Clinician alert: toxigenic diphtheria cases across North Queensland are on the rise

To the Editor: Until recently, detection of locally acquired tox gene carrying diphtheria in Australia was rare. Toxigenic diphtheria had almost disappeared from the Australian landscape, with the widespread uptake of the diphtheria toxoid vaccine. Diphtheria is predominantly caused by toxigenic Corynebacterium diphtheriae and can present as both respiratory and cutaneous diphtheria disease. There have been increasing reports internationally of diphtheria outbreaks primarily in vulnerable migrant populations.1 In Australia, there were 46 diphtheria cases between 1999 and 2019 (eight respiratory diphtheria and 38 cutaneous), with C. diphtheriae accounting for 87% of these cases.2 Since 2020, a genomically linked clone of tox gene carrying diphtheria bacteria has spread across North Queensland. Cases described here are from the Queensland Health's Notifiable Conditions Register. The Townsville Hospital and Health Service Human Research and Ethics Committee provided an ethics exemption (EX/2022/HREC/88895) for this study.

Of the 29 linked cases identified between 2020 and 2022, all have had epidemiological links to North Queensland and 23 were notified in 2022. Three of these cases were detected interstate. Clinically, three cases presented with classic diphtheria, four with mild respiratory diphtheria, and 22 with cutaneous diphtheria. The median age was 21 years (range, 2–59 years). Of the 29 cases, 34% (10/29) were aged 11–20 years and 45% (13/29) were older than 20 years. Further, 38% of cases (11/29) were fully vaccinated, 48% (14/29) were partially vaccinated and 10% (3/29), including two of the classic diphtheria cases, were unvaccinated for their age according to the National Immunisation Program Schedule. All were acquired in Australia, and 86% of patients (25/29) identified as Aboriginal and/or Torres Strait Islander. Isolates in the cluster are closely genomically linked, with between zero and 20 single nucleotide polymorphism differences found on whole genome sequencing. All cases were managed with penicillin or azithromycin, vaccination, and diphtheria antitoxin administered when deemed appropriate by infectious disease physician assessment. 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.

A North Queensland public health working group has recently been established to develop a consensus on further 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. We recommend clinicians be aware of these cases and consider diphtheria among their differentials in patients from, or who recently travelled to, North Queensland.

Classic diphtheria is characterised by a sore throat, fever, and membrane on the back of the throat, which may cause difficulty in breathing and swallowing. Cutaneous diphtheria usually presents as a non‐healing ulcerative lesion.2 The toxoid vaccine protects against the toxin effects rather than the infection itself. Unvaccinated individuals are therefore at highest risk of severe disease, including classic diphtheria, myocarditis, and neuropathies. Vaccination remains imperative and timely vaccinations are essential. The National Immunisation Program Schedule currently recommends a diphtheria‐containing vaccine for children at two, four, six and 18 months, and four years, and adolescents at 11–13 years. A diphtheria‐containing vaccine booster is recommended for adults at 50 years.3 Clinicians across the state and nation should be vigilant for future cases.