Figure 1

Figure 1. Location of central Geelong suburbs in Victoria, Australia, and prior known endemic areas in the Bellarine Peninsula and Surf Coast (higher case numbers dark pink, lower case numbers in light...

We conducted a descriptive epidemiologic study, including a spatiotemporal clustering analysis of laboratory-confirmed and probable Buruli ulcer cases notified to the Department of Health Victoria during January 1, 2011–December 31, 2022, to identify new endemic areas in suburbs of Geelong, a city of ≈250,000 persons in southwest Victoria, Australia (Figure 1). We examined aggregate case numbers and incidence per 100,000 estimated resident population (11) at Australian Bureau Statistics (ABS) Statistical Areas 2 (SA2) for the central Geelong suburbs and known endemic areas of the Bellarine Peninsula and Surf Coast. We examined geographic clustering of Geelong cases, mapped to ABS Meshblocks (small geographic areas, 3–40 dwellings), in a space-time analysis using Poisson models in SatScan (http://www.satscan.org). The models scan across geographic locations over time to detect areas and time periods where reported case numbers significantly exceed expected case numbers.

We used data from a systematic possum fecal survey conducted in Geelong suburbs in 2020 (10), as well as follow-up surveys conducted in a more restricted area of Geelong at 3, 6, and 24 months later and several samples from Belmont in the presurvey period in September 2019 (Appendix Figure 1). Fecal survey collection methods have been published previously (10,12). We used the data to examine the proximity and timing of human Buruli ulcer cases notified in 2020–2022 to feces from M. ulcerans–infected possums by calculating distance from Buruli ulcer case residence (in meters) and timing (in months) from the most proximal possum fecal sample collection to human case at notification.

Figure 2

Figure 2. Distribution, clustering, and timing of Buruli ulcer cases in relation to Mycobacterium ulcerans–positive possum fecal samples, Victoria, Australia. A) Number and home location of human Buruli ulcer cases...

During 2011–2022, a total of 402 Buruli ulcer cases were reported in the Barwon South West (BSW) catchment, including 80 cases within the SA2 locations of interest in central Geelong. We observed increasing case numbers and incidence in Belmont during 2017–2022 and in Highton and Newtown areas during 2020–2022 (Table 1); we identified 3 clusters where reported case numbers exceeded the number expected (Figure 2, panel C).

Of the 49 Buruli ulcer cases diagnosed since 2020 in Geelong suburbs, 39 case-patients resided within areas included in the 2020 possum excreta survey (Figure 2). The median distance from case-patient residence to M. ulcerans–positive possum feces was 283 meters (Table 2). In all, 16/39 (41%) of the cases resided within 200 meters of a positive possum sample. The median distance to a positive possum sample was lower for cases within the identified clusters in Belmont, Highton, and Newtown (199 meters; 50% of case-patients resided within 200 meters of a positive possum sample) than cases who were not within the identified clusters (median 1,177 meters; 23% within 200 meters; p = 0.040 by rank-sum test). For cases within the central areas sampled in both 2020 and 2022 fecal surveys, the median distance from a case residence to a prior positive possum sample was only 108 meters (interquartile range [IQR] 82–263 meters).

The most geographically proximal detections of M. ulcerans in possum fecal samples preceded the emergence of human Buruli ulcer cases by up to 39 (IQR 8−29) months (Table 2). For cases in identified clusters, the most geographically proximal positive possum fecal samples to cases occurred with an IQR of 7–31 months before the case diagnosis, apart from 2 cases in which the closest positive detection (<100 meters) occurred after case diagnosis. For those cases, other positive possum feces within 250 meters of the case residence were detected 16 and 18 months before the case diagnosis. Of note, we observed few Buruli ulcer cases in the northern suburbs of Geelong during2011–2022. Possum population density was lower in these suburbs; few to no possum feces were present during the 2020 excreta survey (Appendix Figure 1). No cases were reported in North Geelong-Bell Park SA2; 1 of the 2 cases reported in 2018 in Corio-Norlane SA2 resided within 344 meters of the subsequent single M. ulcerans–positive possum fecal detection in 2020. Analysis examining the timeliness of diagnosis in BSW demonstrated delays in Buruli ulcer diagnosis in emerging endemic areas.

Buruli ulcer endemicity in Victoria, Australia, is evolving. Our multidisciplinary investigation of human Buruli ulcer cases and M. ulcerans shedding in local possums describes the speed and pattern of spread in a large noncoastal urban center in Victoria. Since 2017, Buruli ulcer cases have increased and clustered within 3 Geelong suburbs, predominantly near possums shedding M. ulcerans in feces. Possum detection precedes human cases by months to years, suggesting that possum fecal surveillance for M. ulcerans may be useful in identifying emerging areas before the onset of human cases. Our findings support a recently published statistical model to predict the location of future human Buruli ulcer cases based on previous human cases and the location of positive possum fecal specimens (10); here, we defined the temporal relationship whereby infection appears in possums before humans in an emerging disease area. Furthermore, we quantified the time between the positive detections in possum populations and subsequent human cases. Our results suggest that possum fecal positivity may provide an early warning signal of emerging at-risk areas for Buruli ulcer. In Geelong, the Barwon South West Public Health Unit, established in late 2020, uses this information (13), to inform targeted efforts to enhance community awareness and clinician knowledge of the condition in relation to recommended behaviors aimed at disease prevention (7) and for testing for more timely diagnosis (14). Evidence that such recommendations reduce Buruli ulcer incidence is limited; further research is required to investigate this possibility. As human cases emerge, M. ulcerans detections in possum feces may help delineate new areas of local transmission from travel-related acquisition within known endemic areas, assisted by research into the ecology and transmission pathways of Buruli ulcer to develop interventions to reduce disease in both human and wildlife populations. Early detection of M. ulcerans shedding in possums may inform decisions on target areas to implement and evaluate any such potential future interventions in Victoria.

This article was preprinted at https://medrxiv.org/cgi/content/short/2024.05.03.24306731v1.