Initial reports came primarily from hospitalized cases. Pertussis is a highly contagious upper respiratory illness with symptoms that can linger six to ten weeks. As the disease progresses, coughing often comes in spasms interspersed with a characteristic whoop on inspiration of air and may be followed by vomiting or cyanosis. Fever is absent or minimal. In infants less than six months of age, apnea is a common manifestation, and the whoop may be absent.
Pertussis is most severe in young infants, who experience complications more frequently than adults.
Secondary bacterial pneumonia is the cause of most pertussis-related deaths. Other complications include seizures, encephalopathy, otitis media, and conditions resulting from pressure effects of severe paroxysmal coughing, including pneumothorax, epistaxis, subdural hematomas, hernias, and rectal prolapse. The classic symptoms of pertussis may be inapparent especially in adolescents and adults whose only symptom may be a cough lasting 14 days or longer. A high index of suspicion is therefore necessary for the early diagnosis of pertussis. Although pertussis vaccine usually provides adequate protection against pertussis, immunity wanes over time, leaving most adolescents and adults susceptible.