What is Equine Protozoal Myeloencephalitis?

Equine Protozoal Myeloencephalitis, commonly known as EPM, is a serious and debilitating disease that occurs in horses. EPM affects the central nervous system (brain and spinal cord) and, if left untreated, could result in death.

What causes EPM?

EPM is usually caused by the protozoa Sarcocystis neurona but another protozoan, Neospora hughesi, can also be the culprit. Treatment is not dependent on the type of protozoa that causes the infection since the drug protocols are the same. A horse can become exposed to the protozoa by ingesting feed or water contaminated by the feces of an opossum, a known host for S. neurona.  EPM is not transmissible between horses.

Once a horse is exposed, one of two things happens: the horse’s immune system eliminates the protozoa, or the protozoa reproduces and survives, usually infecting the central nervous system (CNS).  If the exposed horse eliminates the infection, they will always have antibodies but are not considered actively infected. It is only when the protozoa survives the immune response, generally due to a weakened immune system (caused by stress, injury, extensive trailering, showing, pregnancy, or poor nutrition) that an active infection takes place. In an active infection, the protozoa infest the horse and kill the host cells when they replicate. Lesions, or large areas of dead cells, form in the CNS and visible neurological symptoms become apparent and worsen over time. It is for this reason that recognizing the symptoms early is extremely important. If left untreated death can occur.   

What are the symptoms of EPM?

Signs of EPM may be outwardly evasive for months because the horse tries to compensate for any loss of muscle coordination. Additionally, the horse’s immune system continually tries to fight the protozoa so all that may be visible is tiredness or occasional clumsiness before the more severe neurological symptoms become apparent.

Because EPM affects the CNS, basically any neurological symptom can be diagnostic. However, poor motor coordination due to atrophy of the biceps, gluteals, and other muscles is common, as well as trunk and limb weakness, which can lead to recumbency. Rectal and bladder paralysis may occur, causing geldings to drop when not urinating or to cause urine dribbling. Changes in manure, usually towards diarrhea, have also been reported. Topically, the skin may spontaneously sweat in patches and be non-responsive to reflexive stimuli. Other common symptoms of the body include:

  • Uncoordinated movement of the rear feet, usually worse on one side (asymmetrical ataxia)
  • Recurring lameness issues that switch sides
  • Lethargy
  • Hind end weakness – worse on sloping ground
  • Trouble balancing with a lifted hoof
  • Circling, slipping, or falling while walking
  • Muscle atrophy, commonly over rump or shoulders
  • Leaning on a wall for balance
  • Not standing square, hoof cocked in or out
  • Dragging a hoof, especially while turning
  • A sore back, saddle fits irregularly
  • Unusual sweating patterns or times
  • Carrying the tail to one side or away from the body
  • Decrease or increase of sensitivity in skin or hooves
  • Falling in herd dominance order

Other common symptoms of the head include:

  • A drooping lip or repeated facial twitch
  • Change in vision
  • Dropped feed or trouble swallowing
  • Head tilt
  • Drooping ear
  • Changes in behavior

How is EPM diagnosed?

EPM can mimic other diseases so first those diseases must be ruled out. Such diseases include, Wobblers, Equine Herpes Virus 1, Lyme disease, West Nile Virus, Cushings disease, Selenium deficiency, and orthopedic lameness. A detailed history will help the vet understand your horse’s condition and aid in diagnosis so be as specific as possible. The vet will also perform an EPM neurological examination and grade any physical symptoms according to the Mayhew Scale, which is a way to determine the severity of the neurological deficit observed. Finally, laboratory tests can be conducted to confirm the disease. If a blood sample is collected and is determined to have antibodies to the protozoa, then a cerebrospinal fluid (CSF) tap must be conducted to determine if there is an active infection in the CNS. If the vet determines that an active infection is present according to their exam and laboratory tests, then they will usually begin treatment.

How is EPM treated?

In general, once EPM is diagnosed, the vet will begin treatment immediately. The goal of treatment is to reduce the population of protozoa to a degree where the immune system can take over and eradicate the infection. Therefore the drugs approved for treatment of EPM aim to control the population of protozoa, not eliminate it. Those drugs are: Oroquin-10 (undergoing field trial as of May 2011), Sulfadiazine-Trimethoprim-Pyrimethamine Combination, Diclazuril (Protazil), and Ponazuril (Marquis). A prescription from a licensed veterinarian is required for these medications.

It is also recommended that, during treatment, the horse is not ridden or worked, in order to support the recovering immune system. The vet may also prescribe anti-inflammatory medications, immune stimulants, and vitamin E for regenerating nerve cells and decreasing inflammation. Other therapies, including acupuncture, massage therapy, and the use of herbs and probiotics, in conjunction with the EPM drugs can be very beneficial for both the immune system recovery and the mental well-being of the horse. As always, if you suspect your horse has EPM, please contact you vet immediately for an examination and treatment options. Do not administer any medications without the approval of your vet. 


"EPM Disease Summary." Equine Protozoal Myeloencephalitis. EPMhorse.org, DEC 2011. Web. 6 Jan 2012. <http://www.epmhorse.org/The_Disease/The_Disease.htm>.

Equine Protozoal Myeloencephalitis