Final Ento 310 paper - Peter Somoza and Mary Schied Ento...

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Peter Somoza and Mary Schied Ento 310 Entamoeba Histolytica: A Parasitic Interaction with Humans November 29, 2010
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Entamoeba histolytica is an anaerobic protozoan from the genus Entamoeba. The pathogenic nature of E. histolytica was first reported in 1873 by a Russian man named Friedrich Losch. Losch’s early observations came from a case of a young farmer who had been suffering from chronic dysentery. In his diagnosis, Losch discovered a large amount of amoebas in the young man’s stool, which he associated to be the origin of the severe dysentery (Rees 10). As a result, it was concluded that E. histolytica is a parasite that acts as a symbiont in its definitive human host. In addition, it has also been observed that E. histolytica may modulate the virulence of certain human viruses and is itself a host for its own viruses (Rees 15). E. histolytica primarily affects humans. However, domesticated animals, such as dogs and cats can become infected transiently, but are not thought to contribute significantly to transmission (Rees 8). The primary reason E. histolytica invade humans is to obtain adequate nutrients, such as bacteria found in the gastrointestinal system.
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E. histolytica has a relatively simple life cycle that alternates between trophozoite and cyst stages (Rees 21). The trophozoite stage is an actively metabolizing stage, while the cyst stage is dormant (Rees 22). The trophozoite stage is also known as the pathogenic stage, while the cyst stage in an infective stage. The trophozoite stage is readily killed in the environment and is not able to survive the passage through the acidic environment of the stomach to cause infection. Trophozoites reside in the colon, where they proliferate. In contrast, the cysts are excreted in stool. Ingested cysts release trophozoites which colonize the large intestine this process is known as excystation (Dutta 24). Some trophozoites invade the bowel and cause ulceration, predominately occurring in the cecum and ascending colon. The trophozoites are short-lived outside the human body. They are also not important in the transmission of the disease. In contrast, the cysts are infective to humans and remain viable for several days in contaminated conditions (Dutta 26). Morphologically, E. histolytica lacks mitochondria and their endoplasm is encased with a nucleus, which has a very distinctive rim of finely beaded chromatin lining the nuclear membrane, as well as food vacuoles. Subsequently, bacteria or erythrocytes, red blood cells, from its human host are commonly found in these vacuoles (Martinez-Palomo 16). Reproduction of E. histolytica , like other protozoa, is based upon binary fission. The pathogenesis of E. histolytica begins through fecal-oral transmission of a mature cyst obtained through contaminated food or water, resulting in lytic necrosis. Therefore, the primary portal of entry is the mouth. Lytic necroses are flask shaped holes found in the gastrointestinal tract (Dutta 30). An individual may spread the disease by improper hand washing following lavatory use.
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Final Ento 310 paper - Peter Somoza and Mary Schied Ento...

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