EPM in Horses - Equine Protozoal Myeloencephalitis

Updated May 8, 2023

Equine Protozoal Myeloencephalitis (EPM) is a disease of the central nervous system (brain and spinal cord) caused by the protozoa Sarcocystis neurona. EPM is transmitted by the opossum through intermediate hosts, also known as carriers, which may include birds, cats, and other animals.

opossum that causes epm in horses

Symptoms of EPM

Clinical signs of EPM can present similar to other diseases or neurological conditions. Symptoms can vary depending on where the protozoa organism is in the nervous system – from the brain to the tail. EPM may be characterized by:

  • Ataxia (incoordination) and weakness – Typically in the hind legs with symptoms worsening if the horse goes up or down slopes. Ataxia may be more pronounced when the horse’s head is up, and they may stand splay-footed or lean against a wall for support.
  • Asymmetry (worse on one side of the body than the other).
  • Atrophy (loss of muscle mass) – Generally in the hind limbs or topline, but may occur in the neck, face or front limbs.
  • Facial nerve paralysis - Displayed as a droopy eyelid or lip, abnormal eye movements, difficulty chewing or swallowing. The horse may have a head tilt (indicates the brain stem is involved).
  • Abnormal gait or lameness.
  • Frequent circling.
  • Attitude changes.
  • Seizures or collapsing.

Diagnosis and Treatment of EPM

In addition to a complete physical examination, your veterinarian will perform a series of specific neurological tests such as crossing legs, pulling the tail, blindfolding, etc. Your vet may take blood, however, blood tests only indicate whether the horse has been exposed to the parasite, but do not confirm infection.

There are many conditions that can cause similar neurological signs. Therefore, additional tests such as x-rays or a spinal tap to analyze cerebrospinal fluid may be required to rule out things like Wobblers Disease or Equine Herpes Virus. Your vet may also give antibodies and see if your horse responds positively to this treatment within a month.

Early diagnosis and prompt treatment is crucial fighting the disease and making your horse as comfortable as possible. Depending on your unique horse’s situation, a vet may prescribe antibiotics to inhibit the parasite from further replicating, and anti-inflammatories to control swelling and inflammation.

Supplements that May Lend Support

Many veterinarians suggest horses undergoing treatment for EPM be supplemented with Vitamin E, folic acid, and thiamine to support their muscle and nervous tissue. Other antioxidants as well as natural anti-inflammatories such as MSM, Omega 3 Fatty Acids and certain herbs may be beneficial in reducing damage to cells as the protozoa are destroyed.

Prescription Medications Available

There are FDA-approved treatments for EPM (antiprotozoals and/or antibiotics) on the market, such as:

  • Marquis® (ponazuril)
  • Navigator® paste (nitazoxanide)
  • ReBalance™ (sulfadiazine and pyrimethamine)
  • Protazil (diclazuril)

In addition, some veterinarians recommend the prescription medications bute (phenylbutazone), Banamine® (flunixin meglumine) or PrednisTabs® (corticosteroid) as anti-inflammatories, to help manage pain during treatment.

Management Suggestions to Reduce Risk of Infection

The opossum is the definitive host of the protozoa that causes EPM. The parasite is spread in the opossum’s feces. Birds, cats, and other animals can be carriers. Your horse may ingest the organism’s eggs in their hay, grain, or water and become infected. This parasite can remain in your horse’s central nervous system for years without causing signs of the disease.

Unfortunately, there is currently no vaccine for EPM. Implementing measures to keep these animals away from your horses’ food and water sources may reduce the risk of infection. Other tips for preventing infection include:

  • Keep feed rooms closed and containers sealed.
  • Clean up any grain that has dropped as soon as possible.
  • Provide fresh, clean water.
  • Use feeders to minimize spills.
  • Clean up after spilled bird feeders and fallen fruit, which can attract opossums.
  • Keep hay sheds or storage rooms closed to discourage pests from making it their home.
  • Schedule regular appointments with your veterinarian.

SmartPak strongly encourages you to consult your veterinarian regarding specific questions about your horse's health. This information is not intended to diagnose or treat any disease, and is purely educational.