In 1964, this condition was described as segmental myelitis, noted Kenton Morgan, DVM, Dipl. ACT, a Bayer Animal Health technical service veterinarian at the Horseman's Day seminar during the AAEP Convention. He added that necropsy exams showed sections of spinal cord affected, but at that time scientists didn't know what was causing the problem. In 1976, the problem was given its current name, equine protozoal myeloencephalitis (EPM). "Myelo" refers to spinal cord; "encephalo" refers to brain; "itis" refers to inflammation/infection.

EPM is a protozoal disease that causes inflammation in the brain and spinal cord of the horse. In the late 1970s to early 1980s, Morgan said there was debate whether a toxoplasma or Sarcocystis organism was the cause. They are closely related members of the protozoan family. In the early 1990s, an organism was grown from affected equine spinal cord and given the name Sarcocystis neurona.

S. neurona is what causes EPM in the horse. There is a Neospora organism that can cause EPM, but only in a very small number of cases, and it is considered insignificant as a cause.

This is a disease of the Americas; however, there is one report of a native horse from France suffering from EPM that is under investigation. Previously, every horse which developed EPM in Europe had at some time been in he United States.

How the Life Cycle Works
The Sarcocystis group of protozoa have a complex two-host life cycle, explained Morgan. Much has been learned about the life cycle of S. neurona in the last eight years. It wasn't until 1995 that researchers learned the opossum was the definitive host.

A two-host life cycle means a definitive host sheds the infective parasite in its feces, and intermediate hosts--in this case the nine-banded armadillo, striped skunk, raccoon, and cat--complete the life cycle. That intermediate host does not shed an infective form of the parasite. The life cycle is completed only if the intermediate host dies and is eaten by the opossum because the parasite is in muscle tissue in the intermediate host.

An opossum sheds sporocysts (the infective form) in its feces, he noted. One of intermediate hosts picks up the parasite, which goes through a replication process while in the intermediate host. This results in sarcocysts in the muscle of the intermediate host, which can live for years with the parasite in its muscle tissue. When those intermediate hosts die and an opossum eats the infected muscle, the opossum ingests sarcocysts and the parasite undergoes additional replication and completes its life cycle.

Horses and EPM
The horse is thought to be a dead-end host. That means the parasite is not passed to other animals (although there is still debate on this point whether horse muscle contains a form of the parasite that could be passed to the definitive host; more on this later). The horse orally picks up the parasite from grazing or drinking around infected opossum feces. Replication of the parasite occurs somewhere in the horse, although that is still being investigated.

A recent case at Michigan State University indicates that the horse might be a true intermediate host rather than a dead-end host, said Morgan. A four-month-old foal died after being admitted to the hospital and researchers found sarcocysts in the tongue of this foal. That was the first time the sarcocyst stage of the parasite had been found in a horse. However, this finding might have been because the foal was so sick. (For more information, see "The Latest on EPM".)

In the horse, the parasite is ingested and somewhere in the horse goes through replication and changes form from sporocyst to merozoite, Morgan explained. These are the infective forms that invade cells. This stage of parasite can cross the blood-brain barrier by gaining access into white blood cells. They invade a nerve cell and through asexual replication form a sack-like structure (schizont) that ruptures and kills the host cell and releases more parasites to infect other host cells. The flower-like image of schizonts often seen in magazines is a group of merozoites.

Most horses which ingest sporocysts clear the infection, but they are positive for antibodies to the parasite in their blood. A small number of horses don't clear the parasite, and they are candidates to develop neurologic signs of EPM, he said. There is no horse-to-horse transmission.

Morgan said it is difficult to diagnose or confirm EPM. The only 100% confirmation is if on post-mortem examination the veterinarian finds parasites in the brain or spinal cord. In the live animal, there isn't a confirmative test.

Clinical signs include asymmetrical ataxia (incoordination) with or without atrophy (muscle wasting). Morgan explained that means one side is not affected the same as the other.

Most other central nervous system (CNS) problems in horses produce symmetrical clinical signs (which are the same on both sides of the horse).

Bayer Animal Health was the first company with a licensed treatment for EPM. It is called Marquis, said Morgan. The active ingredient is ponazuril. This product is a paste formulation that is given once daily for 28 consecutive days. The drug has demonstrated effectiveness in treating horses with EPM since its approval in 2001, and it has a favorable safety profile.

Morgan also mentioned another drug that just received FDA approval for the treatment of EPM. That drug is called Navigator. The active ingredient of this treatment is nitazoxanide. It is sold by IDEXX and is also a paste medication given for 28 days. (For more on this medication, see "More on the New EPM Treatment".)

He said veterinarians occasionally might use other treatments, including a sulfadiazine and pyrimethamine combination generally given once a day for three to six months. Supportive therapy can include anti-inflammatory products and vitamin E.

Morgan stressed that horse owners can use good management practices to reduce the risk factors that contribute to disease incidence. Keeping feed sources clean and free of opossums is important, as is protecting your hay from rodent infestation. Dispose of any animal carcasses on or near your property. Stress has been shown to increase the risk of EPM onset, so minimize stress, especially during transport. There is a licensed vaccine available; talk to your veterinarian about its use in your situation.

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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