The protozoan parasite Entamoeba histolytica, possibly other Entamoeba spp.
Fecal–oral route, either by eating or drinking fecally contaminated food or water or person-to-person contact (such as by diaper changing or sexual activity).
Amebiasis is distributed worldwide, particularly in the tropics, most commonly in areas of poor sanitation. Long-term travelers (duration >6 months) are significantly more likely than short-term travelers (duration <1 month) to develop E. histolytica infection. Recent immigrants and refugees from these areas are also at risk. Outbreaks among men who have sex with men have been reported. People at higher risk for severe disease are those who are pregnant, immunocompromised, or receiving corticosteroids; associations with diabetes and alcohol use have also been reported.
Most patients have a gradual illness onset days or weeks after infection. Symptoms include cramps, watery or bloody diarrhea, and weight loss, and may last several weeks. Occasionally, the parasite may spread to other organs (extraintestinal amebiasis), most commonly the liver. Amebic liver abscesses may be asymptomatic, but most patients present with fever, right upper quadrant abdominal pain, and weight loss, usually in the absence of diarrhea. Men are at higher risk of developing amebic liver abscess than are women for reasons not fully understood.
Microscopy does not distinguish between E. histolytica (known to be pathogenic), E. bangladeshi, E. dispar, and E. moshkovskii. E. dispar and E. moshkovskii have historically been considered nonpathogenic, but evidence is mounting that E. moshkovskii can cause illness; E. bangladeshi has only recently been identified, so its pathogenic potential is not well understood. More specific tests such as enzyme immunoassay or PCR are needed to confirm the diagnosis of E. histolytica. Additionally, serologic tests can help diagnose extraintestinal amebiasis.
For symptomatic intestinal infection and extraintestinal disease, treatment with metronidazole or tinidazole should be followed by treatment with iodoquinol or paromomycin. Asymptomatic patients infected with E. histolytica should also be treated with iodoquinol or paromomycin, because they can infect others and because 4%–10% develop disease within a year if left untreated.
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