Amebae
Amebae - OMPATH
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## **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.
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## **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 s