## Entamoeba histolytica: Life Cycle and Infective Stage ### Infective Stage **Key Point:** The CYST is the infective stage of Entamoeba histolytica. Cysts are environmentally stable, resist gastric acid, and are transmitted via the fecal-oral route (contaminated food and water). ### Why Cysts Are Infective - Cysts have a **thick, rigid cell wall** composed of chitin and other polysaccharides - This wall protects the organism from: - Gastric acid and bile salts - Desiccation - Environmental stress - When ingested, cysts excyst in the terminal ileum and cecum, releasing trophozoites ### Life Cycle Overview | Stage | Location | Characteristics | Transmissibility | | --- | --- | --- | --- | | **Cyst** | Intestinal lumen (in stool) | 4 nuclei, thick wall, dormant | **Highly infectious** | | **Trophozoite** | Intestinal mucosa/liver | Motile, 1 nucleus, fragile | Non-infectious (destroyed in stomach) | | **Pre-cyst** | Intestinal lumen | Intermediate stage, 2 nuclei | Not infectious | | **Metacyst** | Not a recognized stage | — | — | **High-Yield:** Cysts survive passage through the stomach because their thick wall is resistant to HCl and pepsin. Trophozoites are destroyed in the stomach, which is why they cannot transmit infection. **Clinical Pearl:** Asymptomatic carriers shed cysts in their stool and are the main reservoir for transmission in endemic areas. This is why proper sanitation and water treatment are crucial for disease control. **Mnemonic:** **CYST = Contagious, Yet Stable, Transmissible** — remembers that only the cyst form spreads disease.
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