Entamoeba histolytica
by Cindy Gode
General Description
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.
Morphology
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.
Transmission
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.
Virulence
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.
Diagnosis
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 involved in spotting E. histolytica in slides prepared from stool specimens. The CDC Morbidity and Mortality Weekly Report from March of 1985 discusses a "pseudo-outbreak" that occurred in California in 1983. 38 patients over the course of 3 months were diagnosed with E. histolytica, in comparison to a previous frequency of about 1 case per month. No connection between the patients was found, nor were the patients part of the high-risk population of travelers or homosexual males. Instead it was found that 36 of these cases were actually misdiagnosed. Two of the patient samples were positive for protozoa other than E. histolytica, and 34 of these stool samples contained polymorphonuclear neutrophils and/or macrophages, which can be visually confused with E. histolyticacysts.
Control and Treatment
E. histolytica is anaerobic and is sensitive to metronidazole when in the trophozoite form. Metronidazole is not effective against the cyst form of the organism, and therefore is followed up with iodoquinol or paromomycin to target the cysts. Dehydroemetine, a treatment that requires hospitalization due to the need for close supervision, and Diloxanide furoate, which is used in conjunction with other treatments in systematic cases, are only available through the Center for Disease Control and Prevention. If liver infection occurs Cholorquine may be used, in the event that Metronidazole is ineffective.
E. histolytica is a typical example of diseases that impact poor populations in developing countries. This makes it far more difficult to fund research and development for new treatments or vaccines, in spite of the fact that research has shown some possibilities for vaccines. The presence of IgA antibodies against E. histolytica indicates that a vaccine that brings about a mucosal immune response could be effective. However, the lack of projected profit limits the interest of pharmaceutical and biotechnology companies. Sanitation and hygiene are effective controls but often cannot be applied in many poor nations. Until a vaccine is created and distributed, E. histolytica will remain an important disease in mortality rates, especially among children in developing countries.