Although several protozoan species in the genus Entamoeba colonize humans, not all of them are associated with disease. Entamoeba histolytica is well recognized as a pathogenic amoeba causing amebiasis, which can involve both intestinal and extraintestinal infections. The other Entamoeba species are important because they may be confused with E. histolytica in diagnostic investigations. Two morphologically identical but genetically distinct and apparently largely nonpathogenic species, E. dispar and E. moshkovskii, are now believed to account for most asymptomatic Entamoeba infections in humans (Pritt and Clark 2008).
Entamoeba histolytica has a worldwide distribution, with a higher incidence of amebiasis in developing countries. Risk groups in industrialized countries include homosexual males, travelers and recent immigrants (although disease may develop months to years after exposure), and institutionalized populations. Infection by E. histolytica typically occurs by ingestion of mature cysts in fecally contaminated food, water, or hands. Cysts can survive for days to weeks in the external environment. Transmission can also occur through exposure to fecal matter during sexual contact (in this case not only cysts but also the far less durable trophozoite stage could prove infective). (Source: Centers for Disease Control Parasites and Health website)
Entamoeba histolytica is responsible for an estimated 35 to 50 million cases of symptomatic disease and around 100,000 deaths annually, apparently as a result of parasite destruction of host tissue. The majority of morbidity and mortality occurs in Asia, Central and South America, and Africa. Children are especially vulnerable as they can suffer from malnourishment and stunted growth as a result of repeated infection. (Ralston and Petri 2011 and references therein)
Entamoeba hemolytica are generally considered to be anaerobic since they can be grown in vitro only under conditions of reduced oxygen tension. However, metabolically these parasitic protozoa have been found to be microaerobic or microaerophilic, consuming oxygen to a certain extent and produce toxic oxygen derivatives. Given this fact, it is notable that they lack some or all of the usual antioxidant defense mechanisms present in aerobic and other aerotolerant cells, a phenomenon reviewed and discussed by Tekwani and Mehlotra (1999).
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