Entamoeba histolytica (amoebiasis, amoebic dysentery)

General: Most commonly found in institutions or developing countries with poor sanitation infrastructure or practices.

Clinical: Only about 1 in 10 people who have ingested cysts become symptomatic, and the incubation time is 1-4 weeks. Most infections cause mild gastrointestinal upset, although the classic association is with gastrointestinal hemorrhage and severe bloody diarrhea. Occasionally trophozoites may enter the circulation and infect other organs, typically the liver (hepatic amoebiasis), and very rarely they may cause peritonitis or spread to other organs.

• Normal:
• Abnormal: Cysts and trophozoites in the stool.

Resistance:

Morphology: The primary diagnostic method is via microscopic examination of stool specimens. Entamoeba dispar is morphologically very similar and much more common, but causes no symptomatic infection; there are several other amoebic forms which can be easily confused.

• Trophozoite: Large, with almost perfectly round nucleus containing a central endosome and even peripheral chromatin distribution.
• Cyst: Irregular chromatid bars, particularly the immature cysts. Mature cysts have four nuclei, though often less than four are visible.

Growth characteristics: The normal life cycle is:

• Cysts ingested from contaminated food or water
• Ingested cysts "excyst" in the small intestine, releasing trophozoites
• Trophozoites colonize the colon, possibly invading to produce hemorrhagic colitis (and may rarely enter the circulation or peritoneal cavity)
• Trophozoites reproduce by simple division, may produce cysts
• Trophozoites & cysts eliminated in feces

Common/important pathogens: