The Eimeria life cycle has basically two stages: the exogenous phase (sporogony) and the endogenous phase (schizogony and gametogony). The initial infective unit of all Eimeria spp. is the sporozoite stage, which is a banana-shaped motile cell. The sporozoite of every apicomplexan parasite is characterized by a unique complex of structures specialized in the invasion of the host cells. The sporozoite is the beginning and the end of the life cycle of any coccidian . Oocysts are ovoid and vary in size and shape according to the species.

After the exogenous phase (sporogony), sporulated oocysts can initiate replication once they are ingested orally by a susceptible host, in this case the chicken . Inside the intestines of the host, the sporozoites are released from oocysts under the influence of digestive enzymes and mechanical disruption. For the emergence of the sporozoite, two separate stimuli must be present: first, stress by carbon dioxide (CO₂), which causes the rupture of the micropyle and increase in the permeability in the oocyst. This leads to a collapse of the contents of the oocyst in a hypertonic salt solution . The temperature is also essential for the liberation of infective sporozoites (i.e., body temperature). Secondly, the action of compounds, such as trypsin and bile , activate the sporozoites inside the sporocyst and digest the Stieda body generating a hole in the sporocyst membrane. Bile can either facilitate entrance of digestive enzymes through the altered micropyle into the oocyst, or can alter lipoproteins of the Stieda body of Eimeria oocysts . Although bile is not strictly necessary for activation of sporozoites, it has been demonstrated that lack of bile for many Eimeria spp. Up to this moment of egress, no damage is done to the host. During the excystation and invasion of the host cell, the sporozoite uses its stored amylopectin for its energy requirements. Vetterling and Doran (1969) observed that during the 30 min period of excystation at 42.9°C, carbohydrate reserve levels decreased 2/3 in activated sporozoites. This is also correlated with the consumption of oxygen and lipid compounds .

Free sporozoites infect intestinal cells of the gut and develop inside a parasitophorus vacuole (PV) into a rounded and growing organism called the trophozoite, which becomes a meront during the first merogony generation. As the sporozoite grows, the endothelial cell becomes hypertrophic and its nucleus undergoes alterations, becoming larger with an enlarged nucleolus with scattered chromatin; its cytoplasm is organized in two concentric zones and is not vacuolated appearance. Merogony begins with multiple nucleus divisions of the Eimeria trophozoite without the division of the cytoplasm, resulting in the formation of ellipsoidal structures called blastophores with a peripheral layer of nuclei. Merozoites I have a polar ring containing a conoid with fibers grouped in a narrow helix.

The anterior region contains abundant micronemes, 22 subpelicular tubes, of which three have granules of glycogen, many ribosomes, one or two mitochondria, a micropore and an endoplasmic reticulum

After the maturation of second meront stages, released merozoites II invade adjacent epithelial cells undergoing sexual gamogony. During the gamogony, most merozoites II develop into a single, large, mononuclear, spheroid cell, the female macrogamete. The macrogametes have characteristic eosinophilic granules [outer granule layer containing glycoproteins and an inner granule layer containing protein-rich molecules; both commonly known as “wall forming bodies” (WFB1, WFB2)]. Few merozoites II develop into large, polynucleated cells (male microgamonts) which form many spindle-shaped cells with two flagella, the microgametes .

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