In traditional protist taxonomy, most parasitic protists were placed in the class Sporozoa. This group has since been found to include protists from a number of unrelated lineages, and has been dropped from current usage. However, many of the protists in the old Sporozoa share certain structural features, in particular an apical complex of microtubules within the cell. These protists have now been grouped in the Apicomplexa, probably the largest and best-known taxon of parasitic protists. There are about 4,000 known species, but this is almost certainly a gross underestimate of the actual number.
There are no known fossil apicomplexans. However, the group is a very important part of the living biota. Apicomplexans infect both invertebrates and vertebrates; they may be relatively benign or may cause serious illnesses. Species in the genus Plasmodium cause malaria in humans and other animals; an estimated 300 million people in over 90 countries are infected with malaria, and over 1 million die from it each year. Other apicomplexans cause serious illnesses, such as coccidiosis and toxoplasmosis, in humans and domestic animals. On the other hand, apicomplexans that infect insects have been used experimentally to control populations of insect pests.
Apicomplexans have complex life cycles, and there is much variation among different apicomplexan groups. Both asexual and sexual reproduction are involved, although some apicomplexans skip one or the other stage. The basic life cycle may be said to start when an infective stage, or sporozoite, enters a host cell, and then divides repeatedly to form numerous merozoites. Some of the merozoites transform into sexually reproductive cells, or gamonts. Gamonts join together in pairs and form a gamontocyst (pictured above). Within the gamontocyst, the gamonts divide to form numerous gametes. Pairs of gametes then fuse to form zygotes, which give rise by meiosis to new sporozoites, and the cycle begins again. Apicomplexans are transmitted to new hosts in various ways; some, like the malaria parasite, are transmitted by infected mosquitos, while others may be transmitted in the feces of an infected host, or when a predator eats infected prey.
For additional information:
Biomedical information about Plasmodium and malaria is available from the Malaria Database maintained at Monash University, Australia. A good epidemiological overview of human malaria is presented by the World Health Organization.
Information on apicomplexans that infect domestic animals is available from Oklahoma State University College of Veterinary Medicine or from the Cryptosporidium Research lab at Kansas State University. General information on various pathogenic apicomplexans is available through MicroWeb.
Life stages of gregarine apicomplexans are portrayed in this 19th-century zoological chart preparted by the great zoologist and parasitologist Rudolph Leuckart.