A major role for mammals in the ecology of Mycobacterium ulcerans

Abstract

Background: Mycobacterium ulcerans is the causative agent of Buruli ulcer (BU), a destructive skin disease found predominantly in sub-Saharan Africa and south-eastern Australia. The precise mode(s) of transmission and environmental reservoir(s) remain unknown, but several studies have explored the role of aquatic invertebrate species. The purpose of this study was to investigate the environmental distribution of M. ulcerans in south-eastern Australia.

Methodology/principal findings: A range of environmental samples was collected from Point Lonsdale (a small coastal town southwest of Melbourne, Australia, endemic for BU) and from areas with fewer or no reported incident cases of BU. Mycobacterium ulcerans DNA was detected at low levels by real-time PCR in soil, sediment, water residue, aquatic plant biofilm and terrestrial vegetation collected in Point Lonsdale. Higher levels of M. ulcerans DNA were detected in the faeces of common ringtail (Pseudocheirus peregrinus) and common brushtail (Trichosurus vulpecula) possums. Systematic testing of possum faeces revealed that M. ulcerans DNA could be detected in 41% of faecal samples collected in Point Lonsdale compared with less than 1% of faecal samples collected from non-endemic areas (p<0.0001). Capture and clinical examination of live possums in Point Lonsdale validated the accuracy of the predictive value of the faecal surveys by revealing that 38% of ringtail possums and 24% of brushtail possums had laboratory-confirmed M. ulcerans skin lesions and/or M. ulcerans PCR positive faeces. Whole genome sequencing revealed an extremely close genetic relationship between human and possum M. ulcerans isolates.

Conclusions/significance: The prevailing wisdom is that M. ulcerans is an aquatic pathogen and that BU is acquired by contact with certain aquatic environments (swamps, slow-flowing water). Now, after 70 years of research, we propose a transmission model for BU in which terrestrial mammals are implicated as reservoirs for M. ulcerans.

Figure 1. Map of central coastal Victoria, showing places referred to in the text or associated references.

Figure 2. Photographs of Point Lonsdale, common brushtail possums and common ringtail possums.

A. Point Lonsdale streetscape showing typical possum habitat. B. Common brushtail possum. C. Common ringtail possum. D. Brushtail possum faeces (left) and ringtail possum faeces (right). E. Ringtail possum tail lesion. F. Ringtail possum nose lesion.

Figure 3. Distribution and estimated bacterial load of M. ulcerans-positive ringtail faecal samples in two towns.

Map shows results of faecal surveys conducted in Point Lonsdale (approx. 81 human cases 2005–09) in August 2008 and Queenscliff (approx. 6 human cases 2005–09) in November 2008.

Figure 4. Variable number tandem repeat (VNTR) typing of M. ulcerans DNA in possum faeces demonstrates identity with human outbreak strain.

Numbers represent VNTR loci . At each locus: left PCR product, Victorian human patient isolate; right PCR product, DNA extracted from brushtail possum faeces collected in Point Lonsdale.

Figure 5. Proposed transmission pathways of M. ulcerans between the environment, mosquitoes, possums and humans.

1. Possums ingest M. ulcerans from the environment and/or infected by an insect vector. 2. Possums amplify and shed M. ulcerans into the environment. 3. Insect vectors become contaminated with M. ulcerans from the environment and/or from contact with infected possums. 4. M. ulcerans transmitted to humans via insect vector and/or direct contact with contaminated environment.