Which organism is the most common causative agent of orbital cellulitis in children?

Explanation

## Microbiology of Orbital Cellulitis **Key Point:** *Haemophilus influenzae* type b (Hib) is the classic and most commonly cited causative organism of orbital cellulitis in children in standard Indian ophthalmology and NEET PG textbooks (Khurana's Comprehensive Ophthalmology, Parson's Diseases of the Eye). This reflects the epidemiological reality in resource-limited settings where Hib vaccination coverage remains incomplete. ### Organism-Specific Features | Organism | Age Group | Frequency | Source | Notes | | --- | --- | --- | --- | --- | | *Haemophilus influenzae* type b | Children (peak 2–5 yr) | Most common (classic/board answer) | Hematogenous spread, sinusitis | Gram-negative coccobacillus | | *Streptococcus pneumoniae* | All ages | Common | Sinusitis, bacteremia | Gram-positive diplococci | | *Staphylococcus aureus* | All ages | Common | Direct inoculation, sinusitis | Gram-positive cocci; MRSA emerging | | *Pseudomonas aeruginosa* | Immunocompromised | Rare | Trauma, contact lens wear | Gram-negative rod; poor prognosis | ### Pathogenesis Routes 1. **Hematogenous spread** — from upper respiratory tract or bacteremia (most common in Hib) 2. **Direct extension** — from paranasal sinusitis (especially ethmoid sinusitis) 3. **Traumatic inoculation** — from penetrating injury or surgery 4. **Dacryocystitis** — from infected lacrimal sac ### Epidemiological Nuance - **Pre-vaccination / India context:** Hib is the textbook most common cause of orbital cellulitis in children and remains the standard answer for NEET PG / INI-CET examinations, consistent with Khurana and Parson's. - **Post-Hib vaccination era (developed nations):** With widespread Hib immunization, *Streptococcus pneumoniae* and *Staphylococcus aureus* have become relatively more prevalent in countries with high vaccine coverage. Some recent Western literature now lists *S. aureus* as the leading isolate in post-vaccination cohorts. - **India-specific reality:** Hib vaccination was introduced into India's Universal Immunization Programme only in select states from 2011 onward and is not yet universally administered nationwide; therefore, Hib remains clinically and epidemiologically significant in the Indian paediatric population. **High-Yield:** For NEET PG and INI-CET, *Haemophilus influenzae* type b is the answer for "most common cause of orbital cellulitis in children." In post-vaccination Western settings, *S. aureus* and *S. pneumoniae* are increasingly common, but this does not change the standard Indian board answer. **Clinical Pearl:** Hib orbital cellulitis typically presents with rapid onset (hours to 1–2 days), high fever, proptosis, and chemosis — the hallmark of acute hematogenous infection in young children aged 2–5 years. Blood cultures are often positive, distinguishing it from sinogenic orbital cellulitis.