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Recurrent dysentery, with loose, mucoid, blood-stained stools, headache and nausea are common symptoms in amebic colitis. Pain is most common in the right lower abdomen, but can occur with tenderness to palpation over any portion of the colon. Almost any variety of large bowel complaint can be caused by amebiasis, which must therefore be considered in differential diagnosis and evaluated by serologic testing, repeated adequate examinations of fresh stool specimens, and/or sigmoidoscopy coupled with biopsy (repeated if necessary). If serological tests are negative, stool samples and biopsy may be unnecessary, but if positive, then the activity of the disease must be assessed.

Ulcerative amebic proctocolitis. This diarrhea-dysentery syndrome is characterized by mucosanguinous watery bowel movements associated with colic and tenesmus. There are four to six episodes of bloody diarrhea per day without fever or systemic manifestations. This syndrome has been termed "ambulatory dysentery" and resolves promptly with appropriate medical therapy. Mortality is 0.5%, but increases to 20% to 40% with bowel perforation and peritonitis, predominantly occurring in children.

Fulminant colitis and toxic megacolon. This is a grave and rapidly evolving disease with 20 or more episodes of bloody diarrhea during a 24-hour period. It is accompanied by severe abdominal pain and intense and constant tenesmus. There is nausea, anorexia, fever, rapid pulse, and hypovolemic hypotension. The patient suffers dehydration and prostration and may present to the emergency room in a state of shock. Massive thrombosis of the colonic wall venules with intestinal ischemia, infarction, and ischemic tissue necrosis are some of the features encountered in patients with severe colonic distention and abdominal hyperesthesia from toxic megacolon. Intestinal perforation worsens the prognosis. There is a high mortality despite palliative or radical surgical procedures.

Ameboma. These pseudotumoral amebic granulomas exhibit necrosis, inflammation and edema of the mucosa and submucosa of the colon. Clinically, there is a bloody dysentery with abdominal pain and associated palpable colonic mass.

Amebic appendicitis. In most patients, there are no distinguishing features between amebic appendicitis and conventional acute appendicitis. However, a preceding episode of bloody diarrhea may herald an amebic etiology.

Chronic amebic colitis. This term has been applied to patients with nonspecific symptoms such as spasmodic colonic pain, and alternating periods of diarrhea and constipation, with occasional mucus and E. histolytica cysts in the stool. These patients may simply have an irritable colon or may be carriers of amebiasis. A diagnosis of amebic disease should be based on the identification of an intestinal wall lesion, a demonstrable trophozoite, and positive serologic tests.

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