A 5-year-old boy from Mumbai presents with severe bloody diarrhoea, fever (39.2°C), and abdominal pain for 4 days. His mother reports he attended a school with poor sanitation. On day 3 of illness, he developed acute kidney injury (serum creatinine 2.8 mg/dL, baseline 0.6 mg/dL) and microangiopathic haemolytic anaemia (Hb 7.2 g/dL, schistocytes on blood smear, LDH 1200 U/L, reticulocyte count 8%). Stool culture grows a Shigella species. Which Shigella species is most likely responsible for this haemolytic-uraemic syndrome (HUS)?

Explanation

## Diagnosis: Shigella dysenteriae Type 1 with HUS ### Clinical Presentation of HUS **Key Point:** Haemolytic-uraemic syndrome (HUS) is a triad of: 1. Microangiopathic haemolytic anaemia (schistocytes, elevated LDH, low Hb) 2. Thrombocytopenia (not mentioned but often present) 3. Acute kidney injury (elevated creatinine, oliguria) This patient has all three components, indicating a Shiga toxin-producing organism. ### Shigella Species & Toxin Production | Species | Shiga Toxin | HUS Incidence | Severity | Geographic Distribution | |---------|-------------|---------------|----------|-------------------------| | **S. dysenteriae type 1** | Shiga toxin (Stx) | 5–15% | Severe; epidemic | Endemic in India, Africa | | **S. flexneri** | Enterotoxins (no Stx) | Rare (<1%) | Mild–moderate | Most common in India | | **S. boydii** | No Shiga toxin | Very rare | Mild | Sporadic | | **S. sonnei** | No Shiga toxin | Very rare | Mild | Developed countries | **High-Yield:** Only *Shigella dysenteriae type 1* produces **Shiga toxin (Stx)**, which causes HUS. This is the most virulent Shigella species and causes epidemic dysentery with high mortality (10–15% in untreated cases). ### Pathogenesis of HUS in S. dysenteriae Type 1 ```mermaid flowchart TD A[Shigella dysenteriae type 1 infection]:::outcome --> B[Shiga toxin production]:::action B --> C[Toxin enters bloodstream]:::action C --> D[Binds to Gb3 receptor on endothelial cells]:::action D --> E[Inhibits protein synthesis]:::action E --> F[Endothelial cell damage]:::urgent F --> G[Platelet activation & microthrombi]:::action G --> H[Mechanical RBC destruction - schistocytes]:::outcome H --> I[Microangiopathic haemolytic anaemia]:::outcome F --> J[Renal capillary thrombosis]:::action J --> K[Acute kidney injury]:::outcome ``` **Clinical Pearl:** HUS in shigellosis is a medical emergency. Unlike STEC-HUS (E. coli O157:H7), Shiga toxin-mediated HUS occurs during the acute diarrhoeal phase, not after resolution of diarrhoea. ### Why This Patient? - Severe bloody diarrhoea with fever → invasive Shigella - Acute kidney injury + microangiopathic haemolytic anaemia → Shiga toxin-producing organism - School with poor sanitation → epidemic potential of *S. dysenteriae type 1* - Age 5 years → children are at highest risk for HUS **Mnemonic:** **SHIGA** = *Shigella dysenteriae* produces **Shiga toxin** → HUS, AKI, Anaemia [cite:Park 26e Ch 23; Harrison 21e Ch 139]