http://www.aaep.org/info/horse-health?publication=752 "Equine Protozoal Myeloencephalitis (EPM) is a master of disguise. This serious disease can be difficult to diagnose because its signs often mimic other health problems in the horse and signs can range from mild to severe. More than 50 percent of all horses in the United States may have been exposed to the organism that causes EPM. The causative organism is a protozoal parasite called Sarcocystis neurona. ... DIAGNOSIS Diagnosis of EPM is difficult to make because there is no specific assay for this disease and because clinical signs of EPM mimic other neurological diseases. Your veterinarian will first conduct a thorough physical examination to assess your horse's general health and identify any suspicious signs. One notable clue is the disease often tends to affect one side or part of the horse more than another. If your equine practitioner suspects EPM, he or she may order blood and cerebrospinal fluid (CSF) analysis. Cerebrospinal fluid may be collected by way of a special needle inserted into the spinal canal either in a site on the lower back or at the poll. Potential risks are involved with the procedure that should be discussed with your veterinarian. A positive blood test only means the horse has been exposed to the parasite, not that it has or will develop clinical disease. Prompt, accurate diagnosis is essential and treatment should begin immediately. TREATMENT The sooner treatment begins, the better the horse’s chances are for recovery. Sixty to 70 percent of EPM cases aggressively treated show significant or complete reversal of symptoms. Many horses are able to return to normal activity...."