Buruli ulcer (BU) is a rare, neglected tropical disease caused by Mycobacterium ulcerans that can lead to severe skin ulcers. To determine the epidemiology of BU in Victoria, Australia, during 2017-2022 we analyzed surveillance data. A total of 1,751 cases of BU were notified; 968 (55%) patients were male and 781 (45%) female (2 were missing sex data), and 984 (56%) resided in established BU-endemic areas, although an increasing number were in new BU-endemic areas. Most cases (83%, 1,301) were classified as category I. Multivariate modeling demonstrated that factors for severe BU included being male, being older, and living in a new BU-endemic or non-BU-endemic area. A relatively shorter interval between first visit to a clinician and receipt of diagnosis was protective against severe disease. The expansion of BU-endemic areas throughout Victoria remains a public health concern and calls for targeted action, particularly for patients and clinicians in new BU-endemic areas.

Buruli ulcer (BU) is a devastating skin and tissue infection caused by Mycobacterium ulcerans (1). BU is prevalent mainly in tropical sub-Saharan Africa, although ≈30 countries have reported cases (2). Although the number of cases has decreased worldwide, local epidemics in Australia have countered that trend (3,4). BU-endemic regions in Australia include the Daintree Rainforest and the Capricorn region in tropical Queensland and the East Gippsland and metropolitan Greater Melbourne/Bellarine regions in the state of Victoria in southeastern Australia (3,5,6). The climate in the southeastern state of Victoria is temperate; temperature and weather vary substantially throughout the year (7).

BU is usually exhibited initially as a painless skin nodule that predominantly affects the distal limbs and, if left untreated, forms a characteristic ulcer with undermined edges (8). The average incubation period for BU is ≈4-5 months, and the average delay between symptom onset and diagnosis is 1-2 months (9). Although the BU mortality rate is low, the illness can result in substantial socioeconomic effects on individual persons and communities (2,10).

Residence in or visitation to a BU-endemic area remains a significant risk factor for M. ulcerans acquisition; previous BU outbreaks have occurred as geographically defined infections (11). In the temperate climates of Australia, transmission research has focused on mosquitoes as vectors and small Australia native marsupials (possums) as animal reservoirs (12,13). Mosquitoes are infected by biting possums that carry the bacteria, after which they directly inoculate humans, causing clinical disease (13,14). An environmental study has shown a correlation between rainfall and BU, as is seen for other vectorborne diseases in the region, including Barmah Forest and Ross River fevers, which further supports the role of mosquitoes (15). Definitive evidence was provided through an extensive field survey and genomic analysis that indicated that mosquitoes transmit M. ulcerans in southeastern Australia from a reservoir of possums (16).

BU was first identified in Victoria in 1948, and only 50 cases were recorded before 1990 (17). Since then, the pattern of disease has changed substantially, from low numbers in fixed geographic regions to more widespread transmission (3,18). New areas of endemicity have emerged, and cases have increased continually since 2011 (11,19). Within Melbourne, the emergence, continued propagation, and expansion of BU-endemic areas remains a public health concern (10).

Using routinely collected surveillance data, we analyzed the epidemiology of BU in Victoria during 2017-2022, identifying factors that influence disease severity and mapping the ongoing spread of the disease. Ethics approval was provided by the Australian National University Human Research Ethics Committee (protocol 2017/909).