Pertussis is a highly contagious respiratory infection caused primarily by Bordetella pertussis, a small Gram-negative bacterium adapted to the human respiratory tract. It is commonly called whooping cough because severe disease can produce repeated coughing followed by an inspiratory “whoop”, although the characteristic sound is not always present, particularly in adults, vaccinated people and young infants. The illness is characterised by prolonged paroxysmal coughing and can persist for weeks. The organism produces several virulence factors and toxins that interfere with respiratory epithelial function and immune responses. Pertussis is particularly dangerous for infants because they can develop severe respiratory complications even when they have not yet developed the classic prolonged cough.
Transmission occurs predominantly through respiratory droplets and close-range respiratory exposure. B. pertussis attaches to ciliated respiratory epithelial cells, where its virulence factors contribute to impaired mucociliary clearance and local tissue effects. The incubation period is usually several days to a few weeks. Early disease can resemble an ordinary upper respiratory infection, which facilitates transmission because the diagnosis may not initially be recognised. Infectiousness is generally greatest during the early stages and decreases after effective antibiotic treatment has been given for an appropriate period. Immunity after vaccination or previous infection is not necessarily lifelong, which contributes to recurrent circulation of the organism.
Pertussis classically progresses through catarrhal, paroxysmal and convalescent phases, although real-world presentations vary. The catarrhal phase involves symptoms such as runny nose, mild cough and low-grade fever and can be difficult to distinguish from other respiratory infections. The paroxysmal phase features repeated rapid coughs that may occur in bouts, sometimes followed by vomiting or an inspiratory whoop. The convalescent phase involves gradually decreasing cough frequency and severity but can last for weeks or longer. The prolonged course explains the historical term “hundred-day cough” used in some contexts, although duration varies considerably.
Infants can present differently from older children and adults. Instead of a prominent whoop, very young infants may develop episodes of apnoea, cyanosis, poor feeding, exhaustion or respiratory distress. Pneumonia is an important complication and is a major contributor to severe outcomes. Other complications can include seizures, encephalopathy, dehydration, weight loss and complications related to forceful coughing. Adults and adolescents often experience a prolonged cough without the classic whoop, meaning that clinical suspicion can be important when a persistent cough occurs during community transmission. Severe disease is most strongly associated with very young infants who have not completed their primary immunisation course.
Diagnosis can involve polymerase chain reaction testing from a suitable respiratory specimen, particularly during the earlier stages when bacterial nucleic acid is detectable. Culture remains useful in appropriate circumstances and is highly specific, although sensitivity depends on timing, specimen quality and prior antibiotic exposure. Serological testing can have a role later in illness, particularly when prolonged cough makes direct detection less useful, although interpretation depends on age, vaccination history and the assay used. Because diagnostic yield changes over time, the choice of test should correspond to the stage of illness.
Antibiotics are primarily important for reducing transmission and may have the greatest effect when started early, before prolonged paroxysmal coughing is established. Once the characteristic cough is well developed, antibiotics may not rapidly reverse the symptoms because much of the clinical syndrome reflects established toxin and epithelial effects rather than ongoing extensive bacterial multiplication. Supportive management is particularly important in infants, who may require hospital care, respiratory monitoring, nutritional support and management of apnoea or hypoxia. Cough suppressants should not be assumed to provide effective treatment for severe Pertussis, and clinical management should be guided by age and severity.
Prevention depends on vaccination with pertussis-containing vaccines. Modern vaccines are commonly acellular and contain selected purified components of B. pertussis, usually combined with other antigens. Protection decreases over time, which contributes to the rationale for booster doses and vaccination during pregnancy in programmes that recommend it. Maternal immunisation can transfer antibodies to the fetus and provide early protection during the period when infants are most vulnerable. Post-exposure management can include antibiotics for selected close contacts, especially those at high risk of severe disease. High vaccination coverage reduces disease burden but does not prevent all infections or transmission because immunity can wane.
Pertussis should be distinguished from an ordinary nonspecific cough because its epidemiology, duration, complications and public-health implications differ. It is also distinct from diseases such as viral bronchiolitis and croup, although symptoms can overlap. The term whooping cough can be misleading because the whoop may be absent. Historically, Pertussis was a major cause of childhood morbidity and mortality before widespread vaccination. Its continued circulation reflects a combination of waning immunity, incomplete vaccination, transmission from older age groups and the ability of vaccinated individuals to experience milder but still transmissible infection.
