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A 35-year-old man who recently traveled to a third world country develops chronic, severe dysentery.
Colonoscopy demonstrates ulceration of the cecum, and a cecal biopsy reveals 15-to-40 micron amoebae with
ingested erythrocytes and small nuclei with distinctive tiny central karyosomes. Which of the following organisms
is the most likely culprit?
A. Acanthamoeba sp.
B. Balantidium coli
C. Entamoeba histolytica
D. Giardia lamblia
E. Naegleria fowleri
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The correct answer is C.Entamoeba histolytica is the usual cause of intestinal amebiasis, and has the
microscopic features described in the question stem. A particularly helpful (but not always present) feature of
this organism is the presence of ingested red blood cells within the amoebae. These amoebae cause
flask-shaped ulceration of the intestinal mucosa and submucosa, with a particular propensity for involving the
cecum and ascending colon. The disease manifestations range from none (asymptomatic carriers) to mild
chronic diarrhea, to severe, purging dysentery. In symptomatic cases, the liver may develop destructive
amoebic liver abscesses that tend to become secondarily (and potentially life-threateningly) infected by
bacteria.
Acanthamoeba(choice A) is a free-living amoebae that can cause amoebic meningocephalitis.
Balantidium coli(choice B) is a large ciliated intestinal parasite that can occasionally cause colonic disease
resembling that caused by Entamoeba histolytica.
Giardia lamblia(choice D) is a small intestinal protozoa with a distinctive pear-shaped morphology that appears
to have a "face."
Naegleria fowleri(choice E) is a free-living amoebae that can cause amoebic meningoencephalitis.
