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The life cycles of Eimeria spp. are direct. Unsporulated oocysts are passed in the feces from an infected host and develop into the infective stage (sporulated oocysts) in the environment. The original single cell of Eimeria divides, forming four sporoblasts, each of which develops into one sporocyst, and within each sporocyst two sporozoites develop (1 : 4 : 2 configuration of the oocyst for Eimeria). When ingested, the wall of the oocyst breaks down, and sporocysts and sporozoites are released. The sporozoites then enter epithelial cells.
Once within the cells, the sporozoites transform to merozoites, which then undergo asexual replication (schizogony or merogony) and produce first-generation schizonts, which contain many merozoites. After the schizont matures, the merozoites are released by rupture of the epithelial cell. New epithelial cells are again invaded, and second-generation and third generation schizogony occur. The second generation and/or third-generation schizonts (depending on Eimeria species) are deeper in the mucosa than first-generation schizonts and usually lead to the sloughing of the epithelium, associated hemorrhage, and tissue destruction; therefore these schizonts cause pathogenic effects and lead to enteritis (bloody in severe infections) and clinical disease, but also induce immunity. Following schizogony, the final merozoites that are released invade epithelial cells and “switch” to produce sexual stages, called the macrogametocyte (female) and the microgametocyte (male) during the phase of gametogony. The microgametocyte eventually produces microgametes, which fertilize macrogametes (from microgametocyte; within the mucosa) to produce zygotes. These zygotes become oocysts, which slough from the epithelium and are excreted in the feces. The sloughing of the epithelial layer during gametogony can also lead to bleeding. The prepatent period varies depending on the species of coccidian.
Once within the cells, the sporozoites transform to merozoites, which then undergo asexual replication (schizogony or merogony) and produce first-generation schizonts, which contain many merozoites. After the schizont matures, the merozoites are released by rupture of the epithelial cell. New epithelial cells are again invaded, and second-generation and third generation schizogony occur. The second generation and/or third-generation schizonts (depending on Eimeria species) are deeper in the mucosa than first-generation schizonts and usually lead to the sloughing of the epithelium, associated hemorrhage, and tissue destruction; therefore these schizonts cause pathogenic effects and lead to enteritis (bloody in severe infections) and clinical disease, but also induce immunity. Following schizogony, the final merozoites that are released invade epithelial cells and “switch” to produce sexual stages, called the macrogametocyte (female) and the microgametocyte (male) during the phase of gametogony. The microgametocyte eventually produces microgametes, which fertilize macrogametes (from microgametocyte; within the mucosa) to produce zygotes. These zygotes become oocysts, which slough from the epithelium and are excreted in the feces. The sloughing of the epithelial layer during gametogony can also lead to bleeding. The prepatent period varies depending on the species of coccidian.