At first, veterinarians and researchers were scrambling to discover anything about the “new” neurologic disease that was causing such a stir in the horse industry. The disease, equine protozoal myeloencephalitis (EPM), was discovered to be caused by a one-celled protozoal parasite called Sarcocystis neurona. This parasite does not include the horse in its normal life cycle (which means horses can’t pass it to other horses). It is known to migrate into the spinal cord or brain of the horse and cause a multitude of asymmetric symptoms, ranging from hardly noticeable to life-threatening.

Looking for antibodies with a blood test revealed an incredible number of positive horses (up to half the equine population or more in some areas, which is still true today). In the beginning, it was thought that a positive blood test meant a horse had EPM. Then researchers realized that a positive blood test only meant the horse had been exposed to the protozoal parasite and had developed antibodies against it.

A new Western blot assay using cerebrospinal fluid was developed that was specific for S. neurona and did not react for the other sarcocystis organisms. Treatments (pyrimethamine and a sulfonamide, usually trimethoprim-sulfamethoxazole or sulfadiazine) were aimed at stopping replication of the organism in the horse’s body until the horse could mount a suitable defense. The use of medication in that timespan is advocated at this time. The life cycle of S. neurona was discovered, and prevention in the form of farm management and animal (opossum and birds) control was added to the arsenal. Folic acid supplementation was recommended, especially for mares, because of the mode of operation of drugs used in treatment. It now is thought that folic acid supplementation is not in the best interest of the health of some horses, especially pregnant mares and unborn foals.

Then, the FDA allowed two drugs (Diclazuril and Toltrazuril, both anti-protozoal medications) to be imported from Canada by veterinarians under a special provision of the U. S. government