Equine Protozoal Myeloencephalitis:
The Life Cycle of the Protozoa

This neurologic disease was first recognized in the 1970s. S. neurona was confirmed as the causative organism in 1991. Equine Protozoal Myeloencephalitis is a protozoal disease that causes inflammation of the brain and/or spinal cord in horses, leading to various degrees of neurological deficits. The primary organism responsible for this disease is Sarcocystis neurona.1 Neospora hughesi (pronounced Hughes-see-I) is sometimes identified as the causative organism, and very rarely, Neospora caninum. All of these organisms are in the coccidia family. Both S. neurona and N. hughesi use the opossum for the definitive host.

Interestingly, EPM is a disease of the "Americas" or western hemisphere. As far as we know now, it does not occur in horses in Europe or Australia, unless those animals have spent time in North or South America.

It is generally thought that up to 53% of horses can be sero-positive where the opossum is present in the US, meaning they have been exposed to the disease, even if they don't show clinical signs.5 The protozoa is a thin shelled organism – it doesn't survive well in some environments, especially arid or cold climates.

Regional prevalence of sero-positive horses that are showing signs of Neurologic Disease3

–California 35%
–Oklahoma 80%
–Texas 65%
–Kentucky 66%
–Florida 56%
–New York 57%

Horses develop clinical disease more when they are stressed – shipping around in trailers, competing at shows or participating in events, etc. However less than 1% develop the disease.1

Like many other animal diseases that are affected by environmental influences, some regions of the country have a higher incidence of the disease than others. While 53% of the general horse population can be sero-positive5, the number of those horses showing neurologic disease will vary greatly. As you might expect, areas like Oklahoma that are warm and humid will have a higher incidence of sero-positive horses with clinical symptoms of neurologic disease. California is arid and therefore not conducive to the survival of the protozoa in the environment. There have also not been reports of many opossum in this region. However, there are many horses that travel to California for racing, breeding and horse shows, that have been exposed to EPM in other regions of the country. That is why you will see a significant number of sero-positive horses with neurologic disease in this area of the country.

The opossum is the definitive host for S. neurona. This means the opossum will ingest the parasite where it will reproduce in the intestine of this host and release sporocysts in the opossum's feces. Sporocysts are the infective stage of the parasite to horses.


There are several intermediate hosts for this parasite and they do not shed an infective form of the parasite. The intermediate host will ingest sporocysts that hatch in gut (excyst) releasing sporozoites that invade the vasculature of host. Organisms are carried to muscle tissue of the intermediate host where they invade muscle cells, asexually divide and form tissue Sarcocysts (muscle-cysts).1 The intermediate hosts include the armadillo, skunk, raccoons and cats.1

intermediate hosts

The opossum is a scavenger that will feed on the carcasses of dead intermediate hosts. An important point to remember is that trying to kill all the cats and raccoons around your barn may actually present the opossum with sarcocyst infected tissue on which to feed. The horse is an aberrant host, meaning it does not form the sarcocysts in its muscle tissue. Horses can not spread the disease to another horse.1

In review, the life cycle of EPM requires both a definitive host and an intermediate host.

cycleAn intermediate host is one in which the parasite lives, but does not reproduce or shed an infective form of the parasite. In the case of EPM, the intermediate host ingests the sporocyst – the sporocyst hatches in the gut of the intermediate host and releases sporozoites. These sporozoites then invade the vasculature of the host and are carried to muscle tissue where they form muscle cysts, called Sarcocysts.

Intermediate hosts for EPM include the nine banded armadillo, the striped skunk, raccoons and domestic cats. A study has shown that 59% of raccoons have tested positive to S. neurona. Opossum become infected by eating the road-killed carcass of naturally infected raccoons.4

The definitive host of EPM is the opossum. A definitive host is one in which the parasite reproduces in the intestine and releases an infective form of the organism (in the case of EPM, the infective form of the organism is the egg, called a sporocyst).1

Route of infection

This illustration shows the route of infection in the horse.

  1. The horse will ingest the sporocysts while consuming forage or grain contaminated with opossum feces.
  2. The organism will pass into the small intestine where it will "excyst". The resulting sporozoites will enter the blood stream.
  3. This stage can replicate in the endothelial cell of the blood vessel to create merozoites
  4. Merozoites migrate to the neural tissue of the horse.
  5. Once the merozoite enters the CNS, it invades cells within the spinal cord and or brain (neurons and microglia cells). At this point it undergoes another round or replication and forms a small, sac-like structure we call a schizont.
  6. The developing schizonts can put pressure on the nervous tissue.
  7. Their slow growth can result in destruction of the nervous tissue to result in permanent damage. This structure eventually ruptures and releases many more merozoites which can then repeat the process. Damage to the nervous tissue is from the direct invasion by the parasite as well as the space-occupying effects of the schizont. This damage can occur at a single site or at multiple sites along the spinal cord as well as within the brain.5

Risk Factors Associated with EPM

There are several risk factors that can affect the development of this disease. The competency of the immune system of the horse can certainly play a role in the disease – the immune system can be compromised during times of stress and the horse could begin to exhibit signs of the disease. Therefore younger horses with developing immune systems or older horses with compromised immune systems can be at risk.

Also horses who are in heavy training or whose immune systems are stressed from being shipped long distances for competitive events can be at risk.

Of course those horses who have a higher exposure to the sporocysts due to the presence of opossum are at risk.

Horses residing on premises where EPM was previously diagnosed have an increased risk for developing EPM.

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IMPORTANT SAFETY INFORMATION: The safe use of MARQUIS in horses used for breeding purposes, during pregnancy, or in lactating mares, has not been evaluated. In animal safety studies, loose feces, sporadic inappetence, lost weight, and moderate edema in the uterine epithelium were observed.

Caution: Federal (U.S.A.) law restricts this drug to use by or on the order of a licensed veterinarian.
Residue Warning: For use in horses only. Do not use in horses intended for food.
Precaution: The safe use of MARQUIS in horses used for breeding purposes, during pregnancy, or in lactating mares, has not been evaluated.

1. Reed S. Neurology is Not a Euphemism for Necropsy: A Review of Selected Neurologic Diseases Affecting Horses, Proceedings of the 54th Annual Convention of the AAEP, San Diego, CA December 2008;78-109
2. Saville WJ, Reed SM, Granstrom DE, et al, Seroprevalence of antibodies to Sarcocystis neurona in horses residing in Ohio. J Am Vet Med Assoc 1997;210(4):519-524
3. West, C. Equine Protozoal Myeloencephalitis: Less Common Than We Thought?, TheHorse.com, March 22, 2005, Article #5608. ©2005 TheHorse.com
4. Lindsay DS, Dubey JP, Kennedy TJ Determination of the Activity of Ponazuril against Sarcocystis neurona in Cell Cultures. Vet Para 2001;100(3-4):131-134.
5. Dubey, J.P., Lindsay, D.S., Saville, W.J.A., Reed, S.M., Granstom, D.E., Speer, C.A. A review of Sarcocystis neurona and equine protozoal myeloencephalitis (EPM), Vet Parasitol 95 2001:89-131