The outbreak of 131 cases reveals how a vaccine-preventable disease has regained a foothold in the Northern Territory, highlighting the urgent need for booster vaccinations, rapid treatment, genomic surveillance and better housing.
Study: Diphtheria outbreak in Australia’s Northern Territory, 2025-2026. Image credit: Chatchouliya / Shutterstock
In a recent study published in the journal euro monitoringa group of researchers investigated the epidemiology, laboratory characteristics, genomic characteristics, and public health response associated with the 2025-2026 diphtheria outbreak in Australia’s Northern Territory.
background
Despite high rates of diphtheria vaccination among five-year-olds in the Northern Territory, diphtheria outbreaks continue around the world, warning public health officials that the vaccine-preventable disease could return under the right conditions.
Diphtheria is caused by toxigenic Corynebacterium, usually Corynebacterium diphtheriae and rarely Corynebacterium ulcerans. It can lead to severe respiratory illness, heart damage, and death. It is spread through contact with respiratory droplets and wound exudate, and crowded living conditions, limited access to health care, and socio-economic disadvantage can facilitate transmission even in well-vaccinated populations.
It remains important to understand how environmental and social factors influence the spread of disease. Effective control requires vaccination, early antibiotic treatment, contact tracing, laboratory surveillance, and addressing social conditions that enable transmission.
About research
Researchers looked at all notified cases of diphtheria reported in Australia’s Northern Territory between January 2025 and April 2026. Confirmed cases required isolation of toxigenic Corynebacterium diphtheriae or Corynebacterium ulcerans from clinically relevant respiratory or skin specimens with matching symptoms.
The researchers collected and analyzed epidemiological data, including vaccination history and clinical information, and cross-referenced these variables against the geographic distribution of diphtheria using routine health department data.
Laboratory analysis was performed by Territory Pathology. Wound and throat specimens were cultured in selective media and mass spectrometry was used to identify bacterial colonies suspected to be Corynebacterium species. DNA was isolated from each isolate and diphtheria toxin genes were detected using quantitative polymerase chain reaction (qPCR). Gradient diffusion strip method (GDS) was used to test antimicrobial susceptibility to various commonly used antibiotics.
Whole-genome sequencing of 19 toxin-positive skin isolates was performed to determine the genomic characteristics of the circulating strains. The quality of the sequences was inspected before further analysis. The researchers used multilocus sequence typing, screening for antimicrobial resistance, identification of virulence genes, and single nucleotide polymorphisms (SNPs) to confirm the genetic characteristics of the toxin-positive isolates.
We used phylogenetic methods to compare Northern Territory isolates with publicly available genomes from Queensland, Papua New Guinea, and the Solomon Islands. Time-corrected evolutionary analyzes were performed to estimate the evolutionary relationships between the outbreak strains.
Research results
From January 2025 to April 2026, 131 cases of diphtheria were confirmed in the Northern Territory. These included 97 cutaneous cases with skin lesions and 34 respiratory cases. This is the first time in more than 20 years that a recurrence of locally transmitted diphtheria has been recorded in the Northern Territory, with a skin case occurring in May 2025 and a respiratory case occurring in March 2026.
The majority of reported cases occurred among Aboriginal Australians, accounting for 125 of the 131 notifications (approximately 95%). Respiratory infections were more common in central Australia, while skin infections predominated in the Top End region. The outbreak was still ongoing as of April 30, 2026.
Although the majority of infections were relatively mild, probably due to high vaccination coverage, some severe infections were also observed. Twelve respiratory patients required hospitalization, two required admission to intensive care, and one adult died, possibly due to myocarditis associated with diphtheria toxin. The adult who died had completed a series of vaccinations as a child, but had not received a booster shot in the past 10 years. The majority of patients with severe respiratory infections either had not been previously vaccinated or it had been more than 10 years since they last received a diphtheria-containing vaccine.
Common respiratory symptoms included sore throat, tonsillitis, fever, cough, pharyngitis, pharyngeal exudate, and swollen cervical lymph nodes. A small number of patients developed the classic pseudomembrane associated with severe diphtheria. These findings suggest that modern outbreaks may manifest differently than historical accounts, making clinical recognition more difficult.
During the outbreak, the detection of toxin-positive Corynebacterium diphtheriae from skin samples also increased significantly. Most of the positive skin samples were from lesions on the lower extremities. Most of the skin samples tested were polymicrobial, with co-isolated microorganisms including Staphylococcus aureus, group A Streptococcus, Arcanobacterium haemolyticum, and other bacterial species. Although this demonstrates the continued complexity of the health-related challenges faced by affected communities, the researchers noted that the precise pathogenic role of toxigenic H. diphtheriae in polymicrobial skin lesions remains unclear.
Testing of 76 isolates revealed that all isolates were susceptible to erythromycin and, with increasing exposure, penicillin. The lack of detectable clinically relevant antimicrobial resistance supports the use of standard treatment regimens, and no major antimicrobial resistance genes were identified.
Based on genomic characterization, the predominant strain during the outbreak belonged to sequence type 381, and the Northern Territory isolate was genetically closely related but distinct from the Queensland strain isolated during the previous outbreak. The median genetic difference between local isolates was only three SNPs, indicating recent transmission. Time-scale phylogenetic analysis suggested that the outbreak strains share a common ancestor dating back to around 2017.
No significant resistance genes were found, and all 19 strains sequenced carried toxin gene allele type 20. Five strains were confirmed to produce toxins using Elec immunoprecipitation assays at public health laboratories overseas. National surveillance data further supported the finding that sequence type 381 is the predominant strain across multiple jurisdictions in Australia.
conclusion
The findings show that diphtheria can re-emerge even in highly vaccinated populations if social and environmental conditions favor infection. Although vaccination appeared to reduce the severity of the disease, it did not completely prevent infection or spread.
Aboriginal communities with overcrowded housing, poor skin health, and persistent health inequalities were disproportionately affected by the diphtheria outbreak, but the predominant ST381 strain was responsible for most cases, and this strain was susceptible to standard antibiotics used for treatment.
Researchers concluded that future outbreaks can be controlled and reduced through booster vaccinations, early antibiotic treatment, contact tracing, judicious use of diphtheria antitoxin, genomic surveillance, collaboration with Aboriginal community health authorities, and continued improvements in housing, health care access, and education.
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Reference magazines:
- Draper, ADK, Systrom, M., McMahon, K., Duguide, R., Chen, J., Crump, G., Cherian, T., Milli. Nargeshi, M., Stendhal, D., Freeman, K., Lomas, K., Trotta, T., Gunn, J.C., Boyd, R., Hennessy, J., Krieper, T., Harbidge, J., Sarmiento, F., Kerr, R., Yang, J., Francis, J.R., Baird, R., Stratton, H. , Davis, J., Marshall, C.S., Janson, S., Moore, N., Wardell, R., Eisan, E., Drewett, G., Moore, L., Leighton, E., Kumar, U.S., Webby, R., Yadav, A., Krause, V.L., Ravindran, B., and Mewman, E.M. (2026). Diphtheria outbreak, Northern Territory, Australia, 2025-2026. Euro monitoring. 31(23). Doi: 10.2807/1560-7917.ES.2026.31.23.2600443, https://www.eurosurveillance.org/content/10.2807/1560-7917.ES.2026.31.23.2600443
