E. histolytica exists in two forms: a cyst (which can survive outside the body) and a trophozoite (which can’t survive outside the body). Transmission occurs through ingestion of feces-contaminated food or water. The ingested cysts pass through the intestine, where digestive secretions break down the cysts and liberate the motile trophozoites within. The trophozoites multiply and either invade and ulcerate the mucosa of the large intestine (causing colitis, or chronic diarrhea) or simply feed on intestinal bacteria. As the trophozoites are carried slowly toward the rectum, they are encysted and then excreted in feces. Humans are the principal reservoir of infection.

Amebiasis occurs worldwide but is most common in the tropics, subtropics, and other areas with crowded living conditions and poor sanitation and health practices. People with suppressed immune systems are at increased risk for severe amebiasis, as are those suffering from alcoholism, cancer, or malnutrition. In addition, advanced age, pregnancy, and recent travel to a tropical area increase the risk of infection. In the United States, amebiasis is most common among immigrants from developing countries, people who live in institutions, and people who have anal intercourse.

Complications associated with amebiasis affect the GI system and include chronic diarrhea, abdominal pain, extraintestinal abscesses, ameboma, megacolon, intussusception, intestinal strictures, intestinal hemorrhage, and intestinal perforation. The parasite can spread through the blood to the liver, lungs, or brain.

The clinical effects of amebiasis vary with the severity of the infestation. Acute amebic dysentery causes a sudden high temperature of 104° to 105° F (40° to 40.6° C) accompanied by chills and abdominal cramping; profuse, bloody, mucoid diarrhea with tenesmus; excessive flatulence; and diffuse abdominal tenderness due to extensive rectosigmoid ulcers.