A 3-year-old boy from rural Maharashtra presents with a 2-day history of fever, irritability, and neck stiffness. On examination, he is febrile (39.2°C), has a positive Kernig sign, and appears acutely ill. CSF analysis shows: WBC 450/µL (90% neutrophils), protein 180 mg/dL, glucose 25 mg/dL (serum glucose 95 mg/dL), and Gram stain reveals small, pleomorphic gram-negative coccobacilli. Blood culture is pending. The child has not received the Hib vaccine. What is the most likely causative organism?

Explanation

## Diagnosis: Haemophilus influenzae Type b Meningitis ### Clinical Presentation The vignette presents a classic case of bacterial meningitis in an unvaccinated child with: - Age 3 years (peak incidence of Hib meningitis: 6 months–5 years) - Fever, neck stiffness, irritability (classic meningeal signs) - Positive Kernig sign (meningeal irritation) ### CSF Analysis Interpretation | Parameter | Finding | Significance | |-----------|---------|---------------| | WBC count | 450/µL | Bacterial meningitis (typically 100–10,000/µL) | | Differential | 90% neutrophils | Acute bacterial infection | | Protein | 180 mg/dL | Elevated (bacterial: 100–500 mg/dL) | | Glucose | 25 mg/dL | Low (CSF:serum ratio <0.4 = bacterial) | | Gram stain | Pleomorphic gram-negative coccobacilli | **Pathognomonic for Haemophilus** | **Key Point:** The pleomorphic gram-negative coccobacilli morphology on Gram stain is virtually diagnostic of *Haemophilus influenzae*. The organism appears as small, variable-sized rods (0.3–0.5 µm), often described as "pleomorphic" because it lacks a uniform shape. ### Why Haemophilus influenzae Type b? **High-Yield:** Type b (Hib) is the most invasive capsular serotype and the leading cause of bacterial meningitis in unvaccinated children worldwide. The capsule (polyribosyl phosphate) is essential for virulence and invasion across the blood–brain barrier. **Clinical Pearl:** The absence of Hib vaccination in this rural Indian child is a critical epidemiological clue. Hib meningitis has become rare in countries with universal Hib immunization (e.g., USA, Europe) but remains common in low-resource settings with incomplete vaccine coverage. ### Pathophysiology 1. Respiratory acquisition of *H. influenzae* type b 2. Bacteremia and seeding of meninges 3. Capsular antigen evades complement and phagocytosis 4. Inflammatory cascade → purulent meningitis **Mnemonic: HEMO** — *Haemophilus* **E**nds **M**eningitis in **O**lder infants (6 months–5 years, especially unvaccinated) ### Confirmatory Tests - **Culture:** Chocolate agar (requires X and V factors: hemin and NAD) - **Latex agglutination:** Detects type b capsular antigen in CSF - **PCR:** Rapid confirmation of *H. influenzae* and serotype ### Management - **Empiric therapy:** Ceftriaxone or cefotaxime (crosses BBB; covers Hib, *N. meningitidis*, *S. pneumoniae*) - **Dexamethasone:** Reduces hearing loss and neurological sequelae if given before or with first antibiotic dose - **Supportive care:** Fluid management, seizure prophylaxis [cite:Harrison 21e Ch 297]