A poor outcome for a horse with EPM demonstrates the complexity of this neurologic disease

Equine protozoal myeloencephalitis (EPM) results from the protozoan parasites Sarcocystis neurona or, less commonly, Neospora hughesi, invading the horse’s central nervous system (CNS). With S. neurona, horses become infected after ingesting the parasite’s sporocysts (the infective egglike stage of development) that contaminate the environment. The opossum is the definitive host and source of these infectious S. neurona sporocysts, which it passes in its feces.

“After a horse ingests the organism, it begins to multiply in the gut, as well as in vascular endothelial cells throughout the horse’s body,” explains Steve Reed, DVM, Dipl. ACVIM, an internal medicine specialist and shareholder at Rood & Riddle Equine Hospital in Lexington, Kentucky. “The organisms are often cleared by the immune system in most but not all horses. How the parasite winds up in the CNS in some horses remains unclear.”

Horses with EPM have neurologic deficits such as weakness, ataxia (incoordination), muscle atrophy (wasting), or cranial nerve deficits such as head tilt, depression, and facial paralysis. The exact clinical signs vary depending on the part of the brain or spinal cord the parasite damages. 

“Typically, the disease has an insidious onset, but some horses may present with acute neurologic deficits that may be severe from the outset,” says Reed.

Meet Magic

The case of Magic, a 6-year-old Belgian Warmblood gelding, demonstrates just how insidious EPM’s onset can be.

“Magic had already been diagnosed with EPM by the primary practitioner, and treatment had been initiated,” recalls Reed. “But the owners wanted to begin a rehabilitation program under direct veterinary supervision. The horse was therefore referred to me.”

Magic was Grade 4 out of 5 ataxic when Reed first assessed him, meaning the horse would fall or nearly fall while walking. Magic also had antibodies against S. neurona detectable in his bloodstream, which Reed says wasn’t surprising.

“It is common to find antibodies in many horses that live in states that have the definitive host, opossums,” he explains. “Reports indicate while 50% to 75% of horses have been exposed to the parasite, very few of those horses ultimately develop EPM. This means that only measuring a blood titer is not sufficient to make a confirmed diagnosis of EPM