Microbiology and Pathogenesis

Bordetella pertussis, the etiologic agent of pertussis, is a fastidious gram-negative pleomorphic aerobic bacillus that attaches to ciliated epithelial cells of the nasopharynx, multiplies locally, and produces a wide array of toxins and biologically active products.

• B. parapertussis causes a similar, though typically milder, illness. B. holmesii causes up to 20% of pertussis-like syndromes.
• The most important toxin in B. pertussis is pertussis toxin, which has ADP ribosylating activity. The absence of this toxin in B. parapertussis may explain the milder illness.

Epidemiology

Pertussis is highly communicable. In households, attack rates are 80-100% among unimmunized contacts and 20% among immunized contacts.

• Pertussis remains an important cause of infant morbidity and death in developing countries, with ∼63,000 deaths worldwide of children <5 years of age in 2013.
• In the United States, although the incidence of pertussis has decreased by >95% because of universal childhood vaccination, >40,000 cases were reported in 2012, with increasing rates among adolescents and adults.
• Persistent cough of >2 weeks' duration in an adult may be due to B. pertussis in 12-30% of cases.
• Severe morbidity and mortality are restricted to infants <6 months of age.

Clinical Manifestations

After an incubation period of 7-10 days, a prolonged coughing illness begins. Symptoms are usually more severe in infants and young children.

• The initial symptoms (the catarrhal phase) are similar to those of the common cold (e.g., coryza, lacrimation, mild cough, low-grade fever, malaise) and last 1-2 weeks.
• The paroxysmal phase follows and lasts 2-4 weeks. It is characterized by a hallmark cough that occurs in spasmodic fits of 5-10 coughs each. Vomiting or a “whoop” may follow a coughing fit. Apnea and cyanosis can occur during spasms. Most complications occur during this phase.
• During the subsequent convalescent phase, coughing episodes resolve gradually over 1-3 months. For 6-12 months, viral infections may induce a recrudescence of paroxysmal cough.
• Disease manifestations are often atypical in adolescents and adults, with paroxysmal cough and the “whoop” being less common. Post-tussive emesis is the best predictor of pertussis as the cause of prolonged cough in adults.
• Lymphocytosis (an absolute lymphocyte count of >10⁵ /µL) suggests pertussis in young children, but is not common among affected adolescents and adults.

Diagnosis

• Cultures of nasopharyngeal secretions-the gold standard for diagnosis-remain positive in untreated cases of pertussis for a mean of 3 weeks after illness onset. Given that the diagnosis often is not considered until the pt is in the paroxysmal phase, there is a small window of opportunity for culture-proven diagnosis.
  • Secretions must be inoculated immediately onto selective media.
  • Results become positive by day 5.
• Compared with culture, PCR of nasopharyngeal specimens is more sensitive and yields positive results longer in both treated and untreated pts.
  • Reporting of pseudo-outbreaks of pertussis based on false-positive PCR results indicates the need for greater standardization.
• Although serology can be useful in pts with symptoms lasting >4 weeks, interpretation of results is complicated by late presentation for medical care and prior immunization.

Prevention

• Chemoprophylaxis with macrolides is recommended for household contacts of pts, especially if there are household members at high risk of severe disease (e.g., children <1 year of age, pregnant women); however, there is no evidence demonstrating that this regimen leads to a decrease in the incidence of clinical disease.
• In addition to the regular childhood immunization schedule, adolescents and adults should receive a one-time booster with an acellular vaccine. Some countries, including the United States and the United Kingdom, also recommend vaccination for pregnant women in the third trimester.