Diphtheria: Your questions answered

In this explainer, PhD Candidate Lamali Sadeesh Kumar, from the Howden Group and Research Assistant in the Microbiological Diagnostic Unit Public Health Laboratory (Dept of Microbiology & Immunology at the Doherty Institute) answers some of the most common questions about diphtheria, including how it spreads, the symptoms it can cause and how everyone can play a role in preventing further infections.

Diphtheria is an acute bacterial infection caused by diphtheria toxin producing strains of Corynebacterium diphtheriae and less commonly, Corynebacterium ulcerans. Diphtheria has two main clinical presentations: respiratory and cutaneous diphtheria. Respiratory diphtheria affects the respiratory tract, including the nose, throat, and airways, whereas cutaneous diphtheria affects the skin.

Diphtheria is a vaccine preventable disease, and the diphtheria vaccine provides a strong protection against the severe effects of diphtheria toxin. Multiple primary and booster doses are required to develop and maintain immunity. However, vaccination does not consistently prevent carriage or transmission of C. diphtheriae, regardless of whether the strain produces toxin or not. For further information on vaccination against diphtheria, refer to the Australian Immunisation Handbook.

Person-to-person transmission of diphtheria occurs through respiratory droplets or nasal and throat secretions or fluid from infected skin lesions.

In respiratory diphtheria, early symptoms include sore throat, mild fever, malaise, and loss of appetite. This may progress to formation of a greyish-white throat membrane (pseudomembrane) causing symptoms such as difficulty swallowing, difficulty breathing and neck swelling. Respiratory diphtheria may also result in severe complications such as myocarditis, neuropathy, airway obstruction, and in some cases, death.

In cutaneous diphtheria, symptoms include superficial skin ulcers with well-defined edges and a bluish appearance, often on the hands or legs. These lesions are typically slow to heal and may become chronic if left untreated.  Cutaneous diphtheria is generally less severe than respiratory diphtheria.

In Victoria, testing for diphtheria is undertaken at the Microbiological Diagnostic Unit Public Health Laboratory (MDU PHL) at the Doherty Institute.

PCR-based molecular testing is used to detect the diphtheria toxin gene in Corynebacterium diphtheriae or Corynebacterium ulcerans isolates referred from diagnostic laboratories in Victoria. Results help to diagnose diphtheria and guide immediate public health and clinical response.

Diphtheria is treated with a combination of diphtheria antitoxin (DAT) and antibiotic therapy. Timely administration of DAT is crucial to prevent toxin-mediated complications, particularly in respiratory diphtheria. First-line antibiotics to treat diphtheria include penicillin or erythromycin.

People who have never, or not recently, been vaccinated will be offered vaccination.

Diphtheria outbreaks are rarely reported in Australia; however, an ongoing outbreak has been reported since October 2025. As of 20 May 2026, the Australian Centre for Disease Control reports 230 confirmed diphtheria cases across four Australian states and territories: Northern Territory, Western Australia, South Australia and Queensland. The outbreak includes both cutaneous and respiratory diphtheria, with cutaneous infections being more common and over 90% of cases have occurred among Aboriginal and/or Torres Strait Islander people.

Health authorities strongly encourage booster vaccination, particularly for individuals living in or travelling to high-risk areas and advise seeking prompt medical attention if symptoms of diphtheria are suspected.

• Diphtheria is a bacterial infection that manifests as respiratory or cutaneous disease, with respiratory diphtheria generally associated with more severe outcomes.
• Vaccination against diphtheria provides the best protection from severe complications of the disease.
• Person-to-person transmission of diphtheria occurs via respiratory droplets or direct contact with infected skin lesions.
• The ongoing outbreak highlights the importance of maintaining awareness of diphtheria in Australia, particularly for people in or travelling to high-risk areas.

This article was originally published by the Doherty Institute on 29 May 2026.

The Peter Doherty Institute for Infection and Immunity (Doherty Institute) is a joing venture between The University of  Melbourne and The Royal Melbourne Hospital