Diphtheria spreading in the north

Watch out for signs of the highly infectious disease across the country, especially in vulnerable populations.

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Cases of toxigenic diphtheria are rising in north Queensland and experts warn that other states should be on alert for the potential spread of the disease.

“Clinicians across the state and nation should be vigilant for future cases,” the Queensland authors wrote in a letter in MJA.

More than a third of cases in Queensland in the last two years were in fully vaccinated people, nearly half were partially vaccinated and 10% were not up to date with their vaccinations. The average age of patients was 21, the authors reported.

Between 2020 and 2022 there were 29 reported cases of diphtheria in North Queensland – eight respiratory diphtheria and 38 cutaneous diphtheria – compared to 46 cases in the two decades prior.

In that preceding decade, C. diphtheriae accounted for 87% of cases, according to data from Queensland Health’s Notifiable Conditions Register.

“Since 2020, a genomically linked clone of tox gene-carrying diphtheria bacteria has spread across North Queensland,” the authors said.

Three cases presented with classic diphtheria, four with mild respiratory diphtheria and 22 with cutaneous diphtheria.

“All were acquired in Australia, and 86% of patients (25/29) identified as Aboriginal and/or Torres Strait Islander,” the authors said.

All cases were managed with penicillin or azithromycin, vaccination and diphtheria antitoxin when appropriate, they said.

“Azithromycin was given in most cases, as higher minimum inhibitory concentrations to penicillin have been observed with this clone.

“Household contacts were screened for symptoms, administered chemoprophylaxis with penicillin or azithromycin, and offered a diphtheria-containing vaccine if due.”

The authors said a North Queensland public health working group had been established to develop a consensus on public health management.

“This diphtheria outbreak, almost exclusively in Aboriginal and Torres Strait Islander communities, highlights the continuing impact of social determinants on disease in vulnerable populations,” they said.

“We recommend clinicians be aware of these cases and consider diphtheria among their differentials in patients from, or who recently travelled to, North Queensland.”

The authors said unvaccinated people were at highest risk of severe disease, including classic diphtheria, myocarditis and neuropathies.

“Vaccination remains imperative and timely vaccinations are essential,” they said.

MJA 2023, online 20 February