receive this new vaccine?6 6. What type of isolation precautions should have been used while this patient was in the hospital? What therapy (if any) should have been provided to health care workers in close contact with this patient prior to institution of appropriate precautions?
CASE 9 CASE DISCUSSION
1. The patient was infected with Bordetella pertussis, the causative agent of whooping cough. With classic whooping cough, children have paroxysmal coughing, which is a series of coughs during a single expiration. Paroxysms are often accompanied by a “whoop” sound in children due to rapid inspiration through a narrow trachea. (An audio file of a child with pertussis can be found at www.immunizationed.org.) Because of repetitive coughing and resulting disruption of breathing, children will have abnormal oxygen exchange and will often turn red and sometimes blue. The repetitive coughing may also result in vomiting or choking on respiratory secretions. Although the classic “whooping” sound was not described for this child, she did have bouts of coughing leading to increased respiration, decreased oxygenation, and posttussive vomiting. In infants <6 months old, apnea is more common than whooping inspirations. Further, the patient had a lymphocytosis, which is commonly seen in pertussis. Although the etiologic agent is a bacterium, the disease is toxin mediated, explaining the rise in lymphocyte count. Historically, the pertussis toxin (a key virulence factor of B. pertussis) has also been described as the “lymphocytosis-promoting factor.” Clinically, lymphocytosis, often as high as 70 to 80%, is routinely seen in patients with pertussis and is a distinguishing characteristic of this infection.
2. B. pertussis specifically binds to ciliated epithelial cells. This binding is mediated primarily by filamentous hemagglutinin, an important virulence factor of this organism. Since the nasopharynx is lined with ciliated epithelial cells, it is the most sensitive site for the detection of B. pertussis.
Culture has long been the gold standard for the laboratory diagnosis of pertussis owing to its superior specificity (~100%). However, there are many disadvantages to B. pertussis culture. First, the organism is very labile outside of the host. Second, it must be cultivated on specialized media such as Bordet-Gengou or Regan-Lowe agar. These attributes make the bacterium difficult to isolate. Third, it generally takes 7 to 10 days to isolate and identify B. pertussis from culture. In Fig. 9.1, we see an isolate of B. pertussis that grew after 7 days of incubation on a charcoal-containing medium, Regan-Lowe agar. In outbreak settings where B. pertussis can be rapidly spread from person to person, culture is too slow. Lastly, the clinical course of pertussis is complex (see answer 3), and the organism is generally only recovered during the first 2 weeks of illness. Sensitivity of culture during the first 2 weeks of pertussis is 30 to 60%, and it drops dramatically (1 to 3%) by the third week of illness. Sensitivity of culture is also negatively affected by antibiotic administration and prior vaccination. Nonetheless, the Centers for Disease Control and Prevention (CDC) recommends culturing of nasopharyngeal specimens during an outbreak so that specificity is preserved and isolates are obtained for susceptibility testing and epidemiologic studies.
Figure 9.1 Organism infecting this patient.
For many years, direct fluorescent-antibody assay (DFA) for B. pertussis was done. This assay takes ~2 hours, versus 7 to 10 days for culture, but it has a sensitivity of only 50 to 65%, and false-positive results may occur, especially when laboratorians are unaccustomed to reading these DFA smears. DFA is no longer in the CDC’s diagnostic algorithm for pertussis because of these limitations.
NAAT, and in particular PCR, has become the method of choice for diagnosing pertussis. Because PCR does not require that the organisms be alive, it is useful when specimens must be transported long distances. PCR is more rapid than culture, with results often available the same day the specimen was collected. PCR is more sensitive than culture and has a high negative predictive value. There are two FDA-cleared molecular products for the detection of B. pertussis. One is a 20-plex test that detects a number of respiratory viruses and bacteria simultaneously, while the other is a stand-alone test. Many laboratories use laboratory-developed NAATs for the detection of B. pertussis. The performance of these tests varies widely. Sensitivity and specificity are dependent on the target used for amplification, with the most sensitive tests targeting multicopy sequences and the most specific tests detecting multiple targets. The primary concern for PCR-based diagnosis of pertussis is the risk of false-positive results. False-positive PCR results have been the subject of “pseudo” outbreaks of pertussis that have been linked to cross-reacting Bordetella spp. (e.g., B. holmesii), laboratory contamination, and environmental contamination at collection. Interestingly, it has been reported that false-positive results can occur when specimens are collected in the same clinic room where pertussis vaccines (some of which contain genomic DNA) are administered. Since there is no perfect test for the diagnosis of pertussis, the CDC recommends that both culture and PCR be used diagnostically.
3. The clinical course of pertussis is defined by three stages: catarrhal, paroxysmal, and convalescent. The catarrhal phase lasts 1 to 2 weeks, but symptoms are often nonspecific and are similar to those of many respiratory viral illnesses (malaise, low-grade fever, rhinorrhea, and mild cough). Laboratory diagnosis is most sensitive at this phase, but laboratory testing (particularly in adolescents and adults) is often not performed. Even though pertussis is a toxin-mediated disease, appropriate antimicrobial therapy during the catarrhal stage decreases the organism load, thereby reducing the infectiousness of the patient, the duration and severity of symptoms, and the transmission rate. The paroxysmal phase is characterized by the paroxysmal cough, excessive mucus production, posttussive vomiting, and lymphocytosis that may last up to 6 weeks. This is the stage at which most children, adolescents, and adults are likely to seek medical attention and receive antimicrobial therapy. The damage that the B. pertussis cytotoxin causes—ciliostasis and death of the tracheal epithelial cells—is not reversed by the administration of an antibiotic. Thus, the cough persists. The last phase is the convalescent phase, characterized by a chronic cough that may last weeks to months. As with the paroxysmal phase, therapy given at this stage is not effective, with the exception of therapy for secondary bacterial pneumonia that develops as a complication.
Macrolide antibiotics (e.g., azithromycin, clarithromycin, and erythromycin) are the drugs of choice for treating pertussis. In addition to delay in administering antibiotics (as was the case with this child), reasons for a lack of response to therapy might include patient noncompliance. Erythromycin, in particular, is often associated with gastrointestinal intolerance. Secondary bacterial pneumonia, an occasional complication of pertussis, must also be considered in patients with persistent cough, particularly if the patient worsens clinically. Finally, the possibility that the organism is resistant to macrolides must be considered. Although macrolide-resistant B. pertussis isolates have been described, susceptibility surveys suggest resistance is still rare.
4. B. pertussis has many virulence factors that are responsible for mediating attachment to host cells and causing tissue damage. Pertussis toxin acts as both a secreted toxin and an adhesin working synergistically with filamentous hemagglutinin. Pertussis toxin belongs to the classic A-B family of ADP-ribosylating toxins (like cholera toxin and Shiga toxin). Additional toxins include adenylate cyclase-hemolysin, a cytotoxin that inhibits chemotaxis and induces apoptosis of macrophages; tracheal cytotoxin, which eliminates mucociliary clearance by ciliostasis and extrusion of ciliated cells and inhibits DNA synthesis; dermonecrotic toxin, which causes dermal necrosis and vasoconstriction; and lipopolysaccharide endotoxin, which has proinflammatory activity. Taken together, these pathogenic properties result in a grossly damaged respiratory epithelium with decreased mucociliary
