Entamoeba histolytica is an ameba that feeds on cells in the human colon. It is the cause of amebic dysentery (bloody diarrhea) as well as colonic ulcerations. The infection is also referred to as amebiasis. If the organisms spread throughout the body via the bloodstream they may cause abscesses in the liver or, less frequently, other organs.
The organism has two forms. The cyst is round and 10-20 micrometers in diameter, and contains four nuclei when mature. It is resistant to desiccation and stomach acid, and can survive long enough in the environment to be spread to other humans. When the cyst reaches the large intestine, it excysts and the four nuclei present in the cyst become four separate amebae, each of which undergoes binary fission immediately; thus the ingestion of a single cyst leads to 8 trophozoites. The trophozoite, 10-60 micrometers in diameter, is the active form of the organism and it is in this form that the damage is done to the body. In 1994 the CDC recorded 2,983 cases of amebiasis in the United States.
E. histolytica is spread by the fecal-oral route. This is achieved through food or water contaminated with cysts, oral-anal sexual contact, or occasionally directly in childcare centers or institutions for the developmentally challenged. The disease is found far more frequently in people from developing countries or travelers to such areas than in developed countries.
Damage is caused by the lysis of epithelial cells, due in part to the insertion of pore-forming proteins into the membrane of the cell. Neutrophils and non-activated macrophages may also be killed and ingested by the organism, limiting the ability of the immune system to deal with the disease. E. histolytica is also capable of phagocytosing red blood cells. Symptoms of infection vary widely, from an asymptomatic carrier state, to mild discomfort and stools containing some blood or mucous, to full-blown dysentery with bloody and mucoid stools. The incubation period typically lasts between two and four weeks, though it can also vary anywhere from a few days to months or years before symptoms are identified. Encystment begins when desiccation is experienced in the colon. The single nucleus in the immature cyst divides twice to produce a mature cyst bearing 4 nuclei.
E. histolytica is diagnosed by the examination of slides prepared from fecal matter. It can be difficult to diagnose E. histolytica for several reasons. It is morphologically indistinguishable from E. dispar, a nonpathogenic species. There are also difficulties...