CHAPTER-I - Amoebiasis - an Introduction INTRODUCTION

CHAPTER-I - Amoebiasis - An Introduction INTRODUCTION 1.1. The term amoebiasis denotes the condition of harbouring B,histolytica with or without clinical manifestations (WHO, 1969). The aetiological agent of amoebiasis is Entamoeba histolytica. 1,2• Epidemiology Amoebiasis has been reported practically from all over the world and has been estimated to affect 10% of the world population although its prevalence and severity may differ from area to area (WHO, 1969). Thus, it is a public health problem of International importance. In India, the prevalence of amoebiasis varies from 1-58% (Arora et al., 1977) . The highest prevalence rate among the asyrrptomatic people has been recorded to be 47,4% (Greval, 1968), and that in symptomatic patients 58% (Patel/ 1945). Prevalence rates of this disease in the states of Himachal Pradesh, Jammu and Kashmir, Orissa and Sikkim have been reported to be less than 10%. Prevalence rate ranging from 1-20% have been reported from Assam, Delhi, Gujarat and Madhya Pradesh. The states of Andhra Pradesh, Kerela, Rajasthan, U.P. and West Bengal showed prevalence rates upto 30% while in Maharashtra, Tamil Nadu and Chandigarh the rates were more than 30% (Arora et al., 1977). Although E.histolytica is generally more prevalent in the tropics and subtropics. the higher temperature and humidity are not thought to be directly responsible since the higher temperatures are likely to have an adverse effect on cysts in the environment. The increased prevalence in tropical countries is generally considered to reflect poor sanitation and health education rather than climatic conditions. Prevalence of E,histolytica is associated with unsanitary conditions in temperate and even in arctic regions (WHO, 1969). The unhygeinic working and living conditions, insanitary surroundings, improper handling of food, contami nation of food by cockroaches and flies and refervoir or hosts such as dogs, cats etc. (which are natural hosts of E,histolytica) play a very important role in the spread of the disease. Water, food and vegetables are potential carriers of thd disease. 1,3. Classification Kingdom - Protista Subkingdom - Protozoa Phylum - Sarcomastigophora Subphylxim - Sarcodina Superclass - Rhizopoda Class - Lobosa Subclass - Gymnamoebia Order - Amoebida 3 Suborder - Tubulina Family - Entamoebidae Genus and - Entamoeba histolytica species (Schaudinn, 1903). Of the seven species of amoebae which are natural parasites of man, only E.histolytica is truely pathogenic. Of current special interest is the recently discovered fact that free living amoebae of the genus Acanthamoeba (Synonym Harmane11a; Page, 1976) and Naegleria are facultative parasites of man and may be highly pathogenic when invading the meninges and brain via the nasal cavity (Singh, 1975) 1.4. Life cycle The life cycle of E»histolytica involves the, encystment of a trophozoite, followed by release of the tropho zoite from the cyst when the conditions are appropriate. The trophozoite undergoes encystment only within the bowel, usually associated with conditions in the lumen which are not ideal for continued activity of the trophozoite. The young cyst is uninuclear and it contains one large glycogen vacuole and chromatoid body. It is the non motile resting stage. It is round or oval measuring 2,5 to 3,5 p in diameter. The cyst matures either within the lumen of the bowel or outside the body in appropriately moist surroundings. The mature cyst contains four nuclei (the result of 2 nuclear divisions) and a number of chromatoid 4' bodies. When this cyst is ingested, another nuclear division occurs during the cyst 's transit through the small intestine and the resulting eight trophozoites escape through a hole created in the wall of the cyst. Trophozoites migrate to the colon where they persist, undergoing binary fission every eight hours in the trophozoite stage. Encystment of these organisms occur again when the trophozoites are excreted outside the body and environmental conditions not so favour able to continued trophozoite multiplication, the life cycle is complete (Wilcocks et al., 1971). The trophozoites are facultative anaerobes which require complex media for growth. The environment for optimum growth of amoeba in the colon includes a pH 6.0-6.5, a low redox potential, nutrients occuringi found in the colon particularly carbohydrates and the presence of bacteria. The trophozoite is the vegetative, motile stage measuring frcan 15-40 u. This stage is not responsible fot the spread of the disease since it is rapidly destroyed by the gastric pH and enzymes. The encysted stage readily passes this barrier. Cysts may remain viable for weeks or months in appropriate moist surroundings. The cyst, therefore, is the primary reason for the extensive prevalence of the infection throughout the world. The conditions that allow the amoebae to become invasive are not clear, however, virulence has been promoted in experimental animals by irritating the bowel with abrasives, chemicals or invasive bacteria (Lesh (Losch) 1975, Wittner et ^., l97o) . Trophozoites have been shovm to breakdown cement substance between epithelial cells of the intestine and to gain access to the submucosal region by moving between these cells (Tekeuchi et al., 1975). The events that trigger this process are not known but a number of factors in the host will contribute to Invasion such as high cholesterol, low ascorbic acid diet, high carbohydrate, low protein diet, the absence of previous exposure to amoebae, as well as climatic, emotional and genetic factors (Perez - Tamayo et al., 1971). 1,5. History Losch in 1875 discovered motile amoeba with ingested red blood cells in the dysenteric stools of a patient. Later at the autopsy of the patients he detected colonic ulcers containing active amoebae. He is credited with documenting amoeba to be pathogenic by inducing lesions in a dog with dysenteric stool. He named the protozoan Amoeba coli; but he did not regard the organism as the actual cause of the disease. After the discovery of Losch, discussion arose as to whether the amoeba was the aetiological agent of dysentery and this question remained unanswered for a long time. Kartulis, in Egypt in 1886, settled the role of amoeba as a fe cause of intestinal and hepatic lesions in patients with diarrhoea. Koch and Gaffky (1887) and Osier (1890) confinned the findings. Councilman and Lafleur (1891) published a complete study of 15 cases of amoebic dysentery. They named the parasite Amoeba dysenteriae. In 1893, Quincke and Roos demonstrated that cats could be infected either by feeding them with stool containing cysts or by injecting stool containing cysts, per anum. They distinguished amoebae other than "Amoeba coli Losch" by their lack of pathogenicity in cats. These they called "Amoeba coli mitis", found in mild dysentery; and "Amoeba intestinl vulgaris" found in the stools of healthy people. Casagrandi and Barbagallo (1897) described Entamoeba coli from healthy people, thus establishing the generic name Entamoeba as distinct from Endamoeba Leidy, 1897. Strong and Musgrave (1900) differentiated the pathogenic amoeba from non pathogenic one in the human intestine, They observed that in cats typical ulcers were produced by pathogenic amoeba, whereas a non-pathogenic amoeba produced no such lesions in these animals. Hxiber (1903) demonstrated quadrinucleate cysts, but thought that it was a different species other than Entamoeba coli, so he called it Entamoeba tetraqena. Schaudinn (1903) distinguished non pathogenic E.colJ. and the pathogenic haenratophagous E.histolytica. The name 'tetragena ' caused confusion. The concept that the quadranucleate cysts were those of some species other than S.histolytica remained imquestioned for a dozen years. Huber (1909), Hartmann (1908) and others accepted that E.tetragena was a separate species. Viereck (1907) stated that E.tetragena was a strain of E.cqli. Elmassian (1909) gave the name E.minuta for the trophozoites associated with quadranucleated cysts. Craig (1905) considered that the correct title should be B.dysenteriae. Thus in 1910, there were 4 names dysenteriae, tetragena, minuta and histolytica, for what is now considered to be a single entity. The accepted name for the parasite is Entamoeba histolytica (Schaudinn, 1903). Walker and Sellards (1913) demonstrated conclusively that man could be infected with the cysts of E.histolytica. In human volunteers fed with E.coli cysts, no such disease developed; thus it was shown experimentally for the first time, in man that E.histolytica was pathogenic to man. 1.6. Pathogenesis and pathology Commensal trophozoites thrive adjacent to the colonic mucosa where the oxygen tension is moderately low and the pH is between 6.0 and 6.5. Host tissue appears to be damaged only by direct contact with amoebae; lyso-somal enzymes are 8 released, possibly by activating a contact-sensitive surface organelle. The relatively anaerobic and acid conditions of cell necrosis favour further penetration of amoebae into the tissue. The role of intestinal bacteria^ in pathogenesis is still uncertain. They may act mainly by creating suitable conditions for the growth of amoebae. 1.6.1, Amoebic ulceration It is most common in the caecxim and rectosigmoid region, but may affect any part of the large intestine, appendix and terminal ileum. The initial lesions are small, discrete, superficial mucosal erosions which may then extend more deeply through the muscularis mucosae and spread lateraly, producing ulcers that are flasTc-shaped in cross

Load more