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The biology of Sarcocystis

Sarcocystis require 2 hosts to survive, a definitive host and an intermediate host.  The sexual cycle occurs in the definitive host and the asexual cycle occurs in the intermediate host.  Definitive hosts that are important in understanding EPM are the dog and the opossum.  The dog transmits Sarcocystis fayeri to horses, this results in Equine Muscle Sarcocystosis, EMS. The opossum transmits Sarcocystis neurona to horses, but horses are aberrant hosts, S neurona isn’t believed to make cysts in horses. Opossums can also harbor S falcatula, these organisms can look like S neurona to horses. The opossum can have other Sarcocystis species but they aren’t related to equine disease.

Equine protozoal myeloencephalitis (EPM) refers to neuromuscular disease in horses, most often associated with S neurona.  Other protozoa, less often associated with EPM, include Neospora. The reaction to the protozoal infections include antibody production (we test for antibodies in serum and CSF) and inflammation.  That means disease is a syndrome. Understanding the biology, the life cycle of the parasite in it’s hosts, facilitates understanding, diagnosing, and treating EPM.



Definitive Host S fayeri: DOG              Definitive Host S neurona: OPOSSUM                       Definitive Host S falcatula: OPOSSUM

How does a horse get EPM? They ingest parasite “eggs” (oocysts) shed in feces from a definitive host and the horse immune responses set up an inflammatory cycle that benefits parasitic infection. Definitive hosts are infected by ingesting mature parasites found in muscle cysts (sarcocysts). Slow growing parasite stages, bradyzoites, are released from the digested tissues where they penetrate cells in the small intestine . The sexual stage (gametogony) of the parasite life cycle occurs here when microgamonts fuse with macrogamonts. After fertilization a protective wall develops around the newly formed zygote-it’s now an oocyst . This occurs within 24 hours of cyst ingestion. These processes don’t happen at the same time, oocyst development is asynchronous. This is important when treating animals. If a drug doesn’t kill all stages the drug is static, when the drug is removed the life cycle continues. The parasite in the oocyst divide forming 4 sporocysts in each oocyst. They are now “sporulated” and infectious to the intermediate host.





Intermediate host S fayeri: HORSE     Intermediate Host S neurona: ARMADILLO (experimentally: Cat, Raccoon)   Intermediate Host S falcatula: GRACKLE, COW BIRD

Sarcocystis enter the asexual stage of the life cycle in the intermediate host (horse) ultimately forming muscle cysts (sarcocysts), this is equine muscular sarcocystosis or EMS. Toxins are sometimes released from degenerating cysts in horses, these toxins produce signs that look just like EPM!! Horses ingest infectious parasite “eggs” (oocysts that contain 4 sporozoites) that were deposited in feed or water by the definitive host, the dog. Sporozoites show up in 4-7 days in lymph nodes. Precystic replication, “schizogony”, occurs virtually throughout the body and these stages disappear before cysts form. (This is important because repeat exposure from the environment causes “new” infections). The next stage of the asexual parasite life cycle is called a “merozoite”. Merozoites are committed to make a cysts in muscle tissue (shown below). Merozoites that are not in muscle cells die because supportive nutrients are lacking. In muscle cells, the parasite is surrounded by a vesicle where it happily transforms into a metrocyte. A metrocyte divides into many daughter cells filling the muscle cyst (sarcocyst) with dormant, slow metabolizing bradyzoites, shown in a microscopic photo below. After 70 days or so, the sarcocyst is infectious to the definitive host and the cycle can begin again, after the definitive host dines on infected muscle tissues. Most muscle cysts begin to disappear after 3 months. Ruptured cysts can cause inflammation, but bradyzoites do not make new cysts-THERE IS NO REACTIVATION OF CHRONIC INFECTION, IRRESPCETIVE OF THE IMMUNE STATUS OF THE HOST. Sarcocystis fayeri causes cysts in horses, see infected muscle tissue below.


s fayeri muscle

cyst microscopic crop



Sarcocystis neurona schizonts (or merozoites) are found in horse neural tissues.  Horses are considered aberrant hosts because muscle cysts aren’t formed by the asexual merozoites.  A very interesting experiment is intentionally infecting horses with S neurona oocysts from the opossum.  If the horse is given a HUGE dose of oocysts, and the horse has a normal immune system, the merozoites are quickly removed-- because they can’t form cysts.  These horses show neurological disease within days, antibodies are produced against S neurona as well. When the same organisms are used to infect immunodeficient Arabian foals, they are called SCID foals and have abnormal white blood cells, the foals show no clinical signs!!  Very convincing evidence, to me, that disease and clinical signs are due to how the horse handles the infection. Horses make detectable antibodies against S falcatula, another protozoa harbored by opossums.  Our experiments show that blocking some receptors on white blood cells in normal horses, and challenge them with S neurona infection, they don’t show signs, just like an Arabian SCID foal.