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23 February 2022 6 min read

CSIRO scientists expected high numbers of mosquitoes and mosquito-borne diseases this summer, due to high rainfall in areas that are normally much drier.

“We anticipated a difficult mosquito season and it has been,” said Dr Brendan Trewin, who has been researching mosquitoes for more than a decade.

“CSIRO doesn’t routinely monitor mosquito populations, but during my travels in NSW this summer there were mozzies everywhere. It’s still too early to know whether there has been an increase in vector-borne diseases, but we certainly expect some regions to experience increased risk with all this rainfall,” he said.

Mosquito-borne diseases

Pinned mosquito specimen
A female Culex annulirostris mosquito in the Australian National Insect Collection.

There are three primary mosquito-borne diseases endemic to Australia that affect humans. They are Barmah Forest fever, Murray Valley encephalitis and Ross River fever. All three diseases are notifiable. This means data on case numbers can be compared from year to year.

“2015 was a bad season for Ross River fever, the worst in over 20 years. We had a dry period followed by a wet summer leading into 2015. This led to an epidemic of Ross River fever, with around 10,000 notifications nationwide and 1,500 in Brisbane alone,” Brendan said.

As a result, populations of the Australian salt marsh mosquito, Aedes vigilax, boomed in the salt marshes near Brisbane. This species is the primary vector of Ross River virus. This early season peak was followed by large numbers of the common banded mosquito, Culex annulirostris, another vector of Ross River virus, which live in freshwater habitats near the suburbs.

Living in a 1940s era house without fly screens on its windows, Brendan recalls turning off his lights to watch a movie one evening. In the sudden darkness a cloud of a thousand mosquitoes descended from the ceiling searching for a blood meal. One of them was infected with Ross River virus and it had found Brendan.

A long history of dengue fever

Brendan suffered a bout of Ross River fever, a disease with arthritis-like symptoms. It’s often overshadowed by the more serious disease, dengue fever.

The mosquito that spreads dengue virus is Aedes aegypti, originally native to Africa. This highly invasive, deadly species spread across the tropics in the hulls of sailboats hundreds of years ago. It is of great concern to scientists and health authorities.

The Aedes aegypti mosquito occurs on every continent except Antarctica. It spreads mosquito-borne diseases like dengue and Zika.

“One hundred years ago, there were large epidemics of dengue fever in Queensland, with up to 75 per cent of residents effected. However, from the 1930s to the 1960s, governments battled mosquitoes by replacing back yard rainwater tanks with piped water, improving sewers and draining swamps,” Brendan said.

Today, the most southern Ae. aegypti populations are around 150 km north of Brisbane. But places as far south as Perth and Sydney are still vulnerable to invasion by this species. This is particularly due to the popularity of rainwater tanks.

Suppressing mosquito populations

Australian scientists are leading the world in innovative mosquito and vector-borne disease control. Last year they announced results of a trial to suppress populations of Ae. aegypti in North Queensland by releasing millions of male mosquitoes infected with Wolbachia.

The trial was an international collaboration between CSIRO, University of Queensland, Verily Life Sciences, QIMR Berghofer Medical Research Institute and James Cook University.

Wolbachia is a naturally occurring bacteria in insects, with up to 60 per cent of species infected. It is harmless to humans but can manipulate reproduction in mosquitoes and other insects. By taking advantage of this natural system, it is possible to produce males that will mate with wild female mosquitoes and produce sterile eggs.

The trial caused a huge crash in mosquito numbers, showing the technology is highly efficient and capable of eliminating populations of Ae. aegypti.

"Although North Queensland has been protected with Wolbachia-containing Ae. aegypti, more southerly populations still represent a threat and our major cities are vulnerable. For example, an outbreak of dengue in Rockhampton in 2019 was the first in the region in over 60 years. A person flying into Australia with dengue can be bitten by local mosquitoes, which then infect other people. This is how outbreaks start," Brendan said.

“There aren’t a lot of invasive mosquito surveys happening in Australian major cities. It would only take a small invasion of Aedes aegypti or the Asian Tiger mosquito, Aedes albopictus, to create a serious disease risk. Fortunately, we haven’t seen many cases of dengue fever in Australia lately. COVID border closures have prevented travellers bringing it in, but we continue to see increased detections of these invasive mosquitoes at our ports and quarantine facilities. This is a serious concern."

Together with partners, CSIRO researchers are now applying the learnings from controlling dengue mosquitoes to controlling those that spread Ross River virus and protecting vulnerable communities in Australia.

Scientific trials in North Queensland to reduce mosquito numbers caused a huge crash in the populations.

An important part of ecosystems

Mosquitoes are flies. They belong to the Order Diptera, which has around 250,000 species worldwide. They’re not the only flies that bite people, an honour shared by midges, march flies and horse flies.

“There are around 3,000 species of mosquitoes in the world, but only around 5 to 10 can kill humans by transmitting disease. The rest make a good contribution to ecosystems. They pollinate flowering plants and are a food source for other organisms like fish and insects,” Brendan said.

Mosquito eggs can lay dormant for a year waiting for rain, then hatch when exposed to water. The larvae live underwater, pupate underwater and emerge as flying adults.

Only females bite, thus only females pose a risk to humans. This is because the protein-rich meal of our blood supports their egg production. Different species specialise in biting different creatures, such as birds or even reptiles. Unfortunately, the species that live in our backyards like to bite us. The aptly named Australian backyard mosquito, Aedes notoscriptus, is one.

Hidden breeding sites around your home

a terracotto dish filled with water with leaves in the background
Mosquitoes can breed in water bowls left out for pets and birds.

Simple things like installing fly screens on doors and windows, wearing long sleeves and pants, using a repellent like Aerogard, staying indoors at dawn and dusk and keeping away from salt marshes will help prevent mosquito-borne diseases. So will preventing mosquitoes from breeding around your home.

“Around 10 years ago as part of my early mosquito studies, we surveyed 5,000 homes in and around Brisbane. Although we found no dengue or Asian Tiger mosquitoes, we did find a lot of great mosquito breeding habitats in people’s backyards," Brendan said.

“Buckets are one of the biggest culprits in backyards. They might have garden clippings or other organic matter in them. They collect water when it rains and mosquitoes love to breed in the water.

“Mosquito life cycles are around 9 days, which means even your dog’s water bowl could be providing breeding habitat. Empty bird baths, chook water and pot plant bases once per week," he said.

Backyard rainwater tanks[Link will open in a new window] are very high-risk habitat for invasive mosquitoes such as the Asian Tiger mosquito. This species lays eggs in pools of water on items awaiting export by ship, such as large machinery tyres. If it rains at sea, dormant eggs can complete their life-cycle and invade new countries.

Brendan recommends checking home rainwater tanks regularly. They should have sieves that are clean and sealed to prevent the entry and exit of mosquitoes.

“Studies overseas have shown that fly screens and air conditioning are two of the best ways to prevent mosquito-borne diseases. This is because people tend to keep their doors and windows closed while using air conditioners, preventing entry of mosquitoes,” he said.

If mosquitoes bother you at night, Brendan suggests a fan blowing over you can help. That's because the strong air currents are too much for the mosquitoes’ tiny wings.

Read more about mosquito trials at CSIRO.

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