A 6-month-old infant from rural India presents with a 2-week history of paroxysmal cough with a characteristic 'whooping' sound during inspiration. Nasopharyngeal culture is positive. What is the most common causative organism of this clinical presentation?

Explanation

## Causative Organism of Pertussis **Key Point:** Bordetella pertussis is the causative agent of classic pertussis (whooping cough), responsible for >90% of cases worldwide. ### Clinical Features Supporting B. pertussis 1. **Paroxysmal cough with inspiratory 'whoop'** — pathognomonic for B. pertussis infection 2. **Nasopharyngeal culture positivity** — B. pertussis is best isolated from nasopharyngeal secretions, especially in the catarrhal and early paroxysmal stages 3. **Age group (6 months)** — peak incidence of pertussis is in infants <1 year, particularly those unvaccinated or incompletely vaccinated ### Bordetella Species Comparison | Feature | B. pertussis | B. parapertussis | B. bronchiseptica | |---------|---|---|---| | **Cause of classic pertussis** | Yes (>90%) | Rare (<5%) | No | | **Whoop sound** | Characteristic | Milder/absent | Absent | | **Severity** | Severe, prolonged | Mild, shorter duration | Rare in humans | | **Vaccine coverage** | Included in DPT | Not included | Not included | | **Culture growth** | Slow (3–5 days) | Faster than B. pertussis | Faster than B. pertussis | **High-Yield:** B. pertussis produces pertussis toxin (PTX), which causes lymphocytosis and is responsible for the characteristic paroxysmal cough and systemic toxicity. B. parapertussis does not produce PTX, resulting in milder disease. **Clinical Pearl:** In unvaccinated or partially vaccinated infants, B. pertussis causes the most severe form of pertussis, with risk of apnea, seizures, and secondary bacterial pneumonia. Early recognition and isolation are critical. **Mnemonic:** **B-PER** = **B**ordetella **PER**tussis = **PER**tussis (whooping cough) [cite:Park 26e Ch 13]