Understanding EPM

Widespread throughout the United States, the degenerative disease Equine Protozoal Myeloencephalitis (EPM) affects a horse’s central nervous system. Caused by unicellular protozoa Sarcocystis neurona, EPM causes inflammation in the nerve tissues of the brain and spinal cord. Another protozoan species, Neospora hughesi, can also cause EPM in horses but with less incidence.

Sarcocystis neurona has a two-host life cycle, with opossum as the definitive host and other animals including raccoons, skunks, cats, and birds as intermediate hosts. Horses are a dead end for the parasite, as they can be infected by it but don’t spread it.

Infected horses cannot transmit EPM to other horses, but horses stabled or pastured together may get it from the same feed or water. Photo by anakondasp/stock.adobe.com

Clinical signs include:

  • Incoordination of movement
  • Stilted movement
  • Lameness 
  • Muscular atrophy
  • Muscular paralysis of mouth, eyes, eyelids, and ears
  • Difficulty swallowing
  • Abnormal sweating
  • Head tilt 
  • Seizures 
  • Collapsing

Early diagnosis and prompt treatment increase chances of recovery. A veterinary exam may include neurological tests and a blood draw, though blood testing can confirm only that the horse has been exposed to the parasite, but not whether he’s been infected. A spinal tap to analyze cerebrospinal fluid can help rule out wobblers syndrome or equine herpes virus, antibiotics can prevent the parasite from replicating, and anti-inflammatories can help reduce swelling.


Several FDA-approved treatments exist including Marquis, Protazil, and ReBalance. Horses treated for EPM may also be supplemented with vitamin E, folic acid, and thiamine to support muscle and nervous tissue. 

With optimal diagnosis and treatment, up to 25% of horses recover completely, and up to 80% show significant improvement. Relapses occur in 10% to 20% of horses that have shown symptoms. If untreated, EPM can lead to permanent physical damage and death. 

Tips for EPM prevention:

  • Limit grazing
  • Keep feed rooms closed
  • Keep feed in sealed containers
  • Quickly clean up any spilled feed
  • Utilize safe rodent/vermin control methods around stable and pastures
  • Promptly remove animal carcasses 
  • Keep horses’ drinking water fresh and clean

[Can a horse with EPM be ridden again?]

Related Articles
HR_24BON Grooming Feature_01
Groom to Win
Grooming Secrets to Show Like the Pros
HR_24BON_Conformation Clinic_Performance-Mares_01
Conformation Clinic: Performance Mares
HR_24BON_Crabbe Dont Call it Maintenance_crabbe_01
To Inject or Not to Inject
Don’t Call It Maintenance! 
Palomino horse with long blond mane
Mane-tain Those Locks
Maximum Mane Care Tips
Receive news and promotions for Horse & Rider and other Equine Network offers.

"*" indicates required fields


Additional Offers

Additional Offers
This field is for validation purposes and should be left unchanged.