Clinical Features:
A wide spectrum, from asymptomatic infection ("luminal amebiasis"), to invasive intestinal amebiasis (dysentery, colitis, appendicitis, toxic megacolon, amebomas), to invasive extraintestinal amebiasis (liver abscess, peritonitis, pleuropulmonary abscess, cutaneous and genital amebic lesions).

Laboratory Diagnosis:
Entamoeba histolytica must be differentiated from other intestinal protozoa including: E. coli, E. hartmanni, E. gingivalis, Endolimax nana, and Iodamoeba buetschlii (the nonpathogenic amebas); Dientamoeba fragilis (which is a flagellate not an ameba); and the possibly pathogenic Entamoeba polecki.  Differentiation is possible, but not always easy, based on morphologic characteristics of the cysts and trophozoites.  The nonpathogenic Entamoeba dispar, however, is morphologically identical to E. histolytica, and differentiation must be based on isoenzymatic or immunologic analysis.  Molecular methods are also useful in distinguishing between E. histolytica and E. dispar and can also be used to identify E. polecki.  Microscopic identification of cysts and trophozoites in the stool is the common method for diagnosing E. histolytica.  This can be accomplished using:

• Fresh stool: wet mounts and permanently stained preparations (e.g., trichrome).
• Concentrates from fresh stool: wet mounts, with or without iodine stain, and permanently stained preparations (e.g., trichrome).  Concentration procedures, however, are not useful for demonstrating trophozoites.

In addition, E. histolytica trophozoites can also be identified in aspirates or biopsy samples obtained during colonoscopy or surgery.

Diagnostic findings:

• Microscopy
• Immunodiagnosis
• Molecular methods for discriminating between E. histolytica and E. dispar
• Morphologic comparison with other intestinal parasites
• Bench aid for E. histolytica

Treatment:
For asymptomatic infections, iodoquinol, paromomycin, or diloxanide furoate (not commercially available in the U.S.) are the drugs of choice.  For symptomatic intestinal disease, or extraintestinal, infections (e.g., hepatic abscess), the drugs of choice are metronidazole or tinidazole, immediately followed by treatment with iodoquinol, paromomycin, or diloxanide furoate.  For additional information, see the recommendations in The Medical Letter (Drugs for Parasitic Infections).