In the 126 years since it was first described, scientists have learned a great deal about Buruli ulcer, (an ulcerating skin infection caused by the environmental bacterium, Mycobacterium ulcerans), including how to treat it. Yet, there is 1 question about this neglected tropical disease that has remained unanswered: how is M. ulcerans transmitted to humans? In southeastern Australia, where Buruli ulcer is endemic and cases are increasing, researchers are putting together the M. ulcerans transmission puzzle. The more they learn, the better equipped they will be for managing infections.

What is Buruli Ulcer?

After tuberculosis and leprosy, Buruli ulcer is the most common mycobacterial disease in the world. When M. ulcerans (the causative agent of the disease) is introduced into the subcutaneous tissue, it produces a cytotoxic and immunosuppressive toxin (mycolactone) that causes skin ulcerations, which tend to cluster on the arms and legs. While these lesions look like the definition of pain, they are, in fact, painless—the toxin also binds cellular receptors (type II angiotensin receptors) that prevent the release of neurotransmitters involved in pain perception. 

Buruli ulcer is treatable with antibiotics (i.e., rifampicin and clarithromycin or moxifloxacin), and in some cases, infections resolve by themselves. However, surgery in the form of wound debridement and skin grafting may be required, and, if treated too late, the disease can lead to lifelong disability and disfigurement.  

Cases of Buruli ulcer have been reported in 33 countries, though West Africa and southeastern Australia are endemic hot spots. In general, the number of global reported cases has decreased in recent years (which may reflect a lack of reporting and/or COVID-19-mediated disruptions in detection rather than a true reduction in cases). Still, the broad picture does not capture trends of individual regions, such as those in Australia. “We had about 20-30 cases [of Buruli ulcer] per year in the early 2000s until about 2010, when [cases] took off,” said Tim Stinear, Ph.D., a professor of microbiology and immunology at the University of Melbourne. Australia has reported near or above 300 cases of the disease every year since 2017 (for reference, there were only 14 reported cases in 2003), mostly in the southeastern region of the country.

With that in mind, the World Health Organization (WHO) has included Buruli ulcer in its list of neglected tropical skin diseases, calling for greater understanding and control of the disease. Such control requires uncovering the answer to a question that scientists have been asking for years: how do people get Buruli ulcer in the first place? 

M. ulcerans Transmission in Southeastern Australia: The Possibility of Possums

For Stinear, who studies M. ulcerans transmission in his native Australia, the answer is rooted in possum poop.

Previous research suggested that Australian native possums can be infected with M. ulcerans and may spread the pathogen to humans. Indeed, while M. ulcerans has been isolated from various animals in endemic regions, carriage among possums is common.“Possums are incredibly well adapted to life in urban Australia,” Stinear said during a scientific session at ASM Microbe 2023. “They live all around us, perhaps a bit like squirrels [in the U.S.]. Their fecal material literally rains down from the sky at night…We are sort of swimming in possum fecal material.” 

Stinear and his colleagues have used that possum poop pool to their advantage. “We walk around the suburbs of Melbourne [where Buruli ulcer cases generally cluster], picking up possum excrement from the ground—every 200 meters we stop, we pick up a fecal sample, we take it back to the lab and we do a PCR test for M. ulcerans,” he explained, noting that the bacterium cannot be cultured from environmental samples due to its fastidious culture requirements and the presence of sample contaminants that hinder its growth. In this way, Stinear’s team collected over 2,200 possum poop samples, 11% of which were positive for M. ulcerans-specific DNA. They could then “build spatial risk maps of where humans are going to get M. ulcerans in the following transmission season just based on these possum fecal surveys.”

Stinear noted that there are 2 takeaways from this study, the first being that there is a strong link between what’s happening in possums and what’s happening in people. “[The other] is that we can use the information from the possums to inform our public health response,” he said. “Instead of relying on where humans are getting Buruli ulcer to let us know where we should be intervening with public health messaging or other interventions, we can use our fecal surveys without waiting for people to get infected.” 

Those Pesky Mosquitoes

But how exactly are humans contracting the pathogen from possums? M. ulcerans must be introduced directly to the skin to cause disease—it does not spread from person-to-person, and mere contact with possum poop wouldn’t be enough to establish infection. Mosquitoes, however, could be just the thing. Cases of Buruli ulcer are linked with aquatic ecosystems, namely freshwater bodies where mosquitoes breed. Moreover, 1 study showed that the odds of being diagnosed with Buruli ulcer in southeastern Australia came down to 2 factors: use of insect repellent and being bitten by mosquitoes on the lower legs.  

Stinear and his collaborators wanted to formally test the hypothesis that mosquitoes are transmitting M. ulcerans to humans. They collected over 66,000 mosquitoes from the Mornington Peninsula (a region south of Melbourne, where Buruli ulcer is endemic) and determined that over 5 mosquitoes were PCR-positive for M. ulcerans per 1000 tested. By overlaying the genomes of M. ulcerans obtained from the mosquitoes with those from possum feces and humans, the scientists showed that “the genomes from the insects and from the possums were indistinguishable from humans from this region,” suggesting a link between all 3 factors in M. ulcerans transmission. In addition, there was significant overlap between clusters of human Buruli ulcer cases and mosquitoes and possum poop harboring the pathogen. The scientists also identified individual mosquitoes that had fed on both humans and native possums, and a separate study experimentally confirmed that mosquitoes can transmit the pathogen using a mouse model.

Collectively, these findings build “a hierarchy of evidence that implicates mosquitoes in the transmission of M. ulcerans to humans,” Stinear explained. Essentially, “you need 4 things to get Buruli ulcer transmission in southeastern Australia: you need possums that are infected with M. ulcerans (that’s the first 2 things), you need mosquitoes in sufficient numbers that are going to be the mechanical vector to spread [the pathogen] and you need people in sufficient numbers living in those areas where we see transmission. And we think that when we have all of these elements in place, we see cases.”

The next order of business is delineating how mosquitoes are acquiring and transmitting M. ulcerans: are they feeding on possums and then humans, or are the mosquitoes breeding in areas loaded with possum feces and acquiring the pathogen that way? The answers to those questions will rely on further investigation. 

Stinear also acknowledged that it may be possible for people to become infected by M. ulcerans sans mosquitoes, such as via contamination of a skin injury. However, he posits that mosquito transmission probably explains more than 90% of human cases in southeastern Australia. 

Transmission in West Africa: A Different Picture

Though there is evidence for a possum-mosquito-human M. ulcerans transmission chain in Australia, the same cannot be said in other endemic regions, such as in West Africa. For example, there is little evidence that mosquitoes in this region harbor M. ulcerans. Estelle Marion, Ph.D., a research associate at Inserm (a public scientific and technological institute in France) and her colleagues conducted several environmental field studies in Benin, where Buruli ulcer is endemic. They collected flying insects and samples of aquatic organisms and tested them for the presence of M. ulcerans DNA. “We were only able to find some positive samples in the aquatic environments, in aquatic plants or macroinvertebrates or vertebrates, but we were unable to detect in flying insects, including mosquitoes,” she said. The findings mean that “in this specific environment, in this area, at this moment, mosquitoes do not play a major role in the dissemination of M. ulcerans.”  

Moreover, based on current research, there “does not appear to be an analogous reservoir to the possum [in West Africa],” Stinear noted. Generally, Buruli ulcer transmission in West Africa is tied to direct contact with slow-flowing or stagnant water bodies containing various aquatic organisms, a number of which could be positive for the pathogen. A more detailed transmission model, like that emerging in southeastern Australia, has yet to be characterized. Still, even without this knowledge, Marion and her colleagues are pinpointing human behaviors that increase and decrease infection risk, such as prolonged walking in water or using water from wells, respectively, which may be useful for mitigating disease.

Putting Data to Work

Back in Australia, however, the identification of possums as environmental reservoirs of M. ulcerans is proving useful on the public health front. Andrew Buultjens, Ph.D., a research officer in microbiology and immunology at the University of Melbourne, is using M. ulcerans genomes from Buruli ulcer patients and possum fecal material to predict where transmissions are coming from. “This is important information for public health agencies because it enables them to understand the riskier places [people are] picking up this disease,” he said.

The method relies on machine learning—the genome of a clinical M. ulcerans isolate is fed into a computer model, which then spits out the likely location of acquisition, based on possum fecal survey data. Buultjens noted that most of the model’s predictions “aren’t too bad.” In fact, 77% of predictions are within 10 km or less of their known origins. With that prediction error, “a public health official could gauge where a specific case was derived from, where a patient picked up this bacterium. They could tease apart, and essentially quantify, where the bulk of transmissions are occurring,” Buultjens stated.  

He pointed to a recent example where the tool was put to work—in 2020, there were 19 cases of Buruli ulcer in Essendon, a region north of Melbourne where the disease had never been reported before. By field-sampling possum fecal material and comparing recovered M. ulcerans genomes with those of clinical isolates, scientists could confirm that local transmission was happening in the area. The Victoria Department of Health “was then able to act upon that [information] by issuing a health advisory with key messages around early prevention and early diagnosis” of Buruli ulcer,” Buultjens said. The example highlights how adequate knowledge of M. ulcerans transmission, coupled with computational tools, can help improve disease management. 

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Research in this article was presented at ASM Microbe, the annual meeting of the American Society for Microbiology, held June 15-19, 2023, in Houston. Ready to share your science at ASM Microbe 2024?