Amebic dysentery is prevalent in regions where human excrement is used as fertilizer; in some such regions over half the population probably harbors the amebic cyst. The cyst is the inactive, resistant stage in which the ameba is transmitted from one host to another; the active form is that which causes damage. Both cysts and active amebas are excreted in the feces of an infected person, but only the cysts are hardy enough to survive outside the body. A person recovering from the infection, or one with an inactive case, passes mostly cysts; such a person is a more likely source of contamination than one with an active case. When cysts are ingested with contaminated food or water they are transformed in the intestine into active amebas. If these remain within the lumen of the intestine they are relatively innocuous, but if they invade the intestinal wall they cause ulceration, dysentery, and usually pain. In severe cases the resulting dehydration may lead to prostration.
Amebic dysentery may occur in acute or chronic form. In prolonged infections the amebas may invade the blood vessels of the intestine and be carried to other parts of the body, where they cause amebic abcesses. Abcesses of the liver and brain are especially dangerous; destruction of liver tissue is the most frequent complication of amebic dysentery. Infection by amebas, whether of the intestine alone or of other parts of the body, is called amebiasis. Infections are diagnosed by finding cysts or active amebas in the feces. However, the disease is easily misdiagnosed for several reasons. Entamoeba histolytica may be harbored without causing symptoms (although it may be passed on and cause the disease in others); it is easily confused with harmless amebas of the human intestine, especially Entamoeba coli ; it commonly coexists with bacteria that may in some cases be the cause of the symptoms.
A combination of drugs is generally used to treat amebic dysentery: an amebicide (metronidazole or tinidazole) to eliminate the organism from the intestinal tract, an antibiotic to eradicate associated bacterial infection, and a drug to combat infection of the liver and other tissues. Preventive measures include the protection of water supplies from contamination and the washing of hands by food handlers.