Figure 1. Life cycle of a typical eimeriid parasite. Sporozoites (A) excyst from sporocysts in the digestive tract of the definitive host. Sporozoites penetrate host cells and initiate merogony (B). Merozoites penetrate additional host cells and undergo further rounds of merogonic replication (C–D). After one or more cycles, merozoites penetrate cells (E) and produce macrogametes (F) or microgametes (G). The latter locate and fertilize (H) the mature macrogamete. The resulting zygote, an unsporulated oocyst, is shed in the faeces (I). Sporogonic development occurs in the environment to produce a fully sporulated, infective oocyst (I–L).

Figure 2. Typical eimeriid oocyts. (a) Sporulated oocysts of an Eimeria sp. containing four sporocysts which each contain two sporozoites. (b) Sporulated oocyst of an avian Isospora sp. with two sporocysts that each contain four sporozoites. Bar, 10 m.

Figure 3. Progression of gross pathological lesions in chicken caeca at 3 to 8 days post inoculation (DPI) caused by infection with 50 000 oocysts of Eimeria tenella. Caeca at 3 DPI (a) appear relatively normal but begin to show gross pathological changes at 4 DPI (b). Haemorrhage is noticeable at 5 DPI (c) and prominent at 6 DPI (d) as the second merogonic generation is completed. (e) Haemorrhage begins to resolve at 7 DPI. (f) By 8 DPI much of the debris and oocysts in the caecal core have been shed in the faeces and the caeca begin to recover their normal appearance. (g) A caecal pouch from 6 DPI opened to reveal the eroded and congested mucosa, and the core of blood and necrotic debris within the lumen.