A 6-month-old infant from rural Maharashtra presents with a 3-week history of paroxysmal cough with the characteristic 'whooping' sound during inspiration. The child is unvaccinated. Which is the most common causative organism of pertussis in this age group?

Explanation

## Causative Organism of Pertussis **Key Point:** Bordetella pertussis is the most common and clinically significant causative organism of pertussis (whooping cough) worldwide, accounting for >95% of cases. ### Epidemiology - **B. pertussis:** Obligate human pathogen; causes epidemic pertussis - **B. parapertussis:** Accounts for 5–10% of cases; milder disease, shorter duration - **B. bronchiseptica:** Primarily an animal pathogen; rare in humans - **H. influenzae type b:** Causes other respiratory infections; NOT a cause of classic pertussis ### Clinical Features of B. pertussis Infection | Feature | B. pertussis | B. parapertussis | |---------|-------------|------------------| | Incidence | >95% of pertussis cases | 5–10% of cases | | Severity | Severe, prolonged (6–8 weeks) | Milder, shorter duration | | Paroxysmal cough | Yes, with "whoop" | Yes, but less pronounced | | Post-tussive vomiting | Common | Less common | | Complications | Apnea, seizures, encephalopathy | Rare | **High-Yield:** B. pertussis is a Gram-negative coccobacillus that produces pertussis toxin (PT) and filamentous hemagglutinin (FHA), which are the primary virulence factors responsible for the characteristic paroxysmal cough and systemic manifestations [cite:Park 26e Ch 28]. **Clinical Pearl:** In unvaccinated infants <6 months old, pertussis is most severe and life-threatening, with the highest risk of complications (apneic episodes, secondary bacterial pneumonia, seizures). This infant's presentation—unvaccinated, 3-week paroxysmal cough with "whoop"—is classic for B. pertussis. ### Pathogenesis 1. Respiratory droplet transmission → colonization of ciliated respiratory epithelium 2. Pertussis toxin (PT) → impaired ciliary clearance and systemic toxemia 3. Filamentous hemagglutinin (FHA) → adhesion and invasion 4. Paroxysmal cough phase → 2–8 weeks duration **Mnemonic:** **BPPF** — **B**ordetella **P**ertussis = **P**ertussis toxin + **F**ilamentous hemagglutinin.