Has Your Horse Been Exposed to a Parasite that Causes EPM?

Be aware of the signs of this debilitating neurological disease.

Nearly 80% of U.S. horses studied have been exposed to the parasite that causes EPM.

When it comes to the neurological disease equine protozoal myeloencephalitis (EPM), the sooner a diagnosis is made and treatment can begin, the better the horse’s chances are for recovery.

EPM is a serious neurological disease primarily caused by Sarcocystis neurona, a protozoan parasite that invades the brain, brainstem and spinal cord. Neospora hughesi is a less common cause of EPM.

One study indicated that overall, in the United States, 78 percent of horses have antibodies against Sarcocystis neurona and 34 percent have antibodies against Neospora hughesi. However, less than 1 percent exposed to EPM will develop clinical signs.

Sarcocystis neurona is spread to horses from a definitive host, in this case, an opossum. Horses become infected with EPM through contact with opossum feces through grazing or contaminated feed.The definitive life cycle of Neospora hughesi is not yet fully understood.

In a horse’s life, there can be a time when he is considered seropositive—meaning the horse has been exposed to the parasite that causes EPM. Some horses harbor the parasite for months or years, then slowly or suddenly develop clinical signs, while others might never develop signs of EPM.

Because horses have such a high chance of being exposed to the parasites Sarcocystis neurona or Neospora hughesi, horse owners should be aware of the subtle early signs of EPM so they can promptly consult with their veterinarian for diagnosis and treatment.

When it comes to neurological diseases such as EPM, missing the subtle early signs could mean lost time before veterinary diagnosis and treatment.

Horse owners should look for:

  • Stumbling or tripping
  • Tilted head
  • Asymmetrical muscle loss
  • Lameness or gait abnormality
  • Leaning against walls

While virtually every horse is at risk, some horses are at greater risk than others, with age, time of year, location, training and history all potentially affecting a horse’s susceptibility. 

For those horses that are exposed to the protozoal parasites Sarcocystis neurona or Neospora hughesi, stress from things such as travel or training can compromise the immune system, thus potentially activating the disease.

“Early detection by horse owners and farm managers, diagnosis by a veterinarian and an effective treatment plan are the keys to stopping the progression of the disease,” said Sarah Reuss, VMD, DACVIM, equine professional services veterinarian for Boehringer Ingelheim. “The faster treatment begins, the better the chance for the horse to recover.”

The importance of prompt treatment of EPM cannot be overstated. 

Marquis (15% w/w ponazuril) is formulated to reach steady state in as little as 24-48 hours when using the loading dosage of 15 mg/kg. Marquis kills the parasite Sarcocystis neurona to stop it from inflicting further damage to the central nervous system.

Your horse might have been exposed to the parasites causing EPM. Make sure you’re aware of the subtle signs of this debilitating neurological disease so you can promptly contact your veterinarian.


The safe use of MARQUIS in horses used for breeding purposes, during pregnancy, or in lactating mares, has not been evaluated. In animal safety studies, loose feces, sporadic inappetence, lost weight, and moderate edema in the uterine epithelium were observed. For use in animals only. Not for use in horses intended for food. Not for human use. Keep out of reach of children. Marquis is a Merial product. Merial is now part of Boehringer Ingelheim.


Morgan, K. EPM: Understanding this debilitating disease. American Association of Equine Practitioners. 2016. Accessed May 1, 2018.

James, K.E.; Smith, W.A.; Packham, A.E.; et al. Seroprevalences of anti-Sarcocystis neurona and anti-Neospora hughesi antibodies among healthy equids in the United States. JAVMA. 2017;250(11):1291-1301.

Reed, S. Neurology is not a euphemism for necropsy: a review of selected neurological diseases affecting horses. American Association of Equine Practitioners. 54th Annual Convention Proceedings. AAEP. 2008:78-109.

Marquis Freedom of Information Summary supplement.

Lindsay, D.S.; Dubey, J.P.; Kennedy, T.J. Determination of the activity of ponazuril against Sarcocystis neurona in cell cultures. Vet Parasitol. 2000;92(2):165-169.

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