A 6-year-old child presents with a 2-week history of paroxysmal cough with a characteristic 'whoop' sound, post-tussive vomiting, and apnoeic spells. Nasopharyngeal swab culture on Bordet–Gengou medium is positive for Bordetella pertussis. What is the drug of choice for treatment?

Explanation

## Drug of Choice for Pertussis **Key Point:** Azithromycin is the first-line antibiotic for Bordetella pertussis infection in children and adults. ### Mechanism & Rationale Azithromycin (a macrolide) achieves excellent intracellular and respiratory tract penetration, eradicating B. pertussis from the nasopharynx and reducing communicability. It is effective across all stages of pertussis but is most beneficial when given in the **catarrhal or early paroxysmal stage** before toxin-mediated tissue damage occurs. ### Dosing in Pediatrics - **Azithromycin:** 10–12 mg/kg once daily for 5 days (or 10 mg/kg on day 1, then 5 mg/kg daily for 4 days) - **Alternative macrolides:** Erythromycin (40–50 mg/kg/day divided into 4 doses for 14 days) — less commonly used now due to GI intolerance and shorter half-life **Clinical Pearl:** Azithromycin is preferred over erythromycin in modern practice because of better tolerability, once-daily dosing, shorter course (5 days vs. 14 days), and lower incidence of pyloric stenosis in infants. **High-Yield:** Prophylaxis of close contacts (household, daycare, school) with azithromycin (same dose as treatment) within 21 days of symptom onset in the index case reduces secondary attack rates significantly. ### Why Macrolides Work 1. Intracellular penetration into respiratory epithelium 2. Inhibit bacterial protein synthesis 3. Eradicate nasopharyngeal carriage 4. Reduce transmission to susceptible contacts **Warning:** By the time the paroxysmal stage is well established (>3 weeks), antibiotics do not alter the clinical course but still reduce communicability — cough is due to toxin-mediated ciliary damage, not active infection. ### Comparison with Alternatives | Drug | Efficacy | Penetration | Tolerability | Duration | Notes | |------|----------|-------------|--------------|----------|-------| | **Azithromycin** | Excellent | Very good | Excellent | 5 days | **First-line** | | Erythromycin | Excellent | Good | Poor (GI upset) | 14 days | Older standard; risk of pyloric stenosis in infants <1 month | | Trimethoprim-sulfamethoxazole | Adequate | Moderate | Fair | 14 days | Second-line; resistance emerging | | Amoxicillin | Poor | Moderate | Good | — | **Not effective**; B. pertussis is naturally resistant | | Cefixime | Moderate | Moderate | Good | — | Less data; not preferred | **Key Point:** β-lactam antibiotics (amoxicillin, cephalosporins) are **NOT effective** against B. pertussis because the organism produces β-lactamase and has intrinsic resistance to penicillins. ### Summary Azithromycin is the **gold standard** for pertussis treatment and prophylaxis due to superior efficacy, tolerability, and short duration of therapy.