Which site is the most common site of infection and inflammation in Shigella dysentery?

Explanation

## Anatomical Site of Shigella Infection **Key Point:** Shigella species preferentially invade and cause inflammation in the **sigmoid colon and rectum**, the distal large intestine, leading to the characteristic bloody diarrhea of bacillary dysentery. ### Pathophysiology of Shigella Infection ```mermaid flowchart TD A[Shigella ingestion]:::outcome --> B[Passage through stomach and small intestine]:::action B --> C[Arrival at colon]:::action C --> D{Site of invasion?}:::decision D -->|Sigmoid colon & rectum| E[Epithelial invasion via M cells]:::action D -->|Other colonic sites| F[Rare/minimal invasion]:::outcome E --> G[Mucosal ulceration & inflammation]:::action G --> H[Bloody mucoid diarrhea]:::outcome E --> I[Shiga toxin production<br/>Systemic effects]:::action ``` ### Mechanism of Localization to Distal Colon | Feature | Mechanism | |---------|----------| | **Acid resistance** | Survives gastric pH; reaches colon intact | | **Mucus penetration** | Produces mucinase; penetrates colonic mucus layer | | **M-cell targeting** | Invades through M cells in colonic epithelium | | **Epithelial invasion** | Type III secretion system injects virulence factors | | **Inflammatory response** | Massive PMN infiltration → mucosal ulceration | **High-Yield:** The **sigmoid colon and rectum** are affected in nearly 100% of Shigella dysentery cases. The rectosigmoid region shows: - Friable, bleeding mucosa - Shallow ulcers with raised edges - Mucopurulent exudate - Intact submucosa and muscularis (unlike typhoid) **Clinical Pearl:** The involvement is **limited to the mucosa and submucosa** (non-invasive colitis), unlike invasive pathogens such as *Salmonella* or *Campylobacter*, which cause deeper inflammation. This explains why perforation is rare in uncomplicated shigellosis. ### Why Other Sites Are Spared - **Small intestine (jejunum, ileum):** Shigella does not efficiently invade small intestinal epithelium; transit is too rapid for mucosal adherence and invasion. - **Ascending colon & cecum:** Less commonly affected; inflammation is primarily in the distal colon. - **Duodenum:** Not a site of Shigella tropism; no mucosal invasion occurs here. **Mnemonic:** **RECT-SIG for SHIG** — Shigella affects the RECTum and SIGmoid colon.