The
Apicomplexa are a large group of
protistProtists , are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista but this group is no longer recognized in modern taxonomy...
s, most of which possess a unique
organelleIn cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid membrane....
called
apicoplastThe apicoplast is a relict, non-photosynthetic plastid found in most Apicomplexa, including malaria parasites such as Plasmodium falciparum, but not in others such as Cryptosporidium.It is proposed that it evolved via secondary endosymbiosis...
and an
apical complex structure involved in penetrating a host's cell. They are unicellular, spore-forming, and exclusively parasites of animals. Motile structures such as
flagellaA flagellum is a tail-like structure that projects from the cell body of certain prokaryotic and eukaryotic cells, and functions in locomotion. There are some notable differences between prokaryotic and eukaryotic flagella, such as protein composition, structure, and mechanism of propulsion...
or
pseudopods are absent except in certain
gameteA gamete is a cell that fuses with another gamete during fertilization in organisms that reproduce sexually...
stages. This is a diverse group including organisms such as
coccidiaCoccidia are microscopic, spore-forming, single-celled parasites belonging to the apicomplexan class Conoidasida. Coccidian parasites infect the intestinal tracts of animals, and are the largest group of apicomplexan protozoa....
, gregarines, piroplasms, haemogregarines, and
plasmodiaThe Plasmodiidae are a family of apicomplexan parasites, including the type genus Plasmodium, which is responsible for malaria. This genus was created in 1903 by Mesnil.They are given their own order - the Haemosporida.- Diagnostic criteria :...
; some diseases caused by apicomplexan organisms include:
- Babesiosis
Babesiosis is a malaria-like parasitic disease caused by Babesia, a genus of protozoal piroplasms. After trypanosomes, Babesia are thought to be the second most common blood parasites of mammals and they can have a major impact on health of domestic animals in areas without severe winters...
(BabesiaBabesia is a protozoan parasite of the blood that causes a hemolytic disease known as Babesiosis. There are over 100 species of Babesia identified; however only a handful have been documented as pathogenic in humans....
)
- Malaria
Malaria is a vector-borne infectious disease caused by a eukaryotic protist of the genus Plasmodium. It is widespread in tropical and subtropical regions, including parts of the Americas, Asia, and Africa. Each year, there are approximately 350–500 million cases of malaria, killing between one and...
(PlasmodiumPlasmodium is a genus of parasitic protists. Infection by these organisms is known as malaria. The genus Plasmodium was discovered in 1885 by Marchiafava and Celli. Currently over 200 species of this genus are recognized and new species continue to be described.Of the over 200 known species of...
)
- Coccidia
Coccidia are microscopic, spore-forming, single-celled parasites belonging to the apicomplexan class Conoidasida. Coccidian parasites infect the intestinal tracts of animals, and are the largest group of apicomplexan protozoa....
n diseases including:
- Cryptosporidiosis
Cryptosporidiosis, also known as crypto, is a parasitic disease caused by Cryptosporidium, a protozoan parasite in the phylum Apicomplexa. It affects the intestines of mammals and is typically an acute short-term infection...
(Cryptosporidium parvumCryptosporidium parvum is one of several species that cause cryptosporidiosis, a parasitic disease of the mammalian intestinal tract.Primary symptoms of C. parvum infection are acute, watery, and non-bloody diarrhoea. C. parvum infection is of particular concern in immunocompromised patients, where...
)
- Cyclosporiasis
Cyclosporiasis is an infection with the protozoan Cyclospora cayetanensis, a pathogen transmitted by feces or feces-contaminated fresh produce and water. Outbreaks have been reported due to contaminated raspberries. It is not spread from person to person...
(Cyclospora cayetanensisCyclospora cayetanensis is a protozoan that causes disease in humans, and perhaps other primates. It has been linked in the United States from fecally-contaminated imported raspberries and was virtually unknown before about 1990, but has been on the rise since...
)
- Isosporiasis
Isosporiasis is a human intestinal disease caused by the parasite Isospora 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...
(Isospora belliIsospora belli is a species of internal parasites classified under Coccidia.It is responsible for the condition isosporiasis.Autofluorescence aids detection....
)
- Toxoplasmosis
Toxoplasmosis is a parasitic disease caused by the protozoan Toxoplasma gondii. The parasite infects most genera of warm-blooded animals, including humans, but the primary host is the felid family. Animals are infected by eating infected meat, by ingestion of faeces of a cat that has itself...
(Toxoplasma gondiiToxoplasma gondii is a species of parasitic protozoa in the genus Toxoplasma. The definitive host of T. gondii is the cat, but the parasite can be carried by all known mammals. Toxoplasmosis, the disease of which T...
)
While "Apicomplexa" is not synonymous with the older term "Sporozoa", there is significant overlap between the species included in the two groupings.
History
The first apicomplexan protozoon was seen by Antony van Leeuwenhoek who in 1674 saw
oocystAn oocyst is the thick-walled spore phase of certain protists , such as Cryptosporidium and Toxoplasma. This state can survive for lengthy periods outside a host and is very resistant....
s of
Eimeria stiedaeEimeria stiedae is a species of Eimeria that causes hepatic coccidiosis in rabbits.- Life cycle:The rabbits ingest sporulated oocysts. Sporulated oocyst contain four sporozoites that hatch and travel via the hepatic portal vein to the liver and eventually penetrate the bile duct epithelium, where...
in the gall bladder of a
rabbitRabbits are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world. There are seven different genera in the family classified as rabbits, including the European rabbit , Cottontail rabbit , and the Amami rabbit...
. The first member of the
phylumIn biology, a phylum ["Phylum" is adopted from the Greek phylai, the clan-based voting groups in Greek city-states.] is a taxonomic rank below Kingdom and above Class...
to be named (by Dufour in 1828) was
Gregarina ovata in
earwig Earwig is the common name given to the insect order Dermaptera, characterized by membranous wings folded underneath short forewings, hence the literal translation of the order being "skin wings". The abdomen extends beyond the wings, and frequently ends in a pair of forceps-like structures called...
s. Since then many more have been identified and named. During the quarter century 1826-1850, 41 species and 6 genera of Apicomplexa were named. In the quarter century 1951-1975, 1873 new species and 83 new genera were added.
By 1987 a comprehensive survey of the phylum was completed: in all 4516 species and 339 genera had been named. They consisted of:
- the gregarines (subclass Gregarinasida) with 1624 named species and 231 named genera
- the hemogregarines (family Haemogregarinidae) with 399 species and 4 genera
- the eimeriorins (order Eimeriorida) with 1771 species and 43 genera
- the hemospororids (order Haemospororida with 444 species and 9 genera
- the piroplasmids (order Piroplasmorida) with 173 species and 20 genera
- and a few others (105 species and 32 genera)
Although there has been considerable revision of this phylum it seems likely these numbers are still approximately correct.
Evolution
Many
Coccidiomorpha have an intermediate host as well as the primary host, and the evolution of hosts proceeded in different ways and at different times in these groups. In some the original host has become the intermediate host while in others it has become the definitive host. In the genera
Aggregata,
Atoxoplasma,
Cystoisospora,
Schellackia and
Toxoplasma the original is now definitive while in
AkibaAkiba Also pronounced Akiva can refer to:*The Aramaic form of the name Jacob*Akiba-Schechter Jewish Day School, a Jewish school in Hyde Park, Chicago*Akiva, a tannaitic Rabbi*Akiba Hebrew Academy, a Jewish school in Merion, Pennsylvania...
,
Babesiosoma,
BabesiaBabesia is a protozoan parasite of the blood that causes a hemolytic disease known as Babesiosis. There are over 100 species of Babesia identified; however only a handful have been documented as pathogenic in humans....
,
HaemogregarinaHaemogregarina is a genus of hemoprotozoans, parasitic mainly in cold-blooded vertebrates. It was described in 1885 by Danilewsky from the european pond turtle . They are unicellular organisms which are parasitic in the red blood cells....
,
HaemoproteusHaemoproteus is a genus of protozoa that are parasitic in birds, reptiles and amphibians. The genus created was by Kruse in 1890. Its name is derived from Greek: Haima - blood and Proteus - a sea god who had the power of assuming different shapes...
,
HepatozoonHepatozoon is a genus of Apicomplexan protozoa which incorporates over 300 species obligate intraerythrocytic parasites. Species have been described from all groups of tetrapod vertebrates, as well as a wide range of haematophagous arthropods, which serve as both the vectors and definitive hosts of...
,
KaryolysusKaryolysus is a genus of coccidia.It includes the species Karyolysus lacerate....
,
LeukocytozoonLeukocytozoon is a genus of parasitic protozoa belonging to the phylum Apicomplexia. The parasites were first seen by Danilewsky in 1884. The genus was created by Ziemann in 1898...
,
PlasmodiumPlasmodium is a genus of parasitic protists. Infection by these organisms is known as malaria. The genus Plasmodium was discovered in 1885 by Marchiafava and Celli. Currently over 200 species of this genus are recognized and new species continue to be described.Of the over 200 known species of...
,
SarcocystisSarcocystis is a genus of protozoa. Species in this genus infect reptiles, birds and mammals. The name is dervived from Greek: sarx = flesh and kystis = bladder.There are about 130 recognised species in this genus...
and
TheileriaTheileria is a genus of parasitic protozoan that belongs to the phylum Apicomplexa and is closely related to Plasmodium. Two Theileria species, T. annulata and T. parva, are important cattle parasites. T. annulata causes tropical theileriosis and T. parva causes East Coast fever. Theileria are...
have original hosts are now intermediate.
Similar strategies to increase the likelihood of transmission have evolved in multiple genera. Polyenergid
oocystAn oocyst is the thick-walled spore phase of certain protists , such as Cryptosporidium and Toxoplasma. This state can survive for lengthy periods outside a host and is very resistant....
s and tissue cysts are found in representatives of the orders Protococcidiida and Eimeriida. Hypnozoites are found in
Karyolysus lacerate and most species of
PlasmodiumPlasmodium is a genus of parasitic protists. Infection by these organisms is known as malaria. The genus Plasmodium was discovered in 1885 by Marchiafava and Celli. Currently over 200 species of this genus are recognized and new species continue to be described.Of the over 200 known species of...
; transovarial transmission of parasites occurs in life cycles of
KaryolysusKaryolysus is a genus of coccidia.It includes the species Karyolysus lacerate....
and
BabesiaBabesia is a protozoan parasite of the blood that causes a hemolytic disease known as Babesiosis. There are over 100 species of Babesia identified; however only a handful have been documented as pathogenic in humans....
.
Life cycle
Most members have a complex life-cycle, involving both asexual and sexual reproduction. Typically, a host is infected via an active invasion by the parasites (similar to
entosisEntosis is a form of cell death that involves the cell dying as a result of becoming engulfed by a neighboring cell. The process was discovered by Overholtzer, et al. as reported in Cell....
), which divide to produce
sporozoiteIn the life-cycle of apicomplexan protozoa, sporozoites are cells that infect new hosts. In the parasites that cause malaria , for instance, the sporozoites are cells that develop in the mosquito's salivary glands, leave the mosquito during a blood meal, and enter the liver where they multiply...
s that enter its cells. Eventually, the cells burst, releasing
merozoites which infect new cells. This may occur several times, until
gamonts are produced, forming gametes that fuse to create new cysts. There are many variations on this basic pattern, however, and many Apicomplexa have more than one host.
The apical complex includes vesicles called rhoptries and
micronemeMicronemes are cellular organs, or organelles, possessed by Apicomplexa protozoans. They are specialized secretory organelles important for gliding motility and host cell invasion....
s, which open at the anterior of the cell. These secrete enzymes that allow the parasite to enter other cells. The tip is surrounded by a band of
microtubuleMicrotubules are one of the components of the cytoskeleton. They have a diameter of 25 nm and length varying from 200 nanometers to 25 micrometers. Microtubules serve as structural components within cells and are involved in many cellular processes including mitosis, cytokinesis, and vesicular...
s, called the polar ring, and among the Conoidasida there is also a funnel of rods called the conoid.. Over the rest of the cell, except for a diminished mouth called the micropore, the membrane is supported by vesicles called alveoli, forming a semi-rigid pellicle.
The presence of alveoli and other traits place the Apicomplexa among a group called the
alveolateThe alveolates are a major line of protists.-Phyla:There are three phyla, which are very divergent in form, but are now known to be close relatives based on various ultrastructural and genetic similarities:...
s. Several related flagellates, such as
PerkinsusPerkinsus marinus is a prevalent pathogen of oysters, causing massive mortality in oyster populations. The disease it causes is known as "Dermo" ,and is characterized by proteolytic degradation of oyster tissues...
and
Colpodella have structures similar to the polar ring and were formerly included here, but most appear to be closer relatives of the
dinoflagellateThe dinoflagellates are a large group of flagellate protists. Most are marine plankton, but they are common in fresh water habitats as well. Their populations are distributed depending on temperature, salinity, or depth. About half of all dinoflagellates are photosynthetic, and these make up the...
s. They are probably similar to the common ancestor of the two groups.
Another similarity is that apicomplexan cells contain a single
plastidPlastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...
, called the
apicoplastThe apicoplast is a relict, non-photosynthetic plastid found in most Apicomplexa, including malaria parasites such as Plasmodium falciparum, but not in others such as Cryptosporidium.It is proposed that it evolved via secondary endosymbiosis...
, surrounded by either 3 or four membranes. Its functions are thought to include tasks such as lipid synthesis, it appears to be necessary for survival. They are generally considered to share a common origin with the chloroplasts of dinoflagellates, and evidence generally points to an origin from
red algaeThe red algae are one of the oldest groups of eukaryotic algae, and also one of the largest, with about 5,000–6,000 species of mostly multicellular, marine algae, including many notable seaweeds...
rather than green.
The Apicomplexa comprise the bulk of what used to be called the Sporozoa, a group for parasitic protozoans without flagella, pseudopods, or cilia. Most of the Apicomplexa are motile however. The other main lines were the
AscetosporeaThe Ascetosporea are a group of protists that are parasites of animals, especially marine invertebrates. There are two groups, the haplosporids and paramyxids, which are not particularly similar morphologically but consistently group together on molecular trees, which place them near the base of...
, the
MyxozoaThe Myxozoa are a group of parasitic animals of aquatic environments. Over 1300 species have been described and many have a two-host lifecycle, involving a fish and an annelid worm or bryozoan. The average size of a Myxosporea spore usually ranges from 10μm to 20μm and Malacosporea up to 2mm...
(now known to be derived from
animalAnimals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously...
s), and the
MicrosporidiaThe microsporidia constitute a phylum of spore-forming unicellular parasites. Loosely 1500 of the probably more than one million species are named now. Microsporidia are restricted to animal hosts, and all major groups of animals host microsporidia. Most infect insects, but they are also...
(now known to be derived from
fungiA fungus is any member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. The Fungi are classified as a kingdom that is separate from plants, animals and bacteria...
). Sometimes the name Sporozoa is taken as a synonym for the Apicomplexa, or occasionally as a subset.
Blood borne genera
Within the Apicomplexa there are three groups of blood borne parasites. These species lie within in three suborders.
- suborder Adeleorina
Adeleorina is a suborder of Apicomplexa.Blood parasites belonging to the suborder Adeleorina are collectively known as haemogregarines. Currently their sister group is thought to be the piroplasms....
- 8 genera
- suborder Haemosporina - all genera in this suborder
- suborder Eimeriorina
Eimeriorina is a suborder of apicomplexa....
- 2 genera (Lankesterella and Schellackia)
Disease Genomics
As noted above, many of the apicomplexan parasites are important pathogens of human and domestic animals. In contrast to
bacteriaThe bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
l pathogens, these apicomplexan parasites are
eukaryoteA eukaryote is an organism whose cells contain complex structures enclosed within membranes. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried...
s and share many metabolic pathways with their animal hosts. This fact makes therapeutic target development extremely difficult – a drug that harms an apicomplexan parasite is also likely to harm its human host. Currently there are no effective
vaccineA vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains a small amount of an agent that resembles a microorganism...
s or treatments available for most diseases caused by these parasites. Biomedical research on these parasites is challenging because it is often difficult, if not impossible, to maintain live parasite cultures in the laboratory and to genetically manipulate these organisms. In the recent years, several of the apicomplexan species have been selected for
genome sequencingGenome Sequencing may refer to:* DNA sequencing* Full genome sequencing...
. The availability of
genomeIn modern molecular biology the genome refers to all of its hereditary information encoded in DNA .The genome includes both the genes and the non-coding sequences of the DNA. The term was adapted in 1920 by Hans Winkler, Professor of Botany at the University of Hamburg, Germany...
sequences provides a new opportunity for scientists to learn more about the
evolutionIn biology, evolution is change in the genetic material of a population of organisms from one generation to the next. Though changes produced in any one generation are normally small, differences accumulate with each generation and can, over time, cause substantial changes in the population, a...
and biochemical capacity of these parasite. A NIH-funded database,
ApiDB.org, provides public access to currently available genomic data sets. One possible target for drugs is the plastid, and in fact existing drugs such as
tetracyclineTetracyclines are a group of broad-spectrum antibiotics whose general usefulness has been reduced with the onset of bacterial resistance. Despite this, they remain the treatment of choice for some specific indications....
s which are effective against apicomplexans seem to operate against the plastid.
Most apicomplexans have plastid genomes as well as nuclear ones, although
Cryptosporidium spp. and possibly gregarines are exceptions as they are thought to have lost plastids after the diverging last common ancestor of apicomplexans.