In most infected individuals,
parasitizes the lumen of the
gastrointestinal tract and causes few or no symptoms or sequelae
. The 2 most
common forms of disease caused by
invasion of the intestinal mucosa, and
amebic liver abscess
with dissemination of
the parasite to the liver.
Two morphologically identical but genetically distinct species of Entamoeba
commonly infect humans. Entamoeba dispar, the more prevalent species, is
associated only with an asymptomatic carrier state.
E. histolytica, the
pathogenic species, can become invasive, causing symptomatic disease
Infection is established by
ingestion of parasite cysts, which measure 10–18 mm
in diameter and contain 4 nuclei.
Cysts are resistant to environmental
conditions such as low temperature and the concentrations of chlorine
commonly used in water purification but can be killed by heating to 55°C
After ingestion, the cyst, which is resistant to gastric acidity and digestive
enzymes, excysts in the small intestine to form 8 trophozoites. These large,
actively motile organisms colonize the lumen of the large intestine and may
invade the mucosal lining.
Infection is not transmitted by trophozoites because of
their rapid degeneration outside the body and especially in the low pH of normal
gastric contents if swallowed.
Prevalence of infection with E. histolytica varies greatly depending on region and
socioeconomic status. Most prevalence studies have not distinguished between E.
histolytica and E. dispar, so the true prevalence of E. histolytica infection from
these studies is not known. It is estimated that infection with E. histolytica leads to
50 million cases of symptomatic disease and 40,000–110,000 deaths annually.
Amebiasis is the 3rd leading parasitic cause of death worldwide
studies have demonstrated that
4–10% of individuals infected with E. histolytica
develop amebic colitis, and <1% of individuals develop disseminated disease,
such as amebic liver abscess
. These numbers vary by region: in South Africa and
Vietnam, liver abscesses form a disproportionately large number of the cases of
invasive disease due to E. histolytica.
Amebic liver abscesses are rare in children
and occur equally frequently in male and female children, whereas in adults,
amebic liver abscesses occur predominantly in men.
Amebiasis is highly endemic in Africa, Latin America, India, and Southeast
Asia. In the United States, amebiasis is seen most frequently in immigrants
from or travelers to developing countries
Residents of institutions for the
mentally retarded and men who have sex with men are also at increased risk
for invasive amebiasis, although most Entamoeba infections in the latter group
are E. dispar.
Food or drink contaminated with Entamoeba cysts and direct
fecal-oral contact are the most common means of infection. Untreated water
and human feces used as fertilizer are important sources of infection