Amebae migrate into the adjacent mucosa and submucosa, which becomes edematous. Perforation through the serosa is very uncommon, but can occur when tissue destruction is rapid and extensive (Fig. 1.7). Invasion of venules and lymphatics predisposes to thrombosis and septic embolization. Cysts form in the lumen of the gut, but not in tissue. Only cysts persist free in nature and can survive for long periods outside the body.

Fig. 1.7 Amebic colitis with perforation--autopsy specimens from two patients with colonic perforations leading to fatal amebic peritonitis. (A) Deep amebic ulcer which penetrated through the muscularis and serosa to cause a peritonitis. There were numerous other ulcers throughout the same colon. Note the markedly swollen mucosa around the ulcers caused by extensive mucosal and submucosal edema. AFIP 67-13734-A. (B) Diffuse ulceration of the colon with hemorrhage around many of the ulcers and perforation of the colon (top center).

Toxic megacolon. In acute, overwhelming infections, there may be massive distension and thickening of the colon with a great deal of destruction, edema, venous thrombosis, and perforation with an accompanying purulent peritonitis and very high mortality.

Ameboma. This is an uncommon pathological event of intestinal amebiasis, occurring most often in the cecum and ascending colon. A mass, often apple-core in appearance and resembling a carcinoma, originates from the colonic wall; it may measure 5 to 30 cm in diameter and may be multiple. How an ameboma develops is still a mystery; some authors believe that it is an amebic granuloma. Histologically, it shows abundant granulation tissue, edema, neovascularity, inflammatory cells and live Entamoeba histolytica. The mucosa is generally ulcerated and there is marked thickening of the colonic wall with edema and hemorrhage.

Amebic appendicitis. In Pakistan, Amhed reported that 13 of 1,400 histologically examined cases of appendicitis were of amebic origin. This is an uncommon entity, recognized postoperatively by the pathologist. The appendix shows suppurative inflammation with nodular mucosal ulcers. Once the etiology has been identified, conventional therapy for intestinal amebiasis must follow.