Amy Johnson, DVM, DACVIM-LAIM and Neurology from the University of Pennsylvania’s New Bolton Center, described a syndrome of equine degenerative myeloencephalopathy (EDM) that causes bilateral degeneration of the brainstem, spinal cord grey matter, and spinal cord white matter.
Since the 1970s, this disease has changed in frequency when it was common in about ¼ of horses presented to the Pennsylvania hospital but became rare in the ’00s. Then, cases began climbing again in 2014 and 2015 and now Johnson sees this syndrome in least 25% of neurologic cases each year. Two differences have developed from the original 1970s literature reports: Now, it is occurring in more middle-aged (5–15 years old) horses in the peak of their athletic career. Most are Warmbloods although it affects a variety of breeds. And, now 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. Owners report things like: Not the same horse; Never used to be naughty; Unpredictable; Irritable; Aggressive to horses or people; Dull; Cognitive dysfunction; Doesn’t recognize me; It’s like a switch flips. Affected horses change the way they perceive sensory input—either with loss of feeling or they become weirdly sensitive.
On exam, Johnson remarks that 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 (incoordination) along with proprioceptive deficits. Diagnosis of neurodegenerative disease in a living horse is one of exclusion so other reasons must first be ruled out, like spinal cord compression, EPM, and vitamin E deficiency.