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Equine Protozoal Myeloencephalitis

By: Dr. Philip Johnson

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Equine protozoal myeloencephalitis or "EPM" is an infectious disease of horses caused by a microscopic protozoan parasite, which has been named Sarcocystis neurona, and is spread by the opossum. The opossum passes the organism in its feces, which are then ingested by horses when they eat contaminated feed, hay or water. This parasite gains access to the horse's brain and spinal cord where it causes inflammation and destruction of nerve tissues.

The incidence of EPM is high in many parts of the USA. It certainly represents one of the most common diagnoses for horses with neurological disease. Our knowledge about EPM changes frequently. In particular, recommendations regarding diagnostic testing, treatment options, and how to determine when to stop treatment change rather frequently as new information is learned about EPM.

Hopefully, increased awareness of EPM will lead to earlier disease recognition and treatment since treatment will likely be more successful when instituted early in the course of the disease.

How Infection Occurs

Horses become infected with S neurona by ingesting food material (grain, hay, grass) or drinking water which has been contaminated by the feces of the opossum. Bird droppings may also be an important source for the infective sporocysts. In the horse, the Sarcocystis neurona parasites migrate to the CNS (brain and spinal cord). *Not all infected horses develop recognizable symptoms of neurological disease.

Recent epidemiological surveys, using a blood test for Sarcocystis neurona, suggest that approximately 50 to 60 percent of all horses in the Midwest have been infected by, or exposed to, Sarcocystis neurona. Fortunately, only a relatively small number of those horses will actually ever develop neurological disease. Unfortunately, we do not presently know which exposed horses are likely to develop signs of EPM.

Sarcocystis neurona parasites set up tiny colonies in the CNS. The actual signs of abnormality depend on the location and the extent of these colonies. The parasites cause damage to nervous tissue and provoke an inflammatory response. When evident, signs of neurological dysfunction in affected horses are attributable to either the destruction of nerve tissue by parasites or interference with normal nerve impulse conduction by the horse's inflammatory reaction against the parasite.

What to Watch For

Essentially, EPM causes signs of neurological dysfunction in affected horses. EPM may cause any neurological sign or combination of signs: any neurological sign or combination of neurological signs could be due to EPM.

Signs of neurological dysfunction associated with EPM may be very minor and nonspecific, and include such problems as mild lethargy, relatively poor body condition, change of attitude, reduced exercise tolerance and/or difficulty with complex gaits. Alternatively, EPM may cause some rather striking and clear-cut neurological abnormalities such as:

  • Incoordination
  • Inability to stand up
  • Lameness (ill-defined lameness)
  • Blindness
  • Head tilt (vestibular dysfunction)
  • Inability to chew food/swallow
  • Facial paralysis
  • Abnormal eye movements
  • Stumbling/falling
  • Seizures/convulsions
  • Headshaking
  • Urinary incontinence
  • Muscle tremors
  • Inappropriate patchy sweating
  • Collapse
  • Discrete loss of specific muscle groups (neurogenic atrophy)


    It is important to recognize that EPM can cause ANY neurological abnormality or combination of abnormalities; therefore, EPM must be considered by the veterinary diagnostician whenever he/she is presented with equine neurological problems. It is also important to consider EPM because it is so common. However, it should be carefully noted that the diagnosis of EPM is commonly made without proper justification and/or consideration of other possibilities. It is easy to implicate this disease without a proper clinical evaluation. Lots of horses are treated for EPM when they are not actually affected with this disease.

    Classically, EPM has been associated with the following three neurological features, and if these three features are evident, the affected horse is very likely affected with EPM:

  • Multifocal neurologic disease: Symptoms can only be explained by damage to more than one area of the central nervous system. This type of damage, in contrast to focal damage of one area of the spinal cord, has been explained by the random migratory movements of the parasite through nervous tissue. The multifocal nature of EPM is important in distinguishing it from other CNS diseases.

  • EPM often affects one side of the horse to a greater extent than the other side.

  • EPM sometimes leads to focal muscle atrophy.

    The most common clinical manifestation of EPM is ataxia (incoordination) that arises from damage to the spinal cord.


    EPM-affected horses are commonly presented to veterinarians for diagnosis of lameness. BUT, a diagnosis of EPM as a cause for lameness should not be made without first undertaking both a thorough lameness examination and a thorough neurological examination. Many of these horses have musculoskeletal problems, not a neurologic one.

    Because it's a difficult lameness to figure out, it does not indicate the need to look for EPM. Incoordination (associated with EPM OR other causes of spinal dysfunction) might predispose to lameness because the affected horse is prone to falling or spraining joints. It might be difficult to determine whether "lameness" is due to musculoskeletal pain, neurological dysfunction or a combination of both.

    In horses that are lame as a consequence of musculoskeletal pain, the lameness should be less pronounced during treatment with pain-relieving drugs such as phenylbutazone. Veterinarians often elect to use a short course of treatment with phenylbutazone to determine whether the observed problem is associated with pain, versus neurologic disease, which does not cause limb pain. If treatment with phenylbutazone causes a reduction in the severity of the lameness, it is less likely that the major problem is a neurological disease (such as EPM). Signs of limb dysfunction associated with EPM do not typically change during treatment with phenylbutazone.

    This disease should be suspected whenever horses develop signs of neurological disease that cannot be readily explained by other obvious events. It must be remembered that EPM can produce any neurological abnormality or combination of abnormalities. Horses living (or that have lived) in areas in which the opossum is found are at the highest level of risk for EPM.

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