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Key Features

Essentials of Diagnosis

  • Acute dysentery: diarrhea with blood and mucus, abdominal pain, tenesmus


  • Chronic nondysenteric diarrhea


  • Hepatic abscess

  • Amoebas or cysts in stool or abscesses; amebic antigen in stool

  • Serologic evidence of amebic infection

General Considerations

  • Defined as an infection with Entamoeba histolytica regardless of symptoms

  • E histolytica is found worldwide but has a particularly high prevalence in areas with poor sanitation and socioeconomic conditions

  • In the United States, infections are seen in travelers to and emigrants from endemic areas

  • Most infections are asymptomatic (> 90%), but tissue invasion can result in amebic colitis, hepatic abscess, and hematogenous spread to other organs

  • Transmission is usually fecal-oral

Clinical Findings

Symptoms and Signs

  • Intestinal amebiasis

    • Asymptomatic cyst passage

    • Acute amebic proctocolitis

    • Chronic nondysenteric colitis

    • Ameboma

  • Acute amebic colitis

    • Typically, there is a 1- to 2-week history of loose stools containing blood and mucus, abdominal pain, and tenesmus

    • Some patients are febrile or dehydrated

    • Pain over the lower abdomen

  • Chronic amebic colitis

    • Causes recurrent episodes of bloody diarrhea over a period of years

    • Clinically indistinguishable from idiopathic inflammatory bowel disease

  • Ameboma

    • Localized amebic infection, usually in the cecum or ascending colon

    • Presents as a painful abdominal mass

  • Amebic liver abscess

    • Typically presents with acute fever and right upper quadrant tenderness

    • Pain may be dull, pleuritic, or referred to the right shoulder

    • Physical examination reveals liver enlargement in less than 50% of affected patients

    • Subacute presentation lasting 2 weeks to 6 months may be seen in some patients

      • Hepatomegaly, anemia, and weight loss are common findings

      • Fever is less common

Differential Diagnosis

  • Acute amebic colitis should be distinguished from other causes of dysentery

    • Bacterial (eg, Salmonella spp, Shigella spp, E coli spp, Campylobacter spp)

    • Parastitic (eg, Schistosoma mansoni, Balantidium coli)

    • Noninfectious (eg, inflammatory bowel disease, diverticulitis, ischemic colitis)

  • Chronic amebic colitis has to be distinguished from

    • Inflammatory bowel disease

    • Cyclospora

  • Liver abscess must be distinguished from echinococcal hydatid cyst


Laboratory Findings

  • ELISA assays are positive in approximately 95% of patients with extraintestinal amebiasis, 70% with intestinal E histolytica disease, and 10% of asymptomatic patients shedding E histolytica cysts

  • However, these antibodies persist for years, and a positive result does not distinguish between acute and past infection

  • Polymerase chain reaction (PCR)–based testing has the highest sensitivity and specificity for the diagnosis of E histolytica


  • Ultrasonography and CT scanning

    • Sensitive techniques to detect hepatic abscesses

    • Can be used to guide fine-needle aspiration to obtain specimens for definitive diagnosis

  • Because an amebic abscess may take up to 2 years to completely resolve on CT scans, imaging techniques are not recommended for therapeutic evaluation

Diagnostic ...

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