Amebae: Medical Parasitology Overview — Notes OmpathStudy

--- An Overview Introduction The term "ameba" originates from the Greek word amibe , meaning "change." Amebae are structurally simple protozoans without a fixed shape. Taxonomic Classification : Phylum : Sarcomastigophora Subphylum : Sarcodina Superclass : Rhizopoda Order : Amebida Structure The cytoplasm is enclosed by a membrane and differentiated into: Ectoplasm : The outer layer. Endoplasm : The inner layer. Pseudopodia :Formed by thrusting out ectoplasm followed by endoplasm. Function in locomotion and food capture (phagocytosis). Reproduction Primarily occurs by: Fission : Splitting into two. Budding : A smaller outgrowth develops and detaches. Cysts :Formed under unfavorable conditions. Serve as the infective stage for vertebrate hosts (e.g., Entamoeba histolytica ). Classification Amebae are categorized into: Intestinal Amebae :Found in the alimentary canal. Includes species like: Entamoeba histolytica (pathogenic) Entamoeba dispar Entamoeba coli Entamoeba polecki Entamoeba hartmanni Entamoeba gingivalis Endolimax nana Iodamoeba butschlii Note : Only Entamoeba histolytica is pathogenic. Free-Living Amebae :Opportunistic pathogens that can cause infections. Includes species like: Naegleria fowleri Acanthamoeba spp. Balamuthia mandrillaris Entamoeba histolytica History and Discovery Discovered in 1875 by Losch in St. Petersburg, Russia, in dysenteric feces. Key developments:1890: William Osler reported dysentery leading to liver abscess. 1891: Councilman and Lafleur introduced the terms "amebic dysentery" and "amebic liver abscess" and studied its pathogenesis. Distribution Worldwide prevalence, more common in tropical regions. Found in poor sanitation areas, across all climates:From Alaska (61°N) to the Straits of Magellan (52°S). Infection statistics:~10% of the global population infected. ~50% prevalence in developing countries. ~1% prevalence in affluent countries like the USA. Clinical Impact :80–99% of infections are asymptomatic. Invasive amebiasis:Affects 50 million people annually. Causes 50,000 deaths yearly, especially in tropical regions (Asia, Africa, Latin America). Third leading parasitic cause of death after malaria and schistosomiasis. Epidemiology in India India can be divided into three zones based on prevalence: High Prevalence ( 30%) :Chandigarh, Tamil Nadu, Maharashtra. Moderate Prevalence (10–30%) :Punjab, Rajasthan, Uttar Pradesh, Delhi, Bihar, Assam, West Bengal, Andhra Pradesh, Karnataka, Kerala. Low Prevalence (<10%) :Haryana, Gujarat, Himachal Pradesh, Madhya Pradesh, Odisha, Sikkim, Puducherry. --- Morphology of Entamoeba histolytica Entamoeba histolytica occurs in three distinct forms: Trophozoite Precystic stage Cystic stage 1. Trophozoite Stage Definition : The trophozoite is the vegetative, actively growing stage of the parasite and the only form found in tissues. Characteristics : Shape and Size :Irregular shape; size ranges from 12–60 µm , with an average of 20 µm . Actively motile in freshly passed dysenteric stools, while smaller in convalescent and carrier individuals. When present as a commensal in the intestinal lumen, the trophozoite is smaller (~15–20 µm) and is called the minuta form. Cytoplasm : Ectoplasm : Clear, transparent, and refractile. Endoplasm : Granular with a "ground glass" appearance. Contains:Nucleus Food vacuoles Ingested erythrocytes (diagnostic feature) Occasionally leukocytes and tissue debris. Pseudopodia :Finger-like projections formed by sudden jerky movements of ectoplasm followed by streaming of endoplasm. Enable ameboid motility (crawling or gliding movement). Movement requires attachment to a surface or particle. Inhibited at low temperatures. Nucleus :Spherical, 4–6 µm in size. Contains: Central karyosome surrounded by a clear halo. Fine radiating fibrils (Linin network), giving a cartwheel appearance. Nuclear membrane is lined with evenly distributed chromatin granules. Best visualized in stained preparations (e.g., iron hematoxylin). Additional Features :Trophozoites divide by binary fission every 8 hours. Trophozoites from acute dysenteric stools contain phagocytosed erythrocytes, a diagnostic feature not seen in other intestinal amebae. Survival:Up to 5 hours at 37°C . Killed by drying, heat, and chemical sterilization. Non-infective stage: Rapidly destroyed in the stomach if ingested. 2. Precystic Stage Definition : Transitional stage as the trophozoite prepares for encystment in the intestinal lumen. Characteristics :The trophozoite extrudes its food vacuoles and becomes round or oval (~10–20 µm in size). Contains:A large glycogen vacuole . Two chromatoid bodies : Cigar-shaped, refractile rods with rounded ends. Encystment occurs only in the intestinal lumen and not in tissues or outside the host body . 3. Cystic Stage Definition : Infective, non-motile, and resistant form of the parasite. Characteristics : Shape and Size :Spherical; size ranges from 10–20 µm . Structures : Early Cyst :Single nucleus. Contains:A mass of glycoge