• Clinical science



Diphtheria is an infectious disease caused by Corynebacterium diphtheriae, which is usually transmitted via respiratory droplets. The clinical features of diphtheria are caused by a toxin produced by C. diphtheriae after it colonizes the upper respiratory tract. Patients initially present with fever, malaise, and sore throat. Within a few days, a grayish white pseudomembrane develops over the tonsils, posterior pharyngeal wall, and/or larynx. Other manifestations include cervical lymphadenopathy, soft tissue swelling of the neck, stridor, and/or difficulty breathing as a result of partial airway obstruction. Systemic absorption of the toxin can result in myocarditis, acute tubular necrosis, and/or polyneuropathy. Even before culture reports come back positive, patients should be promptly treated with penicillin and antitoxins, as untreated diphtheria is associated with a high mortality rate. Since the introduction of routine immunization against diphtheria in the 1920s, the incidence of disease has decreased dramatically in the US.


  • Incidence: 0–5 cases/year
  • Most cases occur in patients 20 years of age or older.

Epidemiological data refers to the US, unless otherwise specified.


  • Pathogen: Corynebacterium diphtheriae
    • A gram-positive, non-sporulating, club-shaped bacillus, containing metachromatic granules (AKA volutin granules).
  • Route of infection
    • Droplet transmission
    • Less commonly through direct or indirect contact with open lesions (see extra information for more details on cutaneous diphtheria)
  • Infectious period: variable




Clinical features

Respiratory diphtheria

  • Incubation period: 2–5 days
  • The patient presents initially with prodromal symptoms: fever, malaise, and sore throat
  • 4–5 days after the onset of prodromal symptoms, symptoms due to the local and systemic effects of the toxin occur
    • Local features
      • Anterior nasal diphtheria: bloody rhinorrhea
      • Tonsillar and pharyngeal diphtheria
        • A grayish white pseudomembrane over the posterior pharyngeal wall, and/or tonsils
        • Any attempt to scrape off the pseudomembrane exposes the underlying capillaries and results in heavy bleeding.
        • Bull neck, which may result in airway obstruction
        • Foul mouth odor
      • Laryngeal diphtheria: difficulty breathing, inspiratory stridor
    • Systemic features



  • Cultures of pharyngeal swabs are used to confirm the diagnosis
    • Microscopic examination of pharyngeal swabs or culture isolates reveals multiple C. diphtheriae clustered together.
    • Culture media of choice are cystine-tellurite agar or Löffler medium
  • Once the culture reveals C. diphtheriae, one or both of the following tests are used to identify whether the strain is toxigenic:
  • Test for myocarditis: Conduct multiple ECGs and serial measurement of cardiac markers.

Therapy should be started immediately upon clinical suspicion, even before diagnostic confirmation of diphtheria!

Differential diagnoses

The differential diagnoses listed here are not exhaustive.


Administration of the antitoxin is a critical part of treatment, as the clinical features of diphtheria are not caused by the pathogen itself but rather by the exotoxin that C. diphtheriae produces!



Pre-exposure prophylaxis

Post-exposure prophylaxis

Post-exposure prophylaxis is indicated for close personal contacts and caretakers of a patient with diphtheria.

Diphtheria is a notifiable disease!