Equine degenerative myeloencephalopathy (EDM) is a neurologic disorder in horses that can cause ataxia, stumbling or falling, dragging the feet, or behavioral changes. “As of today, there is no definitive antemortem diagnosis or treatment for EDM,” said Sarah Colmer, VMD, Dipl. ACVIM-LAIM, neurology fellow at the University of Pennsylvania’s New Bolton Center, in Kennett Square, during her presentation at the 2024 Veterinary Meeting and Expo, held Jan. 13-17 in Orlando, Florida. “That’s terribly frustrating for horse owners and veterinarians when faced with potentially euthanizing these animals without a confirmed diagnosis beforehand.”

What is EDM in Horses?

There is a lot that remains elusive about EDM, Colmer said, but she shared what researchers do understand about the disease:

• It is a symmetrical, degenerative neurologic disorder that classically involves ataxia (incoordination) and, in more recent reports, can also include behavioral changes.
• Some horses exhibit signs of the disease between six and 12 months of age, but clinical signs can also occur in older horses.
• There does not appear to be a sex predilection, though geldings are overrepresented in some studies.
• The antioxidant vitamin E might protect the central nervous system from oxidative damage, and researchers have associated prolonged vitamin E deficiency with neuromuscular dysfunction. Some EDM-affected horses have low blood vitamin E levels.
• On necropsy, EDM horses have lesions in the brainstem and the spinal cord. A second condition, called equine neuroaxonal dystrophy (eNAD), can cause identical clinical signs to EDM. However, this disease involves changes in the brainstem but without significant spinal cord lesions.

Recognizing the Signs of EDM in Horses

“Seventy-three percent of horses studied had a history of ataxia,” said Colmer, describing a recently published study on how EDM presents in horses. “It was usually symmetrical and mild (Grade 1-2 out of 5 using the modified Mayhew ataxia scale). Sixty-nine percent had a history of abnormal behavior, which can run the gamut from dull to hyperexcitable or even aggressive.” Less commonly, owners and veterinarians have observed abnormal penis-dropping behavior and occasionally lipofuscin deposits (yellow-brown pigment, typically associated with age) in the retinas of horses with EDM, Colmer said.

Diagnosing EDM in Horses

Since veterinarians do not yet have a diagnostic test or genetic marker for EDM, they can only make a tentative diagnosis in live horses exhibiting clinical signs by ruling out differential diagnoses, such as cervical vertebral stenotic myelopathy (CVSM, aka wobbler syndrome) and equine protozoal myeloencephalitis (EPM).

Diagnostic tools can include a neurologic examination, a spinal tap to rule out evidence of EPM (caused by Sarcocystis neurona or Neospora hughesi) or Lyme (Borrelia burgdorferi) infection based on cerebrospinal fluid and blood tests, and cervical spine radiographs with or without a myelogram to look for evidence of CVSM. Colmer added that hematology might or might not reveal a vitamin E deficiency, with the target serum level being at least 4 ppm.

“An EDM horse may have had a vitamin E deficiency at some point in his life, predisposing him to neuromuscular disease, but that won’t necessarily be the case at the time of presentation,” added Colmer. Researchers are still unsure of the relationship between vitamin E and neurologic disease progression. “Affected horses may have lower levels of the vitamin because they are metabolizing it faster and may experience accumulated oxidative stress over time, but we are unsure.” Unfortunately, she said, all tentative diagnoses of EDM can only be confirmed—or rejected—postmortem by examining the brain stem and spinal cord during necropsy.

Can EDM Be Prevented or Treated in Horses?

“At this time, there is no known treatment or effective intervention for EDM,” said Colmer. “Risk factors identified in a 1990 research survey included exposure of foals to insecticides and wood preservatives and turnout on dirt lots. On the flip side, grass pasture had a protective effect. Fresh grass contains relatively high levels of vitamin E compared to hay. Therefore, efforts to prevent the disease could include providing pasture access, testing individual horses’ vitamin E levels, and then supplementing diets as needed.”

Moving Forward With EDM Research

“With many gaps left to fill in our understanding of EDM, we have three primary goals for future research,” Colmer said. “Understand why the disease happens, develop a definitive premortem diagnostic test, and, last but not least, find a cure.”