Not all apicomplexa develop all the following cellular varieties and division methods. This presentation is intended as an outline of a hypothetical generalised apicomplexan organism.
Apicomplexans (sporozoans) replicate via ways of multiple fission (also known as schizogony). These ways include gametogony, sporogony and merogony, although the latter is sometimes referred to as schizogony, despite its general meaning.[2]
Sporogony is a type of sexual and asexual reproduction. It involves karyogamy, the formation of a zygote, which is followed by meiosis and multiple fission. This results in the production of sporozoites.
Other forms of replication include endodyogeny and endopolygeny. Endodyogeny is a process of asexual reproduction, favoured by parasites such as Toxoplasma gondii. It involves an unusual process in which two daughter cells are produced inside a mother cell, which is then consumed by the offspring prior to their separation.[6]
Endopolygeny is the division into several organisms at once by internal budding.[6]
A sporozoite (ancient Greek sporos, seed + zōon, animal) is the cell form that infects new hosts. In Plasmodium, for instance, the sporozoites are cells that develop in the mosquito's salivary glands, leave the mosquito during a blood meal, and enter liver cells (hepatocytes), where they multiply. Cells infected with sporozoites eventually burst, releasing merozoites into the bloodstream.[8] Sporozoites are motile and they move by gliding.
A merozoite (G. meros, part [of a series] +zōon, animal) is the result of merogony that takes place within a host cell. During this stage, the parasite infects the host's cells and then replicates its own nucleus and induces cell segmentation in a form of asexual reproduction. In coccidiosis, merozoites form the first phase of the internal life cycle of coccidian. In the case of Plasmodium, merozoites infect red blood cells and then rapidly reproduce asexually. The red blood cell host is destroyed by this process, which releases many new merozoites that go on to find new blood-borne hosts. Merozoites are motile. Before schizogony, the merozoite is also known as the schizozoite.[9]
A gametocyte (G.gametēs, partner + kytos, cell) is a name given to a parasite's gamete-forming cells. A male gametocyte divides to give many flagellated microgametes, whereas the female gametocyte differentiates to a macrogamete.[10]
An ookinete (G. ōon, egg + kinētos, motile) is a fertilised zygote capable of moving spontaneously. It penetrates epithelial cells lining the midgut of mosquitoes to form a thick-walled structure known as an oocyst under the mosquito's outer gut lining.[11] Ookinetes are motile and they move by gliding.
A trophozoite (G. trophē, nourishment + zōon, animal) is the activated, intracellular feeding stage in the apicomplexan life cycle. After gorging itself on its host, the trophozoite undergoes schizogony and develops into a schizont, later releasing merozoites.
A hypnozoite (G. hypnos, sleep + zōon, animal) is a quiescent parasite stage that is best known for its "... probable association with latency and relapse in human malarial infections caused by Plasmodium ovale and P. vivax".[12] Hypnozoites are directly sporozoite-derived.[13]
A bradyzoite (G. bradys, slow + zōon, animal) is a sessile, slow-growing form of zoonoticmicroorganisms such as Toxoplasma gondii, among others responsible for parasitic infections. In chronic (latent) toxoplasmosis, bradyzoites microscopically present as clusters enclosed by an irregular crescent-shaped wall (cysts) in infected muscle and brain tissues. Also known as a bradyzoic merozoite.[14]
A tachyzoite (G. tachys, fast + zōon, animal), contrasting with a bradyzoite, is a form typified by rapid growth and replication. Tachyzoites are the motile forms of those coccidians which form tissue pseudocysts, such as Toxoplasma and Sarcocystis. Typically infecting cellular vacuoles, tachyzoites divide by endodyogeny and endopolygeny. Also known as a tachyzoic merozoite (same journal reference as for "bradyzoic merozoite", above).
An oocyst (G. ōon, egg + kystis, bladder) is a hardy, thick-walled spore, able to survive for lengthy periods outside a host. The zygote develops within the spore, which acts to protect it during transfer to new hosts. Organisms that create oocysts include Eimeria, Isospora, Cryptosporidium, and Toxoplasma.
The dynamics of gene loss was studied in 41 apicomplexan genomes.[15] Loss of genes employed in amino acid metabolism and steroid biosynthesis could be explained by metabolic redundancy with the host.[15] Also, DNA repair genes tend to be lost by apicomplexans with reduced proteome size, probably reflecting a reduced need for DNA repair of genomes with smaller information content.[15] Reduced DNA repair may help explain the elevated mutation rates in pathogens with reduced genome size.[15]
The Apicomplexa are a large phylum of mainly parasitic alveolates. Most of them possess a unique form of organelle that comprises a type of non-photosynthetic plastid called an apicoplast, and an apical complex structure. The organelle is an adaptation that the apicomplexan applies in penetration of a host cell.
Toxoplasma gondii is a parasitic protozoan that causes toxoplasmosis. Found worldwide, T. gondii is capable of infecting virtually all warm-blooded animals, but felids are the only known definitive hosts in which the parasite may undergo sexual reproduction.
Plasmodium is a genus of unicellular eukaryotes that are obligate parasites of vertebrates and insects. The life cycles of Plasmodium species involve development in a blood-feeding insect host which then injects parasites into a vertebrate host during a blood meal. Parasites grow within a vertebrate body tissue before entering the bloodstream to infect red blood cells. The ensuing destruction of host red blood cells can result in malaria. During this infection, some parasites are picked up by a blood-feeding insect, continuing the life cycle.
Isosporiasis, also known as cystoisosporiasis, is a human intestinal disease caused by the parasite Cystoisospora belli. It is found worldwide, especially in tropical and subtropical areas. Infection often occurs in immuno-compromised individuals, notably AIDS patients, and outbreaks have been reported in institutionalized groups in the United States. The first documented case was in 1915. It is usually spread indirectly, normally through contaminated food or water (CDC.gov).
Plasmodium falciparum is a unicellular protozoan parasite of humans, and the deadliest species of Plasmodium that causes malaria in humans. The parasite is transmitted through the bite of a female Anopheles mosquito and causes the disease's most dangerous form, falciparum malaria. It is responsible for around 50% of all malaria cases. P. falciparum is therefore regarded as the deadliest parasite in humans. It is also associated with the development of blood cancer and is classified as a Group 2A (probable) carcinogen.
Coccidia (Coccidiasina) are a subclass of microscopic, spore-forming, single-celled obligate intracellular parasites belonging to the apicomplexan class Conoidasida. As obligate intracellular parasites, they must live and reproduce within an animal cell. Coccidian parasites infect the intestinal tracts of animals, and are the largest group of apicomplexan protozoa.
The Plasmodiidae are a family of apicomplexan parasites, including the type genus Plasmodium, which is responsible for malaria. This family was erected in 1903 by Mesnil and is one of the four families in the order Haemospororida.
Plasmodium ovale is a species of parasitic protozoon that causes tertian malaria in humans. It is one of several species of Plasmodium parasites that infect humans, including Plasmodium falciparum and Plasmodium vivax which are responsible for most cases of malaria in the world. P. ovale is rare compared to these two parasites, and substantially less dangerous than P. falciparum.
Eimeria tenella is a species of Eimeria that causes hemorrhagic cecal coccidiosis in young poultry. It is found worldwide.
Plasmodium knowlesi is a parasite that causes malaria in humans and other primates. It is found throughout Southeast Asia, and is the most common cause of human malaria in Malaysia. Like other Plasmodium species, P. knowlesi has a life cycle that requires infection of both a mosquito and a warm-blooded host. While the natural warm-blooded hosts of P. knowlesi are likely various Old World monkeys, humans can be infected by P. knowlesi if they are fed upon by infected mosquitoes. P. knowlesi is a eukaryote in the phylum Apicomplexa, genus Plasmodium, and subgenus Plasmodium. It is most closely related to the human parasite Plasmodium vivax as well as other Plasmodium species that infect non-human primates.
The gregarines are a group of Apicomplexan alveolates, classified as the Gregarinasina or Gregarinia. The large parasites inhabit the intestines of many invertebrates. They are not found in any vertebrates. Gregarines are closely related to both Toxoplasma and Plasmodium, which cause toxoplasmosis and malaria, respectively. Both protists use protein complexes similar to those that are formed by the gregarines for gliding motility and for invading target cells. This makes the gregarines excellent models for studying gliding motility, with the goal of developing treatment options for both toxoplasmosis and malaria. Thousands of different species of gregarine are expected to be found in insects, and 99% of these gregarine species still need to be described. Each insect species can be the host of multiple gregarine species. One of the most-studied gregarines is Gregarina garnhami. In general, gregarines are regarded as a very successful group of parasites, as their hosts are distributed over the entire planet.
Leucocytozoon is a genus of parasitic alveolates belonging to the phylum Apicomplexa.
Megaloschizonts are large schizonts that produce extremely high numbers of merozoites. They are found in various species of the Phylum Apicomplexa. The Apicomplexa phylum contains several parasitic protozoans. They have a very complex life cycle that includes several stages. Megaloschizonts and the smaller schizonts are the part of the life cycle that takes place inside the infected host organism and operates as an asexually reproductive cell. Megaloschizonts appear as grey-white nodules found in the smooth muscle of major organs, such as the heart, liver, lung or spleen.
Plasmodium fieldi is a parasite of the genus Plasmodium sub genus Plasmodium found in Malaysia. This species is related to Plasmodium ovale and Plasmodium simiovale. As in all Plasmodium species, P. fieldi has both vertebrate and insect hosts. The vertebrate hosts for this parasite are primates.
Karyolysus is a genus of coccidia. With the exception of K. sonomae whose vertebrate host is the yellow-legged frog, species in this genus only infect lizards of the genus Lacerta.
Polychromophilus is a genus of obligate intracellular eukaryotic parasites that infect bats from every continent except Antarctica. They are transmitted by bat flies, which act as an insect vector as well as the parasite’s site of sporogeny. Polychromophilus follows a fairly typical Haemospororidian lifecycle, with gametocytes and gametes restricted to the bloodstream of the host and meronts infecting organs – most notably the lungs and the liver. The type species is Polychromophilus melanipherus, and was described by Dionisi in 1898.
The Archigregarinorida are an order of parasitic alveolates in the phylum Apicomplexa. Species in this order infect marine invertebrates — usually annelids, ascidians, hemichordates and sipunculids.
Plasmodium coatneyi is a parasitic species that is an agent of malaria in nonhuman primates. P. coatneyi occurs in Southeast Asia. The natural host of this species is the rhesus macaque and crab-eating macaque, but there has been no evidence that zoonosis of P. coatneyi can occur through its vector, the female Anopheles mosquito.
Hammondia hammondi is a species of obligate heteroxenous parasitic alveolates of domestic cats. Intracellular cysts develop mainly in striated muscle. After the ingestion of cysts by cats, a multiplicative cycle precedes the development of gametocytes in the epithelium of the small intestine. Oocyst shedding persists for 10 to 28 days followed by immunity. Cysts in skeletal muscle measure between 100 and 340 μm in length and 40 and 95 μm in width. Some of the intermediate hosts develop low levels of antibody and some cross-immunity against Toxoplasma.
Plasmodium cynomolgi is an apicomplexan parasite that infects mosquitoes and Asian Old World monkeys. In recent years, a number of natural infections of humans have also been documented. This species has been used as a model for human Plasmodium vivax because Plasmodium cynomolgi shares the same life cycle and some important biological features with P. vivax.
References
↑ Jadwiga Grabda (1991). Marine fish parasitology: an outline. VCH. p.8. ISBN0-89573-823-6.
↑ Herwaldt; Persing, DH; Précigout, EA; Goff, WL; Mathiesen, DA; Taylor, PW; Eberhard, ML; Gorenflot, AF; etal. (1 April 1996). "A Fatal Case of Babesiosis in Missouri: Identification of Another Piroplasm That Infects Humans". Annals of Internal Medicine. 124 (7): 643–650. doi:10.7326/0003-4819-124-7-199604010-00004. PMID8607592. S2CID46733758.
↑ "Schizozoite", Farlex Partner Medical Dictionary, 2012, A merozoite before schizogony, as in the exoerythrocytic phase of the development of the Plasmodium agent after sporozoite invasion of the hepatocyte and before multiple division.
↑ Sinden, RE; Talman, A; Marques, SR; Wass, MN; Sternberg, MJE (August 2010). "The flagellum in malarial parasites". Current Opinion in Microbiology. 13 (4): 491–500. doi:10.1016/j.mib.2010.05.016. PMID20566299.
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