A 32-year-old man from rural Bihar presents with a 3-week history of bloody diarrhoea, abdominal pain, and tenesmus. He reports consuming contaminated water from a hand pump. On examination, he is afebrile with mild abdominal tenderness. Stool microscopy reveals motile trophozoites with ingested RBCs. Blood culture is negative. What is the most likely diagnosis?

Explanation

## Diagnosis: Entamoeba histolytica Intestinal Infection ### Clinical Presentation The patient presents with the classic triad of *E. histolytica* dysentery: - Bloody diarrhoea (haemorrhagic colitis) - Abdominal pain and tenesmus - Afebrile status (distinguishes from bacterial dysentery) ### Diagnostic Clue: Motile Trophozoites with RBCs **Key Point:** The presence of **motile trophozoites containing ingested RBCs** is pathognomonic for *E. histolytica*. This erythrophagocytosis is a hallmark feature that differentiates it from other causes of dysentery. ### Epidemiology **High-Yield:** *E. histolytica* is endemic in rural India, transmitted via faecal-oral route through contaminated water. The incubation period is typically 1–4 weeks, matching this patient's timeline. ### Why Afebrile? **Clinical Pearl:** Unlike bacterial dysentery (Shigella, Salmonella), *E. histolytica* typically causes **afebrile dysentery**. Fever suggests bacterial infection. ### Laboratory Findings | Feature | *E. histolytica* | Shigella | Salmonella | |---------|------------------|----------|------------| | Trophozoites with RBCs | Yes (pathognomonic) | No | No | | Fever | Absent/mild | Present | Present | | Blood culture | Negative | Negative | Positive (early) | | Stool appearance | Blood-stained mucus | Bloody mucus | Bloody mucus | **Key Point:** Negative blood culture rules out bacteraemia (Salmonella typhi), and the presence of RBC-containing trophozoites is diagnostic of *E. histolytica*. ### Pathophysiology *E. histolytica* trophozoites produce proteolytic enzymes and cytolytic toxins that destroy the intestinal mucosa, causing ulceration and haemorrhage. The amoebae ingest RBCs and WBCs during this process, which is visible on microscopy. ### Management Implications Treatment requires: 1. **Tissue amoebicide** (e.g., metronidazole 750 mg TDS × 10 days) for invasive disease 2. **Luminal amoebicide** (e.g., paromomycin 25–35 mg/kg/day × 7 days) to eliminate cysts and prevent relapse [cite:Park 26e Ch 8]