| Entamoeba histolytica | |
|---|---|
 | |
| Entamoeba histolyticatrophozoite | |
Scientific classification | |
| Domain: | Eukaryota |
| Clade: | Amorphea |
| Phylum: | Amoebozoa |
| Family: | Entamoebidae |
| Genus: | Entamoeba |
| Species: | E. histolytica |
| Binomial name | |
| Entamoeba histolytica Schaudinn, 1903 | |
Entamoeba histolytica is ananaerobic parasiticamoebozoan, part of thegenusEntamoeba.[1] Predominantly infecting humans and other primates causingamoebiasis,E. histolytica is estimated to infect about 35-50 million people worldwide.[1]E. histolytica infection is estimated to kill more than 55,000 people each year.[2] Previously, it was thought that 10% of the world population was infected, but these figures predate the recognition that at least 90% of these infections were due to a second species,E. dispar.[3] Mammals such as dogs and cats can become infected transiently, but are not thought to contribute significantly to transmission.
The wordhistolysis literally means disintegration and dissolution of organic tissues.
The active (trophozoite) stage exists only in the host and in fresh loose feces;cysts survive outside the host in water, in soils, and on foods, especially under moist conditions on the latter. The infection can occur when a person puts anything into their mouth that has touched the feces of a person who is infected withE. histolytica, swallows something, such as water or food, that is contaminated withE. histolytica, or swallowsE. histolytica cysts (eggs) picked up from contaminated surfaces or fingers.[4] The cysts are readily killed by heat and by freezing temperatures; they survive for only a few months outside of the host.[5] When cysts are swallowed, they cause infections by excysting (releasing the trophozoite stage) in the digestive tract.The pathogenic nature ofE. histolytica was first reported byFedor A. Lösch in 1875,[1] but it was not given its Latin name untilFritz Schaudinn described it in 1903.E. histolytica, as its name suggests (histo–lytic = tissue destroying), ispathogenic; infection can be asymptomatic, or it can lead toamoebic dysentery or amoebicliver abscess.[6][7] Symptoms can include fulminating dysentery, bloody diarrhea, weight loss, fatigue, abdominal pain, andamoeboma. The amoeba can 'bore' into the intestinal wall, causinglesions and intestinal symptoms, and it may reach the bloodstream or peritoneal cavity.[8] From there, it can reach vital organs of the human body, usually the liver, but sometimes the lungs, brain, and spleen.[9] A common outcome of this invasion of tissues is a liver abscess, which can be fatal if untreated.[8] Ingestedred blood cells are sometimes seen in the amoeba cell cytoplasm.[10]
Poor sanitary conditions are known to increase the risk of contracting amebiasisE. histolytica.[11] In the United States, there is a much higher rate of amebiasis-related mortality in California and Texas (this might be caused by the proximity of those states toE. histolytica-endemic areas, such as Mexico), parts of Latin America, and Asia.[12]E. histolytica is also recognized as an emerging sexually transmissible pathogen, especially in male homosexual relations, causing outbreaks in non-endemic regions.[13] As such, high-risk sex behaviour is also a potential source of infection.[14] Although it is unclear whether there is a causal link, studies indicate a higher chance of being infected withE. histolytica if one is also infected withhuman immunodeficiency virus (HIV).[15][16]
TheE. histolyticagenome was sequenced, assembled, and automatically annotated in 2005.[17]The genome was reassembled and reannotated in 2010.[18] The 20 million basepair genome assembly contains 8,160 predicted genes; known and noveltransposable elements have been mapped and characterized, functional assignments have been revised and updated, and additional information has been incorporated, including metabolic pathways,Gene Ontology assignments, curation of transporters, and generation of gene families.[19]The major group of transposable elements inE. histolytica are non-LTR retrotransposons. These have been divided in three families called EhLINEs and EhSINEs (EhLINE1,2,3 and EhSINE1,2,3).[20] EhLINE1 encodes an endonuclease (EN) protein (in addition to reverse transcriptase and nucleotide-binding ORF1), which has similarity with bacterialrestriction endonuclease. This similarity with bacterial protein indicates thattransposable elements have been acquired from prokaryotes byhorizontal gene transfer in this protozoan parasite.[21]
The genome ofE. histolytica has been found to havesnoRNAs withopisthokont-like features.[22]TheE. histolytica U3 snoRNA (Eh_U3 snoRNA) has shown sequence and structural features similar to Homo sapiens U3 snoRNA.[23]
E. histolytica may modulate the virulence of certain human viruses and is itself a host for its own viruses.[citation needed]
For example,acquired immunodeficiency syndrome (AIDS) accentuates the damage and pathogenicity ofE. histolytica.[16] On the other hand, cells infected with HIV are often consumed byE. histolytica. Infective HIV remains viable within the amoeba, although there has been no proof of human reinfection from amoeba carrying this virus.[24]
A burst of research on viruses ofE. histolytica stems from a series of papers published by Diamondet al. from 1972 to 1979. In 1972, they hypothesized two separate polyhedral and filamentous viral strains withinE. histolytica that caused cell lysis. Perhaps the most novel observation was that two kinds of viral strains existed and that within one type of amoeba (strain HB-301), the polyhedral strain had no detrimental effect but led to cell lysis in another (strain HK-9). Although Mattern et al. attempted to explore the possibility that these protozoal viruses could function like bacteriophages, they found no significant changes inEntamoeba histolytica virulence when infected by viruses.[25]
E. histolytica causes tissue destruction which leads to clinical disease.E. histolytica induces tissue damage by three main events: direct host cell death, inflammation, and parasite invasion. Once the trophozoites are excysted in the terminal ileum region, they colonize the large bowel, remaining on the surface of the mucus layer and feeding on bacteria and food particles. Occasionally, and in response to unknown stimuli, trophozoites move through the mucus layer where they come in contact with the epithelial cell layer and start the pathological process.E. histolytica has alectin that binds to galactose andN-acetylgalactosamine sugars on the surface of the epithelial cells, The lectin normally is used to bind bacteria for ingestion. The parasite has several enzymes such as pore-forming proteins, lipases, and cysteine proteases, which are normally used to digest bacteria in food vacuoles but which can cause lysis of the epithelial cells by inducing cellular necrosis and apoptosis when the trophozoite comes in contact with them and binds via the lectin. Enzymes released allow penetration into the intestinal wall and blood vessels, sometimes onto the liver and other organs. The trophozoites will then ingest these dead cells. This damage to the epithelial cell layer attracts human immune cells and these in turn can be lysed by the trophozoite, which releases the immune cell's lytic enzymes into the surrounding tissue, creating a type of chain reaction and leading to tissue destruction. This destruction manifests itself in the form of an 'ulcer' in the tissue, typically described as flask-shaped because of its appearance in the transverse section. This tissue destruction can also involve blood vessels leading to bloody diarrhea, and amebic dysentery. Occasionally, trophozoites enter the bloodstream where they are transported typically to the liver via theportal system. In the liver a similar pathological sequence ensues, leading to amebic liver abscesses. The trophozoites can also end up in other organs, sometimes via the bloodstream, sometimes via liver abscess rupture orfistulas. Similarly, when the trophozoites travel to the brain, they can cause amoebic brain abscesses.[26]
Diagnosis is confirmed by microscopic examination for trophozoites or cysts in fresh or suitably preserved fecal specimens, smears of aspirates or scrapings obtained by proctoscopy, and aspirates of abscesses or another tissue specimen. A blood test is also available, but it is recommended only when a healthcare provider believes the infection may have spread beyond the intestine to some other organ of the body, such as the liver. However, this blood test may not help diagnose current illness, because the test can be positive if the patient has had amebiasis in the past, even if they are not infected at the time of the test.[27]Stool antigen detection and PCR are available for diagnosis, and are more sensitive and specific than microscopy.[2]
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Several antibiotics are available to treatEntamoeba histolytica. The infected individual will be treated with only one antibiotic if theE. histolytica infection has not made the person sick, and will most likely be prescribed two antibiotics if the person has been feeling sick.[28] Otherwise, below are other options for treatments.
Usuallynitroimidazole derivatives (such as metronidazole) are used, because they are highly effective against the trophozoite form of the amoeba. Since they have little effect on amoebacysts, usually this treatment is followed by an agent (such as paromomycin or diloxanide furoate) that acts on the organism in thelumen.[2]
In addition to targeting organisms in solid tissue, primarily with drugs likemetronidazole andchloroquine, treatment of liver abscess must include agents that act in the lumen of the intestine (as in the preceding paragraph) to avoid re-invasion. Surgical drainage is usually not necessary, except when rupture is imminent.[29]
People without symptoms, otherwise known as asymptomatic carriers, in non-endemic areas, should be treated withparomomycin; other treatments includediloxanide furoate, andiodoquinol.[citation needed]There have been problems with the use of iodoquinol and iodochlorhydroxyquin, so their use is not recommended.Diloxanide furoate can also be used by mildly symptomatic persons who are just passing cysts.[citation needed]
| Genus and species | Entamoeba histolytica |
| Etiologic agent of: | Amoebiasis;amoebic dysentery; extraintestinal amoebiasis, usually amoebic liver abscess; "anchovy sauce"); amoeba cutis; amoebic lung abscess ("liver-colored sputum") |
| Infective stage | Tetranucleated cyst (having 2–4 nuclei) |
| Definitive host | Human |
| Portal of entry | Mouth |
| Mode of transmission | Ingestion of mature cyst through contaminated food or water |
| Habitat | Colon and cecum |
| Pathogenic stage | Trophozoite |
| Locomotive apparatus | Pseudopodia ("false foot"") |
| Motility | Active, progressive and directional |
| Nucleus | 'Ring and dot' appearance: peripheral chromatin and central karyosome |
| Mode of reproduction | Binary fission |
| Pathogenesis | Lytic necrosis (it looks like "flask-shaped" holes in Gastrointestinal tract sections (GIT) |
| Type of encystment | Protective and Reproductive |
| Lab diagnosis | Most common is direct fecal smear (DFS) and staining (but does not allow identification to species level);enzyme immunoassay (EIA); indirect hemagglutination (IHA); Antigen detection – monoclonal antibody;PCR for species identification. Sometimes only the use of a fixative (formalin) is effective in detecting cysts. Culture: From faecal samples – Robinson's medium, Jones' medium |
| Treatment | Metronidazole for the invasive trophozoites PLUS a lumenal amoebicide for those still in the intestine.Paromomycin (Humatin) is the luminal drug of choice, sinceDiloxanide furoate (Furamide) is not commercially available in the United States or Canada (being available only from the Centers for Disease Control and Prevention). A direct comparison of efficacy showed that Paromomycin had a higher cure rate.[30]Paromomycin (Humatin) should be used with caution in patients with colitis, as it is both nephrotoxic and ototoxic. Absorption through the damaged wall of the intestinal tract can result in permanent hearing loss and kidney damage. Recommended dosage: metronidazole 750 mg three times a day orally, for 5 to 10 days followed by paromomycin 30 mg/kg/day orally in 3 equal doses for 5 to 10 days or Diloxanide furoate 500 mg 3 times a day orally for 10 days, to eradicate lumenal amoebae and prevent relapse.[31][32] |
| Trophozoite stage | |
| Pathognomonic/diagnostic feature | Ingested RBC; distinctive nucleus |
| Cyst Stage | |
| Chromatoidal body | 'Cigar' shaped bodies (made up of crystalline ribosomes) |
| Number of nuclei | 1 in early stages, 4 when mature |
| Pathognomonic/diagnostic feature | 'Ring and dot' nucleus and chromatoid bodies |
In sexually reproducingeukaryotes,homologous recombination (HR) ordinarily occurs duringmeiosis. The meiosis-specificrecombinase,Dmc1, is required for efficient meiotic HR, and Dmc1 is expressed inE. histolytica.[33] The purified Dmc1 fromE. histolytica formspresynaptic filaments and catalyzesATP-dependenthomologous DNA pairing and DNA strand exchange over at least several thousandbase pairs.[33] The DNA pairing and strand exchange reactions are enhanced by the eukaryotic meiosis-specific recombination accessory factor (heterodimer) Hop2-Mnd1.[33] These processes are central to meiotic recombination, suggesting thatE. histolytica undergoes meiosis.[33]
Several other genes involved in both mitotic and meiotic HR are also present inE. histolytica.[34] HR is enhanced under stressful growth conditions (serum starvation) concomitant with the up-regulation of HR-related genes.[35] Also, UV irradiation induces DNA damage inE. histolyticatrophozoites and activates the recombinational DNA repair pathway.[34] In particular, expression of theRad51 protein (arecombinase) is increased about 15-fold by UV treatment.[34]
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