Invasive Amebiasis

Amebiasis is an intestinal disease that's typically transmitted when someone eats or drinks something that's contaminated with a microscopic parasite called Entamoeba histolytica. The parasite is an amoeba, a single-celled organism. That's how the disease got its name — amebiasis.

Amebiasis is an infection caused by the protozoal organism Entamoeba histolytica and includes amebic colitis and liver abscess. In developed countries, infection occurs primarily among travelers to endemic regions, recent immigrants from endemic regions, homosexual males, immunosuppressed persons, and institutionalized individuals. Transmission usually occurs by food-borne exposure, particularly when food handlers are shedding cysts or food is cultivated in feces-contaminated soil, fertilizer, or water. Less common means of transmission include contaminated water, oral and anal sexual practices, and direct rectal inoculation through colonic irrigation devices.

Amebiasis occurs worldwide but is more common in areas of poor sanitation and nutrition, particularly in the tropics. The majority of E. histolytica infections, morbidity, and mortality occurs in Africa, Asia, and Central and South America (1,3). Approximately 50 million cases of invasive E. histolytica disease occur each year, with up to 100,000 deaths (2,4). However, only an estimated 10%-20% of infected individuals become symptomatic.

The disease is most commonly transmitted when a person eats food or drinks water containing E. histolytica cysts from human feces. In the digestive tract the cysts are transported to the intestine where the walls of the cysts are broken open by digestive secretions, releasing the mobile trophozoites. Once released within the intestine, the trophozoites multiply by feeding on intestinal bacteria or by invading the lining of the large intestine. Within the lining of the large intestine, the trophozoites secrete a substance that destroys intestinal tissue and creates a distinctive bottle-shaped sore (ulcer).

Invasive amebiasis, including amebic liver abscess, is much more common in adult males than in females. However, amebic liver abscess is equally common in both sexes among prepubertal children. Acuna-Soto and colleagues have noted that asymptomatic E histolytica infection is distributed equally between sexes. Therefore, the higher proportion of men with invasive amebiasis may be due to a male susceptibility to invasive disease.
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Diagnosis of amebiasis can be very difficult. One problem is that other parasites and cells can look very similar to E. histolytica when seen under a microscope. Therefore, sometimes people are told that they are infected with E. histolytica even though they are not. Entamoeba histolytica and another ameba, Entamoeba dispar, which is about 10 times more common, look the same when seen under a microscope. Unlike infection with E. histolytica, which sometimes makes people sick, infection with E. dispar never makes people sick and therefore does not need to be treated.

People exposed to this parasite may experience mild or severe symptoms or no symptoms at all. Fortunately, most exposed people do not become seriously ill. The mild form of amebiasis includes nausea, loose stools, weight loss, abdominal tenderness and occasional fever. Rarely, the parasite will invade the body beyond the intestines and cause a more serious infection, such as a liver abscess.