ProtozoansAmebiasis.docx - Protozoans: Amebiasis In most...

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Protozoans: Amebiasis Definition In most infected individuals, E. histolytica parasitizes the lumen of the gastrointestinal tract and causes few or no symptoms or sequelae . The 2 most common forms of disease caused by E. histolytica are amebic colitis with parasitic invasion of the intestinal mucosa, and amebic liver abscess with dissemination of the parasite to the liver. Etiology Two morphologically identical but genetically distinct species of Entamoeba commonly infect humans. Entamoeba dispar, the more prevalent species, is associated only with an asymptomatic carrier state. E. histolytica, the pathogenic species, can become invasive, causing symptomatic disease . Infection is established by ingestion of parasite cysts, which measure 10–18 mm in diameter and contain 4 nuclei. Cysts are resistant to environmental conditions such as low temperature and the concentrations of chlorine commonly used in water purification but can be killed by heating to 55°C . After ingestion, the cyst, which is resistant to gastric acidity and digestive enzymes, excysts in the small intestine to form 8 trophozoites. These large, actively motile organisms colonize the lumen of the large intestine and may invade the mucosal lining. Infection is not transmitted by trophozoites because of their rapid degeneration outside the body and especially in the low pH of normal gastric contents if swallowed. Epidemiology Prevalence of infection with E. histolytica varies greatly depending on region and socioeconomic status. Most prevalence studies have not distinguished between E. histolytica and E. dispar, so the true prevalence of E. histolytica infection from these studies is not known. It is estimated that infection with E. histolytica leads to 50 million cases of symptomatic disease and 40,000–110,000 deaths annually. Amebiasis is the 3rd leading parasitic cause of death worldwide . Prospective studies have demonstrated that 4–10% of individuals infected with E. histolytica develop amebic colitis, and <1% of individuals develop disseminated disease, such as amebic liver abscess . These numbers vary by region: in South Africa and Vietnam, liver abscesses form a disproportionately large number of the cases of invasive disease due to E. histolytica. Amebic liver abscesses are rare in children and occur equally frequently in male and female children, whereas in adults, amebic liver abscesses occur predominantly in men. Amebiasis is highly endemic in Africa, Latin America, India, and Southeast Asia. In the United States, amebiasis is seen most frequently in immigrants from or travelers to developing countries . Residents of institutions for the mentally retarded and men who have sex with men are also at increased risk for invasive amebiasis, although most Entamoeba infections in the latter group are E. dispar. Food or drink contaminated with Entamoeba cysts and direct fecal-oral contact are the most common means of infection. Untreated water and human feces used as fertilizer are important sources of infection
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This note was uploaded on 02/18/2012 for the course PAS 600 - 601 taught by Professor Garrubba during the Fall '10 term at Chatham University.

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ProtozoansAmebiasis.docx - Protozoans: Amebiasis In most...

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