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

Buruli ulcer (BU) is a neglected tropical disease of skin and subcutaneous tissues caused by Mycobacterium ulcerans . BU-endemic areas are highly focal, and M. ulcerans transmission dynamics vary in different settings. In the south-eastern Australian state of Victoria, BU is an endemic vector-borne zoonosis, with mosquitoes and native possums implicated in transmission, and humans as incidental hosts. Despite the importance of possums as wildlife reservoirs of M. ulcerans , knowledge of BU in these animals is limited. Opportunistic necropsy-based and active trap-and-release surveillance studies were conducted in and around Melbourne and Geelong, Victoria, to investigate BU in possums. Demographic data and biological samples were collected, and when present, cutaneous lesions suggestive of BU were mapped. Samples were tested for the presence of M. ulcerans DNA by IS2404 qPCR. The final dataset included 26 possums: 20 necropsied; 6 trapped and released. Most possums (77%) were common ringtails from inner Melbourne. Nine possums (eight ringtails, one brushtail) had skin lesions, ranging from single and mild, to multiple and severe, exposing bones and tendons in three cases. M. ulcerans was confirmed in 78% (7/9) of clinically affected possums and 65% of possums without lesions (11/17). Possums with moderate and severe disease had widespread systemic internal bacterial dissemination and were shedding M. ulcerans in their faeces. The anatomical distribution of cutaneous lesions and PCR positivity of biological samples suggests possums may be contracting BU from bites of M. ulcerans -harbouring mosquitoes, traumatic skin wounds, ingestion of an unknown environmental source, and/or during early development in the pouch. Ringtail possums appear highly susceptible to infection with M. ulcerans and are important reservoirs of the bacteria in Victoria. A One Health approach is needed to design and implement integrated interventions that reduce M. ulcerans transmission in Victoria, thereby protecting wildlife and humans from this emerging zoonotic disease. Author summary Buruli ulcer (BU), a neglected tropical skin disease, is emerging as a public health concern in the temperate Australian state of Victoria. Here, BU is spread by mosquitoes, and native possums are wildlife reservoirs of the causative bacterium, Mycobacterium ulcerans . Possums can be infected by BU, but knowledge of infection and disease in these animals is limited. We conducted surveillance studies in the two largest cities of Victoria, examining live and deceased possums. We found skin lesions in a third of examined possums and confirmed presence of M. ulcerans in almost three-quarters of the animals. Mouth swabs were positive for the bacteria in nearly two thirds of possums, as were pouch swabs of almost half the females. We also conducted mapping of the bodily distribution of skin lesions and found that paws and the undersides of abdomens and tails were the most affected areas. Our findings add support to the concept that possums, particularly ringtails, are M. ulcerans reservoirs in Victoria, and suggest several possible routes of infection for free-living possums that warrant further research. Improved understanding of BU in possums may allow development of targeted interventions that reduce disease transmission and protect both animal and human health. ### Competing Interest Statement The authors have declared no competing interest.