Understanding EDM/eNAD in Horses

By Sarah F. Colmer, VMD, Dipl. ACVIM, and Amy L. Johnson, DVM, Dipl. ACVIM

Horses can develop ataxia (incoordination) for a variety of reasons, including diseases that involve spinal cord compression (e.g., cervical vertebral stenotic myelopathy, aka wobbler syndrome) and infectious causes (e.g., equine protozoal myeloencephalitis [EPM] or Lyme neuroborreliosis), among others. Similarly, a plethora of factors can cause behavioral changes in horses, from saddle fit and environmental disturbances to pain and stress. A category of disease often comprising both ataxia and behavioral changes that has recently gained attention is neurodegenerative. Two conditions—equine neuroaxonal dystrophy (eNAD) and equine degenerative myeloencephalopathy (EDM)—both plague the central nervous system but differ primarily in where they affect the brain and spinal cord.

While the exact mechanisms remain unknown, veterinarians suspect low vitamin E levels in the blood are associated with the damage that leads to these consequences, which might occur in the horse’s early life. A genetic predilection might also exist; although veterinarians have diagnosed EDM/eNAD in most breeds, certain lines and individuals have been known to produce multiple affected offspring. However, researchers have not identified any associated genes. Most likely, both a genetic predilection and a period of insufficient vitamin E status contribute to disease development, and other factors might also be involved.