Entamoeba histolytica

Entamoeba histolytica

For the infection and disease caused by this parasite, refer to Amoebiasis.

Entamoeba histolytica is an anaerobic parasitic protozoan, part of the genus Entamoeba. Predominantly infecting humans and other primates. E. histolytica is estimated to infect about 50 million people worldwide. Mammals such as dogs and cats can become infected transiently, but are not thought to contribute significantly to transmission.

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. 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.

Pathogenesis

E. histolytica, as its name suggests (histolytic = tissue destroying), is pathogenic; 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.

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 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


See also

References

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