The somewhat evasive neurologic disease equine protozoal myeloencephalitis (EPM) occurs when horses are exposed to certain
protozoal parasites, most commonly Sarcocystis neurona (secondarily, Neospora hughesi), that infect and invade the central nervous system (CNS). The disease is widespread throughout North and South America.
(LEA ROTH/GETTY IMAGES)
EPM is seen any time of year, with an estimated 50 percent of horses exposed to S. neurona, though less than 1 percent develop clinical EPM. Although there's a low incidence of EPM in the general horse population,
14 cases per 10,000 horses per year (23 percent of the horses that died with neurologic signs) showed S. neurona antibodies in their CNS, according to studies done at the University of California-Davis (UC Davis).1
A large variation of neurologic signs is possible with EPM, depending on the degree of CNS damage, which can make this disease
easy to confuse with other neurologic conditions such as wobbler syndrome, trauma, equine herpesvirus myeloencephalopathy,
equine degenerative myeloencephalopathy and West Nile encephalomyelitis.
Photos 1 and 2: Two examples of lower motor neuron signs and muscle atrophy associated with EPM. (Photos courtesy Stephen
Reed, DVM, Rood & Riddle Equine Hospital, Lexington, Ky.)
Depending on the site and extent of CNS damage, horses may show signs suddenly or progressively. Some signs may be almost
imperceptible, such as isolated atrophy of gluteal and masseter muscles. More commonly, neurologic signs occur acutely but
also may progress gradually to a noticeable gait abnormality, functional impairment of one or more limbs, weakness, asymmetric
muscle atrophy (Photos 1 and 2,), incoordination (leaning to one side), head tilt, stiffness, or severe ataxia, as a horse
might suddenly fall over during a race or workout, without any previous apparent signs.
History of research
EPM has been studied since the 1970s. An early pioneer was J.P. Dubey, PhD, a parasitologist and now adjunct professor at
the Virginia-Maryland Regional College of Veterinary Medicine, Blacksburg Va., and Animal Parasitic Diseases Laboratory, Agricultural
Research Service, United States Department of Agriculture, Beltsville, Md. Dubey, who did this work while at The Ohio State
School of Medicine, was one of the early scientists to recognize EPM. It was initially described by James Rooney, DVM, and
colleagues at the University of Pennsylvania as an unusual neurologic syndrome, then called segmental myelitis.
Since then, it's been reported in horses from 2 months to 24 years old, typically affecting horses 1 to 6 years of age.2 EPM is common in Thoroughbreds, Standardbreds and Quarter horses. It's also been found in ponies, but not in mules, donkeys
or other nonhorse equids.