Pertussis is an acute infection of the respiratory tract caused by Bordetella pertussis. The name pertussis means “violent cough,” which aptly describes the most consistent and prominent feature of the illness. The inspiratory sound made at the end of an episode of paroxysmal coughing gives rise to the common name for the illness, “whooping cough.” However, this feature is variable: it is uncommon among infants ≥6 months of age and is frequently absent in older children and adults. The Chinese name for pertussis is “the 100-day cough,” which accurately describes the clinical course of the illness. The identification of B. pertussis was first reported by Bordet and Gengou in 1906, and vaccines were produced over the following two decades.
Of the 10 identified species in the genus Bordetella, only three are of major medical significance. B. pertussis infects only humans and is the most important Bordetella species causing human disease. B. parapertussis causes an illness in humans that is similar to pertussis but is typically milder; co-infections with B. parapertussis and B. pertussis have been documented. B. bronchiseptica is an important pathogen of domestic animals that causes kennel cough in dogs, atrophic rhinitis and pneumonia in pigs, and pneumonia in cats. Both respiratory infection and opportunistic infection due to B. bronchiseptica are occasionally reported in humans. Two additional species, B. hinzii and B. holmesii, are unusual causes of bacteremia; both have been isolated from patients with sepsis, most often from those who are immunocompromised.
Bordetella species are gram-negative pleomorphic aerobic bacilli that share common genotypic characteristics. B. pertussis and B. parapertussis are the most similar of the species, but B. parapertussis does not express the gene coding for pertussis toxin. B. pertussis is a slow-growing fastidious organism that requires selective medium and forms small glistening bifurcated colonies. Suspicious colonies are presumptively identified as B. pertussis by direct fluorescent antibody testing or by agglutination with species-specific antiserum. B. pertussis is further differentiated from other Bordetella species by biochemical and motility characteristics.
B. pertussis produces a wide array of toxins and biologically active products that are important in its pathogenesis and in immunity. Most of these virulence factors are under the control of a single genetic locus that regulates their production, resulting in antigenic modulation and phase variation. Although these processes occur both in vitro and in vivo, their importance in the pathobiology of the organism is unknown; they may play a role in intracellular persistence and person-to-person spread. The organism's most important virulence factor is pertussis toxin, which is composed of a B oligomer–binding subunit and an enzymatically active A protomer that ADP-ribosylates a guanine nucleotide-binding regulatory protein (G protein) in target cells, producing a variety of biologic effects. Pertussis toxin has important mitogenic activity, affects the circulation of lymphocytes, and serves as an adhesin for bacterial binding to respiratory ciliated cells. Other important virulence factors and adhesins are filamentous hemagglutinin, a component of the cell wall, and pertactin, an outer-membrane protein. Fimbriae, bacterial appendages that play a role in bacterial attachment, are the major antigens against which agglutinating antibodies are directed. These agglutinating antibodies have historically been the primary means of serotyping B. pertussis strains. Other virulence factors include tracheal cytotoxin, which causes respiratory epithelial damage; adenylate cyclase toxin, which impairs host immune-cell function; dermonecrotic toxin, which may contribute to respiratory mucosal damage; and lipooligosaccharide, which has properties similar to those of other gram-negative bacterial endotoxins.
Pertussis is a highly communicable disease, with attack rates of 80–100% among unimmunized household contacts and 20% within households in well-immunized populations. The infection has a worldwide distribution, with cyclical outbreaks every 3–5 years (a pattern that has persisted despite widespread immunization). Pertussis occurs in all months; however, in North America, its activity peaks in summer and autumn.
In developing countries, pertussis remains an important cause of infant morbidity and death. The reported incidence of pertussis worldwide has decreased as a result of improved vaccine coverage. However, coverage rates are still <50% in many developing nations (Fig. 148-1); the World Health Organization (WHO) estimates that 90% of the burden of pertussis is in developing regions. In addition, overreporting of immunization coverage and underreporting of disease result in substantial underestimation of the global burden of pertussis. The WHO estimates that there were 254,000 deaths from pertussis among children in 2004.
Before the institution of widespread immunization programs in the developed world, pertussis was one of the most common infectious causes of morbidity and death. In the United States before the 1940s, between 115,000 and 270,000 cases of pertussis were reported annually, with an average yearly rate of 150 cases per 100,000 population. With universal childhood immunization, the number of reported cases fell by >95%, and mortality rates decreased even more dramatically. Only 1010 cases of pertussis were reported in 1976 (Fig. 148-2). After that historic low, rates of pertussis slowly increased, peaking at >25,000 cases annually in 2004 and 2005. In 2007, 10,454 cases of pertussis were reported in the United States.
Pertussis incidence (per 100,000 population) by year—United States, 1976–2006. [From the Centers for Disease Control and Prevention, MMWR Morb Mortal Wkly Rep 55(53):60, 2008.]
Although thought of as a disease of childhood, pertussis can affect people of all ages and is increasingly being identified as a cause of prolonged coughing illness in adolescents and adults. In unimmunized populations, ...