The protist phylum Apicomplexa Levine, 1970, comprises >4000 described species, all obligate parasites, that infect a wide range of vertebrates and invertebrates. Within the phylum, the genus Eimeria Schneider, 1875, (Family Eimeriidae) is the most speciose lineage (>1800 species) of intracellular parasites found in all classes of vertebrates and some invertebrates. All eimerians have direct life cycles that include both asexual (merogony) and sexual (gamogony) reproduction, the latter resulting in the production of resistant propagules, the oocysts, which are discharged into the environment by the infected host. Endogenous development usually occurs in epithelial or endothelial cells of the intestine or in related structures (e.g. gall bladder, bile ducts, renal tubular epithelium, esophageal epithelium, oviduct epithelium uterine wall epithelium, etc.). Some eimerian species are highly pathogenic, especially in domesticated animals, but some also in wild animal species.

Key Concepts:

  • Within the coccidia proper, the Family Eimeriidae comprises about 15 genera, including Eimeria; all are obligate intracellular parasites known to infect every animal group which has been examined for them.

  • Untapped Biodiversity: The genus Eimeria, with >1800 described forms, may be the most speciose group of parasites on Earth, but it has been little studied in the Earth's app. 62 150 vertebrate species.

  • It can be estimated that 98.6% (or more) of the coccidian parasites of vertebrates are yet to be discovered, based on what we know through 2010.

  • Our knowledge of the biodiversity of life on Earth is still badly incomplete, especially of the microbes that inhabit the seas, the soils and the bodies of ‘higher’ organisms that live in and on them.

  • Parasitologists, and most other biologists in general, agree that parasites represent a large proportion of all living species and may, in fact, far exceed the total number of all ‘free‐living’ forms.

  • As human population increases and agricultural development continues to shrink/fragment natural ecosystems, food webs will be disrupted and new and more virulent strains of coccidia may be expected, especially when exacerbated by environmental stressors (pollution), global climate change and invasion of new parasites from edge‐dwelling hosts.

Keywords: apicomplexa; eucoccidia; eimeriidae; Eimeria; intracellular parasites; protista; parasitic protozoa; domestic animals; wild animals; coccidia

Figure 1.

Line drawing showing the major steps in the endogenous and exogenous development of a ‘typical’ Eimeria life cycle: a, entry of the sporulated oocyst into a susceptible definitive host; b, sporozoites released from sporocysts/oocyst; c, sporozoite penetrates host enterocyte and rounds up; d, mitotic nuclear division in the meront; e, merozoite formation; f, first generation merozoites released from host enterocyte seek out new host cells to penetrate; g–i, next merogenous generation continues; j, final generation of merozoites; k, merozoite rounds up in the first stage of developing into a macrogametocyte; l, early macrogamont; m, developing macrogamont with wall‐forming bodies; n, mature macrogamete; o, merozoite rounds up in the first stage of developing into a microgametocyte; p, mitosis; q, microgametogenesis; r, mature microgametocyte with fully formed microgametes around the periphery of cell; s, flagellated microgametes leave host enterocyte to penetrate another enterocyte with fully developed macrogamete; t, after fertilization occurs returning the diploid condition, the unsporulated oocyst leaves the host enterocyte, enters the lumen of the gut; u, unsporulated oocyst, containing the sporoplasm of the next generation, leaves the host in the faeces; v, meiosis takes place and the sporocysts begin to form; w, the pyramid stage of sporocyst development; and x, after the last mitotic division of the nuclei, sporozoite formation within each sporocyst is completed, resulting in the sporulated oocyst.

Figure 2.

Line drawings of the parts of hypothetical sporulated oocysts/sporocysts of Eimeria: (a) completely sporulated oocyst showing major structural features with four sporocysts each with two sporozoites; (b) sporulated sporocyst showing major features, including two sporozoites; (c) end of an oocyst showing other possible structures, a micropyle and micropyle cap, present in some oocysts, especially those of ruminants; (d) another sporulated sporocyst showing a variety of structural features, some of which may be present on the sporocysts of different Eimeria species (a=anterior and p=posterior refractile bodies of sporozoite).



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Further Reading

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Duszynski, Donald W(Apr 2011) Eimeria. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001962.pub2]