Entamoeba histolytica

This amoeba infects humans and other primates. The active (trophozoite) stage exists only in the host and in fresh feces; cysts survive outside the host in water, soils and on foods.. When swallowed they cause infections the excyst ( become the trophozoite stage) in the intestines. Amebiasis (or amoebiasis) is the name of the infection caused by E. histolytica .

Infections sometimes last for years may cause 1) no symptoms, 2) vague gastrointestinal distress, 3) dysentery (with blood and mucus). Most infections occur in the digestive tract but other tissues may be invaded. Complications include 4) ulcerative and abscess pain and, rarely, 5) intestinal blockage.. The amoeba's enzymes help it to penetrate and digest human tissues and it secretes other toxic substances. Eating of one cyst can cause an infection.

Human cases are diagnosed by finding cysts in the stool. Since cysts are not shed constantly, a minimum of 3 stools should be examined. Serological tests exist for long-term infections. It is sometimes difficult to distinguish the E. histolytica cyst from the cysts of non-pathogenic intestinal protozoa by its appearance.

Amebiasis is transmitted by faecal contamination of drinking water and foods, but also by direct contact with dirty hands or objects and by sexual contact.

Usually amoebas remain in the gastrointestinal tract of the hosts but ulceration of the gastrointestinal mucosal surfaces occurs in less than 16% of cases. In fewer cases, the parasite invades the soft tissues, most commonly the liver. Amoebomas are rare masses that lead to intestinal obstruction.