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Accession information: (99)00061-7h.htm (shortcode: fig001wpc); 22 March 1999
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Life cycle of Entamoeba histolytica and the clinical manifestations of infection in humans

Christopher D. Huston, Rashidul Haque and William A. Petri, Jr

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Figure 1. Life cycle of Entamoeba histolytica and the clinical manifestations of infection in humans. E. histolytica is a protozoan parasite that causes amoebic colitis and liver abscess. Its life cycle consists of two stages: cysts and trophozoites. (a) Cysts measure 10–15 mm in diameter and typically contain four nuclei; (b) they are spread via the ingestion of faecally contaminated food or water. (c) During excystation within the lumen of the small intestine, nuclear division is followed by cytoplasmic division, giving rise to eight trophozoites. (d) Trophozoites, which measure 10–50 mm in diameter and contain a single nucleus with a central karyosome, reside in the lumen of the caecum and large intestine, where they adhere to the colonic mucus and epithelial layers. (e) Approximately 90% of individuals infected with E. histolytica are asymptomatically colonised; (f) re-encystation of the trophozoites occurs within the lumen of the colon, resulting in (g) the excretion of cysts in the faeces and continuation of the life cycle. (h) Alternatively, the trophozoites can invade the colonic epithelium, causing amoebic colitis (in ~10% of infected people). Amoebic dysentery usually occurs gradually, with symptoms [such as abdominal pain and tenderness, and painful sudden bowel evacuation (tenesmus) and diarrhoea] developing over a period of one to several weeks, often followed by weight loss. E. histolytica can spread in the bloodstream (haematogenously) after it has penetrated the colonic epithelium (not shown) and can establish persistent extraintestinal infections, most commonly (i) amoebic liver abscess. Liver abscess is overwhelmingly the most common extraintestinal manifestation of amoebiasis. This complication is 10 times more common in adult men than in adult women. The typical patient with amoebic liver abscess is a 20–40-year-old male with a 1–2-week history of fever and diffuse or right, upper quadrant abdominal pain (fig001wpc).

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