AAEP 2021: EDM Causes Poor Performance in Horses
Dr. Amy Johnson said of equine degenerative myeloencephalopathy (EDM): "It’s like a switch flips. Affected horses change the way they perceive sensory input—either with loss of feeling or weirdly sensitive." Brought to you by American Regent Animal Health

Owners of horses affected with EDM often complain of performance decline following a period of good performance. Getty Images

At the 2021 AAEP Convention in Nashville, Tennessee, Amy Johnson, DVM, DACVIM-LAIM and Neurology, from the University of Pennsylvania’s New Bolton Center, spoke about equine degenerative myeloencephalopathy (EDM). She pointed out that EDM is more common than one might think, but there is as yet no antemortem test for diagnosis. 

Equine neuroaxonal dystrophy affects brainstem nuclei and spinal cord grey matter and looks the same as EDM, which causes bilateral degeneration of the brainstem, spinal cord grey matter and spinal cord white matter.

Johnson noted that in the 1970s, only 24% of horses with spinal cord disease had EDM. In the early 2000s, it became very rare. Then things changed. 

By 2014/2015, her clinic received only a few cases a year; by 2018, a dozen cases per year. Now 25% of neurologic cases in their clinic are caused by EDM—42 cases in 2021 alone! 

Two differences have developed from the original 1970s literature reports: Now, it is occurring in more middle-aged (i.e, 5-15-year-old horses) in the peak of their athletic careers. Most are Warmbloods, although it affects a variety of breeds. 

And currently, EDM is accompanied by behavior changes.

Usually an owner complains of performance decline following a period of good performance. The horse’s personality and behavior transform, gait abnormalities develop, and the horse’s physical appearance changes with muscle loss and a dull haircoat—kind of a Cushing’s (PPID) look. 

Johnson emphasized that owner observations are key. They report things like: 

  • Not the same horse
  • Never used to be naughty
  • Unpredictable
  • Irritable
  • Aggressive to horses or people
  • Dull
  • Cognitive dysfunction

It’s like a switch flips. Affected horses change the way they perceive sensory input—either with loss of feeling or weirdly sensitive.

Johnson remarked that when examined, the horses seem inappropriately quiet and dull, not engaged or interactive, and sometimes are unpredictable, erratic or spooky in the hospital. Many have mild ataxia (Grade 1-2) along with proprioceptive deficits. 

Diagnosis of neurodegenerative disease is one of exclusions, so look for other reasons first, she said, such as spinal cord compression, EPM and vitamin E deficiency. Vitamin E levels tend to be normal in EDM individuals. 

The phosphorylated neurofilament heavy-chain (PNF-H) biomarker is not reliable for diagnosis. 

While signs appear like cervical spinal cord disease, the lesions are actually in the brainstem, and that is where an experienced pathologist should look on post-mortem exam.

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