Enteric Fever (Typhoid): Salmonella Pathogenesis & Peyer's Patch Ulcers

**1. Etiological Agent & Transmission** Enteric fever is caused by *Salmonella enterica* serovar Typhi and Paratyphi (A, B, C). It is a Gram-negative, facultatively anaerobic, motile, non-lactose fermenting, hydrogen sulfide (\( \text{H}_2\text{S} \)) producing bacillus. Transmitted exclusively by the fecal-oral route through contaminated drinking water or food. **2. Pathogenesis & Stepwise Progression** * **Invasion of Peyer's Patches**: Bacilli survive the gastric acid barrier and invade enterocytes and **Microfold (M) cells** overlying Peyer's patches in the terminal ileum. * **Macrophage Hijacking & Primary Bacteremia**: Phagocytosed by mononuclear cells, bacilli inhibit phagolysosomal fusion via Type III secretion system effectors (Salmonella Pathogenicity Island-2, SPI-2). They replicate intracellularly and disseminate through mesenteric lymphatics and thoracic duct into the bloodstream (transient primary bacteremia). * **Reticuloendothelial Sequestration**: Organisms multiply within the liver, spleen, and bone marrow during the 10–14 day asymptomatic incubation period. * **Secondary Bacteremia & Clinical Onset**: Massive release of bacilli and endotoxin (LPS) into the systemic circulation produces the classical sustained "step-ladder" fever, bacteremia, and toxemia. * **Gallbladder Sequestration & Chronic Carrier State**: Organisms reinvade the intestinal tract via bile secretions. In chronic carriers (e.g., "Typhoid Mary"), bacilli persist within gallbladder gallstones as a protective biofilm. **3. Pathological Lesions & Complications** * **Peyer's Patch Hyperplasia & Necrosis**: Peyer's patches in the terminal ileum become hyperemic, elevated, and necrotic, sloughing to form **longitudinal ulcers oriented along the long axis of the bowel** (in contrast to tuberculous ulcers, which are transverse). * **Intestinal Perforation & Hemorrhage**: Typically occurs in the **3rd week of illness** due to necrosis of the thin-walled ileal ulcer base, causing catastrophic peritonitis. * **Other Features**: Hepatosplenomegaly, typhoid nodules (reticuloendothelial cell clusters), and "Rose spots" (transient blanching erythematous maculopapular rash on trunk in week 2). **4. 🔬 Chronological Laboratory Diagnostic Schedule (Mnemonic: BASU)** * **Week 1 - Blood Culture**: Positive in 80–90% of patients. Inoculated in Castaneda's biphasic medium or bile broth (contains 0.5% sodium taurocholate). * **Week 2 - Agglutination Test (Widal Test)**: Measures agglutinating antibodies against **O (somatic)** and **H (flagellar)** antigens. Significant titers in endemic regions: Anti-O $\ge$ 1:160 (indicates active infection); Anti-H $\ge$ 1:160 (indicates past infection or vaccination). * **Week 3 - Stool Culture**: Positive on selective media (XLD agar, MacConkey agar showing non-lactose fermenting pale colonies, Wilson-Blair bismuth sulfite showing black colonies with metallic sheen). * **Week 4 - Urine Culture**: Organisms shed in urine. * **Bone Marrow Culture**: Gold standard diagnostic test with highest sensitivity (90–95%), remaining positive even after prior antibiotic therapy. **5. 📖 Standard Textbook Reference** * *Ananthanarayan and Paniker's Textbook of Microbiology*, 11th Ed., Ch. 29: Salmonella, pp. 289–302. * *Robbins and Cotran Pathologic Basis of Disease*, 10th Ed., Ch. 8: Infectious Diseases, pp. 362–365.