Amebiasis Life Cycle |
|
 |
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 (  :
non-invasive 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 ( :
extra-intestinal 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). |