A 6-year-old child from rural India presents with a 2-week history of paroxysmal cough with inspiratory stridor (whooping sound) and post-tussive vomiting. Nasopharyngeal swab culture on Bordet–Gengou medium grows gram-negative coccobacilli. What is the drug of choice for treatment?

Explanation

## Pertussis: Antibiotic Management **Key Point:** Azithromycin is the first-line antibiotic for Bordetella pertussis infection in children and adults. ### Mechanism of Action Azithromycin is a macrolide antibiotic that inhibits bacterial protein synthesis by binding to the 50S ribosomal subunit. It achieves excellent intracellular and respiratory secretion penetration, making it ideal for pertussis treatment. ### Dosing in Pediatrics - **Azithromycin:** 10–12 mg/kg once daily for 5 days (max 500 mg/day) - Alternatively: 10 mg/kg on day 1, then 5 mg/kg daily for 4 days ### Clinical Efficacy - Most effective when given **early** (catarrhal or early paroxysmal stage) - Reduces duration of infectivity from ~3 weeks to ~5 days - Late administration (paroxysmal stage >2 weeks) has limited symptomatic benefit but still reduces transmission ### Prophylaxis in Contacts - **Azithromycin 10 mg/kg once daily for 5 days** is also the regimen of choice for post-exposure prophylaxis in unvaccinated or incompletely vaccinated contacts **High-Yield:** Azithromycin is preferred over erythromycin (older macrolide) because it has better tolerability, fewer drug interactions, and superior tissue penetration. | Feature | Azithromycin | Erythromycin | Amoxicillin | |---------|--------------|--------------|-------------| | **Efficacy in Pertussis** | Excellent | Good | Poor | | **GI Tolerance** | Better | Causes nausea/vomiting | N/A | | **Tissue Penetration** | Excellent | Good | Moderate | | **Drug Interactions** | Moderate | High | Low | | **First-line Status** | Yes | No (second-line) | No | **Clinical Pearl:** Even though B. pertussis is technically susceptible to amoxicillin in vitro, clinical efficacy is poor because amoxicillin does not achieve adequate intracellular concentrations in respiratory epithelium. **Warning:** Do not confuse pertussis treatment with diphtheria. Diphtheria (Corynebacterium diphtheriae) requires erythromycin or penicillin G + antitoxin; azithromycin is not standard for diphtheria.