Equine Protozoal Myeloencephalitis (EPM) in Horses

for the horse to swallow or move its tongue as well as lethargy behavioral changes and a drooping eyelid 2
What Causes EPM in Horses?
EPM is caused by infection of the horses central nervous system with a singlecelled organism called a protozoan. The most common protozoan to cause EPM is Sarcocystis neurona. Less commonly it can be caused by infection with Neospora hughesi 2
Opossums are the primary host animals for Sarcocystis neurona and enable the protozoa to carry out the reproductive part of their lifecycle. An opossum ingests the protozoans which reproduce in the animals digestive system. The opossum doesnt become ill but will pass infectious sporocysts or eggs in its feces.
When a horse ingests the sporocysts from the grass water or ground they enter its digestive system. In a small percentage of horses these sporocysts can mature into a form called merozoites which then migrate through the horses bloodstream into the central nervous system. There they infect the brain andor spinal cord causing the disease called equine protozoal myeloencephalitis 1
Any horse of any age can develop EPM but it is more common in young horses that race or participate in other athletic activities 2 Its important to note that EPM is not contagious from horse to horse nor is the horse contagious to other animals.

How Vets Diagnose EPM in Horses
Diagnosing EPM can be tricky as it can come on slowly and resemble other neurological diseases initially such as viral encephalitis West Nile disease meningitis or even rabies. A definitive diagnosis can only be obtained by examining the brain tissue of an infected horse after its death.
Equine veterinarians diagnose their EPM patients indirectly by observing the characteristic signs and symptoms ruling out other causes of similar symptoms such as trauma or other infectious diseases. They may perform blood tests or tests on the horses spinal fluid to look for antibodies to S. neurona which indicate that the horse has been exposed to the organism 3
Leg Injuries in Horses
How to Treat EPM in Horses
Once the diagnosis has been confirmed the most effective course of treatment can begin. There are currently three different FDAapproved treatments for EPM.
Ponazuril an antiprotozoal drug by mouth for 28 days
Diclazuril also an antiprotozoal by mouth for 28 days
A combination of sulfadiazine an antibiotic and pyrimethamine an antiparasitic drug by mouth for at least 90 days3
Along with one of the above treatments many equine vets also prescribe antiinflammatories
such as steroids vitamin E supplementation or immunesystemmodifying drugs. 
Prognosis for Horses With EPM
With treatment up to 60 percent of horses with EPM will improve but less than 25 percent recover completely. Relapses are also common for as long as two years after the initial treatment with antiprotozoal medications 3
Fortunately your horse has a good chance of regaining at least some of its former health with prompt intervention. If left untreated the prognosis for horses with EPM is very poor as the condition can deteriorate to seizures and death.
How to Prevent Equine Protozoal Myeloencephalitis
There is no vaccine for EPM. Prevention consists of minimizing the presence of opossums on your property as much as possible. Wiremesh fencing can help keep these marsupials out of your stable area but opossums are good climbers so you may have to resort to electrified fencing if theyre especially persistent.
Opossums are generally looking for food and are not picky eaters. They will raid trashcans eat dog food thats left outside consume your horses grains and even eat carcasses of dead animals that they find on the road or within your property. Its essential that all food stores be secure and any animal carcasses buried promptly. Clean up any spilled feed right away and make sure trashcans have tightly fitted lids.
If opossums make their home on your property they should be humanely trapped and removed. Other options are motionactivated lights or sprinklers which can frighten the animals away.