FeedbackThe year 2006 marks the 100-year anniversary of the discovery that the bacterium Bordetella pertussis is the causative agent of whooping cough. In the early- to mid-1900's, pertussis was one of the most common childhood diseases and a major cause of childhood mortality in the United States.
After the introduction of pertussis vaccine in the 1940s, pertussis cases decreased from the average incidence of 150/100,000 to only 1/100,000 of the US population in 1980. However, since the 1980s, the number of cases has been steadily climbing. The pertussis vaccine is far from perfect and cases have been appearing in fully immunized children and adults.
Cases among adults are particularly problematic as the presentation is atypical, with the patient having little more than a chronic cough. Infected adolescents and adults may introduce pertussis into households where susceptible preschool-age children could be exposed.
Laboratory Diagnosis
The diagnosis of pertussis is usually based on a characteristic history and physical examination. However, laboratory tests may be useful with young infants, atypical cases and cases modified by vaccine or previous antibiotic therapy. B. pertussis specifically binds to ciliated epithelial cells. Since the nasopharynx is lined with ciliated epithelial cells, culture of this site has a higher yield than culture from any other specimen source. However, culture is slow; it may take as long as 10 days to isolate this organism. Therefore, it is of limited value in an outbreak setting where the organism can be rapidly spread from person to person via inhalation.
Direct fluorescent-antibody (DFA) for B. pertussis yields a quicker response than culture, but it has a sensitivity of only 50 to 65%, and false-positive results occur. Ordering DFA for pertussis is not recommended.
Polymerase Chain Reaction (PCR) has become the method of choice for diagnosing pertussis. This is especially important in settings where specimens may be delayed in transport, because such transport compromises the sensitivity of culture but not the sensitivity of PCR. PCR is rapid (usually less than 48 hours turn-around-time), is far more sensitive than culture, and has a high negative predictive value. False positives may occasionally occur because of cross-reactivity with organisms similar to B. pertussis. In some situations, it is recommended that culture for B. pertussis be performed in addition to PCR so that the organism is available for susceptibility testing and molecular epidemiology studies.
The optimal specimen for pertussis is a nasopharyngeal swab collected during the first two weeks of disease. A separate NP swab should be collected for PCR and culture.
