Diphtheria returns to regional, remote Australia

Diphtheria has re-emerged in Australia, with outbreaks confirmed in remote and Indigenous areas of the Northern Territory and Western Australia.

Additional cases have been recorded in Queensland and South Australia as vaccination coverage declines following the COVID pandemic.

Key Points

Outbreak at a glance

The NT has recorded 17 cases of respiratory diphtheria in the past month and 60 cases of cutaneous diphtheria over the past year.

In WA’s Kimberley region, 27 cases have been detected in the past month, with one-quarter of those identified as respiratory infections. Two cases have been reported in Queensland and one in South Australia this year.

Historically, diphtheria was a leading cause of childhood death globally. More than 4,000 Australians died from diphtheria between 1926 and 1935. Vaccination began nationally in the 1930s, and the disease has rarely been seen since the 1950s. However, vaccine coverage has waned since the COVID pandemic, and case numbers are rising.

Transmission and clinical course

Diphtheria is caused by a toxin produced by the bacteria Corynebacterium diphtheriae or Corynebacterium ulcerans. Infection can involve the nose, throat and airway (respiratory diphtheria) or the skin (cutaneous diphtheria). It spreads through respiratory droplets from coughs and sneezes, or by direct contact with fluid from infected skin sores.

Early symptoms of respiratory diphtheria include fever, sore throat, malaise and loss of appetite. In the following days, a greyish-white membrane may form over the throat and tonsils, making swallowing and breathing difficult. If unchecked, the membrane can expand.

Cutaneous diphtheria presents as chronic skin ulcers that fail to heal. While it is generally not fatal, the sores can serve as a bacterial reservoir. Transmission from these lesions can infect un-immunised or partially immunised children and adults, leading to respiratory disease.

The toxin associated with respiratory infections can also cause myocarditis (inflammation of the heart) and polyneuropathy (nerve paralysis) days or weeks after illness onset. Such complications occur only rarely in cutaneous infections.

Treatment and antitoxin constraints

Both respiratory and cutaneous diphtheria require prompt antibiotic therapy to clear the organism and limit transmission. In respiratory cases, diphtheria antitoxin may be required early to neutralise circulating toxin. However, once the toxin enters host cells, its effects are not reversible.

Diphtheria is vaccine-preventable. The diphtheria vaccine is delivered as part of a combined formulation that also protects against whooping cough and tetanus (DTP). It stimulates antibodies that protect against diphtheria toxin.

The vaccine is also given during each pregnancy, and a booster is recommended for adults around age 50. In outbreaks, targeted vaccination campaigns can reduce transmission and disease.

One month after completing the three-dose course in infancy, more than 99% of babies reach protective antibody levels that last through childhood. By middle age, at least half of people have low antibody levels that may not protect against disease; a booster dose restores immunity. The vaccine is safe and generally well-tolerated. Mild pain at the injection site and mild fevers are the most common side effects.

Coverage by jurisdiction and community

• Only 91.9 per cent of five year olds in the NT have received all the recommended doses of diphtheria vaccine, the lowest of all States and Territories
• Aboriginal and Torres Strait Islander children in the NT have higher coverage, with 95.35 per cent of five year olds vaccinated against diphtheria
• In WA, 92.2 per cent of five year olds — and 92.8 per cent of First Nations five year olds — are vaccinated against diphtheria

What to do now

To check diphtheria vaccination status, individuals can ask a health provider to access Australian Immunisation Records, or review records via the myGov app. For catch-up vaccinations, people are advised to see a GP, pharmacist or nurse at a community health clinic or Aboriginal medical centre. Catch-up vaccines are free for children and adolescents under 20.