Bordetella pertussis and Bordetella parapertussis are the human pathogens of this genus. The former causes the disease pertussis (also known as whooping cough), and the latter causes a mild pertussis-like illness. Whooping cough is a highly contagious disease and a significant cause of morbidity and mortality worldwide (51 million cases and 600,000 deaths each year). Members of the genus Bordetella are aerobic. They are small, encapsulated coccobacilli that grow singly or in pairs. They can be serotyped on the basis of cell-surface molecules including adhesins and fimbriae.

A. Epidemiology

 The major mode of transmission of Bordetella is via droplets spread by coughing, but the organism survives only briefly outside the human respiratory tract. The incidence of whooping cough among different age groups can vary substantially, depending on whether active immunization of young children is widespread in the community. In the absence of an immunization program, disease is most common among young children (ages 1 to 5 years). Adolescent and adult household members, whose pertussis immunity has disappeared, are an important reservoir of pertussis for young children.

B. Pathogenesis

B. pertussis binds to ciliated epithelium in the upper respiratory tract . There, the bacteria produce a variety of toxins and other virulence factors that interfere with ciliary activity, eventually causing death of these cells (Figure 1).

Fig1. Toxins and virulence factors produced by Bordetella pertussis.

 C. Clinical significance

 The incubation period for pertussis generally ranges from 1 to 3 weeks (Figure 2). The disease can be divided into two phases: catarrhal and paroxysmal.

1. Catarrhal phase: This phase begins with relatively nonspecific symptoms, such as rhinorrhea, mild conjunctival infection (hyperemia, or bloodshot conjunctivae), malaise, and/or mild fever, and then progresses to include a dry, nonproductive cough. Patients in this phase of disease are highly contagious.

2. Paroxysmal phase: With worsening of the cough, the paroxysmal phase begins. The term “whooping cough” derives from the paroxysms of coughing followed by a “whoop” as the patient inspires rapidly. Large amounts of mucus may be produced. Paroxysms may cause cyanosis and/or end with vomiting. [Note: Whooping may not occur in all patients.] Pertussis typically causes leukocytosis that can be quite striking as the total white blood cell count sometimes exceeds 50,000 cells/μL (normal range = 4,500–11,000 white blood cells/μL), with a striking predominance of lymphocytes. Following the paroxysmal phase, convalescence requires at least an additional 3 to 4 weeks. During this period, secondary complications, such as infections (for example, otitis media and pneumonia) and central nervous system (CNS) dys function (for example, encephalopathy or seizures), may occur. Disease is generally most severe in infants.

 D. Laboratory identification

 Presumptive diagnosis may be made on clinical grounds once the paroxysmal phase of classic pertussis begins. Pertussis may be suspected in an individual who has onset of catarrhal symptoms within 1 to 3 weeks of exposure to a diagnosed case of pertussis. Culture of B. pertussis on Bordet-Gengou or Regan-Lowe media (selective and enrichment media) from the nasopharynx of a symptomatic patient supports the diagnosis. The organism produces pinpoint colonies in 3 to 6 days on selective agar medium (for example, one that contains blood and charcoal), which serves to absorb and/or neutralize inhibitory substances and is supplemented with antibiotics to inhibit growth of normal flora. More rapid diagnosis may be accomplished using a direct fluorescent antibody test to detect B. pertussis in smears of nasopharyngeal specimens. Serologic tests for antibodies to B. pertussis are primarily useful for epidemiologic surveys.

E. Treatment

Erythromycin is the drug of choice for infections with B. pertussis, both as chemotherapy (where it reduces both the duration and severity of disease) and as chemoprophylaxis for household contacts (see Figure 2). For erythromycin treatment failures, trimethoprim-sulfamethoxazole is an alternative choice. Patients are most contagious during the catarrhal stage and during the first 2 weeks after onset of coughing. Treatment of the infected individuals during this period limits the spread of infection among household contacts.

Fig2. Summary of Bordetella disease. 1  Indicates first-line drugs;2 indicates alternative drugs.

 F. Prevention

 Pertussis vaccine is available and has had a significant effect on lowering the incidence of whooping cough. It contains proteins purified from B. pertussis and is formulated in combination with diphtheria and tetanus toxoids . To protect infants who are at greatest risk of life-threatening B. pertussis disease (Figure 3), immunization is generally initiated when the infant is 2 months old. Widespread use of pertussis vaccine was followed by a dramatic decrease in reported pertussis in the United States for decades, until the middle of the first decade of the 21st century. However, because neither disease- nor vaccine-induced immunity is durable, there has been a resurgence, with reported cases in 2010 the highest since the 1950’s. A new vaccine, licensed for adolescents and adults, and vaccination of women even during the last trimester of pregnancy, may help to reduce reported pertussis in the United States.

Fig3. Incidence of pertussis by age group, United States.

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