Are mosquitoes to blame for the spread of 'flesh-eating' bacteria?

Those who live and holiday on the coast are no strangers to mosquitoes. Our coastal wetlands are home to abundant mosquito populations. During summer, swarms of these nuisance, biting pests can make life quite unpleasant. They also pose a health risk.

Thousands of Australians fall ill following mosquito bites each year but it’s the suspected link between mosquito bites and an increasing incidence of mysterious “Buruli ulcers” that has Victorian authorities on alert.

What is a Buruli ulcer?

The Buruli ulcer, also known locally as the “Bairnsdale ulcer”, is caused by an environmental pathogen, Mycobacterium ulcerans. The bacterium is typically associated with many wetland areas but there is uncertainty about how individuals become infected. Is it contact with water, soil or plants? Perhaps it is insect bites?

Initially, infection with the bacterium does not cause serious problems but, over a period of many months, can lead to a destructive skin ulcer. A toxin, mycolactone, produced by the bacterium causes much of the problem by inhibiting the body’s immune response.

With an early diagnosis, treatment is straight forward and serious tissue damage can be avoided.

In more serious cases, amputation is required. Antibiotics and surgery are typical treatments but scientists are now investigating the possibility that maggot therapy (where maggots eat away the dead tissue in order for new tissue to grow) could be a useful treatment too.

Where in the world are Buruli ulcers found?

Internationally, the Buruli ulcer is considered an important but neglected disease of tropical and sub-tropical regions, particularly Africa. While it has been detected in more than 30 countries and around 6,000 cases are reported each year, in many regions, the full extent of the problem is unknown.

The Buruli ulcer has been reported in Australia for decades. Since the first case was described in 1948, cases have occurred in Queensland, the Northern Territory, Western Australia and New South Wales. In recent years, numerous cases have been reported from coastal Victoria, particularly the Mornington Peninsula and Bellarine Peninsula, which has sparked concern. Buruli ulcers seems to be on the rise, with around 150 cases reported in Victoria over the past 18 months.

Are mosquitoes responsible for the spread?

Insects and other arthropods can spread pathogens in different ways. “Biological transmission” occurs when the insect itself is infected and passes on the pathogen when it bites. This is the case for Zika virus, which is transmitted in the saliva of an infected mosquito.

Diseases can also spread through “mechanical transmission”, in which a pathogen is carried on the body of the insect between infected hosts or other substances. This is how flies move bacteria between waste and our food preparation areas.

Given reports that ulcers appeared at what was first thought to be an insect bite, could mosquitoes be to blame?

There is no conclusive evidence mosquitoes are involved in the spread of the bacterium that causes the Buruli ulcer but there is growing evidence their role may be one of multiple pathways of infection.

A small 2007 study in Point Lonsdale, Victoria, detected Mycobacterium ulcerans among approximately 11,000 locally collected mosquitoes.

A larger study in the Bellarine Peninsula tested almost 42,000 mosquitoes collected between December 2004 and December 2009. It found that at some locations, up to four in every 1,000 mosquitoes tested positive for the presence of Mycobacterium ulcerans.

Are the mosquitoes spreading the pathogen? If they are, how is it being transmitted? Is the pathogen in the local environment and people simply pick up the infection through contact with soil or water containing the pathogen? Is it just a coincidence mosquitoes are infected?

A recent study demonstrated that when consumed by immature mosquitoes in local waterways, Mycobacterium ulcerans does not continue when the adult mosquitoes emerge. It appears unlikely the mosquitoes are injecting bacteria-infected spit when they bite but what about bacteria found on their bodies, particularly those that stick into us as they suck up our blood?

More laboratory studies are needed to confirm how mosquitoes may be transmitting the bacteria to people from the local environment or wildlife.

What about local wildlife?

There is complexity underlying the transmission of Mycobacterium ulcerans. Generally thought to be an environmental pathogen of slow-moving waterways, there is some evidence native wildlife may also be involved, particularly possums.

Perhaps the pathogen is being circulated between possums, mosquitoes and the local environment and somewhere along this cycle, humans are exposed.

How can we protect ourselves?

It will be tricky for authorities to provide effective advice on avoiding infection until the exact processes of transmission are understood.

If mosquitoes are playing a role, avoiding mosquito bites will be critical. Minimising time in areas where mosquitoes are most active (dawn and dusk close to coastal wetlands), covering up where possible, and wearing topical insect repellents will help reduce the risks of the Buruli ulcer – and more common mosquito-borne diseases such as Ross River virus.

Take care to minimise contact with water or soil around these regions too, particularly if you have cuts or abrasions.

The Buruli ulcer is a mysterious condition with numerous questions about its spread yet to be answered. We need to meet many challenges before we fully understand this unusual pathogen and its relationship with the local environment and wildlife.

Cameron Webb and the Department of Medical Entomology, NSW Health Pathology and University of Sydney, have been engaged by a wide range of insect repellent and insecticide manufacturers to provide testing of products and provide expert advice on biting insect biology. Cameron has also received funding from local, state and federal agencies to undertake research into mosquito-borne disease surveillance and management.