A 28-year-old woman from rural Maharashtra presents with acute onset bloody diarrhea, abdominal cramps, and fever for 3 days. Stool culture grows a gram-negative rod that is non-motile, lactose-fermenting, and produces acid without gas. Which Shigella species is the most common cause of bacillary dysentery in India?

Explanation

## Most Common Shigella Species in India **Key Point:** Shigella flexneri is the most prevalent Shigella species causing bacillary dysentery in developing countries, including India, accounting for approximately 60–70% of all Shigella infections. ### Epidemiological Distribution | Species | Geographic Prevalence | Clinical Severity | Frequency in India | |---------|----------------------|------------------|-------------------| | **S. flexneri** | Developing countries, endemic areas | Moderate to severe | **Most common (60–70%)** | | S. sonnei | Developed countries, sporadic outbreaks | Mild to moderate | ~20–30% | | S. boydii | Sporadic, limited geographic distribution | Moderate | ~5–10% | | S. dysenteriae | Rare, epidemic potential | **Most severe** | <5% (epidemic strains) | ### Clinical Features of S. flexneri **High-Yield:** S. flexneri typically causes: - Bloody diarrhea with mucus (dysentery) - Abdominal cramps and tenesmus - Fever (38–40°C) - Self-limited course (3–7 days untreated) - Fecal-oral transmission in crowded, poor sanitation settings ### Laboratory Identification **Clinical Pearl:** All Shigella species share these biochemical features: - Gram-negative, non-motile rods - Lactose fermentation (slow, variable) - No gas production from carbohydrates - Oxidase-negative - Catalase-positive - Indole-positive (S. flexneri, S. sonnei, S. boydii); S. dysenteriae is indole-negative ### Why S. flexneri Dominates in India 1. **Adaptation to tropical climate** — thrives in warm, humid conditions 2. **Transmission in endemic areas** — sustained circulation in populations with poor sanitation 3. **Multiple serotypes** — S. flexneri has 6 serotypes (1a, 1b, 2a, 2b, 3, 4, 5, 6), allowing immune escape and recurrent infections **Mnemonic:** **FLEX** = **F**lexneri is the **L**eading cause in **E**ndemic areas with poor sanitation, especially in **X** (developing) countries. [cite:Park 26e Ch 31]