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Differential Diagnosis of Amebic Colitis

The differential diagnosis of amebiasis of the colon includes:

1. Ulcerative colitis: Differentiation of amebic colitis from ulcerative colitis, granulomatous colitis, and tuberculosis may often be impossible radiologically. However, ulcerative colitis always involves the bowel in continuity and symmetrically and usually begins in the rectum and progresses proximally towards the cecum. In the late stage of the disease, the presacral space is increased and there is uniform diminution of the lumen and shortening of the colon with fewer flexures and convolutions, resulting in a distinct migration of the splenic and hepatic flexures. The ulcerations may be superficial; aphthous ulcers and, occasionally, "collar button" ulcers may be seen. By contrast, there may be skip areas of involvement in amebiasis, with intervening normal bowel, and the disease is more common in the cecum and right colon than in the distal colon. Deep ulcers, fistulous tracts, skip areas of stenosis and pericolic abscesses are uncommon in ulcerative colitis but frequent in amebiasis.

2. Granulomatous colitis (Crohn's disease): In granulomatous colitis (and also tuberculosis), the ileum is involved in a high percentage of patients in whom there is cecal and right colon involvement, whereas in amebiasis the ileum is only occasionally abnormal even when there is extensive cecal disease. The asymmetry of bowel wall involvement, and a somewhat greater tendency toward skip areas, aphthous ulcers, and a cobblestone mucosal pattern, as well as terminal ileitis, allow the differentiation of Crohn's disease from amebic colitis in many, but certainly not all, patients. Fortunately, Crohn's disease is rare or non-existent in most of the tropical world whereas amebiasis is universal.

The finding of E. histolytica cysts in the stool or trophozoites in biopsy specimens of the bowel may help in diagnosis, but the parasite can be found coincidentally in patients with granulomatous or other colitis. A therapeutic test may establish the correct diagnosis. In doubtful cases where treatment with amebicidal drugs has been instituted, there is no short-term improvement in patients suffering from Crohn's disease or tuberculosis.

3. Tuberculosis. Tuberculosis may mimic amebiasis with a conical, irregularly contracted cecum, but there is often intense spasm of a the ileocecal valve in tuberculosis which prevents reflux into the ileum. If reflux does occur, the ileum is usually abnormal, with areas of ulceration, stenosis, and fistulas; other more proximal portions of the small bowel are also often involved with tuberculosis. Skip lesions or granulomas within the bowel of tuberculous origin cannot be distinguished radiologically from amebiasis or Crohn's disease.

4. Malignancy: An ameboma may simulate and be indistinguishable from carcinoma, and a thickened bowel wall may resemble lymphoma.

5. Ischemic lesions of the colon: A history of bowel infarction is usually typical and the classical "thumbprinting," which changes rapidly even without therapy, permits the diagnosis with more confidence. Ischemic colitis is uncommon in most tropical areas, except when it is a complication of sickle cell disease. Unfortunately, the acute amebic colon may have "thumbprints" indistinguishable from ischemic colitis. Age and geographical location of the patient, response to treatment, and progress of the disease may be distinguishing features. Contrast-enhanced CT studies may at times pinpoint the ischemic nature of the colitis by clearly identifying thrombosis of the superior mesenteric artery or the splanchnic venous circulation.

6. Pseudomembranous colitis: Antibiotic-induced alterations of the intestinal flora allow the growth and potentiation of Clostridium difficile via production of potent endotoxins that induce a severe inflammatory reaction with multiple whitish-yellowish pseudomembranes, representing mucus and inflammatory detritus covering the colonic mucosa. Abdominal pain, diarrhea, fever, and leukocytosis following antibiotic administration during the prior six weeks constitute the clinical picture and are the clues to search for C. difficile in feces or perform the more reliable toxin detection test.

7. Actinomycosis is rare and usually involves the appendix as well as the right colon. Seldom are there lesions elsewhere in the bowel. When sinuses erupt through skin, they are multiple and unlike those seen in cutaneous amebiasis. The mycotic grains and the pathological changes in biopsy or surgically excised specimens clearly distinguish the two diseases.

8. Schistosomiasis involves the rectum and left colon in particular and can cause marked inflammatory polyposis. The cecum is rarely involved. Coincidental infections of schistosomiasis and amebiasis may be seen together in some geographical areas of the tropics.

The frequency of each disease on this list and therefore its weight in differential diagnosis varies throughout the world.

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