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Life-cycle of the
Entamoeba histolytica
For the infection and disease caused by this parasite, see
Amoebiasis.
Entamoeba histolytica is an anaerobic parasitic protozoan, part of the genus Entamoeba.[1] Predominantly infecting humans and other primates, E. histolytica is estimated to infect about 50 million people worldwide. Many older textbooks state that 10% of the world population is infected, but these figures predate the recognition that at least 90% of these infections were due to a second species, E. dispar.[2] Mammals such as dogs and cats can become infected transiently, but are not thought to contribute significantly to transmission.
[edit] Transmission
The active (trophozoite) stage exists only in the host and in fresh loose feces; cysts survive outside the host in water, soils and on foods, especially under moist conditions on the latter. The cysts are readily killed by heat and by freezing temperatures, and survive for only a few months outside of the host.[3] When cysts are swallowed they cause infections by excysting (releasing the trophozoite stage) in the digestive tract. The trophozoite stage is readily killed in the environment and cannot survive passage through the acidic stomach to cause infection.
[edit] Pathogenesis
E. histolytica, as its name suggests (histo–lytic = tissue destroying), is pathogenic; infection can lead to amoebic dysentery or amoebic liver abscess.[1] Symptoms can include fulminating dysentery, bloody diarrhea, weight loss, fatigue, abdominal pain, and amoeboma. The amoeba can actually 'bore' into the intestinal wall, causing lesions and intestinal symptoms, and it may reach the blood stream. From there, it can reach different vital organs of the human body, usually the liver, but sometimes the lungs, brain, spleen, etc. A common outcome of this invasion of tissues is a liver abscess, which can be fatal if untreated. Ingested red blood cells are sometimes seen in the amoeba cell cytoplasm.
[edit] Diagnosis
It can be diagnosed by stool samples but it is important to note that certain other species are impossible to distinguish by microscopy alone. Trophozoites may be seen in a fresh fecal smear and cysts in an ordinary stool sample. ELISA or RIA can also be used.
| 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 |
Quadrinucleated cyst (having 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 lumenal drug of choice, since Diloxanide furoate (Furamide) is not commercially available in the USA or Canada (only being available from the Centers for Disease Control and Prevention). A direct comparison of efficacy showed that Paromomycin had a higher cure rate [4]. Recommended dosage: Metronidazole 750mg tid orally, for 5 to 10 days FOLLOWED BY Paromomycin 30mg/kg/day orally in 3 equal doses for 5 to 10 days or Diloxanide furoate 500mg tid orally for 10 days, to eradicate lumenal amoebae and prevent relapse [5]. |
|
| 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 |
[edit] Additional images
Trophozoites of Entamoeba histolytica with ingested erythrocytes
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[edit] See also
[edit] References
[edit] External links
