Pathophysiology

Intestinal amebiasis results from the ingestion of amebic cysts that contain a substance within their walls called quitina, which protects the parasites from adverse low pH and the enzyme-rich gastrointestinal environment. After passing through the unfavorable acid environment of the stomach, the cyst has its capsule dissolved in the alkaline content of the small intestine in preparation for colonization in the colon. Each cyst usually releases four active trophozoites which are carried along the small bowel (Fig. 1.2). Some are destroyed, but others produce strong proteolytic enzymes and hyaluronidases which can lyse the intestinal epithelium, making it possible for the trophozoites to burrow into the mucosa. This process occurs in association with bacteria (the exact mechanism is not understood) and produces ulcerations which, if they continue to progress, become the classical flask-shaped ulcers of invasive amebiasis. Specimens obtained from patients with invasive disease and from carriers reveal two genotypes of E. histolytica designated as "pathogen" and "nonpathogen", each generating phenotypic isoenzymes called zimodemes . According to Mirelman and other investigators, the existing bacterial flora may contribute to the pathogenic or invasive nature of the parasite. Experimental studies have demonstrated that bacterial-rich cultures increase certain aspects of the virulent functions in vitro. This controversial issue is yet to be resolved.

The majority of individuals infested with Entamoeba histolytica are asymptomatic. No symptoms will occur until there has been tissue invasion and ulceration, which marks the onset of clinical amebiasis. Until this happens, the individual is only an asymptomatic cyst passer or "carrier" because the parasite can and does live in the intestinal contents of many hosts without producing noticeable tissue reaction or symptoms.

Fig. 1.2 Entamoeba histolytica. (A) Trophozoites (1), precysts (2 and 3), and mature cysts (4) (bottom row). (B) Electron microscopy shows a trophozoite with three pseudopods simulataneously phagocytizing erythrocytes. X 2,200. (C) Electron microscopy shows the presence of erythrocytes in phagolysosomes within an amoeba. X 2,400. (B and C Courtesy of Dr. Tsutsumi, Research Institute IPN, Mexico City.) (D) A single trophozoite of E. histolytica is in the center of the photograph. Note the slightly irregular ameboid outline and coarse cytoplasm, the spherical nucleus, and the tiny central karyosome. There is also an ingested erythrocyte in the cytoplasm (arrow). H & E X2000. (Courtesy of Dr. Daniel Connor.) (E) E. histolytica cyst with four nuclei and chromatoidal bars.