When Alice Mika was bitten by a mosquito in Melbourne’s west last summer, she thought nothing of it.
Then, more than five months later, a small raised bump appeared on her ankle and wouldn’t go away. She saw her GP who prescribed antibiotics, believing it to be a spider bite.
But over the course of two months, “it just got worse and worse,” she said. Her ankle began to swell and the scab from the bite eventually erupted into a gaping hole exposing the layer of fat beneath the skin.
Mika’s doctors initially did not think she could have the flesh-eating Buruli ulcer because she told them she had not visited Victoria’s Bellarine Peninsula, one of the coastal areas. where the disease is thought to be limitedwhich also includes the Mornington Peninsula.
However, in recent years it has spread. Since 2019, cases have been reported in the suburbs of Geelong and inner-city Melbourne, including Essendon, Moonee Ponds and Brunswick West.
The Victorian government announced last week a a record 363 cases were reported in 2023contributing to the exponential increase over the past two decades since 12 were reported in 2003.
But how Australians end up with these skin sores has been a mystery since Australian scientists discovered this bacterium more than 80 years ago, said Prof Tim Stinear, the director of the World. Health Organization Collaborating Center for Mycobacterium Ulcerans at the Doherty Institute.
But now research led by Stinear, published on Wednesday Nature Microbiologyclaims to have solved the “transmission mystery” with evidence that mosquito bites spread the infection between possums and humans.
“The research builds on decades of research where we had an idea that mosquitoes were involved in the spread of this infection, but we were met with a lot of skepticism in the field because it is unprecedented for this type of bacteria to be spread by mosquitoes, ” Stinear said.
The researchers trapped and tested more than 65,000 mosquitoes between 2016 and 2021 in the Mornington Peninsula, known to have one of the highest incidences of Buruli ulcer in the world, and built a “hierarchy of evidence” which shows that the insects are responsible for spreading the infection, Stinear said.
Using forensic-level genomics, the researchers found that the bacteria in the mosquitoes matched the bacteria in the humans.
They also sequenced the DNA in the blood in the mosquitoes’ abdomens and found that the insects fed on both Australian native possums and humans.
The researchers also did a spatial mapping and statistical analysis of the areas where people get Buruli ulcer, where possums carry the bacteria, shed it in their faecal pellets, and where mosquitoes also carry the bacteria, and found that those areas overlap in the space.
Prof Rhonda Stuart, the director of Public Health and Infection Prevention at Monash Health, said “this is a very important paper. It fills in the gaps of things we’ve wanted to know about Buruli ulcer for many years, and the link between human disease, possums and mosquitoes as the likely vectors.”
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Stuart said increased mosquito incidence due to torrential rains could be one of the factors contributing to the spread of the Buruli sore.
“We saw it last summer with Japanese encephalitis, where normally Japanese encephalitis mosquitoes were just north, and then we saw them further down into southern New South Wales and northern Victoria,” Stuart said.
Stinear hopes the research’s findings will lead to better control of this infection, including greater awareness of how individuals can prevent mosquito bites, stop mosquitoes from breeding around homes and public health measures.
The infection has a very long incubation period, Stinear warned. From the time you are bitten by the mosquito to the time symptoms begin to appear, starting with a bite or pimple that doesn’t heal, is usually about four to five months, he said.
“It can be painless, so people ignore it and eventually the skin breaks down, and you’re left with an ulcer, but actually underneath the skin is a lot of dead and dying tissue that the victim isn’t aware of.”
If a GP can diagnose Buruli ulcer early, a PCR test can quickly confirm the infection and specific antibiotics that will effectively treat it.
The infection is not fatal, but if untreated, people are left with scars that can affect quality of life during treatment, and afterward when they are left with a lifelong disfigurement or disability, Stinear said.
Mika said her ulcer is healing and “going in the right direction.”
“Hindsight is 2020. GPs can’t know everything, but it’s a shame they weren’t aware [the disease] was in Melbourne – it might have been diagnosed earlier.”
