During a surveillance study of enteric fever in a tertiary hospital in Delhi, blood cultures from 150 confirmed cases were analysed. Which site is the most common source of bacteraemia in enteric fever?

Explanation

## Pathogenesis and Source of Bacteraemia in Enteric Fever **Key Point:** Peyer's patches in the terminal ileum and proximal colon are the primary site of bacterial invasion and multiplication in enteric fever. This is the initial focus from which bacteraemia originates. ### Sequence of Pathological Events 1. **Ingestion and intestinal invasion:** Salmonella organisms penetrate the intestinal epithelium via M cells overlying Peyer's patches in the small intestine (primarily terminal ileum). 2. **Local multiplication:** Bacteria multiply within the lymphoid tissue of Peyer's patches. 3. **Lymphatic dissemination:** Organisms spread to mesenteric lymph nodes, causing hyperplasia and necrosis. 4. **Bacteraemia:** Bacteria enter the bloodstream via lymphatic channels, leading to systemic infection. 5. **Reticuloendothelial seeding:** The liver, spleen, bone marrow, and other organs become infected. ### Anatomical and Microbiological Basis | Site | Role in Pathogenesis | Significance | | --- | --- | --- | | **Peyer's patches** | Primary invasion site; initial bacterial multiplication | Source of bacteraemia | | **Mesenteric lymph nodes** | Secondary site of replication; lymphadenitis | Gateway to systemic spread | | **Gallbladder** | Secondary infection; chronic carriage reservoir | Biliary shedding in 1–5% of cases | | **Spleen** | Target organ for reticuloendothelial seeding | Splenomegaly develops later | | **Bone marrow** | Site of bacterial multiplication | Positive culture in ~90% of cases | **High-Yield:** While mesenteric lymph nodes are the second site of involvement, Peyer's patches are the PRIMARY and INITIAL source of bacteraemia. This distinction is crucial for understanding the pathophysiology. ### Clinical Pearls **Clinical Pearl:** Intestinal perforation (a serious complication) typically occurs in the third week of illness when necrotic Peyer's patches slough off, allowing bacteria to breach the intestinal wall. This complication is more common in untreated cases. **Mnemonic: PIMBS** — **P**eyer's patches → **I**nvasion → **M**esenteric nodes → **B**acteraemia → **S**plenomegaly (and systemic seeding). ### Why Other Sites Are Secondary - **Mesenteric lymph nodes:** Become involved after Peyer's patches; they amplify but do not initiate bacteraemia. - **Gallbladder:** Infected secondarily via haematogenous spread; becomes a reservoir for chronic carriage in 1–5% of patients (chronic carriers shed organisms in bile). - **Spleen:** Seeded from bacteraemia; enlargement occurs as a consequence of reticuloendothelial response, not as the source. [cite:Robbins 10e Ch 8; Harrison 21e Ch 159]