Whooping cough

Overview

•Caused by Bordetella pertussis - gram-negative coccobacillus; transmitted via respiratory droplets; secondary attack rate up to 90% in unimmunised contacts

•Incubation 7-10 days (range 5-21 days)

•Most serious disease and death in infants under 6 months (too young to complete primary vaccination)

Presentation

•Classic three-phase illness: catarrhal (1-2 weeks, coryzal) → paroxysmal (weeks 2-6, violent cough fits) → convalescent (weeks to months, gradual resolution)

•Inspiratory whoop - forceful gasp against partially closed glottis after rapid expiratory coughs

•Post-tussive vomiting - very characteristic; can cause weight loss

•Well appearance between episodes - key feature distinguishing from other respiratory illness

•Subconjunctival haemorrhage - from raised intrathoracic pressure

•Adults/adolescents - whoop often absent; presents as prolonged cough >3 weeks ('100-day cough')

🚨

In infants under 6 months the whoop is often absent - replaced by apnoea and cyanosis. Any infant with paroxysmal cough + apnoea must be assessed urgently for pertussis.

Investigations

•Weeks 1-3 of cough: nasopharyngeal/per-nasal swab for PCR - most sensitive, rapid, does not require viable organisms

•Culture - gold standard for confirmation but lower sensitivity; requires viable organisms; most useful in early catarrhal phase

•After week 3: pertussis serology (anti-PT IgG) - organism no longer reliably detectable; single elevated titre in unvaccinated patient or significant rise is diagnostic

•FBC - characteristic marked lymphocytosis (often >10 x10⁹/L in young children); pertussis toxin blocks lymphocyte egress from blood

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PCR early (weeks 1-3); serology late (after week 3). Sending a swab after 3 weeks will often be falsely negative - request serology instead.

Management

🥇 First-line

•clarithromycin or azithromycin orally for 7 days - macrolides are first-line; start in catarrhal phase to reduce severity and infectivity

🥈 Second-line

•erythromycin orally for 14 days (avoid in infants <1 month); co-trimoxazole for macrolide allergy (avoid in pregnancy and infants <6 weeks)

•Supportive care: ensure hydration and nutrition; hospitalise infants <6 months, those with apnoeas, severe paroxysms, or poor feeding

⚠️

Do NOT use erythromycin in infants under 1 month - associated with infantile hypertrophic pyloric stenosis. Use azithromycin instead.

Complications

•Pneumonia - most common serious complication; leading cause of pertussis-related death

•Apnoea and hypoxia - most dangerous in infants; risk of hypoxic brain injury

•Pertussis encephalopathy - rare; seizures, altered consciousness; from hypoxia and direct toxin effects

•Others: rib fractures (adults), pneumothorax, weight loss/failure to thrive in infants

Public health actions

•Notifiable disease - notify local Health Protection Team (HPT)/UKHSA on clinical suspicion; do not wait for lab confirmation

•Exclusion - exclude from school/nursery until 48 hours of effective antibiotic treatment completed, or 21 days from cough onset if untreated

•Post-exposure prophylaxis (PEP) - offer macrolide to close contacts at high risk within 21 days of exposure: infants <1 year, pregnant women in third trimester, healthcare workers in high-risk settings

Prevention (vaccination)

•Primary series - 6-in-1 vaccine (DTaP/IPV/Hib/HepB) at 8, 12, and 16 weeks

•Pre-school booster - 4-in-1 vaccine (DTaP/IPV) at 3 years 4 months

•Maternal vaccination - offered at 16-32 weeks of pregnancy (ideally 20 weeks); maternal IgG crosses placenta from ~28 weeks, protecting neonate before vaccination - single most impactful intervention for reducing infant deaths

•Acellular vaccines produce less durable immunity than older whole-cell vaccines - waning immunity in adults creates a transmission reservoir