1,500 (660-1,000 pounds)|
|NAD/EDM Affected Horses**||5,000 IU||2,500-3,300 IU|
* Recommended for a veterinarian to provide an injectable vitamin E dose at birth (1mL/100 pounds of body weight)
** Supplementation is unlikely to improve neurologic status once abnormalities are evident
"It is important to realize, however, that the foal may still be mildly affected with neurologic deficits due to NAD/EDM despite adequate vitamin E supplementation," she cautioned.
Finno stressed that breeding changes are the only way to prevent new cases of NAD/EDM from developing. As researchers continue deciphering the disease, horse owners should use caution when breeding affected horses and consider supplementation for pregnant broodmares and susceptible foals.
About the Author
Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in three-day eventing with her OTTB, Dorado.