If you identify fibropurulent exudate overlying colon mucosa, a careful examination for Entamoeba histolytica is warranted, as the exudate is their favorite hiding spot.1 The trophozoites of Entamoeba histolytica have foamy cytoplasm and an eccentrically located nucleus. Note that the identification of ingested red blood cells in their cytoplasm (not well demonstrated in this image) is pathognomonic.2
In the overlying mucinous material are a few more organisms, which show a few intracellular particles suggestive of red cells.
Entamoeba Histolytica is the most common protozoan infection worldwide, as ~10% of the world's population is infected by this organism. Rates of infection are especially high in tropical regions.
The cecum is the most frequent site of infection, although any place along the GI tract, including appendix, may be affected. Early in the infection, there may be a neutrophilic infiltrate, however, as disease progresses, ulcers may form that extend into the submucosa. The overlying fibropurulent debris is a high yield area to spot the organisms.
The unsuspecting pathologist may mistake the amebas for foamy macrophages. The pathognomonic finding of RBCs inside the amebic cytoplasm, identification of adjacent ulcers, clinical history and immunostains, can help you avoid this embarrassing mistake. The amebas stain with trichrome (but not macrophages), and macrophages are positive for alpha-1-antitrypsin and chymotrypsin (but not amebas).2
Clinical presentations may range from asymptomatic, to vague abdominal pain and diarrhea +/- blood, to disseminated disease (especially to the liver). Helpful diagnostic tests include stool examination for parasites and serological tests.
Antiparasitic drugs (metronidazole) are highly effective.
1 Sternberg SS, ed. Diagnostic Surgical Pathology.4th Ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2004: 1490.
2 Iacobuzio-Donahue CA, Montgomery EA. Gastrointestinal and Liver Pathology: Foundations in Diagnostic Pathology. Philadelphia, PA: Elsevier; 2005: 297-299.