Life cycle stages, specific organelles and invasion mechanisms of Eimeria species

Abstract

Apicomplexans, including species of Eimeria, pose a real threat to the health and wellbeing of animals and humans. Eimeria parasites do not infect humans but cause an important economic impact on livestock, in particular on the poultry industry. Despite its high prevalence and financial costs, little is known about the cell biology of these 'cosmopolitan' parasites found all over the world. In this review, we discuss different aspects of the life cycle and stages of Eimeria species, focusing on cellular structures and organelles typical of the coccidian family as well as genus-specific features, complementing some 'unknowns' with what is described in the closely related coccidian Toxoplasma gondii.

Keywords: Cell biology; Eimeria; coccidia; invasion; life cycle stages; secretory organelles.

Fig. 1.

Phylogenetic tree showing the relationships between relevant apicomplexan species. The tree was generated based on data from the following sources: Levine, ; Tenter and Johnson, ; Carreno et al., ; Jirku et al., ; Tenter et al., ; Adl et al., ; Lane and Archibald, ; Golemansky, ; Megia-Palma et al., .

Fig. 2.

Life cycle of Eimeria tenella. Numbers correlate with subsequent stages of the development. (1) Oocyst sporulation in the environment and oral ingestion by the chicken. (2) Release of sporocysts and sporozoites along the transit in the chicken digestive system. (3) Active invasion of sporozoites in the ceaeca epithelium and formation of the intracellular trophozoite within the parasitophorous vacuole. (4) First round of shyzogony and release of first generation merozoites. (5) Second and third rounds of shyzogony and release of second and third generation merozoites, respectively. (6) Development of microgametes and macrogametes (gametogony) and fecundation. (7) Zygote, development of the oocyst and release to the environment as unsporulated oocyst.

Fig. 3.

Sporulated oocysts of E. tenella. The oocysts contain four sporocysts (arrowheads), each containing two sporozoites (asterisks) and a micropyle at each sporocysts apex (arrow).

Fig. 4.

TEMs of E. tenella sporozoite ultrastructures (A. Burrell, unpublished). (A) Sporozoites with two large non-membrane bound organelles known as anterior and posterior refractile bodies (ARB and PRB) situated at either side of the nucleus (N) as well as numerous amylopectin granules (arrowhead). (B) Micronemes and rhoptries (asterisks) occupying most of the cytoplasm in the anterior quarter of the cell. (C) Apex of the cell with a cone-shaped structures composed of helical fibres known as the conoid (arrow) sitting within the APRs (arrowheads). (D) Centrally located nucleus (N) next to which the Golgi apparatus can be observed (arrowhead). (E) Mitochondrion cross-section showing plump cristae. (F) Triple-layered pellicle (arrowheads) consisting of plasma membrane and IMC beneath which sits an array of sub-pellicular microtubules (arrows). (G) Apicoplast with four membrane layers. Scale bars: (A) ~2 µm; (B–G) ~500 nm.

Fig. 5.

Cell structure and organelle content of different coccidian cells. (A) Tachyzoite of Toxoplasma gondii. (B) Sporozoite of Eimeria species. (C). Merozoite of Eimeria species.

Fig. 6.

Schizogony (adapted from Francia and Striepen, 2014). (1) Trophozoite development after sporozoite invasion. (2) Immature schizont, nuclei multiply by several rounds of mitosis. (3) Mature schizont, the last round of division coincides with the merozoites budding at the parasite surface. Merozoites release and initiate a new round of schizogony (or gametogony).