Apicomplexa

Apicomplexa is a large group of single-celled eukaryotic parasites that share a special set of structures at one end of the cell, the apical complex, used to invade host cells. It includes the agents of malaria, toxoplasmosis and cryptosporidiosis, and is one of the most important groups of protozoa.

The best known is Plasmodium, including Plasmodium falciparum and Plasmodium vivax. Toxoplasma gondii infects many warm-blooded animals, with cats as its final hosts. Cryptosporidium causes diarrhea through contaminated water, Babesia infects red blood cells and is carried by ticks, and Eimeria causes serious disease in poultry.

Most have complex life cycles with infective stages called sporozoites that invade host cells, multiply, and then form stages that infect new hosts. In malaria, sporozoites pass from the Anopheles mosquito to humans, multiply in the liver, and then infect red blood cells. Some, such as Plasmodium, cross the placenta in pregnancy.

Most apicomplexans contain the apicoplast, a vestigial plastid derived from an alga that was engulfed by their ancestors in a secondary event described under endosymbiotic theory. It no longer carries out photosynthesis but makes fatty acids and other molecules, and since humans lack it, it is a target for drugs. Malaria parasites are the target of the drug artemisinin, and the whole group belongs to the eukaryote domain.

• Malaria — the disease caused by Plasmodium
• Protozoa — the wider informal group
• Artemisinin — a drug used against its malaria parasites