Submitted to: Journal of Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/15/2004
Publication Date: 4/15/2004
Citation: Lindsay, D.S., Mitchell, S.M., Vianna, M.C., Dubey, J.P. 2004. Sarcocystis neurona (protozoa: apicomplexa): description of oocysts, sporocysts, sporozoites, excystation and early development.. Journal of Parasitology 90:461-465..
Interpretive Summary: Sarcocystis neurona is a single-celled parasite that causes a fatal neurologic disease in horses. Scientists at the Beltsville Agricultural Research Center and the Virginia Tech redescribe the structure of this parasite. The results will be of interest to parasitologists, veterinarians and biologists.
Technical Abstract: Equine protozoal myeloencephalitis is a major cause of neurological disease in horses from the Americas. Horses are considered accidental intermediate hosts. The structure of sporocysts of the causative agent, Sarcocystis neurona, has never been described. Sporocysts of S. neurona were obtained from the intestines of a laboratory-reared opossum fed skeletal muscles from a raccoon that had been fed sporocysts. Sporocysts were 11.3 by 8.2 µm and contained 4 sporozoites. The appearance of the sporocyst residuum was variable. The residuum of some sporocysts was composed of many dispersed granules, while some had granules mixed with larger globules. Excystation was by collapse of the sporocyst along borders of plates. The sporocysts wall was composed of 3 layers, i.e., a thin electron dense outer layer, a thin electron lucent middle layer and a thick electron dense inner layer. The sporocyst wall was thickened at the junctions of the plates. Sporozoites were weakly motile and contained a centrally or posteriorly located nucleus. No refractile or crystalloid bodies were present, but lipid-like globules about 1 µm in diameter were usually present in the conoidal end of sporozoites. Sporozoites contained 2 to 4 electron dense rhoptries and other organelles typical of coccidian zoites. Sporozoites entered host cells in culture and underwent schizogony with in 3 days. Equine protozoal myeloencephalitis (EPM) caused by Sarcocystis neurona is a major neurological syndrome of horses in the America's (Dubey et al., 2001). The parasite was known since the early 1960's but was not named until 1991 when it was isolated and grown in cell culture (Dubey et al., 1991). Horses are considered accidental hosts and no S. neurona sarcocyst stages have been described in their tissues. The present study was done to describe the structure of S. neurona sporocysts from an experimentally infected definitive host, the Virginia opossum, Didelphis virginiana. This is important because the structure of known S. neurona sporocysts has not previously been reported and the opossum is the definitive host for at least 4 species of Sarcocystis. Additionally, we describe the features of excystation and the ultrastructure of S. neurona sporozoites.