Equine Protozoal Myeloencephalitis (EPM) in Horses

Causes of Equine Protozoal Myeloencephalitis

Some protozoa, including Sarcocystis neurona, are capable of causing disease in animals. Typically, these types of protozoa require a host animal in which to carry out the reproductive part of their lifecycle. For S. neurona, that host is the opossum. An opossum ingests the protozoans, which reproduce in the animal's digestive system. The opossum doesn't become ill itself, but does, however, 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. For reasons that are not well understood, in a very small percentage of horses the sporocysts mature into a form called merozoites, which then migrate through the horse's bloodstream into its central nervous system. There, they infect the brain and/or spinal cord, causing the disease called equine protozoal myeloencephalitis.2

It is important to note that EPM is not contagious from horse to horse, nor is the horse contagious to other animals.

Diagnosing 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. However, 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, and performing blood tests or tests on the horse's spinal fluid to look for antibodies to S. neurona, which indicate that the horse has been exposed to the organism.

Treatment

Once the diagnosis has been confirmed, the most effective course of treatment can begin. There are currently three different FDA-approved treatments for EPM.

1. Ponazuril (an anti-protozoal drug) by mouth for 28 days
2. Diclazuril (also an anti-protozoal) by mouth for 28 days
3. A combination of sulfadiazine (an antibiotic) and pyrimethamine (an anti-parasitic drug) by mouth for at least 90 days3

Along with one of the above treatments, many equine vets also prescribe anti-inflammatories such as steroids, vitamin E supplementation, or immune-system-modifying drugs.