Statistical modeling based on structured surveys of Australian native possum excreta harboring Mycobacterium ulcerans predicts Buruli ulcer occurrence in humans

Abstract

Background: Buruli ulcer (BU) is a neglected tropical disease caused by infection of subcutaneous tissue with Mycobacterium ulcerans. BU is commonly reported across rural regions of Central and West Africa but has been increasing dramatically in temperate southeast Australia around the major metropolitan city of Melbourne, with most disease transmission occurring in the summer months. Previous research has shown that Australian native possums are reservoirs of M. ulcerans and that they shed the bacteria in their fecal material (excreta). Field surveys show that locales where possums harbor M. ulcerans overlap with human cases of BU, raising the possibility of using possum excreta surveys to predict the risk of disease occurrence in humans.Methods: We thus established a highly structured 12-month possum excreta surveillance program across an area of 350 km2 in the Mornington Peninsula area 70 km south of Melbourne, Australia. The primary objective of our study was to assess using statistical modelling if M. ulcerans surveillance of possum excreta provided useful information for predicting future human BU case locations.Results: Over two sampling campaigns in summer and winter, we collected 2282 possum excreta specimens of which 11% were PCR positive for M. ulcerans-specific DNA. Using the spatial scanning statistical tool SaTScan, we observed non-random, co-correlated clustering of both M. ulcerans positive possum excreta and human BU cases. We next trained a statistical model with the Mornington Peninsula excreta survey data to predict the future likelihood of human BU cases occurring in the region. By observing where human BU cases subsequently occurred, we show that the excreta model performance was superior to a null model trained using the previous year's human BU case incidence data (AUC 0.66 vs 0.55). We then used data unseen by the excreta-informed model from a new survey of 661 possum excreta specimens in Geelong, a geographically separate BU endemic area to the southwest of Melbourne, to prospectively predict the location of human BU cases in that region. As for the Mornington Peninsula, the excreta-based BU prediction model outperformed the null model (AUC 0.75 vs 0.50) and pinpointed specific locations in Geelong where interventions could be deployed to interrupt disease spread.Conclusions: This study highlights the One Health nature of BU by confirming a quantitative relationship between possum excreta shedding of M. ulcerans and humans developing BU. The excreta survey-informed modeling we have described will be a powerful tool for efficient targeting of public health responses to stop BU.Funding: This research was supported by the National Health and Medical Research Council of Australia and the Victorian Government Department of Health (GNT1152807 and GNT1196396).

A transmission chain linkingMycobacterium ulceranswithAedes notoscriptusmosquitoes, possums and human Buruli ulcer cases in southeastern Australia

“…As reported in other sequence enrichment studies, we observed decreasing genome sequence recovery with decreasing pathogen load, as indicated by increasing IS2404 Ct values (Figure 3A) 52 . Nevertheless, DNA sequence reads were obtained from five IS2404 positive mosquitoes and two IS2404 positive possum excreta specimens, the latter specimens collected as part of a large field survey of M. ulcerans in Australian native possum excreta in the region 53 . While DNA sequence reads were obtained across the length of the 5.6Mbp M. ulcerans reference chromosome, for all five mosquitoes and the two possum excreta specimens, only the excreta specimens (IS2404 Ct values <23) and three of the five mosquito DNA extracts (ID: 5675, Ct 32.62; ID: 226, Ct 33.47; ID: 819, Ct 31.20) yielded sufficient M. ulcerans reads for SNP-calling (Figure 3B).…”

“…These results also add to the growing body of evidence demonstrating the utility of pathogen genomics in specifically identifying the sources and transmission pathways of environmental pathogens and suggest that continued surveillance of M. ulcerans genotypes in mosquitoes and possums could be useful in guiding public health interventions that seek to control the spread of Buruli ulcer in the region. We are now exploring the use of genome sequence enrichment to dissect the genomic epidemiology from the perspective of environmental sources, in particular to track the spread of Buruli ulcer in Australian native possums across the region 53 .…”

“…rubrithorax) were also identified as having dual bloodmeals from ringtail possum and human origins (Figure 4). ulcerans, as detected by structured surveys, and clusters of human Buruli ulcer cases 53 . To expand on this finding, we investigated whether further spatial clustering associations could be detected by analysing qualitative qPCR (IS2404) data of trapped mosquitoes using SaTScan.…”

Buruli ulcer surveillance in south-eastern Australian possums: infection status, lesion mapping and internal distribution ofMycobacterium ulcerans

“…In contrast, recent studies have demonstrated that BU is a vector-borne zoonosis in Victoria, Australia, with mosquitoes and native possums as bacterial vectors and reservoirs, respectively [19, 33–37]. While exact mechanisms of M. ulcerans transmission remain unclear, spatial clustering analysis has shown a striking overlap between clusters of human BU cases, M. ulcerans -harbouring mosquitoes and M. ulcerans -positive possum faeces in BU endemic areas of Victoria[33, 37].…”

“…Common ringtail (CRT, Pseudocheirus peregrinus ) and to a lesser extent common brushtail (CBT, Trichosurus vulpecula ) possums, both of which are endemic and abundant in urban areas of Victoria, excrete large quantities of M. ulcerans in their faeces[19, 34], thus providing a potential source of environmental contamination for transmission to humans. Furthermore, the presence of M. ulcerans -positive possum excreta in a geographical area may be predictive of future human BU cases[37] possum excreta positivity in historically negative areas can predate the emergence of human BU cases by up to 31 months [39]. Faecal carriage of M. ulcerans in possums may be transient and can occur without accompanying clinical disease[36, 40].…”