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[ameba + -iasis]
Infection or colonization with amebas, esp. Entamoeba histolytica. About 500 million people in tropical countries are infected. The infection typically begins in the colon but may spread to other organs, e.g., the liver or, less often, the skin or lungs.
SYN: SEE: amebic dysentery; SEE: amebic colitis
SEE: amoebapore; SEE: cyst; SEE: dysentery

Amebiasis is acquired by ingesting contaminated food or drink that contains E. histolytica cysts, which gastric acid does not destroy. The cysts enter the intestines, where they release trophozoites, the feeding form of the organism, which may invade the walls of the colon or spread to the liver by the portal vein. Trophozoites divide to form new cysts, which may subsequently be excreted in stool.

The diagnosis of amebiasis is based on the detection of cysts or trophozoites of E. histolytica in stools and the presence of antibodies to the amebas in the blood. Antiamebic antibodies appear by the seventh day of infection. A colonoscopy may be performed to obtain tissue samples to differentiate amebiasis from inflammatory bowel disease. A liver abscess is diagnosed when a patient has right upper quadrant pain, jaundice, and fever; a mass in the liver (found by ultrasonography or computed tomography); and positive serological tests for E. histolytica.

Most infected patients have no tissue invasion and are asymptomatic. Acute colitis, when it occurs, is marked by bloody diarrhea, abdominal pain, tenesmus, and weakness. The symptoms may be confused with those of ulcerative colitis. The dysentery lasts 3 to 4 weeks. Complications occasionally include toxic megacolon and ulcer perforation. Patients who develop liver abscesses present with severe upper right quadrant pain and fever. Massive diarrhea is not usually present.

Asymptomatic patients are treated with paromomycin (500 mg orally tid for 7 days) or iodoquinol (650 mg orally tid for 20 days). Dysentery and liver abscess are treated with metronidazole (750 mg orally tid for 10 days), followed by iodoquinol (650 mg orally tid for 20 days).

People traveling to developing countries, esp. India and Mexico, should avoid unboiled water, ice, and fresh fruits and vegetables, all of which may be infected with amebic cysts.

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