Medical Parasitology M.Sc. Course 2024-2025 (LEC. 2) PDF

Medical Parasitology Prof. Dr. Malak M. Almusawi M.Sc. Course/ 2024-2025 Lec. 2 Entamoeba histolytica the causative agent of amoebic dysentery. Dysentery is a general term that is used to describe a serious inflammatory disorder affecting the intestines...

Medical Parasitology Prof. Dr. Malak M. Almusawi M.Sc. Course/ 2024-2025 Lec. 2 Entamoeba histolytica the causative agent of amoebic dysentery. Dysentery is a general term that is used to describe a serious inflammatory disorder affecting the intestines that results in intense diarrhoea and is often accompanied by pain and fever. There are two stages in the life cycle of Entamoeba histolytica: the actively growing and feeding stage referred to as the trophozoite form and the transmission stage called the cyst form (Figure1). Figure 1: Entamoeba histolytica trophozoite and mature cyst. Like all other parasitic protozoa (but unlike the free-living forms), Entamoeba histolytica has no contractile vacuole. Reproduction takes place asexually by cell division and through cyst formation. The stimuli that cause the trophozoites to transform into cysts are uncertain, but it is an essential part of the life cycle. This has led to the suggestion that if drugs could be developed that would prevent cyst formation, it might be possible to reduce parasite transmission. The mature cysts contain four nuclei and characteristic bar-shaped chromatoidal bodies. The cell wall contains chitin which provides protection and enables the cyst to survive in the outside environment for prolonged periods. The output of cysts can be enormous and an infected person may excrete over 10 million cysts per day in their faeces. Transmission to a new host is a result of faecal-oral contamination (flies, fingers, faeces, food’) with infective cysts. Once ingested by a suitable host, the amoeba travels to the small intestine, where it emerges from the cyst and then undergoes a complicated series of divisions to produce eight trophozoites. The amoebae are subsequently swept down to the large intestine (colon) where they multiply in the lumen and may also invade the gut wall. 1 Medical Parasitology Prof. Dr. Malak M. Almusawi M.Sc. Course/ 2024-2025 Lec. 2 Avirulent strains of Entamoeba histolytica remain in the lumen of the colon and cause their host no harm. Those that are virulent attack and ingest the epithelial cells lining the gut wall and then proceed to spread through underlying layers. In the process of invasion, flask-shaped ulcers are formed which can become widespread with consequent bleeding over large areas of the intestine. It is therefore not unusual to find ingested red blood cells inside the food vacuoles of the trophozoites of virulent strains, and this can be useful in laboratory diagnosis. The loss of functional mucosal surface within the patient’s gut and the loss of blood and fluid may cause death by dehydration ‫ الجفاف‬and emaciation ‫الهزال‬. A great deal of water is normally re-absorbed in the colon so, with the loss of functional surface, re- absorption is reduced. Patients suffering from amoebic dysentery. The ulcers in the intestine often suffer secondary invasion by bacteria – this extends and deepens the ulcers and leads to increased blood and fluid loss. Secondary ulcers occur as a result of the amoebae damaging the lining of blood vessels and then being swept up in the bloodstream and setting up infections elsewhere in the body. The secondary ulcers can be potentially life-threatening ‫مهددة للحياة‬. They are most common in the liver, although the lungs and brain may also be affected. Entamoeba histolytica infections appear to be limited to humans. The trophozoites voided ‫تفرغ‬ ّ with faeces soon die but the cysts are very resistant to environmental conditions and it is these that are the main source of infection ‫االصابة‬. The cysts are usually transmitted via drinking water (or ice made from contaminated water), as contamination on vegetables grown on land fertilised with human faeces or via the bodies of insects that have moved between faeces and human food. Because of this, the safe and sensible disposal of human faeces is a crucial factor ‫عامل مهم‬in reducing the transmission ‫ لتقليل االنتشار‬of Entamoeba histolytica. Diagnosis Routine Diagnostic Procedures Microscopic examination of a direct saline wet mount may reveal motile trophozoites, which may contain RBCs. However, sometimes these trophozoites with RBCs are present is limited. Definitive diagnosis of liver abscess can be achieved by identification of organisms from liver aspirate material. Antibody Detection. For patients suspected of having extraintestinal disease, serologic tests are much more relevant. For patients suspected of having extraintestinal disease, serologic tests are much more relevant Histology. 2 Medical Parasitology Prof. Dr. Malak M. Almusawi M.Sc. Course/ 2024-2025 Lec. 2 Epidemiology and Prevention Infections with E. histolytica are worldwide in distribution and are generally most prevalent in the tropics ‫المناطق االستوائية‬. Population with a higher incidence of amebiasis include recent immigrants ‫المهاجرين‬and refugees ‫الالجئين‬. Although infections with this organism are usually associated with poor sanitation ‫مستوى ضعيف النظافة‬and underdeveloped areas of the world, epidemiologic studies in both Europe and the United States in the documented that sexual transmission of amebiasis occurred mainly among urban homosexual men. Entamoeba dispar Entamoeba dispar is morphologically indistinguishable from Entamoeba histolytica and has the same life cycle but it is non-pathogenic. It is much more prevalent than Entamoeba histolytica and therefore it is essential to be able to distinguish between the two species in order to avoid a false diagnosis of amoebic dysentery and inappropriate treatement. Naegleria fowleri Although over 30 species of Naegleria have been described, only one of these, Naegleria fowleri, is pathogenic. Like the other members of the genus, Naegleria fowleri is a free-living amoeboflagellate, i.e. it is an amoeba which, in one of its life cycle stages, possesses flagellae. It is a cosmopolitan ‫عالمية االنتشار‬species that is normally found in freshwater ponds and lakes but it has also been recovered from a wide range of wet or moist environments such as swimming pools. There are three-life cycle stages: (1) the active amoeboid trophozoite: (2) the non-feeding flagellate stage ‫الطور المسوط غير المتغذي‬that is produced when the food supply runs low and acts as a dispersal stage‫;مرحلة االنتشار‬ and (3) a cyst stage that is formed in response to adverse environmental ‫ الظروف غير الجيدة‬conditions. Humans usually become infected when they swim in infected water. The trophozoite is the infective stage and it migrates from the nasal mucosa ‫ االغشية المخاطية لالنف‬along the olfactory nerves ‫االعصاب الشمية‬, and then into the brain where it causes a condition called primary amoebic meningoencephalitis ‫ التهاب السحايا‬. This term is used to distinguish it from encephalitis ‫التهاب الدماغ‬caused by Entamoeba histolytica in which invasion of the brain is a secondary consequence of infection in the gut. 3