## Epidemiology of Bacterial Meningitis by Age **Key Point:** Haemophilus influenzae type b (Hib) remains the most common cause of bacterial meningitis in unvaccinated children aged 2 months to 5 years, particularly in resource-limited settings where Hib vaccination coverage is incomplete. ### Age-Related Causative Organisms | Age Group | Most Common Organism | Second Most Common | Notes | |-----------|---------------------|-------------------|-------| | 0–1 month | Group B Streptococcus, E. coli K1 | Listeria monocytogenes | Maternal flora | | 2 months–5 years | **Hib (unvaccinated)** | N. meningitidis | Vaccine-preventable | | 5–18 years | N. meningitidis | S. pneumoniae | Hib rare if vaccinated | | > 18 years | S. pneumoniae | N. meningitidis | Age-dependent immunity | **High-Yield:** In unvaccinated or incompletely vaccinated populations, Hib accounts for 40–50% of bacterial meningitis cases in children aged 2–5 years. ### Haemophilus influenzae Type b — Key Features - **Morphology:** Pleomorphic gram-negative coccobacillus ("pleomorphic" = variable rod-to-coccus morphology) - **Virulence factor:** Polysaccharide capsule (type b = ribose-ribitol phosphate, RRP) - **Vaccine:** Conjugate Hib vaccine (PRP-T, PRP-OMP) — part of routine immunization - **CSF findings in Hib meningitis:** Turbid fluid, elevated protein (100–500 mg/dL), low glucose (CSF:blood ratio < 0.4), pleocytosis with neutrophil predominance **Clinical Pearl:** The pleomorphic appearance on Gram stain is a diagnostic clue — Hib may appear as short rods, coccobacilli, or even cocci depending on growth phase and culture conditions. **Warning:** Do not confuse Hib meningitis with meningococcal meningitis — both present similarly, but epidemiology differs. Meningococcemia (petechial rash) is more common with N. meningitidis; Hib does not typically cause rash. [cite:Park 26e Ch 8, Textbook of Microbiology Ch 32]
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.