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1. Introduction and Literature Review 1. INTRODUCTION AND LITERATURE REVIEW 1.1 Introduction Parasites are the main causative agent of disease in man, where most of the human population are exposed to the risk and vast economic losses due to parasitic infection.(Ambrosio 1990) Intestinal protozoan infections are among the infections causing diarrhea and abdominal symptoms, including cramps, pain, bloating, or tenderness in humans. They have a cosmopolitan existence and cause a significant morbidity and mortality (Rossignol 2001) . Before invasion, E. histolytica may inhabit the human intestine for a long time, extending two years, as asymptomatic infection (Blessmann 2002) . There are 75% of the world's population lives in developing countries where the amoebiasis spreads, and 70% of the world's livestock resources are in the developing countries according to the Food and Agriculture Organisation (FAO) which the situation is very critical (Ambrosio 1990). Entamoeba histolytica is an enteric protozoan parasite that infest half a billion people worldwide. Ten percent of infected individuals develop intestinal amoebiasis and extraintestinal amoebiasis, leading to about 70,000 deaths each year, making it the second disease to malaria as a cause of mortality due to a protozoan parasites .(Pillai and Kain 1999) The remainder being asymptomatic cyst passers. The numerical difference between the incidence of infection and expression of morbidity is based on the existence of pathogenic and nonpathogenic strains of E. histolytica. (Egbert Tannich and Gerd 1991; Dawn Britten 1997) E. histolytica is a pathogen or invasive parasite, whereas E. dispar and E. moshkovskii are nonpathogenic and noninvasive although they are morphologically identical to E. histolytica. There are at least eight amebas (E. histolytica, E. dispar, E. moshkovskii, E. coli, E. hartmanni, E. polecki, Iodamoeba bütschlii, and Endolimax nana) which inhabit the human intestinal lumen. However, these are generally accepted as commensal organisms except for E. histolytica. E. polecki, Dientamoeba fragilis, and I. bütschlii which have occasionally been implicated as causes of diarrheal illness in humans.(Tanyuksel and Petri 2003) 1 1.1.1 Historical Background : Amebiasis was earlier recognized as a deadly disease by Hippocrates (460 to 377 B.C.), who described a patient with fever and dysentery.(Tanyuksel and Petri 2003) The Milestones in the study of E. histolytica and amebiasis were its description by Fedor Lösch in 1873 when a case of a young farmer had been admitted to his clinic in St Petersburg, Russia for diagnosis. The patient was suffering from chronic dysentery and Lösch found very large numbers of amoebae in his faeces and he became convinced that those amoebae were responsible for his illness. The young farmer was the first person known to have died from amoebiasis. In 1875, Fedor Losch isolated E. histolytica from a stool specimen of a patient with dysentery. Afterwards Schaudinn renamed the organism Entamoeba histolytica.(Ackers 2002; Tanyuksel and Petri 2003) Leonard Rogers has chosen emetine as the first effective treatment for amebiasis in 1912. In the next year (1913), Walker and Sellards demonstrated the infective cyst form of E. histolytica. (Tanyuksel and Petri 2003) It was 1925 when Brumpt proposed that two morphologically identical species of Entamoeba produced tetra nucleated cysts measuring 10 mm or more in diameter, could infect humans. One of the species caused disease and the other did not, and he named the nonpathogenic species Entamoeba dispar. In 1961 Diamond performed axenic cultivation of E. histolytica which was considered as a major turning point in understanding the cell biology and biochemistry of E. histolytica.(Tanyuksel and Petri 2003) These studies were not sufficient to distinguish the two morphologically identical parasites until Sargeaunt and his colleagues demonstrated in 1978, that isoenzyme typing could be able to distinguish the two species of Entamoeba which have now been conventionally named Entamoeba histolytica (which is pathogenic, causes invasive amebiasis, and is still frequently referred to as the pathogenic strain of E. histolytica) and E. dispar (which is probably truly nonpathogenic and which is still found to be described as the nonpathogenic strain of E. histolytica). (Louis S. Diamond 1993; Rashidul Haque 1995; Dawn Britten 1997; Yury O. Nu´N˜Ez 2001) 1.1.2 Classification of Entamoeba Parasites: Entamoeba parasite was classified according to Garcia (1993) (Garcia 1993; Tanyuksel and Petri 2003) as follows: 2 Kingdom: Protozoa, ( single celled eukaryotic organism). Phylum: Sarcomastigophora, (Locomotion by flagella or pseudopodia or both). Subphylum: Sarcodina, (Locomotion by pseudopodia, temporary flagellated in some forms). Class: Lobosea, ( pseudopodia lobopodia) Order: Amoebida, ( No flagellated stage in life cycle) Family: Entamoebidae, Genus and Species: Entamoeba histolytica Entamoeba dispar Entamoeba coli Entamoeba polecki Endolimax nana Entamoeba moshkovski Entamoeba hartmanni Dientamoeba fragilis Entamoeba gingivalis Iodamoeba bütschlii Blastocystis hominis The nomenclature Entamoeba histolytica is being used to indicate true pathogenic strain, while E. dispar is now being used to indicate nonpathogenic strain but actually the two organisms cannot be differentiated on the basis of morphological features seen in permanent stained faecal smears.(Garcia 1993) 1.1.3 Life Cycle, Morphology, and Biology of Entamoeba: The life cycle of Entamoeba histolytica is monogenic and humans act as its host. The infected individuals are the main source of transmission through contamination of fresh food or water and infection may be spread by arthropods such as cockroaches and flies. It is thought that there is some sort of zoonotic transmission, but this is not clear. Experimental infections with E. histolytica have been produced in some animals such as dogs, cats, rats, monkeys, and some other laboratory animals.(Tanyuksel and Petri 2003) 3 There may be some animal reservoirs of E. histolytica, but they represent a very small source of human infection compared with humans themselves. However, there are no reports of occasional zoonotic transmission of cases between infected animals and humans, although E. histolytica is most commonly associated with animals such as cats, dogs and primates. (Tanyuksel and Petri 2003) The life cycle of E. histolytica is simple. It has two stages, an infective cyst stage and a vegetative multiplying trophozoite stage. Humans acquire infection by ingesting the infective stages, which pass through the lumen to the small intestine , where the process of excystation occurs giving eight daughter trophozoites from each cyst. The trophozoites are motile forms, which adhere to and invade intestinal mucosa. Trophozoites move by extending creeping projections of cytoplasm, the pseudopodia, which pull the parasite along resulting in amoeboid movement (Tanyuksel and Petri 2003). They also use these pseudopodia to surround and engulf food particles. The cytoplasm frequently contains many red blood cells (RBCs) that have been ingested. The trophozoites of E. histolytica always have a single nucleus. Trophozoites are easily destroyed in the outside environment, degenerating within minutes. The trophozoite of E. histolytica can convert to a precyst form with a nucleus (E. coli precysts have two nuclei), and this form matures into a tetranucleated cyst as it migrates down and out of the colon. The precyst contains aggregates of ribosomes, called chromatoid bodies, as well as food vacuoles that are extruded as the cell shrinks to become a mature cyst. It is the mature cyst that, when consumed in contaminated food or water, is infectious. In the process of becoming tetranucleated, the nucleus of the cyst divides twice. Chromatoid bodies and glycogen vacuoles cannot be seen at this stage.(Tanyuksel and Petri 2003) Cysts can remain viable in the external environment outside the host for weeks or months, especially under wet conditions, but are quickly destroyed at temperatures under 5°C and over 40°C. Cysts are not invasive, but trophozoites can invade the gastrointestinal mucosa and are able to migrate via blood to other organs, causing extraintestinal infections.(Tanyuksel and Petri 2003) Like other protozoa, E. histolytica appears unable of a fresh (de novo) purine synthesis. Biochemical analysis has indicated that glutathione is not present. For this reason, E. histolytica is different from higher eukaryotes. It also uses pyrophosphate instead of ATP. The cytoplasm of the cyst is vacuolated with numerous glycogen 4 deposits, visible by permanent stains such as ironhematoxylin, that decrease in size and number as the cyst matures. (Tanyuksel and Petri 2003) The gene organization of E. histolytica seems quite distinct from that of other eukaryotes. Although the structure of E. histolytica chromosomes is not yet known completely, electrokaryotypic analysis suggests that the chromosomes range in size from 0.3 to 2.2 Mb and give a total haploid genome size of approximately 20 Mb. A complete sequence map of the ribosomal DNA (rDNA) episome has been successfully completed, Sehgal et al (1994). and Bhattacharya et al (1998), found that E. histolytica circular DNA is 24.5 kb. This sequence has proved quite useful for the genotyping of the different enteric amoebae.(Tanyuksel and Petri 2003) 5 1.2 Review of Literature: 1.2.1 Redescription of E. histolytica and E. dispar: Entamoeba histolytica and Entamoeba dispar were first differentiated
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