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United States Department of Agriculture

Agricultural Research Service

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Sarcocystis neurona
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Systematics Success Stories

Life cycle for Sarcocystis neurona, causative agent of equine protozoal myeloencephalitis is distinguished from related non-pathogenic species circulating in opossums based on molecular systematics approaches.

Problem: The coccidian parasite Sarcocystic neurona is the causative agent of equine protozoal myeloencephalitis (EPM), a severe neurological disease of horses. EPM was an enigma prior to linkage of the disease with a specific coccidial pathogens. Understanding how to reduce exposure of horses to infection required an accurate understanding of parasite diversity, host associations and the mode of perpetuation for the parasite at the interface of natural and managed systems.

Solution: Genetic means were employed to identify the parasite as a member of the genus Sarcocystis, which cycle in an obligate manner between herbivores (containing the tissue cyst stage) and carnivores (containing the reproductive stages). Identification of the specific genetic signature for S. neurona was then crucial in identification of opossums as hosts capable of shedding oocysts onto horse pastures. These findings enabled us to identify cyst stages for the first time, that have been critical in documenting the distribution of the parasites through survey of an array of wild hosts.


  • Diagnostic tools have promoted accuracy and sensitivity in the quest to understand the routes for exposure of horses to the causative agent of EPM.

  • Identification of S. neurona as the specific agent of EPM has promoted development of potential mechanisms for treatment and control.

  • Subsequent analyses have identified a suite of morphologically similar and closely related parasites that circulate in marsupials including opossums, but which differ in their pathogenic potential.

Interestingly, the particularly close relationship of these parasites to those in Australian marsupials suggests an archaic association that links these faunas historically and pre-dates the separation and divergence of the Neotropical and Australian faunas over 65 million years ago.

(Contact- Benjamin Rosenthal, J.P. Dubey, Animal Parasitic Diseases Laboratory)

Last Modified: 8/12/2016
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