The incubation period is 2-5 days (range 1-10 days), and the onset of symptoms is gradual. Early symptoms of respiratory diphtheria include malaise, sore throat, difficulty in swallowing, loss of appetite, and a mild fever (rarely >101° F). If the larynx is involved, the affected person may become hoarse. Within 2–3 days, an adherent, gray membrane forms over the mucous membrane of the tonsils, pharynx, or both. Attempts to remove the membrane cause bleeding. In severe cases of respiratory diphtheria, cervical lymphadenopathy and soft tissue swelling in the neck give rise to a “bull-neck” appearance. Extensive membrane formation may result in life-threatening or fatal airway obstruction.
Diphtheria toxin can cause serious systemic complications including myocarditis and neuropathies, if it is absorbed from the site of infection. Cutaneous and nasal diphtheria are localized infections that are rarely associated with systemic toxicity. The case-fatality rate of respiratory diphtheria is 5%-10%.