Life cycle of Entamoeba histolytica

  Cysts are passed in feces  .  Infection by Entamoeba histolytica occurs by ingestion of mature cysts  in fecally contaminated food, water, or hands.  Excystation  occurs in the small intestine and trophozoites  are released, which migrate to the large intestine.  The trophozoites multiply by binary fission and produce cysts  , which are passed in the feces  .  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 can also be passed in diarrheal stools, but are rapidly destroyed once outside the body, and if ingested would not survive exposure to the gastric environment.)  In many cases, the trophozoites remain confined to the intestinal lumen ( : noninvasive infection) of individuals who are asymptomatic carriers, passing cysts in their stool.  In some patients the trophozoites invade the intestinal mucosa ( : intestinal disease), or, through the bloodstream, extraintestinal sites such as the liver, brain, and lungs ( : 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, however not all persons infected with E. histolytica will have invasive disease.  These two species are morphologically indistinguishable.  Transmission can also occur through fecal exposure during sexual contact (in which case not only cysts, but also trophozoites could prove infective).