The causative agent of pertussis, also known as whooping cough, is Bordetella pertussis. Less often, pertussis is caused by infection with Bordetella parapertussis. Bordetella species are fastidious gram-negative coccobacilli.
Pertussis is very contagious. Transmission occurs through direct contact with respiratory droplets discharged by an infected person. If non-immune people are exposed to an infected person who is coughing and sneezing, 80% to 90% of them will become infected.
The incubation period of pertussis is commonly 7 to10 days after exposure but may range from 4 to 21 days. From the onset of symptoms, the disease can take 6 to 8 weeks to resolve.
Pertussis begins with mild upper respiratory tract symptoms and progresses to coughing episodes after 2 to 3 weeks of illness. Fever is absent or low grade. The disease can progress to severe coughing paroxysms, often with a characteristic inspiratory whoop as patients gasp for air. Severe coughing can lead to vomiting and subconjunctival hemorrhage. This stage of disease can last from 2 to 8 weeks. In infants younger than six months, apnea is a common manifestation and the characteristic 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 is 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. Asymptomatic contacts of confirmed cases should not be tested. Bordetella PCR testing is also inappropriate for a test of cure of confirmed cases.
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. 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%.
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.
Bordetella serology may be useful in patients having symptoms for more than two weeks. The presence of Bordetella-specific IgM antibody in a single specimen can indicate acute disease, but IgM may persist for up to 6 months after infection. IgG antibodies are not very useful in diagnosing an acute infection because paired acute and convalescent samples, collected over a 4-week period, must be compared. A significant rise may not always be demonstrated because peak levels of IgG may be reached before the first sample is collected.
Bordetella IgG is 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.
Serologic testing requires one red top tube of blood.
Reference
Nieves DJ, Heininger U, Bordetella pertussis, Microbiology Spectrum, 2016;4, #3, https://doi.org/10.1128/microbiolspec.ei10-0008-2015
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