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PHYLUM APICOMPLEXATA

INTRODUCTION TO THE APICOMPLEXATA

Apicomplexata (a-pi-com-plex-A-ta) is derived from two Latin roots that mean the top (apex) infolds (complexus).  The reference is to a set of organelles at the tip of the spindle-shaped sporozoite, a mobile stage in the life cycles of these organisms.

Apicomplexans are parasites that inhabit a variety of animals.  Perkensus is a parasite of bivalve mollusks and exhibits characters that are also seen in the dinoflagellates (e.g. laterally-inserted heterodynamic flagella; Figure A).   Most of the apicomplexans have a life history which can be quite elaborate and require more than one host.  The fusion of gamonts (Figure B) precedes the formation of a cyst within which multiple eggs and sperm are formed and produce zygotes.   Most are intracellular parasites (Figures C-H).  They are united by the formation of an apical complex made of characteristic conoids, micronemes, and rhopteries in the sporozoite.

Aside from Perkensus, there are three major groups of apicomplexans: gregarines, coccidians, and haemosporidians.  The gregarines, typically parasites of arthropods, mollusks, and annelids, have relatively simple life cycles.  The trophozoite develops within a cell into a plasmodium and divides into merozoites by schizogony.  Merozoites are released by lysing the host cells and then invade other cells.  Typically, these cycles occur in cells that line the lumen of the gut or other ducts.  Eventually, gamonts are formed and  emerge from the host cells to group together in the lumen by a process called syzygy. Together, each gamont forms multiple gametes which fuse and form zygocysts, which are expelled to be taken up by another host.  When ingested, sporozoites emerge from the zygocyst and invade the cells of the lumen wall. [See life histories of Monocystis and Stylocephalus.]

Coccidians have life histories that are similar to those of the gregarines; however, they tend to infect vertebrate hosts.  Like the gregarines, coccidians invade epithelial cells of the gut or other things like the gall bladder duct.  The animal ingests a zygocyst from which sporozoites emerge and invade appropriate cells.  The epithelial cycle continues as in the gregarines, but is different in the formation of the zygote.  The trophozoite enlarges in some of the epithelial cells to form a functional egg, called a macrogamete.  Other gametocytes release multiple sperm (also called microgametes), which fertilize the macrogametes.  The resulting zygocyst emerges from the cell by lysing it and passes outside the body, usually with the feces.  Coccidial diseases are very important disease agents for the domestic animals, particularly dogs, cats, cattle, rabbits, and poultry.  Coccidiosis most often manifests itself by diarrhea and can lead to death of the host. [See the life history of Eimeria.]

Toxoplasma is a coccidial organism that alternates between cats and mice.  The typical epithelial cycle occurs in the cat, but zygocysts which have passed out in the feces of the cat "germinate" when consumed by a mouse.  The sporozoites go into a blood infective stage and feed on rbc's (red blood cells).  When the mouse is eaten by a cat, the merozoites in the bloodstream of the mouse invade the intestinal epithelial cells and the cycle continues.  The danger to humans is that Toxoplasma zygocysts can germinate in the gut of anyone who handles cat feces (cat boxes, or even cats that have recently defecated).  The sporozoites in the human system behave as though we are mice and begin an erythrocytic cycle.  In most cases a healthy human can fight off a Toxoplasma infection.  However, Toxoplasma can cross the placental barrier in a pregnant woman and cause death of the fetus.  [See life history of Toxoplasma.]

Haemosporidians typically have complex life cycles that alternate between an arthropod and a vertebrate host.   Malaria (Plasmodium spp.; Figure F), one of the most important diseases of humankind, is a haemosporidian.  The trophozoite, or feeding stage occurs in the red blood cell (rbc).  Tertian malaria, caused primarily by Plasmodium vivax,  typically invades an rbc as a spindle-shaped cell called a merozoite, which enlarges and forms a plasmodium.  After three days, the plasmodium breaks apart in a process called schizogony and then lyses the rbc to release more merozoites.  Plasmodium vivax is synchronous in its infective cycle.  During day 1 of this cycle, the infected person has chills and fevers as the waste products of the lysed cells flood into the circulatory system.  On day 2, the person feels weak, but improves as the toxins are removed and lost rbc's are replaced.  Day 3 begins with the person feeling almost normal until another round of cell lysis begins.  Some of the developing plasmodia form gametocytes, which concentrate in the peripheral blood.  There, they are taken by a mosquito during its blood meal.  The macrogametocyte forms eggs and the microgametocyte forms sperm in the gut of the mosquito.  Syngamy forms a zygote called an ookinete that breaches the gut wall and forms an oocyst which develops into many sporozoites in the haemolymph.  The sporozoites concentrate in the salivary glands from which they are injected into the blood of another person.  Initially, the sporozoites invade liver cells where they go through a cycle of plasmodium and merozoites as in the erythrocytic stage, but this stage produces almost no symptoms.  They can stay in the liver cycle for up to 20 years before they again break out into the circulatory system and the cycle of chills and fevers begins. [See life history of Plasmodium.]

Malaria is limited on earth mainly by the range of the particular mosquitoes that can serve as hosts.  For Plasmodium, that is the Aedes mosquito, which occurs mainly in tropical and subtropical climates.  The great European powers of the 18th and 19th centuries found that in their acquisition of lands in the building of empires, they had to contend not only with the people who occupied those lands, but with the diseases that they encountered.  In the tropics, one of the most important diseases was malaria.  This was much more than a nuisance.  Britain found that when it tried to occupy areas where tertian malaria was common, as in India, they had to field three times more troops than they would otherwise because many were down for two of the three days.  A "cure" was found in the extract of the bark of the cinchona tree of South America.  The substance was quinine, which the British called tonic [This was the source of gin and tonic].  However, quinine does not really cure the person infected, but it prevents the liver stage from breaking out into the symptomatic erythrocytic stage.  Even today, malaria remains a scourge on humankind.  According to the CDC, 350-500 million cases of malaria occur worldwide, and over one million people die, most of them young children in sub-Saharan Africa.

Babesia follows a life cycle similar to that of Plasmodium and is the causative agent of Texas Fever in cattle.  The vector or intermediate host is the cattle tick.  The erythrocytic cycle occurs in cattle. [See the life history of Babesia.]

SYNOPTIC DESCRIPTION TO THE APICOMPLEXATA

SYSTEMATICS OF THE APICOMPLEXATA

Vivier and Desportes (1990) divided the phylum into three classes: the gregarines, the coccidians, and the hematozoans.  Margulis and Schwartz (1988, Pr-19 and 1998, Pr-9), Sleigh et al. (1984), Lee et al. (1985), and Grell (1976) had a similar organization of the group. Older manuals such as Kudo (1966) lumped the Apicomplexa together with the Microspora and Myxospora in a "protozoan" group called the Sporozoa. 

Ultrastructural details clearly indicate that these groups are not related.  Levine (1984) suggests that the Apicomplexata arose from a dinoflagellate ancestor in the Paleozoic. Clearly, the ancestor was motile since the sperm is flagellated. Also, Vivier and Desportes (1990) suggest the the apical complex could be interpreted as an altered basal body complex. Most recent treatments of the group [e.g. Gajadhar et al. (1991), Walters (1991), Cavalier-Smith (1993), Schlegel (1994), Lipscomb (1991), Patterson (1999), and Taylor (1999)] consider the Apicomplexata to be related to the Pyrrhotista and Ciliotista in a monophyletic group called the Alveolates.  Indeed, more recent work summarized by Baldauf (2003) presents the dinoflagellates and apicomplexans to be sisters.

This system is taken from Clopton, et al. (2000), a modification of Levine (1985) and Vivier and Desportes (1990).

HIERARCHICAL CLASSIFICATION OF THE APICOMPLEXATA

This page was written and maintained by Jack R. Holt.  Last revised: 02/15/2008