COCCIDIAL PARASITES

Coccidia in the genera Cryptosporidium, Eimeria, Isospora, arcocystis, and Toxoplasma are the most economically important group of protozoans in domestic animals in the United States. Historically the structure of the sporulated oocyst, especially the number of sporocysts and sporozoites, was used as a major characteristic to differentiate genera of coccidia.
Eimeria and isospora
· A. Host specificity - these parasities have a narrow host range -(stenoxenous).
· B. Organ specificity - these parasities are usually found in a specific organ.
· C. Site specificity within organ or cell - these parasities inhabit specific sites within an organ or cell.
· D. Monoxenous - coccidia usually parasitize one host, the definitive host.
· E. Reproduction
o 1. Schizogony (Merogony) -multiple fission.
o 2. Gamogony (syngamy) - union of similar gametes (Isogamy)
· F. Structure of sporulated oocyst
o 1. Eimeria - 4 sporocysts, each with 2 sporozoites
o 2. Isospora - 2 sporocysts, each with 4 sporozoites
Detailed Life Cycle of The Coccida Eimeria and Isospora
Infection of the definitive host begins with the ingestion of infective oocysts containing fully developed sporozoites (within sporocysts) which are freed in the lumen of the intestine by enzymic action of digestion upon the oocyst wall. The freed motile sporozoites penetrate epithelial cells of the intestine and round-up to form trophozoites which undergo a form of internal budding or cell division which is termed a schizont at this point (or schizogony the process). The later process (first generation schizonts) is terminated when the round internal cells elongate or become sickle shaped (which are termed merozoites, first generation) and are released (rupture host cells) into the intestinal lumen to enter other cells in their vicinity and repeating a second and/maybe a third generation of schiogony (merogony). Most merozoites of the second generation form macrogamethocytes with macrogametes which undergo no cell division. Those merozoites which develop into microgametocytes undergo further division forming biflagellated microgametes. The later leave host cells to penetrate other cells containing macrogametes resulting in fertilization, forming the zygote. The latter lays down a wall around itself from the eosinophilic plastic granules in its cytoplasm, thus forming the oocysts which breaks out of the host cells to be passed in the feces to sporulate or become infective in about 2-3 days under ideal conditions of temperature (Sporogony).