Classic pertussis (whooping cough) is caused by B. pertussis,a gram-negative pleomorphic bacillus with fastidious growth requirements. B. pertussis infects only humans and is transmitted person to person by coughing.
The incubation period is 7 to 10 days. Patients are most contagious during the first 2 weeks of cough. The peak incidence of pertussis in the United States is among those less than 4 months of age—infants too young to be completely immunized and most likely to have severe complications. The annual rate of pertussis was approximately 100 to 200 cases per 100,000 population in the prevaccination era; worldwide there are an estimated 30 to 50 million cases of pertussis and 300,000 deaths annually. In the United States, the incidence of pertussis decreased after the introduction of vaccine but has increased steadily since the 1980s with more than 27,000 cases reported in 2010. Infections in adolescents have also been rising, likely due to waning immunity from previous vaccines. Likewise when vaccination rates are low, pertussis rates increase: there was a steady decline in the incidence of pertussis until the late 1970s in the United Kingdom, when the incidence increased dramatically as the vaccination rate declined. Epidemics also occur in fully immunized patients, as vaccine-induced immunity wanes.
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Classic pertussis is seen in children between 1 and 10 years old. The progression of the disease is divided into catarrhal, paroxysmal, and convalescent stages. The catarrhal stage is marked by nonspecific signs (increased nasal secretions andlow-grade fever) lasting 1 to 2 weeks. The paroxysmal stage is the most distinctive stage of pertussis and lasts 2 to 4 weeks.Coughing occurs in paroxysms during expiration, causingyoung children to lose their breath. This pattern of coughingis needed to dislodge plugs of necrotic bronchial epithelialtissues and thick mucus. The forceful inhalation against a narrowed glottis that follows this paroxysm of cough producesthe characteristic whoop. Posttussive emesis is common. The convalescent stage is marked by gradual resolution of symptoms over 1 to 2 weeks. Coughing becomes less severe, andthe paroxysms and whoops slowly disappear. Although thedisease typically lasts 6 to 8 weeks, residual cough may persist for months, especially with physical stress or respiratoryirritants.
Infants may not display the classic findings, and the first sign in the neonate may be apnea. Young infants are unlikely to have the classic whoop, more likely to have central nervous system damage as a result of hypoxia, and more likely to have secondary bacterial pneumonia. Adolescents and adults with pertussis usually present with a prolonged bronchitic illness with persistent, nonproductive cough that often begins as a nonspecific upper respiratory tract infection. Generally adolescents and adults do not have a whoop with the cough, although they may have severe paroxysms. The cough may persist many weeks to months.
The diagnosis depends on isolation of B. pertussis or detection of its nucleic acids. Culture on specialized media is usually accomplished during the early phases of illness on specimens from nasopharyngeal swabs or aspirates but can be difficult to accomplish given the organism’s fastidious nature. Polymerase chain reaction, although potentially lacking sensitivity in previously immunized individuals, is likely as sensitive as culture and is available in many clinical laboratories. False positive results can occur. Direct fluorescent antibody staining is not recommended. Serologic tests are not useful for diagnosing acute infection but can be confirmatory in the convalescent phase of illness, though there are no commercial kits approved by the U.S. Food and Drug Administration.