Equine Protozoal Myeloencephalitis

What is Equine Protozoal Myeloencephalitis?

EPM is a protozoal infection of the central nervous system caused by Sarcocystis neurona in most cases. Neospora hughesi can also cause disease. The opossum (Didelphis virginiana in North America and D. albiventris in South America) is the definitive host for S. neurona. Sporocysts are excreted in opossum’s feces and then consumed by an intermediate host. In the horse, which is an incidental-dead end host, the sporocysts are consumed in food or water contaminated by opossum feces.

Clinical Signs: Clinical signs are variable and can mimic most other neurologic diseases as well as musculoskeletal problems. This variability occurs because the parasite can affect all or part of the central nervous system (CNS).
Clinical signs reflect affected areas of the spinal cord and brain and can include the following:

  • ataxia (asymmetric) 
  • weakness
  • muscle atrophy 
  • unusual or atypical lameness
  • reduced or absent sensation
  • alterations in level of consciousness or behavior
  • cranial nerve deficits such as problems eating, facial paralysis and abnormal eye movements. 

EPM is often progressive but can have an acute or insidious onset. The progression can be rapid, or the clinical signs might appear to stabilize only to relapse or worsen later.

Diagnosis: The diagnosis is made by observing clinical signs consistent with spinal cord or brain dysfunction during a neurologic examination and exclusion of other potential causes of these signs using appropriate diagnostic testing.  Immunodiagnostic (serologic) testing on paired serum and cerebral spinal fluid samples is used to confirm antibody production against S. neurona or N. hughesi. A positive serum test indicates exposure to the organism but does not confirm CNS infection. A negative serum test usually indicates that the horse has not been exposed to the organism. Although a positive CSF test is more likely to correlate with an EPM diagnosis than a positive serum test false positives commonly occur.

Treatment: Treatment of horse with clinical signs includes administration of anti-protozoal drugs such as ponazuril and diclazuril. The sooner treatment is initiated; the better the horse’s chances are for recovery.

Prevention: Maintaining strict sanitation in the horses environment helps to decrease the chance contamination with opossum feces.