GENUS SARCOCYSTSIS

INTRODUCTION AND IMPORTANCE: All Sarcocystis species have a obligatory heteroxenous life cycle. Cats are important as definitive hosts for at least 11 named species (Table 5) and are the intermediate host for one species. Sarcocystis (Sarco = muscle, cystis = cyst) was first observed in 1843 by F. Miescher in a house mouse (Dubey BOOK). The muscles of the mouse contained "milky white threads" which came to be known as Miescher's tubules. Latter a species was found in the pig and named Synchytriummiescherianum by Kühn in 1865 and changed to Sarcocystismiescheriana by Labbé in 1899. Therefore the correct type species is Sarcocystismiescheriana (Kühn, 1865) Labbé 1899. It was not until the early 1970's that the two-host life cycle was described (Fayer, 1972; Rommel and Heydorn, 1972; Rommel et al., 1972).

SARCOCYSTIS SPECIES LIFE CYCLE: The Sarcocystis life cycle is obligatorily a two-host cycle. The intermediate hosts becomes infected by ingestion of sporocysts from the environment. The sporozoites excyst from the sporocysts in the intestinal tract. The sporozoites leave the intestinal tract and undergo first-generation merogony in endothelial cells of arteries usually in mesenteric lymph nodes (Dubey et al., 1989). A second generation of merogony occurs in capillaries or small arteries in many tissues throughout the body. The meronts are usually most numerous in glomeruli of the kidneys. The merozoites from the last generation are released in to the circulation and can occasionally be found intracellularly in unidentified mononuclear cells (Dubey et al., BooK). Limited multiplication may occur at this stage of infection. Eventually these merozoites will penetrate cells and develop into the sarcocyst stage that contains bradyzoites. The first- and second-generation meronts develop directly in the host cell cytoplasm, whereas the bradyzoites develop within a parasitophorous vacuole.

The developing sarcocyst contains a stage called a metrocyte that divides by endodyogeny to produce the bradyzoites. A mature sarcocyst can contain thousands of bradyzoites and be grossly visible. The presence of grossly visible sarcocysts of S. gigantea in sheep and S. hirsuta in cattle is a cause for condemnation of the carcass (Dubey, Leek, Fayer, 1986; Dubey et al., 1990).