Entamoeba histolytica
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 Entamoeba histolytica cyst
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For the infection and disease caused by this parasite, refer to Amoebiasis.
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Entamoeba histolytica is an anaerobic parasitic protozoan, part of the genus Entamoeba. It infects predominantly humans and other primates. It is estimated that about 50 million people are infected with the parasite worldwide. Diverse mammals such as dogs and cats can become infected but are not thought to contribute significantly to 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. When cysts are swallowed they cause infections by excysting (releasing the trophozoite stage) in the digestive tract.
E. histolytica, as its name suggests (histo–lytic = tissue destroying), causes disease; infection can lead to amoebic dysentery or amoebic liver abscess. Symptoms can include fulminating dysentery, diarrhea, weight loss, fatigue, abdominal pain, and amebomas. 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.
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 | Cyst |
| 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. 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 is the most widely used) |
| 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 |
External links
- CDC DPDx Parasitology Diagnostic Web Site
- LSHTM Entamoeba Homepage
- Public domain Entamoeba histolytica article from the Bad Bug Book, from which the original version of this article was adapted.
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