Mycobacterium ulcerans not detected by PCR on human skin in Buruli ulcer endemic areas of south eastern Australia

Introduction Mycobacterium ulcerans (MU) causes Buruli ulcer (BU), a geographically restricted infection that can result in skin loss, contracture, and permanent scarring. Lesion-location maps compiled from more than 640 cases in south eastern Australia suggest biting insects are likely involved in transmission, but it is unclear whether MU is brought by insects to their target or if MU is already on the skin and inoculation is an opportunistic event that need not be insect dependent. Methods We validated a PCR swab detection assay and defined its dynamic range using laboratory cultured M. ulcerans and fresh pigskin. We invited volunteers in Buruli-endemic and non-endemic areas to sample their skin surfaces with self-collected skin swabs tested by IS2404 quantitative PCR. Results Pigskin validation experiments established a limit-of-detection of 0.06 CFU/cm2 at a qPCR cycle threshold (Ct) of 35. Fifty-seven volunteers returned their self-collected kits of 4 swabs (bilateral ankles, calves, wrists, forearms), 10 from control areas and 47 from endemic areas. Collection was timed to coincide with the known peak-transmission period of Buruli. All swabs from human volunteers tested negative (Ct ≥35). Conclusions M. ulcerans was not detected on the skin of humans from highly BU endemic areas. Author Summary Buruli ulcer (BU) incidence is increasing in temperate south eastern Australia. We have yet to develop public health programs to assist people avoid BU partly because the precise mode of transmission is contentious. Recent research has shown that environmental contamination with M. ulcerans (the cause of BU) is widespread in endemic areas as a result of faecal shedding from infected possums that live close to humans, although direct human-possum contact is rare. We investigated the possibility that the skin of humans in endemic areas could become transiently contaminated with M. ulcerans while outdoors. If this were the case then BU prevention programs could be developed around skin protection and regular washing/showering. To study this possibility, we developed a sensitive skin swab PCR-assay that we tested using a pigskin laboratory model so we could be confident in our results. We asked volunteers to collect their own skin swabs after spending at least 4 hours outside during the known period of peak Buruli transmission. Fifty seven volunteers returned swab sets for testing. Our results were negative. We did not find evidence that humans in our endemic zone have M. ulcerans contamination on their skin. ### Competing Interest Statement The authors have declared no competing interest.