Pertussis is caused by Bordetella pertussis and, less frequently, by Bordetella parapertussis. Both are fastidious gram-negative aerobic bacilli that require special media for growth.

The epidemiology of reported clinical pertussis is very different from the epidimiology of B. pertussis infection, resulting in much confusion relating to the epidemiology of pertussis. The epidemiology of reported pertussis is affected extensively by the degree of vaccine use. Pertussis occurs in all parts of the world, and humans are the only known hosts of B. pertussis. Transmission occurs from person to person by respiratory secretion droplets, and contagion is extremely high in nonimmunized populations. Spread occurs from children, adolescents, and adults with disease to susceptible contacts; asymptomatic carriers are not important in transmission. Adults with protracted cough illnesses (i.e., unrecognized pertussis) are an important source of B. pertussis infection among nonimmunized or partially immunized children.

In nonvaccinated populations, approximately 10% of reported cases occur in children younger than 1 year of age, 40% in children 1 to 4 years old, 45% in children 5 to 9 years old, and 5% in persons older than 9 years of age. In highly immunized populations today, such as in the United States, 30% of the reported cases occur in the first year of life, another 11% occur in children younger than age 5 years, 10% occur in children between 5 and 9 years of age, 29% occur in children and adolescents, and the remaining 20% are reported in adults.

Reported pertussis occurs in epidemic cycles with intervals of 2 to 5 years. In the prevaccine era in the United States, the average yearly reported attack rate was 157 per 100,000 persons. Immunization reduced this rate of reported pertussis to fewer than 1 per 100,000 in the 1970s. Since 1984, a modest increase in reported pertussis, from less than 1 to 8 cases per 100,000, has occurred. The major incidence of mortality from pertussis occurs among infants. Of reported cases of pertussis in the United States, 0.6% of the patients younger than 1 year
of age die. The clinical attack and mortality rates of pertussis are higher for female than for male patients.

In contrast with the cyclic nature of reported pertussis, the epidemiology of B. pertussis infection is not cyclic but endemic in adolescents and adults. B. pertussis infection is the cause of 13% to 20% of prolonged cough illnesses in adults. Serologic survey data suggest that infection rates in adolescents and adults are between 1% and 8%. The rate in adolescents and adults of cough illness caused by B. pertussis infection is in the range of 0.4% to 1.5%.

Pertussis is predominantly a disease of the ciliated epithelium of the respiratory tract. The B. pertussis organism has many unique, biologically active antigens, and studies have suggested roles for these antigens in the pathogenesis of disease. In the pathogenesis of pertussis, four steps are important: attachment, evasion of host defenses, local damage, and systemic manifestations.

After the airborne transmission of respiratory secretions containing B. pertussis occurs, the organisms attach to the cilia of the respiratory epithelial cells of the new host. Filamentous hemaggluttinin (FHA), pertussis toxin (PT), pertactin, and fimbriae (types 2 and 3) are B. pertussis antigens that are most important in the attachment process. Of these four proteins, pertactin is the most important adhesion. After attachment, the infection proceeds because of the profound adverse effect on host immune effector cell function by adenylate cyclase toxin and PT.

Tracheal cytotoxin is likely to be the main cause of local tissue damage of the ciliated respiratory epithelium, and this damage may be, at least in part, the cause of the paroxysimal cough. Pertussis is a unique disease in that systemic manifestations are rare. The characteristic lymphocytosis is caused by PT.

Classic pertussis is a lengthy illness, commonly lasting 6 to 12 weeks and characterized by three stages: catarrhal, paroxysmal, and convalescent. The catarrhal stage has its onset after an incubation period of 7 to 10 days. The onset of illness is subtle and resembles a mild upper respiratory tract infection with coryza, mild conjunctival injection, and mild cough. The upper respiratory symptoms continue, and, during the next 7 to 10 days, coughing becomes more persistent and frequent. Mild fever may occur during the catarrhal stage.

The paroxysmal stage is manifested by increasingly forceful coughing in the form of episodic paroxysms, which occur particularly frequently at night. In classic pertussis, episodes of repetitive severe coughing are followed by a single sudden massive inspiration. The characteristic whoop sound results from the forceful inhalation and a narrowed glottis. Each coughing paroxysm consists of 10 to 30 forceful coughs in a series. The patient’s face becomes increasingly cyanotic; the tongue protrudes to the maximum; and mucus, saliva, and tears stream from the nose, mouth, and eyes, respectively. Episodes of paroxysmal cough may be singular, or several may occur in rapid succession. Twenty to 200 or more sessions of paroxysmal cough may occur each day. The paroxysmal episodes are exhausting, and young children appear apathetic and dazed after attacks. Paroxysms are precipitated by eating, drinking, and any physical activity. Between attacks, patients usually show few signs of illness, and fever is not characteristic of uncomplicated cases. In young infants, apnea and bradycardia may accompany coughing fits, and a whoop is less likely to occur after a paroxysm.

After the paroxysmal stage, which lasts from 1 to 4 weeks or more, the convalescent stage is heralded by a lessening in the severity and frequency of paroxysms. The duration of the convalescent stage varies. Paroxysmal-type coughing often reoccurs for 6 months or more after a child has recovered from pertussis in association with other respiratory infections. Weight loss or failure to gain weight is a conspicuous feature of severe pertussis, especially in infants. Studies indicate that only 60% of pertussis cases in children have the classic picture; 40% of children have milder disease, with a total duration of cough of less than 4 weeks and generally less frequent and less severe paroxysms. Children in whom vaccine has failed also tend to have less severe disease.

Complications of pertussis occur commonly and can be grouped into three categories: respiratory, central nervous system, and secondary pressure effects. The rate of complications is inversely related to age. Bronchopneumonia, the most common complication, is caused by secondary infection with common respiratory pathogens (i.e., Haemophilus influenzae, Streptococcus pneumoniae, S. pyogenes, and Staphylococcus aureus), or it can be caused by a more extensive B. pertussis infection. If pneumonia is the result of secondary infection, significant fever and tachypnea are the usual findings. Other respiratory complications include atelectasis, bronchiectasis, interstitial or subcutaneous emphysema, and pneumothorax. Although atelectasis may persist for months after illness, carefully performed follow-up studies have not demonstrated permanent pulmonary sequelae. Young infants with B. pertussis pneumonia often have pulmonary hypertension, which often leads to death in spite of aggressive respiratory management. Otitis media is a frequent complication, especially in infants.