Coccidiosis is principally an enteric disease of vertebrates that results from infection with eimeriid parasites of the genus Eimeria or Isospora (Apicomplexa; Coccidia; Eimeriidae). Coccidiosis is a frequent cause of diarrhoea, morbidity and mortality in domesticated birds and mammals that can be alleviated by use of prophylactic or therapeutic antiparasitic drugs or, less commonly, by vaccination.

Keywords: protist; Apicomplexa; disease; immunity; coccidia; parasite; pathogenesis; parasitism

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 μm.

Figure 3.

Progression of gross pathological lesions in chicken caeca at 3 to 8 (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.



Ball SJ, Pittilo RM and Long PL (1989) Intestinal and extraintestinal life cycles of eimeriid coccidia. Advances in Parasitology 28: 1–54.

Barker IK (1993) Pathological processes associated with coccidiosis. In: Barta JR and Fernando MA (eds) Proceedings of the VIth International Coccidiosis Conference, University of Guelph, Guelph, Canada, pp. 81–94.

Barta JR, Martin DS, Liberator PA et al. (1998) Phylogenetic relationships among eight Eimeria species infecting domestic fowl inferred using complete small subunit ribosomal DNA sequences. Journal of Parasitology 83: 262–271.

Duszynski DW, Couch L and Upton SJ The Coccidia of the World Parent site:[]; mirror site:[].

Lillehoj HS (1998) Role of T lymphocytes and cytokines in coccidiosis. International Journal for Parasitology 28: 1071–1081.

Soete M, Hettman C and Soldati D (1999) The importance of reverse genetics in determining gene function in apicomplexan parasites. Parasitology 118 (supplement): S53–61.

Williams RB and Catchpole J (2000) A new protocol for a challenge test to assess the efficacy of live anticoccidial vaccines for chickens. Vaccine 18: 1178–1185.

Yun CH, Lillehoj HS and Lillehoj EP (2000) Intestinal immune responses to coccidiosis. Developmental and Comparative Immunology 24: 303–324.

Further Reading

Lee JJ (1999) An Illustrated Guide to the Protozoa, 2nd edn. Lawrence, KS: The Society of Protozoologists.

Long PL (1990) Coccidiosis of Man and Domestic Animals. Boca Raton, FL: CRC Press.

Shirley MM and Tomley FI (eds) (1998) VIIth International Coccidiosis Conference: 1–5 September 1997, Keble College, Oxford, UK. International Journal for Parasitology 28: 1005–1144.

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Barta, John R(Apr 2001) Coccidiosis. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0001947]