How we kept disease-spreading Asian Tiger mozzies away from the Australian mainland

Spring floods and summer heatwaves. There is nothing mosquitoes love more than warm weather and water. In many regions of Australia, these extreme conditions can increase the risk of mosquito-borne disease.

Outbreaks of Ross River virus in some parts of the country have been as record-breaking as the heatwaves. The virus has even started encroaching on suburban Sydney and Melbourne.

While increasing climate variability and extreme weather events may heighten the risks of home-grown mosquito-borne disease, the real game changer will be the arrival of new mosquitoes to Australian backyards. The Asian tiger mosquito, Aedes albopictus, is the type of greatest concern.

Why worry about the Asian tiger mosquito?

This mosquito has already invaded most continents, bringing with it outbreaks of mosquito-borne disease. But it’s not just the risk of disease that’s a concern. This mosquito is also one of the most important pest mosquito species on the planet because they bite relentlessly during the day. In a recent study of over 120 people in the US, almost 60% claimed the Asian Tiger Mosquito prevented them enjoying time outside.

Having adapted to life around our homes, the mosquito lays its eggs in water-holding containers rather than wetlands. It’s not just restricted to tropical regions and is just as much at home in cooler climates.

So the tiger mosquito could become a common feature of life in major metropolitan regions of Brisbane, Sydney, Melbourne, Adelaide or Perth. Once it’s here, the risk of outbreaks of dengue, chikungunya and Zika viruses will increase as, at the moment, the common mosquitoes in our cities cannot transmit these pathogens. But the tiger mosquito can.

A serious pest on our doorstep

This mosquito has been spread around the world with the movement of humans and their belongings. Although it has been detected many times at our border, it hasn’t become established on mainland Australia. However, more than a decade ago, the mosquito was discovered on islands in the Torres Strait and, given the invasive nature of this mosquito, authorities were concerned this would just be a stepping stone to the suburbs of our major metropolitan regions.

Between 2005 and 2008, a government-funded eradication program attacked the mosquito on many fronts. Teams were dispatched from Cairns into the Torres Strait for weeks at a time to assess and respond to the detection of the mosquito.

The teams searched water-filled containers looking for mosquito wrigglers every day. Genetic analysis of specimens was also undertaken to ensure there was no confusion between the detection of Aedes albopictus and other local mosquito species.

Insecticides were applied to water-filled containers and other habitats, while rubbish and other potential habitats (such as discarded containers or tyres) were removed, destroyed or placed under cover and out of reach of mosquitoes. Unfortunately, eradicating a mosquito isn’t easy. The job was made more difficult due to suspected re-introductions into the region by boat traffic and the continual accumulation of rubbish that the mosquito loves.

More worrisome was that the mosquito invaded both Horn Island and Thursday Island, which are the major transport hubs between Torres Strait and the mainland. If the mosquito became established there, the chance of them hitching a ride into Queensland would increase.

Strategic shifts on mosquito control efforts required

In response, authorities took the fight to where the mosquitoes were hiding out. Well-shaded leaf litter under thick vegetation that provided protection from wind and sunshine was a perfect place for mosquitoes to rest. It was also the perfect place to target control efforts.

In an approach known as “harbourage spraying”, insecticides were applied to these mosquito resting spots. The insecticides offered residual control, in much the same way surface sprays used in our kitchen keep cockroaches at bay. Once mosquitoes went looking for these hiding places, they were exposed to the insecticide and died.

Supporting the control efforts was a strategic surveillance program, tracking the presence of this mosquito at key locations. The mosquito control proved so successful that there was a steady decline in the detection of the mosquito, to the point that it is now undetectable in up to 90% of surveys on Horn and Thursday Islands.

This success is great news on two fronts. Firstly, it has undoubtedly reduced the risk this mosquito will make it to mainland Australia. Testimony to the effectiveness of the program is there has never been an established population of Aedes albopictus detected on the mainland in the more than ten years since its first detection in the Torres Strait.

Secondly, the experience gained from the Aedes albopictus eradication program provides a framework for practical management of this species during disease outbreaks, both within Australia or internationally. When outbreaks of dengue, chikungunya or Zika viruses occur, the approaches shown to be effective in suppressing populations in the Torres Strait can be employed to reduce the burden of disease. These approaches may not work against the “wetland” mosquitoes spreading Ross River virus, but they look to be perfect for “backyard” mosquitoes such as Aedes albopictus.

At a time when emerging technologies are expected to radically change the way we control mosquito populations, the lessons from Torres Strait are a reminder there will never be a “one size fits all” approach. Strategic and flexible approaches will provide the best outcomes in the battle against mosquito-borne disease.