Travellers could trigger mosquito disease outbreak in Australia as weather warms

New tropical diseases could become established in Australia as the weather gets hotter and wetter.

Australia’s most annoying insects are bad now, and their impact could be about to get worse. International travel combined with hotter weather will likely elevate the emergence of new mosquito-borne diseases across Australia.

This week, a reminder was issued about infections including Kunjin virus, Ross River fever, Barmah Forest virus, Japanese encephalitis and Murray Valley encephalitis in NSW after heavy rain. Residents were told to beware of “serious symptoms” including tiredness, headaches, rashes, along with sore and swollen joints.

While there are several fatal mosquito-borne diseases in Australia, the population isn’t yet threatened by the worst of them. Dengue fever, chikungunya and Zika virus aren't common because the mosquito species that carry them aren’t established on the mainland, but that could be about to change.

Background - children jumping into water. Inset - mosquito bites on a girl's neck.
This year has been the world's hottest year on record, and authorities are concerned changing climate could increase the spread of mosquito-borne diseases. Source: AAP/Getty

How dengue fever could become established across Australia

As a leading researcher in medical entomology, Cameron Webb from the University of Sydney believes health authorities around the country are mindful that climate change will have an impact on mosquito-borne viruses and bacterial infections.

“With a warmer world, Australia becomes more suitable for these mosquitoes that are prevalent throughout tropical parts of the world,” he told Yahoo News Australia. “The yellow fever mosquito or the tiger mosquito may one day make their way to mainland Australia. Then that's a game changer in terms of the mosquito-borne disease risk to our major cities.”

Most mosquito-borne diseases also require mammal or bird hosts to complete their lifecycle and spread. If mosquitos capable of carrying these diseases were to become established on the mainland, then anyone who enters Australia with an infection could pose a health risk to the local population.

“It would mean every traveller that returns to Australia that's been infected with dengue while on holidays in Bali, the Pacific, South America or Southeast Asia, has the potential to trigger a local outbreak of dengue,” Webb said.

“At the moment there are no mosquitoes in Sydney for instance that can transmit it. So even though we might see a thousand travellers come back every year infected with dengue, there's no risk that they're going to trigger a local outbreak.

“But once you've got these mosquitoes in your in your cities, or towns or suburbs, that are kind of like the tinder in the forest just waiting for that spark of infected traveller to trigger an outbreak.”

In India, fumigation is required to reduce the risk of dengue fever. Source: Getty
In India, fumigation is required to reduce the risk of dengue fever. Source: Getty

Experts preparing to limit spread of mosquito-borne diseases

In recent years, Australia has experienced a string of noteworthy mosquito summers caused by the La Niña weather event which caused heavy rainfall. It’s caused major outbreaks of Japanese encephalitis and Murray Valley encephalitis — pathogens that have caused fatalities.

There have been higher occurrences of Ross River virus — around 5,000 across Australia each year. And while this disease does not result in death it can be extremely debilitating.

Flooding, above-average rainfall, and excessive inundation of coastal wetlands by king tides will increase numbers of mosquitoes, but that won’t necessarily mean they will trigger increased disease in humans.

For a disease to complete its lifecycle, vertebrate hosts are often needed. Ross River virus is most commonly spread through macropods, while encephalitis is primarily transmitted between mosquitoes and waterbirds.

To limit the spread of mosquitoes in cities, Webb and his colleagues are particularly interested in ensuring trends towards greening metropolitan centres don’t result in increased habitat for the insects.

Experts are also examining how to respond to extreme weather events in regional centres. This would include alerting local populations to the presence of disease and controlling their spread.

While use of fumigation similar to what occurs in South East Asia could be required in future, Webb believes its use in Australia would remain limited where there is widespread flooding. "One of the best things we can to inform health authorities is doing surveillance to understand how the mosquitoes respond to floodwaters," he said.

"Sometimes there's too much water and it will flush away mosquitoes. Then it's only after the floodwaters recede, and you start to get more stagnant water that their populations increase."

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