Equine Protozoal Myeloencephalitis, or EPM, is a disease that affects horses. This disease can cross the blood/brain barrier and attack the Central Nervous System (CNS) and can become debilitating to the animal. The horse does not feel any pain but the symptoms the horses may exhibit can go from weakness, muscle loss/wasting, un-coordination to death.
EPM is caused by one to two protozoa from the feces in some of the opossum animals. The two protozoa are “Sarcocystis neurona” or S.neurona, and “Neospora hughesi” or N.hughesi and the majority of horses afflicted with EPM have the S.neurona protozoa. Horses can contract EPM by eating food stuffs that are infected with these protozoa, whether it be grain, hay, turn out fields or even contaminated drinking water. It is not passed horse to horse as horses are the dead-end carrier.
Not all horses who have been exposed to S.neurona will develop EPM. If the horse has a good immune system built up, some horses will be able to keep the infection at bay if it has been exposed. It will carry the protozoa for life but it may never manifest itself. However, if the immune system has been compromised by either stressful events, trauma, pregnancy or inadequate nutrition, the disease can certainly manifest. If left untreated, the protozoa will begin to duplicate and spread to areas in the spine and brain – this causes lesions in the CNS which then will begin to cause the neurological symptoms a lot of people expect when they hear of EPM.
Horses can harbor the protozoa for a few years before it finally manifests its symptoms. Research has shown the incubation period to be between two weeks to two years until the horse begins to show symptoms. If left untreated, as mentioned before, it can result in death to the animal either naturally or by euthanasia.
Information taken from http://www.epmhorse.org/. Thank you!