There is, however, recent evidence that many asymptomatic "cyst passers" have tiny shallow amebic ulcers of the cecum (Connor, 1998). In tropical Africa postmortem examinations of indigenous peoples have revealed, on careful inspection of colonic mucosa, minute amebic ulcers. These are shallow, only a few millimeters across, usually in the cecum and frequently in the recesses between folds of plicae. Their detection requires close inspection. Clinically amebiasis is unsuspected in these patients and the ulcers would probably not be demonstrated by barium enema or colonoscopy. Patients harboring these small amebic ulcers may well be "asymptomatic carriers". Many other amebae (E. coli, E. moshkovskii, E. polecki, Iodamoeba bütschlii, Endolimax nana and others) may occasionally inhabit the human bowel without producing lesions or symptoms. Invasive amebiasis is confirmed by identifying cysts or hematophagous trophozoites in the stool (Fig. 1.2), or when biopsy specimens from involved bowel reveal trophozoites in the margins of ulcers and in the fibrin-rich exudate covering the base of the ulcers (Figs. 1.3, 1.4, 1.5).

Amebae within the lumen of the gut do not provoke significant antibodies, but those amebae which are invasive do, and this provides a useful serological test; once present, these antibodies persist long after invasion has ceased. Thus, they may indicate past rather than current infection but, if they are absent, clinical amebiasis can be excluded.

Fig. 1.3 Amebic infection begins when trophozoites attach to the epithelial cells of the colonic mucosa. Here trophozoites line the base of a shallow ulcer.

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Fig. 1.4 E. histolytica trophozoites in tissue. (A) Numerous trophozoites are present in the submucosa of the colon. Those that enter lymphatics or venules may be carried to the liver or other organs and form the nidus of infection there. AFIP (Armed Forces Institute of Pathology) 625901. (B) Trophozoites surrounded by inflammatory cells in a submucosal abscess. X440. AFIP 53-18759. (C and D) E. histolytica amid destroyed liver cells in hepatic abscesses. (C) X240. AFIP 53-18758. (D) X440. AFIP 53-3146.

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Fig. 1.5 Amebic ulcers-histological sections. (A) Early superficial erosion of the mucosa. The exudate contains numerous E. histolytica trophozoites. X108. AFIP 69-7842. (B) Early amebic ulcer in the colon at low magnification. (C) Classical flask-shaped amebic ulcer of the colon extending through the mucosa into the submucosa with undermining. The extensive infiltrate of neutrophils surrounding the ulcer suggests secondary bacterial infection. (D) Section of an amebic ulcer of the colon showing characteristic undermining. The muscularis is partly eroded and the serosa is edematous and hyperemic. X430. AFIP 58-15140.