Equine protozoal myeloencephalitis (EPM) is a disease of the nervous system of the horse. It is caused by single celled microorganisms (protozoa) which damage regions of the brain and spinal cord. The parasitic protozoan Sarcocystis neurona is responsible for most cases of EPM. A smaller number are caused by Neospora hughesi. Horses are considered the dead end host of this parasite, once inside the horse it can no longer be passed to another animal.
Opossums are the parasite’s definitive host. A definitive host is the host organism in which the parasite matures and their oocysts (eggs) are secreted via the feces. Horses ingest the sporocysts, (the resting stage of the oocyst) from grain, hay and pasture contaminated with opossum feces. The sporocysts leave the gastrointestinal tract through the blood and lymphatic systems. For a horse to become infected with EPM, the sporocysts must cross the blood brain barrier. Most horses’ immune systems are able to effectively fight this infection. Only one percent of the horses who ingest the EPM causing protozoa get the disease.
The symptoms of EPM vary considerably because any region of the brain or spinal cord can be affected. Common symptoms are uncoordinated movement especially in the rear end, stumbling, tiredness, lack of balance, muscle atrophy, loss of or heightened sensation throughout the body, declining dominance in the pasture, partial facial paralysis, personality changes, and sweating at unusual times. Horse’s symptoms can be subtle or acute, and they may come and go. In addition, EPM looks like many other equine maladies that compromise the central nervous system. Diagnosing EPM includes blood tests to rule out other diseases and a neurological exam. Other diseases that affect the central nervous system are Wobblers Syndrome, Equine Herpes Virus 1, Lyme Disease, Cushings Disease, West Nile Virus. The horse could also be selenium deficient or have a lameness issue. A horse with a fever or cough before the onset of neurological symptoms generally does not have EPM. A lame horse that improves when administered Bute generally does not have EPM.
Blood tests can show that a horse has been exposed to the parasite that causes EPM because the antibodies made by the immune system will be detected. To confirm a horse has an active EPM infection, however, the cerebrospinal fluid will have to be tested. This proves the parasite did cross the blood brain barrier and was not killed off by the horses immune system. Treating EPM is most successful if the disease is caught early. The disease will continue to progress and can eventually cause death if left untreated. If permanent damage has been done to the central nervous system it can not be reversed, but treatment can halt the progress of the disease. Treatment for horses with EPM consists of administering anti-protozoal drugs and supportive treatment to help the horses immune system fight the parasite. Treatment usually lasts for 90 to 120 days and may need to be repeated depending upon the severity of the disease. Horses can learn to compensate for loss of sensation due to nerve damage and in time may regenerate some nerves. Rehab is often required for these horses, and it takes time and patience. There is a wealth of information on the internet to able horse owners better understand this disease.
References:
EPM: Etiology,diagnosis and treatment
EPM in Horses