Entamoebidae

Classification: Taxonomic ranks under review (cf. Illustrated Guide to Protozoa, 2000. Allen Press)

Protista (unicellular eukaryotes)
Sarcomastigophora (with pseudopodia and/or flagella)
Sarcodina (amoeboid protista)
Rhizopodea (lobopodia, filopodia, reticulopodia)
Lobosea (locomotion by broad lobopodia)
Amoebida (naked amoebae with simple life-cycles)

Family: Entamoebidae
These rhizopod amoebae form broad lobopodia and do not produce fruiting bodies like the mycetozoa (or slime molds). They are naked amoebae with simple life-cycles and do not form temporary flagellated stages. Most members are parasites or endocommensals in the digestive tracts of arthropod or vertebrate hosts. Individual species are differentiated on the basis of nuclear structure but all are characterized by the possession of a vesicular nucleus with a central endosome. Trophozoites form single lobopodia and they form cysts.

Entamoeba histolytica [causes amoebic dysentery in man and primates]

Parasite morphology: The trophozoites are 20-30 µm in diameter and contain a vesicular nucleus with a central endosome, peripheral chromatin and radial achromatic fibrils (imparting a ‘cart-wheel’ appearance). The cysts are spherical measuring 10-15 µm in diameter and have 4 nuclei.

Host range: Entamoeba histolytica is predominantly found in primates (including humans) and occasionally in dogs, cats, cattle and pigs. The parasite has a worldwide distribution and is prevalent in tropical and subtropical countries. However, it is readily confused with Entamoeba dispar, an identical species but apparently not pathogenic. With the “wisdom” of hindsight, asyptomatic infections in Australia are thought to be due to E. dispar. Another species Entamoeba polecki has occasionally been found in association with disease in pigs, monkeys and sometimes humans. The species Entamoeba invadens is considered to be a serious pathogen of snakes and lizards (especially captive animals).

Human enteric amoebae	Cyst diameter	Number of nuclei per cyst	Nuclear characteristics
Entamoeba histolytica	10-15 µm	4	cartwheel
Entamoeba polecki	10-15 µm	4	cartwheel
Entamoeba dispar	10-15 µm	4	cartwheel
Entamoeba hartmanni	7-9 µm	4	cartwheel
Entamoeba coli	15-30 µm	8	cartwheel
Iodamoeba buetschlii	9-15 µm	1	1
Endolimax nana	7-9 µm	1	1
Dientamoeba fragilis	no cyst formed	no cyst formed

Site of infection: Trophozoites generally infect the large intestinal mucosa but under certain conditions they may perforate the gut and invade other organs (especially liver, lungs and brain).

Pathogenesis: Many infections remain asymptomatic whereas others cause severe diarrhoea (amoebic dysentery), ulceration and perforation of the colon, and secondary lesions in other organs. Virulence factors are not yet known.

Mode of transmission: Trophozoites passing posteriad condense into spherical precysts (containing chromatoidal bars) which then mature into cysts (containing 4 nuclei). The cysts are very resistant to environmental conditions and are usually ingested with contaminated food or water.

Differential diagnosis: Infections are diagnosed by repeated stool examinations for trophozoites and cysts. Considerable expertise is required to differentiate pathogenic species from harmless commensals on the basis of nuclear and cyst morphology.

Treatment and control: Patients may be treated with luminal, hepatic and/or tissue amoebicides as warranted (metronidazole appears most effective). Control may be facilitated by maintaining high standards of hygiene and ensuring proper water and sewage treatment.