Mosquitoes in certain regions of Australia might be carrying a perilous cargo from possums to humans – flesh-eating bacteria known as Mycobacterium ulcerans. This microbe is responsible for Buruli ulcer, a disfiguring skin disease prevalent in parts of Australia and Africa. While antibiotics can help heal ulcers, untreated cases can lead to scarring, permanent disfigurement, and disability.
It is not just humans who are susceptible to this disease; native possums in Australia, such as the common ringtail possum, also develop ulcers and shed the bacteria through their feces. Researchers have long suspected that mosquitoes interacting with infected possums could play a role in transmitting the bacteria to people, but establishing a definitive link between possums, mosquitoes, and human infection has been a challenge.
Recent surveys of mosquitoes in southeastern Australia have finally provided the missing link. Analysis of captured mosquitoes has shown that a small number of these insects had recently fed on both possums and humans, according to molecular microbiologist Timothy Stinear, who presented the findings at the ASM Microbe 2023 meeting. Furthermore, genetic analysis has revealed that the M. ulcerans bacteria found in mosquitoes, possums, and humans are identical. This discovery strongly supports the transmission chain between these species.
The research suggests compelling evidence that mosquitoes may indeed be transmitting the disease in Australia. However, M. ulcerans is a slow-growing microbe, and symptoms of the flesh-eating disease may not appear until several months after transmission, making it challenging to pinpoint the exact source of infection.
During the study, Stinear and his team captured over 72,000 mosquitoes from the Mornington Peninsula in southeastern Melbourne, an area where the disease is prevalent. When approximately 18,000 individuals were tested for M. ulcerans, nearly all positive cases were found in one species of day-biting mosquito called Aedes notoscriptus.
Among the tested mosquitoes, two of the A. notoscriptus species had recently fed on both a ringtail possum and a human. Although this number is relatively small, it is likely that such mosquitoes are rare. Approximately 200 to 300 cases of Buruli ulcer are reported in Australia each year, with numbers steadily rising. In 2022, the World Health Organization recorded around 2,100 cases from 11 countries.
The study also revealed overlapping areas where infected mosquitoes, possum feces contaminated with bacteria, and diagnosed Buruli ulcer cases in humans were found. However, the exact mechanism by which mosquitoes transmit M. ulcerans from animals to humans remains unclear.
One possibility is that A. notoscriptus, while feeding on possum ulcers, physically carries the bacteria to their next victim. However, other mosquito species that also feed on possums do not seem to pick up the bacteria. Another theory suggests that possum feces may contaminate the small artificial containers where A. notoscriptus lays its eggs, infecting the water and subsequently infecting the developing insects.
The transmission of the disease in Australia appears distinct from what occurs in Africa. In western Africa, water bug bites can inject the bacteria into the skin. However, it is unknown whether these bugs transmit the bacteria to humans from another animal.
Although M. ulcerans from Australia and Africa share genetic similarities, it is surprising that the transmission cycle may differ. Thus far, surveys in western Africa involving mosquitoes and small mammals have not uncovered signs of M. ulcerans. It is possible that further studies in Africa may reveal missing pieces of the puzzle.