Eimeria species are coccidians within the Apicomplexa, and are most closely related to the genus Isospora. Both Eimeria and Isospora species are highly host specific. The most significant difference between Eimeria and Isospora is that the latter may use a paratenic host in its life cycle, whereas the former does not.
The life cycle stages of Eimeria species visible by standard microscopy include those associated with asexual (merogony) and sexual (gametogony and fertilization) reproduction within the enterocytes of the small intestine, and the oocysts. The intestinal stages – meronts, merozoites, macrogametocytes, microgametocytes, gamonts and oocysts, are typical for the genus, and can usually be seen in histological sections.
Oocysts of E. leuckarti passed in feces are very characteristic. They are oval to pear-shaped, measure up to approximately 88 µm by 60 µm, and have a very thick, dark brown shell and a micropyle at the narrower end.
Freshly passed oocysts are unsporulated and contain one or two cells. After a few weeks, depending on external environment temperatures, each oocysts will sporulate and become infective. Each sporulated oocyst of E. leuckarti contains four sporocysts, each containing two sporozoites.
Host range and geographic distribution
Life cycle - direct
Eimeria species undergo asexual (merogony) and then sexual (gametogony and fertilization) reproduction in the enterocytes of the small intestine. The infective stage is the sporulated oocyst which, following ingestion by a suitable host, hatches to release eight sporozoites. These sporozoites enter enterocytes and divide rapidly to form merozoites enclosed within a meront, which can come to occupy most of a host cell. The infected cell then bursts, releasing the meronts into the intestinal lumen, from where they penetrate new enterocytes. The number of generations of merogony for E. leuckarti is not known.
Eventually, merozoites entering host cells do not divide to produce meronts, but instead form microgametocytes ("male") and microgametocytes ("female") within the enterocytes. Each microgametocyte contains several microgametes, but each macrogametocyte contains only a single macrogamete. Next, the microgametocyte disintegrates, releasing the microgametes, which fertilize the macrogametocytes, forming gamonts which develop into unsporulated oocysts.
The oocysts burst from the enterocytes and are passed in feces. In the environment the oocysts sporulate and are then infective. For E. leuckarti, sporulation under ideal conditions takes approximately three weeks. Infection of horses is by ingestion of sporulated oocysts.