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It is generally assumed that, in the proper host, after one or more generations of schizonts have developed, merozoites invade striated muscles, transform to metrocytes, and repeatedly divide to form bradyzoites (in a cyst). Immature uni-zoite sarcocysts have been reported and are associated with a particular merozoite generation in some Sarcocystis species. A biphasic “second schizongonic” peak at 14 days post infection is not uncommon in sarcocystis infections. (Entzeroth 1983)
Electron microscopic study of merogony preceding cyst formation in at least one species of Sarcocystis indicated that single merozoites, but no meronts, were found in liver, spleen, and lymph nodes—transforming merozoites and meronts were found in myofibroblasts, satellite cells, and endothelial cells of muscle tissue—and they were surrounded by two membranes. (Entzeroth 1983) It is interesting that the parasite-host cell relationship differs during various stages of infection and in different organs.
Whether the variation of host cell-parasite contact zone depends on the species characteristics of Sarcocystis or it is influenced by the type of host cell involved remains unknown. The merozoites that were observed in the spleen and liver in the Entzeroth study were in the cytoplasm of the cell without an obvious separating membrane and that indicated to them that the cells were still motile. The merozoites found in the myoblasts, satellite cells, and endothelial cells were separated from the host cell cytoplasm by a unit membrane-the inner membrane complex of the merozoite disappeared. The importance is that the parasite-host cell contact zone was seen to consist of two membranes, the plasmalemma of the parasite and the membrane formed by the host cell. The parasite isn’t in direct contact with the host cell cytoplasm, but is separated from the cytoplasm by a regular narrow space of the parasitophorous vacuole. It was intracellular and extra-cytoplasmic.
Understanding the parasitophorous vacuole formation by S. neurona could be important to elucidating pathogenicity—the ability of this organism to induce pathology in one host but remain non-pathogenic in another. A grant was funded by USDA in 2003-2005 (A. E. Marsh and A. Wilson) to investigate these questions. Unfortunately, the project was not completed and no information was published.
Parasitophorous vacuole formation was mentioned by David Lindsay in a paper that examined penetration of equine leukocytes by merozoites. (David Lindsay 2006) This study was conducted to evaluate the interactions of S. neurona merozoites with equine leukocytes using transmission electron microscopy (TEM). It was our paper demonstrating an equine model of EPM that produces clinical signs and identifiable parasites in the tissues of experimentally infected horses that led to Dr. Lindsay’s interest in leukocyte penetration by S. neurona. Our hypothesis that merozoites were entering leukocytes, and that these infected leukocytes were migrating across the menengies and entering the CNS ,was evident to us when we readily induced EPM– whereas others that used extracellular parasites didn’t. (Ellison 2004) David Lindsay’s research using electron microscopy confirmed that merozoites enter equine lymphocytes. They observed the infected cells for 3 days and didn’t see schizogony. They also didn’t see a parasitophorous vacuole. Either is isn’t present in these infections or it forms and dissolves very quickly.
And why is this important? Our drugs need to be in the right cell at the right time to kill parasites. The long held pharmacokinetics view of Oroquin-10 and the current view of the presence and distribution of S. neurona in the horse don’t predict success for the treatment of disease. Surprisingly, just like our model, treatment is very successful. Our success means that what needs to change is the understanding of parasite infections in the horse, drug synergy, or the definition of EPM.
Our work centers on the objective measurement of specific antibodies and the reduction of these antibodies that is documented after treatment. We rely on no less than 200 veterinarians that selected possible cases of EPM, many horses that didn’t respond to alternative treatments, and they determined a response to treatment for us. We can’t use data unless the antibody testing is present. Now we know it works, we and others are working on the how. Did we the cart before the horse and get ahead of ourselves again? Perhaps. Some think so. We are happy that we have stimulated more research on the important topics associated with EPM. And so far, there are hundreds of happy horses and their owners because we did.
David Lindsay, Sheila Michell, Jibing Yang, J. P. Dubey, Robert Gogal, and Sharon Witonsky. “Penetration of equine leukocytes by merozoites of S. neurona.” Vet Parasit 138 (2006): 371-376.
Ellison, S. P., Greiner, E., Brown, K K., Kennedy, T. “Experimental infection of horses with S. neurona merozoites as a model for Equine Protozoal Myeloencephalitis.” J App Res Vet Med 2, no. 2 (2004): 79-89.
Entzeroth, Rolf. “Electron Microscope Study of Merogony Preceding Cyst Formation of Sarcocystis sp. in Roe Deer.” Parasitenkunde 69 (1983): 447-456.
Mayhew, Charles F. Simpson and Ian G. “Evidence for Sarcocystis as the etiologic agent of equine protozoal myeloencephalitis.” J Protozool 27 (1980): 288-292.
T. Jakel, C. Archer-baumann, A. M. boehmler, I . Sorger, M. Henke, D. Kliemt, U. Mackenstedt. “Identification of a subpopulation of merozites of S. singaporensis that invades and partially develps inside muscle cells in vitro.” Parasitology 111 (1999): 235-44.
