Apicomplexa

Apicomplexa is a large phylum of obligate intracellular parasitic protists within the group Alveolata. Members are distinguished by an apical complex used to invade host cells, a set of secretory organelles (micronemes and rhoptries), and often a gliding motility mechanism. Many species also possess a nonphotosynthetic plastid called the apicoplast, derived from secondary endosymbiosis, which contains essential biosynthetic pathways. Most apicomplexans lack flagella in their mature forms, though male gametes of some species are flagellated.

Life cycles in Apicomplexa are typically complex, frequently alternating sexual and asexual generations and involving one or more hosts. Transmission forms include sporozoites shed from oocysts in the environment and merozoites produced after asexual replication that invade host cells. Several genera are notable for causing disease in humans and animals, including Plasmodium (malaria), Toxoplasma gondii (toxoplasmosis), Cryptosporidium (cryptosporidiosis), Babesia, Eimeria, and Theileria. In malaria, sporozoites reach the liver, replicate to form merozoites, which then invade red blood cells; the parasite’s life cycle and antigenic variation contribute to disease severity and transmission.

The apicoplast hosts pathways absent in animals, making it a key drug target. Antibiotics that disrupt plastid translation can inhibit parasite growth. The mitochondrion in many apicomplexans is reduced, with a compact genome, and parasites rely on distinctive metabolic processes that differ from their vertebrate hosts. Understanding their diverse life cycles and organellar biology informs disease control and treatment strategies.