Pertussis is an infection caused by the gram-negative bacteria Bordetella pertussis. In children it often presents with respiratory symptoms, including the classic whooping cough, but in neonates it tends to have an atypical and more severe presentation.
The overall incidence of pertussis has been increasing in recent years, with the highest incidence seen in infants <6 months of age in the United States. There are an estimated 300,000 deaths due to pertussis worldwide each year, mostly affecting young infants. The incidence of pertussis in pregnant women is thought to be similar to that of the general population.
Transmission of Bordetella pertussis occurs through the respiratory route via close contact with respiratory secretions or aerosolized droplets. The parents of infants are estimated to be the source of infection in 25% of cases, and household members are thought to be the source in about 80% of cases. Bordetella pertussis produces multiple toxins including the pertussis toxin, which inhibits neutrophil migration to the lungs, and tracheal cytotoxin, which damages cilia in the respiratory epithelium via a nitric oxide synthase (NOS)-dependent pathway. The lymphocytosis seen in cases of pertussis is attributed to the potent mitogen, lymphocytosis-promoting factor. The aggregation of leukocytes in the pulmonary circulation is thought to cause the refractory pulmonary hypertension seen in severe cases of neonatal pertussis.
Infants <6 months of age are at highest risk for severe pertussis and complications or death from pertussis, especially if they are unvaccinated. Other risk factors include preterm (<37 weeks' gestation) and low birthweight infants. Infection during pregnancy does not appear to increase either maternal or fetal morbidity.
The 3 phases of classic pertussis, namely catarrhal (1–2 weeks), paroxysmal (2–6 weeks), and convalescent (2–6 weeks), are not typically observed in infants. Neonatal cases tend to present with paroxysmal cough, gagging, bradycardia, gasping, apnea, and cyanotic spells, but not fever and tachypnea. They do not have the characteristic “whoop” due to lack of prolonged inspiratory effort at the end of a paroxysm. Infants <6 months of age tend to have a shorter catarrhal and longer convalescent phase. The following complications may be observed in neonates and young infants with pertussis:
Secondary infections. Pertussis can be complicated by secondary infections such as pneumonia (22–25% of cases), meningoencephalitis, and otitis media.
Ophthalmologic complications. The forceful coughing paroxysms characteristic of pertussis can also result in ophthalmologic complications such as subconjunctival, scleral, and rarely retinal hemorrhages.
Central nervous system manifestations. Intraventricular and subarachnoid hemorrhages may result from increased intracranial pressure secondary to the Valsalva effect of paroxysmal coughing. Seizures (2–4% of cases) are attributed to hypoxemia from apnea or relentless coughing, but may also be due to hyponatremia secondary to pneumonia-induced syndrome of inappropriate antidiuretic hormone secretion. Encephalopathy is estimated to occur in 0.5–1% of cases and usually occurs during the paroxysmal stage. It can present with fever, convulsions, focal neurological signs including blindness and deafness, pareses and plegias, and altered mental state progressing to coma. Pertussis encephalopathy is fatal in one-third of patients and leads to neurodevelopmental delay in another third of patients.
Respiratory complications. Infants with pertussis are at increased risk for severe pulmonary hypertension due to pulmonary vasoconstriction from hypoxia and acidosis secondary to recurrent prolonged apnea, as well as restriction of pulmonary blood flow from leukocyte thrombi. Neonates with pertussis have a greater need for mechanical ventilation due to frequent apnea, respiratory compromise during paroxysms of coughing, and pulmonary hypertension.