Medical protozoology: Apicomplexa

A note on taxonomy
Adl et al. 2012 The revised classification of eukaryotes. The Journal of Eukaryotic Microbiology. 59(2), 429 – 493.
Taxonomy is the science of defining groups of biological organisms on the basis of shared characteristics and giving names to those groups.
Practical application
Understanding evolution of life

Morphology-based taxonomy
Depending on the locomotion mode protozoa divided into four major groups:
Phylum Flagellates (move by means of flagella)

Morphology-based taxonomy
Depending on the locomotion mode protozoa divided into four major groups:
Phylum Amoebae (by pseudopodia)

Morphology-based taxonomy
Depending on the locomotion mode protozoa divided into four major groups:
Phylum Ciliates (by cilia)

Morphology-based taxonomy
Depending on the locomotion mode protozoa divided into four major groups:
Phylum Sporozoa (lacking any obvious means of locomotion)

Parasitic Apicomplexa
(former Sporozoa)
The apicomplexa are a monophyletic group composed almost entirely of parasitic (ie, no free-living) species.
Apicomplexa, along with ciliates and dinoflagellates, form a higher order group known as Alveolata.
A major defining characteristic of the this group are flattened vesicle-like structures, called cortical alveolae, which are found just underneath the plasma membrane.
Formerly the apicomplexa were part of a group called Sporozoa and this name is still sometimes used.

Parasitic Apicomplexa
A defining characteristic of the apicomplexa is a group of organelles found at one end--called the apical end--of the organism.
This is a type of plastid called an apicoplast, and anapical complex structure involved in penetrating a host's cell.

Parasitic Apicomplexa
This 'apical complex' includes secretory organelles known as micronemes and rhoptries, polar rings composed of microtubules, and in some species a conoid which lies within the polar rings.

Parasitic Apicomplexa
At some point during their life cycle, members of the apicomplexa either invade or attach to host cells.
It is during this invasive (and/or motile) stage that these apical organelles are expressed as well as the subpellicular membranes, which are actually cortical alveoli.
The apical organelles play a role in interaction of the parasite with the host cell and the subsequent invasion of the host cell.
Motile forms of apicomplexa crawl along the substratum in a non-ameboid fashion known as gliding motility. Many apicomplexan species have flagellated gametes.

Parasitic Apicomplexa
The apicomplexa have complex life cycles that are characterized by three distinct processes: sporogony, merogony and gametogony .

Parasitic Apicomplexa
Although most apicomplexa exhibit this overall general life cycle the details can vary between species.
Furthermore, the terminology used to describe these various life cycle stages vary between the species.
The life cycle consists of both asexually reproducing forms and sexual stages.

Parasitic Apicomplexa
In monoxenous species all three of these processes will be carried out in a single host and often in a single cell type or tissue.
Whereas, in heteroxenous species the various processes will be carried out in different hosts and generally involve different tissues.

Parasitic Apicomplexa
Sporogony occurs immediately after a sexual phase and consists of an asexual reproduction that culminates in the production of sporozoites.
Sporozoites are an invasive form that will invade cells and develop into forms that undergo another asexual replication known as merogony.

Parasitic Apicomplexa
Merogony and the resulting merozoites are known by many different names depending of the species. In contrast to sporogony, in which there is generally only one round of replication, quite often there are multiple rounds of merogony.
In other words, the merozoites, which are also invasive forms, can reinvade cells and initiate another round of merogony.

Parasitic Apicomplexa
As an alternative to asexual replication merozoites can develop into gametes through a process variously called gametogony, gamogony or gametogenesis.
As in other types of sexual reproduction, the gametes fuse to form a zygote which will undergo sporogony.

Parasitic Apicomplexa
The apicomplexa are an extremely large and diverse group (>5000 named species). Seven species infect humans.
Plasmodium
Babesia
Cryptosporidium
Isospora
Cyclospora
Sarcocystis
Toxoplasma

Parasitic Apicomplexa
Plasmodium, as the causative agent of malaria, has the greatest impact on human health.
Babesia is a relatively rare zoonotic infection.
The other five species are all classified as coccidia. However, recent molecular data indicates that Cryptosporidium is more closely related to the gregarines than to the coccidia. The coccidia are generally considered opportunistic pathogens and are often associated with AIDS.

Parasitic Apicomplexa
Several apicomplexan parasites are also important in terms of veterinary medicine and agriculture. Most notable are Babesia and Theileria in cattle and Eimeria in poultry.

Parasitic Apicomplexa
Diseases caused by apicomplexan organisms include, but are not limited to:
Forms of coccidiosis including:
Cryptosporidiosis (Cryptosporidium parvum)
Cyclosporiasis (Cyclospora cayetanensis)
Isosporiasis (Isospora belli)
Toxoplasmosis (Toxoplasma gondii)
Malaria (Plasmodium)
Babesiosis (Babesia)