Submitted to: Journal of Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 10, 1998
Publication Date: N/A
Interpretive Summary: Sarcocystis neurona is a protozoan (single celled) parasite. It causes equine protozoal myeloencephalitis (EPM) in horses. EPM is the most common cause of neurological disorders in horses. Affected horses may develop lameness, paralysis, and some die acutely. Sarcocystis neurona is found in the brain and spinal cord of horses. The opossum (Didelphis virginiana) is considered the reservoir host for S. neurona and the resistant stage of the parasite (oocyst) is shed in opossum feces. However, S. neurona has not been isolated from opossum feces. Scientists at Beltsville Agricultural Research Center, Montana State University and Virginia Tech have distinguished S. neurona from a closely related S. falcatula, also found in opossum feces. In addition, they have discovered a new S. neurona-like species in opossum feces. These results will be of use to biologists, parasitologists and veterinarians.
Opossums (Didelphis virginiana) were found to be hosts for 3 species of Sarcocystis: Sarcocystis falcatula with an avian intermediate host, S. neurona with an undetermined intermediate host, and a third unnamed species. Sporocysts from the intestines of 2 opossums (Nos. 26 and 47) were fed to budgerigars (Melopsittacus undulatus), nude mice and gamma-interferon knock out (KO) mice. Sporocysts of S. falcatula were not infective to nude or KO mice. Sporocysts of S. neurona induced encephalitis in KO and nude mice; only schizonts and merozoites were found in tissues of mice and they reacted with anti-S. neurona serum raised against the SN-2 isolate of S. neurona originally obtained from tissues of a paralyzed horse. All 3 species of Sarcocystis were present in opossum 47. Sarcocystis neurona was isolated in cell culture from this opossum. Sporocysts from opossum 47 were lethal to budgerigars indicating S. falcatula infection. Only 1 species of Sarcocystis (the third species) was found in opossum No. 26. Sporocysts of the third species were infective to KO and nude mice. Schizonts and merozoites of this species were predominantly in the liver but were also found in other tissues; schizonts did not react with anti-S. neurona serum. Merozoites of the third species were structurally distinct from S. falcatula and S. neurona merozoites. Sarcocysts were found in leg muscles of 2 mice killed 50 and 54 days after feeding sporocysts from opossum No. 26. These sarcocysts had steeple-shaped protrusions on the cyst wall and were distinct from sarcocysts of S.