Pertussis, or whooping cough, is an acute infectious disease caused by the bacterium Bordetella pertussis. Pertussis is primarily a toxin-mediated disease. The bacteria attach to the cilia of the respiratory epithelial cells, produce toxins that paralyze the cilia, and cause infelammation of the respiratory tract, which interferes with the clearing of pulmonary secretions. Pertussis antigens appear to allow the organism to evade host defenses, in the lymphocytosis is promoted but chemotaxis is impaired. Until recently it was thought that B pertussis did not invade the tissues. However, recent studies have shown the bacteria to be present in alveolar macrophaes.
The incubation period of pertussis is commonly 7-10 days, with a range of 4-21 days, and rarely may be as long as 42 days. The clinical course of the illness is divided into three stages. The first stage, the catarrhal stage, is characterized by the insidious onset of coryza (runny nose), sneezing, low-grade fever, and a mild, occasional cough, similar to the common cold. The cough gradually becomes more severe, and after 1-2 weeks, the second, or paroxysmal stage, egins. Fever is generally minimal throughout the course of the illness.
It is during the paroxysmal stage that the diagnosis of pertussis is usually suspected. Characteristically, the patient has bursts, or paroxysms, of numerous, rapid coughs, apparently due to difficults expelling thick mucus from the tracheobroncial tree. At the end of the paroxysm, a long inspiratory effort in usually accompanied by a characteristic high-pitched whoop. During such an attack, the patient may become cyanotic (turn blue). children and young infants, especialy, appear very ill and distressed. Vomiting and exhaustion commonly follow the episode. The person does not appear to be ill between attacks.
Paroxysmal attacks occur more frequeintly at night, with an average of 15 attacks per 24 hours. During the first 1 or 2 weeks of this stage, the attacks increase in frequency, remain at the same level for 2 to 3 weeks, and then gradually decrease. The paroxysmal stage usually lasts 1 to 6 weeks but may persist for up to 10 weeks. Infants yourger than 6 months of age may not have the strength to have a whop, but they do have daroxysms of coughing.
In the convalescent stage, recovery is gradual. The cough becomes less paroxysmal and disappears in 2 to 3 weeks. However, paroxysms often recur with subsequent respiratory infections for many months after the onset of pertussis.
Adolescents and adults and children partially protected by the vaccine may become infected with B. pertussis but may have milder disease than infants and young children. Pertussis infection in these persons may be asymptomatic, or present as illness ranging from mild cough illness to classic pertussis with persistent cough (i.e., lasting more than 7 days). Inspiratory whoop is not common.
Even though the disease may be milder in older persons, those who are infected may transmit the disease to other susceptible persons, including unimmunized or incompletely immunized infants. Older persons are often found to have the first case in a household with multiple pertussis cases, and are often the source of infection for children.
Culture is considered the gold standard laboratory test and is the most specific of the laboratory tests for pertussis. However, fastidious growth requirements make B. pertussis difficult to culture. The yield of cuture can be affected by speciment collection, transportation, and isolation techniques. Specimens from the posterior nasopharynx, no the throat, should be obained using Dacron or calcium alginate (not cotton) swabs. Isolation rates are highest during the first 3 to 4 weeks of illness (catarrhal and early paroxysmal stages). Cultures are variable positive (30%-50%) and may take as long as 2 weeks, so results may be tooo late for clinical usefulness. cultures are less likely to be positive if performed later in the course of illness (more than 2 weeks after cough onset) or on speciments from persons who have received antibiotics or have been vaccinated. Since adolescents and adults have often been coughing for serveral weeks before they seek medical attention, it is often too late for culture to be useful.
Because of the increased sensitivity and faster reporting of results of PCR, many laboratories are now using this method exclusively. PCR should be used in addition to, and not as a replacement for culture. No PCR product has been approved by the Food and Drug Administration (FDA), and there are no standardized protocols, reagents, or reporting formats for pertussis PCR testing. Consequently, PCR assays vary widely among laboratories. Specificity can be poor, with high rates of false-positive results in some laboratories. Like culture, PCR is also affected by specimen collection. An inappropriately otained nasopharyngeal swab will likely be negative by both culture and PCR. PCR is less affected by prior antibiotic therapy, since the organism does not need to be viable to be positive by PCR. Continued use of culture is essential for confirmation of PCR results.
Serologic testing could be useful for adults and adolescents who present late in the course of their illness, when both culture and PCR are likely to be negative. However, there is no FDA-approved diagnostic test. The currently available serologic tests measure antibodies that could result from either infection or vacination, so a positive serologic response simple means that the person has been exposed to pertussis by either recent or remote infection or by recent or remote vaccination. Since vaccination can indue both IgM and IgA antibodies (in addition to IgG antibidies), use of such serlogic assays cannot differentiate infection from vaccine response. At this time, serologic test results should not be relied upon for case confirmation of pertussis infection.
An elevated white blood cell count with a lymphacytosis is usually present in classical disease of infants. the absolute lymphocyte count often reaches 20,000 or greater. However, there may be no lymphocytosis in some infants and children or in persons with mild or modified cases of pertussis. More information on the laboratory diagnosis of pertussis is available at: http://www.cdc.gov/vaccines/pubs/surv-manual/default.pdf