These nonspecific symptoms are followed by paroxysms of coughing and whooping that persist for six weeks or more. Efforts to expel thick sputum may end in vomiting. Infection by adenovirus, Chlamydia , and He mophilus may cause a similar illness. Pertussis can be severe, particularly in the very young, and often requires hospitalization. Periods of apnea occur in 40 percent of patients younger than one year.
Seizures occur in about two percent of patients, most of whom are younger than six months.
Encephalopathy is rare. The major causes of mortality are encephalopathy, which is caused by hypoxia or pertussis toxin, and bacterial superinfection. Although trimethoprim-sulfamethoxazole, tetracycline, and erythromycin have been used to treat acute infections, their effectiveness is difficult to document. Erythromycin has been used for chemoprophylaxis in close contacts younger than 18 months and in incompletely immunized close contacts younger than seven years. However, erythromycin does not provide complete prophylaxis to exposed individuals.