The catarrhal state may be indistinguishable from a viral upper respiratory tract infection. The infection damages respiratory epithelium, producing respiratory obstruction and paroxysmal coughing, often with a characteristic whoop. There is little fever. Apnoea, seizures and encephalopathy may occur. Infants aged less than six months and adults often do not have the characteristic whoop.
Paroxysms frequently end with the expulsion of clear, tenacious mucus, often followed by vomiting.
Pneumonia is the most common cause of death. Fatal encephalopathy, probably hypoxic, and inanition from repeated vomiting occasionally occur. IgA in serum, or B. pertussis antigen in a nasopharyngeal specimen using immunofluorescence with a history of clinically compatible illness. It is a distressing and often serious illness, particularly in children under one year of age. The mortality rate is 0.5 per cent in infants under six months.