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Amoebas at the start

Were not complex;

They tore themselves apart

And started sex.

—Arthur Guiterman, The Light Guitar

The Sarcomastigophora include both the amebas and flagellate groups. Because of their divergent organization and medical importance they are considered in separate chapters. The amebas are characterized by movement involving cytoplasmic streaming dependent upon pseudopodia formation. These projections of the relatively solid ectoplasm are formed by streaming of the inner, more liquid endoplasm. They move the ameba forward and, incidentally, engulf and internalize food sources found in its path. Amebas multiply by simple binary fission. Most amebas, when faced with a hostile environment, can produce an external cyst wall that surrounds and protects them. These cysts may survive for prolonged periods under conditions that would rapidly destroy the motile trophozoite. The majority of amebas belong to free-living genera. They are widely distributed in nature, being found in literally all bodies of standing fresh water. Few free-living amebas produce human disease, although two genera, Naegleria and Acanthamoeba, have been implicated occasionally as causes of meningoencephalitis and keratitis.

Several genera of amebas, including Entamoeba, Endolimax, and Iodamoeba, are obligate commensalistic parasites of the human alimentary tract and are passed as cysts from host to host by the fecal–oral route. Most amebas are amitochondriate, presumably because of the anaerobic conditions under which they exist in the colon. Only one, Entamoeba histolytica, regularly produces disease; it has been recently subdivided into two morphologically identical but genetically distinct species, an invasive pathogen that retains the species appellation “histolytica” and a commensal organism, now designated E dispar. The two species can be differentiated by isoenzyme analysis, antibodies to surface antigens, and DNA markers.



Entamoeba histolytica is found throughout the world and the causative agent of diarrhea and amebic dysentery. Infections may spread to extraintestinal sites and become life- threatening. Close to 500 million people are thought to be infected at any one time, but the majority of these are likely due to the morphologically identical E dispar. Because methods are now available to distinguish E histolytica from E dispar, the figure of 500 million infected with E histolytica may actually be closer to 50 million. Transmission is fecal–oral, either directly, or through contaminated water.


Humans are the principal hosts and reservoirs of E histolytica. Transmission from person to person occurs when a cyst passed in the stool of one host is ingested directly or indirectly by another. Human hosts may pass up to 45 million cysts daily. Although the average infective dose exceeds 1000 organisms, ingestion of a single cyst has been known to produce infection. After passage through the stomach, the cyst eventually reaches the distal small bowel. Here, the cyst wall disintegrates, ...

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