Entamoeba and Entamoeba histolytica

Abstract

Entamoeba species are unicellular eukaryotes that parasitise all classes of vertebrates and some invertebrates. Only three species of Entamoeba have been proven to cause disease and sometimes death in their hosts: Entamoeba histolytica, a parasite of humans, Entamoeba nuttalli, a parasite of nonhuman primates and Entamoeba invadens, a parasite of reptiles. Other species appear to live as commensals in their hosts and do not cause evident disease. Amoebiasis is defined as infection with E. histolytica regardless of symptoms. Most cases of amoebiasis are asymptomatic, but E. histolytica can rarely cause intestinal or disseminated disease. Identification of E. histolytica is complicated by the existence of two morphologically identical amoebae that may also colonise the human intestinal tract, Entamoeba dispar and Entamoeba moshkovskii, neither of which are thought to cause disease. Entamoeba spp. are commonly studied to gain further insight into protozoal evolution, amoeboid locomotion and cell‐killing ability, among other topics.

Key Concepts:

  • Entamoeba species are unicellular eukaryotes in the supergroup Amoebozoa.

  • Entamoeba species parasitise all classes of vertebrates and some invertebrates.

  • The life cycle of most Entamoeba species consists of a motile, feeding and reproductive trophozoite stage and an environmentally resistant cyst stage responsible for transmitting the infection.

  • Most Entamoeba species do not cause disease in their host and are considered commensal organisms.

  • Entamoeba histolytica infection is referred to as amoebiasis regardless of whether or not symptoms are present.

  • Most infections with E. histolytica in humans are asymptomatic, but E. histolytica can rarely cause dysentery or disseminated disease and is associated with significant human morbidity and mortality worldwide.

  • Diagnosis and epidemiologic studies of E. histolytica are complicated by the presence of morphologically identical amoebae in human faeces.

  • Entamoeba species are studied to gain insights into aspects of biology, including human disease, parasitism, evolution, cell locomotion and cell‐killing mechanisms.

Keywords: Entamoeba; Entamoeba histolytica; amoebiasis; protozoa; parasitism; parasite

Figure 1.

Trophozoites of Entamoeba histolytica in fluid drained from the chest of a patient suffering from amoebic empyema. Note the nuclear morphology, a distinguishing feature of the genus. Trichrome stain. The scale bar (lower right) represents 10 μm. From the collection of Kerrison Juniper Jr, MD (deceased).

Figure 2.

A cyst of Entamoeba histolytica seen in faeces from an infected asymptomatic carrier. Three of the four nuclei are clearly visible. Note the large, dark‐staining chromatoid bodies. These are composed of ribosomal aggregates. Iron haematoxylin stain. The cyst is 14 μm in diameter. Courtesy of John E Williams, London School of Hygiene and Tropical Medicine.

Figure 3.

Trophozoites of Entamoeba histolytica in exudate from a rectal ulcer biopsy of a patient with bloody diarrhoea suffering from acute amoebic dysentery. The dark‐staining bodies within the cytoplasm of some of the parasites are ingested red blood cells. Trichrome stain. The scale bar (lower right) represents 10 μm. From the collection of Kerrison Juniper Jr, MD (deceased).

Figure 4.

Classic ‘flask‐shaped’ ulcer of amoebiasis, in which the base is wider than the apex. This is created as trophozoites invade through the intestinal epithelium and invade laterally in the submucosa. Haematoxylin and eosin stain, 200 times original magnification. Reproduced by permission of Mayo Clinic Proceedings.

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Pritt, Bobbi S, and Clark, C Graham(Oct 2011) Entamoeba and Entamoeba histolytica. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001963.pub3]