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The causative agent of pertussis, also known as whooping cough, is Bordetella pertussis, or occasionally B. parapertussis. Bordetella species are fastidious gram-negative coccobacilli. The incidence of pertussis continues to rise in the U.S. According to Centers for Disease Control (CDC) data, both Missouri and Kansas have experienced significant increases in pertussis in 2005 (MMWR, Vol. 54, No. 43). Immunity due to either infection or vaccination is not long lasting. Pertussis is thought to be the cause of 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 infants and 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 transmission of the disease. From the onset of symptoms, the disease can take 6-8 weeks to resolve. Pertussis is highly communicable, infecting 80-90% of susceptible contacts. The incubation period is four to 21 days after exposure. Making a specific diagnosis of pertussis in patients with clinical evidence of infection can be challenging. All suspected cases of pertussis should have a nasopharyngeal aspirate or swab obtained from the posterior nasopharynx. Throat and anterior nasal swabs are insufficient for the recovery of B. pertussis. Because of the low yield of direct smears and cultures for the organism, PCR has become the preferred method for diagnosis of acute disease. The sensitivities for detection of the organism from respiratory specimens are reportedly as follows: culture, 15%; DFA, 52%; PCR, 93%. Saint Luke's Regional Laboratories refers Bordetella PCR testing to Mayo Medical Laboratories where testing is performed daily. Mayo's PCR assay detects both B. pertussis and B. parapertussis. Bordetella serology is both sensitive and specific, and is a reasonable alternative to diagnosis when collection of a nasopharyngeal specimen is not possible. However, diagnosis by serology 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. Specific 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. Special transport media and swabs are necessary for PCR. Serologic testing requires one serum gel tube. |
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