Equine Protozoal Myeloencephalitis

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Also Known As

EPM, Protozoal myeloencephalitis


Equine protozoal myeloencephalitis (EPM) is a common neurological disease of horses found throughout the United States, southern Canada and countries in Central and South America.

The disease affects the central nervous system of the horse and usually begins gradually with few symptoms, but sometimes has a severe onset with immediate signs of neurological damage evidenced by a lack of limb coordination and loss of reflexes. Signs of spinal cord involvement are more common than signs of brain disease.

Without treatment, EPM often causes the horse to lose his ability to stand followed by other severe symptoms and death. The progression of the disease may take a few hours or can occur over weeks, months or years. Progression may occur steadily or may stop for a period of time, only to begin again.

At least 60% of horses improve with treatment, but less than 25% ever recover completely. Relapses are common in horses that continue to test positive, but rare in those that test negative after treatment.


  • Asymmetric or symmetric weakness
  • Inability to control one or more limbs
  • Muscle atrophy
  • Areas of spontaneous sweating
  • Loss of reflexes
  • Depression
  • Tilting of head
  • Facial paralysis
  • Seizures
  • Behavioral abnormalities
  • Recumbency and death


Most cases of EPM are caused by the protozoa Sarcocystis neurona, first identified in the 1980's. A few cases of EPM in the Americas and in Europe have been associated with Neospora hughesi, an organism that is closely related to S. neurona.

The predator host of the protozoa is believed to be the opossum. Opossums become infected by eating muscle tissue from infected prey and infectious cysts containing the protozoa are passed in the opossum's feces. The horse is infected when it eats forage contaminated by the opossum's feces. It is believed that armadillos, skunks, raccoons, sea otters, seals and domestic cats can also be intermediate hosts of the protozoa.

The horse is considered a dead-end host of Sarcocystis neurona, meaning that the horse does not pass the disease on to other animals, including humans.

Studies have found that 22 to 65% of horses in the United States, depending on the geographic location, test positive for S. neurona antibodies which means they have ingested forage containing the disease-causing organism. Little is known about the incubation period between exposure and development of the clinical disease. 


No proven preventative is available. A conditionally-approved vaccine is marketed, but the efficacy has not been fully evaluated.

Although there is no way to completely prevent EPM in areas where there is exposure to wild life, it is important that feed and bedding supplies are protected from animals that might carry the infection. Any carcasses of small animals found on property where horses are kept or ridden should be removed immediately upon discovery.

Possums and other small wild animals should be caught and "rehomed," away from where horses are kept either by the local county agencies responsible, the humane society or wildlife and game agencies. Keeping property and areas where horses are ridden or graze free of small  animals such as opossums, will reduce the risk of a horse catching EPM.

To prevent wild animals from coming on horse property, all garbage and litter should be confined in metal containers. Other attractions for wild life such as bird feeders and fallen fruit should be removed from horse areas.


Once infected, horses usually begin to display clinical signs, some of which are consistent with other neurologic diseases. It is important that a veterinarian diagnose the disease as soon as possible. A neurological exam along with a blood or a cerebrospinal fluid test is usually done.

Unfortunately, these tests determine only if the horse has been exposed to a causative agent of the disease. The IFAT test has been shown to help establish a probability of whether a horse has EPM, but no test is 100% accurate.

Currently, two FDA-approved products are available for treating EPM. These are ponazuril and diclazuril. Ponazuril is marketed under the trade name Marquis and is a paste that is administered orally over a 28 day period. Few side effects have been noted with its use.

Diclazuril is marketed under the trade name Protazil, and comes in an FDA-approved pellet form and is used as a top-dressing on the horse's grain ration, which makes administration of the drug easier in some cases.

Other treatments that are used to aid horses with clinical signs of EPM include dexamethazone, flunixin, meglumine (Benamine) and dimethyl sulfoxide (DMSO). Some veterinarians also recommend adding natural vitamin E to the horse's diet because, in some cases, it helps reduce neurological signs.

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EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.