Scientists say they have found conclusive evidence linking the rise in Australian Buruli ulcer cases to mosquito bites, answering a question that has confounded public health control efforts for almost a century.

It comes after Victorian health authorities reported a record 363 cases in 2023, with patients spreading beyond the traditional Mornington and Bellarine Peninsula hotspots to Geelong and even suburban Melbourne.

For the study, microbiologists conducted extensive field survey analyses of more than 73,580 mosquitoes, mostly trapped over four months in 2019 and 2020 on the Mornington Peninsula, about 90km south of Melbourne.

PCR screening revealed a significant association between the Buruli ulcer-creating bacteria Mycobacterium ulcerans and one mosquito species; Aedes notoscriptus.

And crucially, the bacterial genomic analysis then confirmed identical single-nucleotide-polymorphism profiles in both the mosquitoes and human cases, reported the team led by Professor Time Stinear of the Doherty Applied Microbial Genomics Centre

Taken together with spatial scanning statistics, these findings provided strong evidence of the mosquitos as vectors for the bacteria, the researchers said.

These profiles were also shared with examples of M. ulcerans found in the excreta of possums, already established as a local wildlife reservoir for the virus, they noted.

“Our key findings were that M. ulcerans was almost exclusively associated with one mosquito species in this region, Ae. notoscriptus, at a frequency of 0.87%,” they wrote in Nature (link here).

The team added: “Where and how M. ulcerans is contaminating (or infecting) these mosquitoes remains to be determined.”

“However, we made the somewhat unexpected observation that the most abundant mosquito species on the Mornington Peninsula, Culex molestus, was consistently IS2404 PCR negative.”

“This observation might be explained by a difference in the ecology of these two mosquito species, such as how (or where) they are encountering M. ulcerans.”

While Buruli ulcer had been described in more than 32 countries worldwide, and was an ongoing public health issue in West and Central Africa, the Victorian surge had been unexpected and remained partially unexplained, the authors noted.

“How humans contract Buruli ulcer is a central question that has confounded public health control efforts and intrigued scientists since the discovery of M. ulcerans from patients in the Bairnsdale region of Australia in the 1930s and across Africa shortly thereafter,” they wrote.

“Buruli ulcer epidemiology can be unpredictable, with a 4–5 month median incubation period and outbreaks emerging in specific geographical areas and then disappearing over a number of years.”

“It is also very challenging to isolate the bacterium in pure culture from the environment, presumably due to its very slow growth, although it can be isolated from human skin lesions. These factors combined have made it incredibly challenging to establish how M. ulcerans is spread to humans, despite global research efforts over more than 80 years.”