A 6-month-old female infant from rural Maharashtra presents with a 3-week history of paroxysmal cough. The mother reports that the cough is worst at night and is followed by a high-pitched inspiratory sound (whoop). The infant has not received any vaccinations. On examination, she is afebrile, with mild conjunctival injection and scattered crackles on auscultation. A nasopharyngeal swab culture on Bordet–Gengou medium grows small, gram-negative coccobacilli. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis of Pertussis ### Key Clinical Features **Key Point:** Pertussis (whooping cough) classically presents in three stages: catarrhal (1–2 weeks of mild upper respiratory symptoms), paroxysmal (2–8 weeks of characteristic paroxysmal cough with post-tussive vomiting or apnea), and convalescent (gradual recovery over weeks to months). This infant is in the **paroxysmal stage**, with the hallmark features: - Paroxysmal cough (sudden, violent bursts) - Post-tussive whoop (high-pitched inspiratory sound due to laryngeal spasm) - Absence of fever (cough is disproportionate to systemic illness) - Conjunctival injection (from increased intrathoracic pressure during coughing) ### Microbiological Identification | Feature | Bordetella pertussis | Bordetella parapertussis | |---------|---------------------|------------------------| | **Culture medium** | Bordet–Gengou (potato-glycerol agar) | Bordet–Gengou | | **Growth rate** | Slow (3–6 days) | Faster (2–3 days) | | **Gram stain** | Small, gram-negative coccobacillus | Small, gram-negative coccobacillus | | **Oxidase test** | Positive | Positive | | **Urease test** | Negative | Positive | | **Clinical severity** | More severe, longer duration | Milder, shorter duration | **High-Yield:** The organism grows on Bordet–Gengou medium as small, gram-negative coccobacilli. Pertussis is **urease-negative**, whereas parapertussis is **urease-positive** — this is the key biochemical differentiator. ### Pathophysiology 1. **Adhesion:** Filamentous hemagglutinin (FHA) and pertactin bind to ciliated respiratory epithelium. 2. **Toxin production:** Pertussis toxin (PTX) and tracheal cytotoxin cause ciliary paralysis and epithelial damage. 3. **Lymphocytosis:** PTX prevents lymphocyte homing to lymph nodes, causing marked peripheral lymphocytosis (often >15,000/μL). 4. **Cough paroxysms:** Result from airway inflammation and loss of mucociliary clearance. ### Diagnosis in This Case **Clinical Pearl:** In an unvaccinated 6-month-old with 3 weeks of paroxysmal cough, whoop, and conjunctival injection, pertussis is the most likely diagnosis. The isolation of small gram-negative coccobacilli on Bordet–Gengou medium confirms *Bordetella pertussis*. **Key Point:** Pertussis is most severe in infants <6 months and in unvaccinated children. Complications include apneic spells, secondary bacterial pneumonia, seizures (from hypoxia), and encephalopathy. ### Management Implications - **Antibiotic:** Azithromycin (first-line) or trimethoprim–sulfamethoxazole for 5 days. - **Supportive care:** Oxygen, nutritional support, monitoring for apneic episodes. - **Isolation:** Respiratory isolation for 5 days after starting antibiotics. - **Vaccination:** Complete the primary series (3 doses at 6, 10, 14 weeks) once acute illness resolves.