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  • Organisms or antigen present in stools or abscess aspirate.

  • Positive serologic tests with colitis or hepatic abscess, but these may represent prior infections.

  • Mild to moderate colitis with recurrent diarrhea.

  • Severe colitis: bloody diarrhea, fever, and abdominal pain, with potential progression to hemorrhage or perforation.

  • Hepatic abscess: fever, hepatomegaly, and abdominal pain.


The Entamoeba complex contains three morphologically identical species: Entamoeba dispar and Entamoeba moshkovskii, which are avirulent, and Entamoeba histolytica, which may be an avirulent intestinal commensal or lead to serious disease. Disease follows penetration of the intestinal wall, resulting in diarrhea, and with severe involvement, dysentery or extraintestinal disease, most commonly liver abscess.

E histolytica infections are present worldwide but are most prevalent in subtropical and tropical areas under conditions of crowding, poor sanitation, and poor nutrition. Of the estimated 500 million persons worldwide infected with Entamoeba, most are infected with E dispar and an estimated 10% with E histolytica. The prevalence of E moshkovskii is unknown. Mortality from invasive E histolytica infections is estimated at 100,000 per year.

Humans are the only established E histolytica host. Transmission occurs through ingestion of cysts from fecally contaminated food or water (eFigure 35–15), facilitated by person-to-person spread, flies and other arthropods as mechanical vectors, and use of human excrement as fertilizer. Urban outbreaks have occurred because of common-source water contamination.

eFigure 35–15.

Life cycle of Entamoeba histolytica. Cysts and trophozoites are passed in feces

image. Cysts are typically found in formed stool, whereas trophozoites are typically found in diarrheal stool. Infection by Entamoeba histolytica occurs by ingestion of mature cysts
image in fecally contaminated food, water, or hands. Excystation
image occurs in the small intestine and trophozoites
image are released, which migrate to the large intestine. The trophozoites multiply by binary fission and produce cysts
image, and both stages are passed in the feces
image. Because of the protection conferred by their walls, the cysts can survive days to weeks in the external environment and are responsible for transmission. Trophozoites passed in the stool are rapidly destroyed once outside the body and would not survive exposure to the gastric environment if ingested. In many cases, the trophozoites remain confined to the intestinal lumen (
image: noninvasive infection) of individuals who are asymptomatic carriers, passing cysts in their stool. In some patients, the trophozoites invade the intestinal mucosa (
image: intestinal disease) or through the bloodstream, extraintestinal sites such as the liver, brain, and lungs (
image: extraintestinal disease), with resultant pathologic manifestations. It has been established that the invasive and noninvasive forms represent two separate species, respectively E histolytica and E dispar. These two species are morphologically indistinguishable unless E histolytica is observed with ingested RBCs (erythrophagocystosis). Transmission can also occur through exposure to fecal matter during sexual contact (in which case not ...

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