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The Apicomplexa is one of the largest phyla amongst the protozoa and includes many important parasites of humans and domestic animals (Table 3.1).

As with many other protozoa, the taxonomic arrangements within the Apicomplexa are in a state of flux. All members of the phylum are obligate intracellular parasites of invertebrates and vertebrates. A common feature shared by all apicomplexans is the presence within their invasive stage of a unique intracellular structure called the apical complex that is composed of a group of secretory organelles called the micronemes and rhoptries (Figure 3.9). The apical complex lies at the anterior apex of the cell where it is associated with a region called the oral structure. The secretions of the micronemes and the rhoptries play an important role in the invasion of red blood cells by the malaria parasites (Suarez et al. 2019).

Table 3.1 Representative examples of parasitic protozoa belonging to the phylum Apicomplexa to illustrate the wide variety of hosts and transmission strategies.

Genus	Example	Host	Transmission	Disease
Plasmodium	Plasmodium falciparum	Humans	Vector: Anopheline mosquitoes	Malaria
Toxoplasma	Toxoplasma gondii	All warm‐blooded animals	Contamination, congenital, ingestion of infected flesh	Toxoplasmosis
Neospora	Neospora caninum	Dogs, cattle	Contamination, congenital	Neosporosis
Cyclospora	Cyclospora cyetanensis	Humans	Contamination	Cyclosporosis
Eimeria	Eimeria tenella	Poultry	Contamination	Coccidiosis
Theileria	Theileria parva	Cattle	Vector: Rhipicephalus ticks	East Coast Fever
Babesia	Babesia bigemina	Cattle	Vector: Rhipicephalus ticks	Texas Fever
Isospora	Isospora belli	Humans	None	Isosporosis
Cryptosporidium	Cryptosporidium hominis	Humans	Contamination	Cryptosporidiosis

Figure 3.9 Generalized diagram of the invasive stage of an apicomplexan parasite. Abbreviations: Ac: apical complex (conoid + polar ring); Ap: apicoplast; Dg: dense granules; Mic: micronemes; Mit: mitochondrion; Mp: micropore; N: nucleus; Rho: rhoptries.