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William C Marquardt, Colorado State University, Fort Collins, Colorado, USA CA Speer, Montana State University, Bozeman, Montana, USA
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.

Secondary article
Article Contents
. Introduction . Characterization . Place in Overall Taxonomic Scheme . Phylogenetic and Evolutionary Considerations . Host–Parasite Interactions . Major Taxa and Important Species

The Apicomplexa comprises a phylum of protists that have structures referred to as the apical complex and in which the lifecycles have both asexual and sexual phases. All members are parasitic. Included in the phylum are several thousand described species that parasitize members of all of the major animal phyla. They are parasites of common invertebrate phyla such as molluscs, arthropods and annelids as well as of some lesser-known phyla. Apicomplexans are parasites of members of all the classes of the phylum Chordataincluding sea squirts or tunicates, fish, amphibia, reptiles, birds and mammals. Members of the Apicomplexa cause some of the most devastating diseases of mankind and cause enormous economic losses in domesticated animals and poultry. The most prominent apicomplexans are Plasmodium spp., which cause malaria in humans; even after one hundred years of concerted control programmes, malaria still takesmore than 2 million human lives each year. Coccidiosis is a complex of intestinal diseases of livestock and poultry that are common wherever animals are raised for food and wool. Global losses to coccidiosis reach into many millions of dollars (US) annually. Poultry cannot be raised in current high concentration methods without the continuous use of drugs to prevent the development of thecausative agents of coccidiosis. Piroplasmoses are tickborne diseases that affect livestock on all continents and may be the single most important group of infectious agents of grazing animals.

(Figure 2), often referred to as a micropore. All species are parasitic. Schizogony is an asexual process of multiplication in which multiple nuclear replications take place without

Membersof the phylum have one or more elements of the apical complex (Figure 1) at some stage of the life cycle: polar ring, rhoptries, micronemes and conoid. Sexuality is by syngamy. Multiplication is by schizogony or endodyogeny; 9 1 2 flagellar form is present in some gametes; locomotion is by body flexion or gliding; subpellicular tubules are present in most species. Feeding is by osmotrophy orphagotrophy, the latter by a cytostome

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 thesubpellicular tubules which extend about two-thirds of the length of the zoite. (Courtesy of C. A. Speer.)

ENCYCLOPEDIA OF LIFE SCIENCES © 2001, John Wiley & Sons, Ltd. www.els.net



cytokinesis until all the progeny have formed. Then, cytokinesis occurs simultaneously forming all of the progeny (Figure 3). Endodyogeny is a process in which two progeny form within a parental cell(Figure 4); when the offspring are fully formed, the parental cell breaks down, releasing them. Apicomplexans have three stages in their life cycles:

organisms such as Plasmodium spp. Merogony is also a schizogonic process and the main one by which the apicomplexans multiply within a host. In malaria, this process occurs first in the liver and then in the red blood cells. Gamogony...
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