The phylum Apicomplexa includes several thousand parasitic protists that cause major diseases of mankind and of domestic and companion animals. Among the diseases that they cause are malaria, coccidiosis, toxoplasmosis, sarcocystosis, cyclosporosis, cryptosporidiosis and piroplasmosis.

Keywords: Cryptosporidium; Eimeria; gregarine; immunity; Perkinsus; Plasmodium; schizogony; Toxoplasma; vaccination

Figure 1.

Structure of apicomplexans as revealed by electron microscopy. Upper left: a metrocyte, or mother cell, in which the elements of the apical complex are minimally expressed. Upper right: a zoite in which all of the elements of the apical complex are seen. Lower: A detailed view of the conoid and associated elements such as the polar rings and the subpellicular tubules which extend about two‐thirds of the length of the zoite. (Courtesy of C. A. Speer.)

Figure 2.

The cytostome or micropore of Toxoplasma gondii as seen in transmission electron microscopy. The cytostome ingests food provided by the parasite's host cell. (Courtesy of C. A. Speer.)

Figure 3.

A meront of Sarcocystis sp. in a hepatocyte of a mouse as seen in transmission electron microscopy. (A) Merozoites that have completed budding from (B) a centrally located residual body. (Courtesy of C. A. Speer.)

Figure 4.

Endodyogeny in zoites of Neospora caninum, an economically important parasite of domestic animals. (A) A zoite not undergoing endodyogeny. (B) Partially formed progeny cells in which the nucleus is being divided into two progeny cells. (C) Two progeny cells completely formed and probably ready to be released. (Courtesy of C. A. Speer.)

Figure 5.

The life cycle of Eimeria tenella of the chicken. This apicomplexan is typical of those coccidia that parasitize the intestinal tracts of their hosts. Sporogony takes place outside of the host, and both merogony and gamogony occur within cells of the intestine. (From Marquardt et al., .)

Figure 6.

An oocyst of Eimeria magna of the rabbit, Oryctolagus cuniculi, as seen in interference contrast light microscopy. The oocyst is infective if ingested by a rabbit. In the genus Eimeria, each oocyst has four sporocysts each of which contains two sporozoites. (A) Oocyst wall. (B) Sporocyst. (C) Sporozoite. (Courtesy of C. A. Speer.)



Marquardt WC, Demaree RS and Grieve RB (1999) Parasitology and Vector Biology, 2nd edn. San Diego: Academic Press.

Speer CA and Dubey JP (1999) Ultrastructure of schizonts and merozoites of Sarcocystis falcatula in the lungs of budgerigars (Melopsittacus undulatus). Journal of Parasitology 85: 630–637.

Further Reading

Barta J (1989) Phylogenetic analysis of the class Sporozoa (phylum Apicomplexa Levine, 1970): evidence for the independent evolution of heteroxenous life cycles. Journal of Parasitology 75: 195–206.

Clark DP and Sears CL (1996) The pathogenesis of cryptosporidiosis. Parasitology Today 12: 221–225.

Corliss JO (1994) An interim utilitarian (‘user‐friendly’) hierarchical classification and characterization of the protists. Acta Protozoologica 33: 1–51.

Doyle PS, Crabb J and Petersen C (1993) Anti‐Cryptosporidium parvum antibodies inhibit infectivity in vitro and in vivo. Infection and Immunity 61: 4079–4084.

Fourmaux MN (1996) Toxoplasma gondii microneme proteins: gene cloning and possible function. Current Topics in Microbiology and Immunology 219: 55–58.

Jensen JB (ed) (1983) In Vitro Cultivation of Protozoan Parasites. Boca Raton, FL: CRC Press.

Levine ND (1985) Apicomplexa. In: Lee JJ, Hutner SH and Bovee EC (eds) An Illustrated Guide to the Protozoa. Lawrence, KS: Society of Protozoologists.

Levine ND (1988) The Protozoan Phylum Apicomplexa. Boca Raton, FL: CRC Press.

Margulis L, Corliss JO, Melkonian M and Chapman DJ (1990) Handbook of the Protoctista. Boston: Jones & Bartlett Publishers.

Perlmann P, Perlmann H, Berzins K and Troye‐Blomberg M (1998) Selected problems of malaria blood stage immunity. Tokai Journal of Experimental Clinical Medicine 23: 55–62.

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Marquardt, William C, and Speer, CA(Apr 2001) Apicomplexa. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0001956]