The diphtheria outbreak in Australia shows the importance of boosters, antibiotics and better housing

The 131-case outbreak reveals how a vaccine-preventable disease has regained a foothold in the Northern Territory, exposing the urgent need for boosted vaccination, rapid treatment, genomic surveillance and better housing.

Study: Diphtheria outbreak, Northern Territory of Australia, 2025 to 2026. Image credit: Chatchouliya / Shutterstock

In a recent study published in the magazine Euro controla group of researchers investigated the epidemiology, laboratory characteristics, genomic features and public health response associated with the 2025-2026 diphtheria outbreak in the Northern Territory, Australia.

background

Despite high diphtheria vaccination coverage among 5-year-old children in the Northern Territory, outbreaks continue to emerge around the world, reminding public health authorities that vaccine-preventable diseases can make a comeback under the right conditions.

Diphtheria is caused by the usually toxic Corynebacterium bacterium Corynebacterium diphtheriae And less common Corynebacterium ulcerans. It can lead to severe respiratory illness, heart damage, and death. It is spread through respiratory droplets or contact with wound secretions, and crowded living conditions, limited access to health care, and socioeconomic deprivation can facilitate transmission even in highly immunized populations.

Understanding how environmental and social factors influence disease spread remains essential. Effective control requires vaccination, early antibiotic treatment, contact tracing, laboratory monitoring, and action on social conditions that enable transmission.

About the study

The researchers investigated all cases of diphtheria reported in Australia’s Northern Territory between January 2025 and April 2026. The confirmed case required isolation of the toxin. Corynebacterium diphtheriae or Corynebacterium ulcerans From a clinically relevant respiratory or skin specimen with compatible symptoms.

The researchers collected and analyzed epidemiological data, including vaccination history and clinical information, and compared these variables with the geographic distribution of diphtheria using routine health department data.

Laboratory analyzes were performed by the district pathology department. Wound and throat samples were cultured on selective media, and bacterial colonies were suspected Corynebacterium Species were identified using mass spectrometry. DNA Each of the isolates was isolated and quantitative polymerase chain reaction was performed (qPCR) was used to detect the diphtheria toxin gene. Gradient diffusion bar method (Global distribution system) has been used to test antimicrobial susceptibility to a group of commonly used antibiotics.

Whole genome sequencing of 19 toxin-positive skin isolates was performed to examine the genomic characteristics of circulating strains. Sequence quality was checked before further analysis. The researchers used multilocus sequence typing, antimicrobial resistance screening, virulence gene identification, and single nucleotide polymorphisms (polymorphism) to confirm the genetic characteristics of toxin-positive isolates.

Phylogenetic methods were used to compare Northern Territory isolates to publicly available genomes from Queensland, Papua New Guinea and the Solomon Islands. Time-normalized phylogenetic analyzes were performed to estimate evolutionary relationships between outbreak strains.

Study results

Between January 2025 and April 2026, 131 cases of diphtheria were identified in the Northern Territory. These cases included 97 skin cases involving skin lesions and 34 respiratory cases. This was the first documented recurrence of locally acquired diphtheria in the Northern Territory in more than 20 years, with skin cases appearing in May 2025 and respiratory cases in March 2026.

The majority of reported cases were among Indigenous Australians, who accounted for 125 of the 131 notifications (about 95%). Respiratory infections They were most common in central Australia, while skin infections were predominant in the Top End region. The outbreak was ongoing until April 30, 2026.

The majority of infections were relatively mild, likely due to high vaccination rates, but some severe infections were observed. There were 12 respiratory patients who required hospitalization, 2 required intensive care admission, and 1 adult died, most likely from myocarditis associated with diphtheria toxin. The deceased adult had completed the childhood vaccination series but had not received a booster dose within the previous 10 years. The majority of patients with acute respiratory infections either have not had any prior vaccination or it has been more than 10 years since they last received a diphtheria-containing vaccine.

Respiratory symptoms typically include sore throat, tonsillitis, fever, cough, pharyngitis, pharyngeal discharge, and swollen cervical lymph nodes. Only a few patients developed the classic pseudomembrane associated with severe diphtheria. These findings suggest that recent outbreaks may appear differently from historical descriptions, making clinical recognition more difficult.

During the outbreak, there was also a significant increase in the detection of toxic substances Corynebacterium diphtheriae In skin samples, most positive skin samples originated from lesions in the lower extremities. Most of the skin samples tested were polymicrobial, including isolated organisms Staphylococcus aureusgroup A Streptococcus, arcanobacterium haemolyticum, And other bacterial species. This demonstrates the continuing complexity of health-related challenges faced by affected communities, although researchers note that the precise pathogenic role of toxic agents C. diphtheria In polymicrobial skin lesions it remains uncertain.

Testing on 76 isolates showed that all isolates were susceptible to erythromycin and susceptible to penicillin with increasing exposure. The lack of clinically relevant and detectable antimicrobial resistance supports the use of standard treatment regimens, and no key antimicrobial resistance genes have been identified.

Based on genomic characterization, the dominant strain in the outbreak belonged to sequence type 381, and the Northern Territory isolates were genetically closely related to but distinct from the Queensland strains isolated during previous outbreaks. The average genetic difference among local isolates was only three SNPs, indicating recent transmission. Phylogenetic analysis on a time scale indicates that the outbreak strain shares a common ancestor dating back to approximately 2017.

No significant resistance genes were found, and all 19 isolates sequenced carried the toxin type 20 gene. Five isolates were confirmed to produce toxin using the Elek immunoprecipitation assay at an overseas public health laboratory. National surveillance data also supported the finding that sequence type 381 was the dominant strain across multiple jurisdictions in Australia.

conclusion

The results show that diphtheria can re-emerge even in populations with high vaccination coverage when social and environmental conditions support transmission. Although vaccination appears to reduce the severity of the disease, it does not completely prevent infection or spread.

Indigenous communities with overcrowded housing, poor skin health, and persistent health inequalities were disproportionately affected by diphtheria outbreaks, while the dominant ST381 strain was responsible for most cases, and the strain was sensitive to the standard antibiotics used in treatment.

The researchers concluded that future outbreaks can be controlled and reduced through enhanced vaccination, early antibiotic treatment, contact tracing, judicious use of diphtheria antivenoms, genomic surveillance, collaboration with Indigenous community-controlled health organizations, and sustainable improvements in housing, access to health care, and education.

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