Bordetella Pertussis PCR
The causative agent of pertussis, also known as whooping cough, is Bordetella pertussis, or occasionally B. parapertussis. Bordetella species are fastidious gram-negative coccobacilli. Transmission occurs through direct contact with discharge from respiratory mucous membranes of an infected person. The incubation period of pertussis is commonly 7 to10 days after exposure but may range from 4 to 21 days. Pertussis is highly communicable, infecting up to 90% of susceptible contacts. From the onset of symptoms, the disease can take 6-8 weeks to resolve.
Pertussis can cause severe disease in very young children. It begins with mild upper respiratory tract symptoms and progresses to cough, and can further progress to severe paroxysms, often with a characteristic inspiratory whoop followed by vomiting. Fever is absent or minimal. In infants younger than six months, apnea is a common manifestation and whoop may be absent.
Immunity due to either infection or vaccination is not long lasting. Pertussis is thought to cause 12% to 26% of cough illness in adults. Adult infections should be suspected when an illness that begins with cold symptoms is prolonged and associated with a non-productive paroxysmal cough that worsens at night. Clinical diagnosis is complicated by the fact that the characteristic cough (whoop) is rarely observed in adults. Although mortality from this disease is low among adolescents and adults, these populations are a potential reservoir for pediatric infections. Pertussis can be very severe in young infants and early diagnosis is essential to limit complications and minimize disease transmission.
Making a specific diagnosis of pertussis can be challenging. PCR has become the test of choice, due to the rapid availability of results compared to culture and PCR’s capability of detecting non-viable bacteria. The sensitivity of PCR for detection of the organism from respiratory specimens has been reported to be 93%. However, both false-positive and false-negative results can occur with PCR.
Only patients with signs and symptoms consistent with pertussis should be tested to confirm the diagnosis. When possible, PCR should be done during the first 3 weeks of cough when bacterial DNA is still present in the nasopharynx. The quantity of bacterial DNA rapidly diminishes after the fourth week of cough, increasing the risk of obtaining a false-negative result. PCR testing after 5 days of antibiotic therapy is unlikely to be beneficial, because antibiotic therapy can also result in false-negative results. The optimal specimen for PCR testing is a nasopharyngeal aspirate or swab obtained from the posterior nasopharynx. Throat and anterior nasal swabs are insufficient for the recovery of B. pertussis. Asymptomatic contacts of confirmed cases should not be tested. Bordetella PCR testing is also inappropriate for a test of cure of confirmed cases.
Some pertussis vaccines have been found to contain PCR-detectable B. pertussis DNA (list available at www.cdc.gov/pertussis). Environmental sampling has identified B. pertussis DNA from these vaccines in clinic environments. Accidental transfer of the DNA from environmental surfaces to a clinical specimen can result in specimen contamination and false-positive results. Clinics should follow the CDC guidelines for preparation and administration of vaccine to prevent cross-contamination.
Nasopharyngeal swab specimens for Bordetella PCR should be collected on a rayon swab with aluminum shaft and placed in Stuart’s or Amies medium with charcoal for transport. Nasopharyngeal aspiration specimens are also acceptable for testing and are less likely to produce false positive results from contaminant DNA because the aspiration kit is a closed system. Some PCR assays detect both B. pertussis and B. parapertussis. Cross-reactivity with other Bordetella species, including B. holmseii and B. bronchiseptica is possible, however the prevalence of these organisms is less than1%.
Bordetella serology may be useful in patients having symptoms for more than two weeks. However, serologic diagnosis generally requires the comparison of acute and convalescent samples collected over a 4-week period. The presence of Bordetella-specific IgM antibody in a single specimen can indicate acute disease, but may persist for up to 6 months after infection. IgG antibodies are only of use in active infection when a rise in antibody level is seen in paired sera. A significant rise may not always be demonstrated as peak levels of IgG may be reached before the first sample is collected. Whooping cough due to Bordetella parapertusis is not detected.
Bordetella IgG is also useful to assess immune response to vaccine, or evaluate prior exposure to the organism. IgA does not appear to contribute significantly to the diagnostic sensitivity of paired sera for acute infection.
In summary, the test of choice for the diagnosis of pertussis is PCR from a nasopharyngeal specimen. Collection swabs and transport media are available from the performing laboratory. Serologic testing requires one red top tube.
