<P> Entamoeba histolytica is an anaerobic parasitic amoebozoa, part of the genus Entamoeba . Predominantly infecting humans and other primates causing amoebiasis, E. histolytica is estimated to infect about 50 million people worldwide . Previously, it was thought that 10% of the world population was infected, but these figures predate the recognition that at least 90% of these infections were due to a second species, E. dispar . Mammals such as dogs and cats can become infected transiently, but are not thought to contribute significantly to transmission . </P> <P> The word histolysis literally means disintegration and dissolution of organic tissues . </P> <P> The active (trophozoite) stage exists only in the host and in fresh loose feces; cysts survive outside the host in water, in soils, and on foods, especially under moist conditions on the latter . The infection can occur when a person puts anything into their mouth that has touched the feces of a person who is infected with E. histolytica, swallows something, such as water or food, that is contaminated with E. histolytica, or swallows E. histolytica cysts (eggs) picked up from contaminated surfaces or fingers . The cysts are readily killed by heat and by freezing temperatures, and survive for only a few months outside of the host . When cysts are swallowed they cause infections by excysting (releasing the trophozoite stage) in the digestive tract . The pathogenic nature of E. histolytica was first reported by Lösch in 1875, but it was not given its Latin name until Fritz Schaudinn described it in 1903 . E. histolytica, as its name suggests (histo--lytic = tissue destroying), is pathogenic; infection can be asymptomatic or can lead to amoebic dysentery or amoebic liver abscess . Symptoms can include fulminating dysentery, bloody diarrhea, weight loss, fatigue, abdominal pain, and amoeboma . The amoeba can actually' bore' into the intestinal wall, causing lesions and intestinal symptoms, and it may reach the blood stream . From there, it can reach different vital organs of the human body, usually the liver, but sometimes the lungs, brain, spleen, etc . A common outcome of this invasion of tissues is a liver abscess, which can be fatal if untreated . Ingested red blood cells are sometimes seen in the amoeba cell cytoplasm . </P> <P> It is more common in people who live in tropical areas with poor sanitary conditions, male homosexuals, travelers and recent immigrants, and institutionalized populations . </P>

Where does excystation of mature entamoeba histolytica cysts occur