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Equine Protozoal Myeloencephalitis (EPM) Overview
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Equine protozoal myeloencephalitis, or EPM, is a disease cause by a protozoal infection of the central nervous system of horses. Once ingested, the sporocysts migrate from the intestinal tract into the bloodstream and cross the blood/brain barrier. There they begin to attack the horse's central nervous system. The onset of the disease may be slow or sudden. If left undiagnosed and untreated, EPM can cause devastating and lasting neurologic damage. As many as 50 percent of all horses in the United States may have been exposed to the organism that causes EPM.
EPM Transmission: The disease is not transmitted from horse to horse, in fact horses cannot transmit the disease to each other. Rather, the protozoa are spread by the definitive host, opossums, which acquires the organism from infected birds. In the laboratory, raccoons, cats, armadillos, skunks, and sea otters have been shown to be capable of being intermediate hosts. For the most part the opossum is the definitive host of the disease.  The infective stage of the organism, the sporocysts, are passed in the opossum's feces.  The causative organism is a protozoa parasite named Sarcocystis neurona. In order to complete its life cycle this parasite needs two hosts, a definitive and an intermediate. Horses most commonly contract comes into contact with the infective sporacysts of EPM from grazing or watering in areas where an opossum has recently defecated.
EPM Symptoms: Equine Protozoal Myeloencephalitis (EPM) is a master of disguise. This serious disease can be difficult to diagnose because its symptoms often mimic other health problems in the horse and signs can range from mild to severe.
As a result the clinical signs of EPM can be quite varied. Clinical signs are almost always asymmetrical (not the same on both sides of the horse). Actual symptoms may depend on the severity and location of the lesions that develop in the brain, brain stem or spinal cord.
Symptoms may include:

  1. Ataxia (incoordination), Spasticity (stiff, stilted movements), abnormal gait or lameness
  2. Incoordination and weakness which worsens when going up or down slopes or when head is elevated
  3. Muscle atrophy, most noticeable along the topline or in the large muscles of thehindquarters, but can sometimes involve the muscles of the face or front limbs
  4. Paralysis of muscles of the eyes, face or mouth, evident by drooping eyes, ears or lips
  5. Difficulty swallowing
  6. Seizures or collapse
  7. Abnormal sweating
  8. Loss of sensation along the face, neck or body
  9. Head tilt with poor balance; horse may assume a splay-footed stance or lean against stall walls for support
  10. Fatigue or narcolepsy - horse may seem to suddenly fall asleep, or lie down for extended periods
  11. Another common side effect of EPM is back soreness, which can be severe.

Three things seem to influence progression of the disease:
1.The extent of the infection (i.e. the number of organisms ingested)
2.How long the horse harbors the parasite prior to treatment
3.The point(s) in the brain or spinal cord where the organism localizes and damage occur