Rain (and roos) help the spread of mosquito-borne illnesses in Australia’s south

Widespread rain and flooding along parts of southern Australia have created a perfect breeding ground for mosquito species to introduce rare illnesses to parts of the continent.

In recent months, Ross River virus and Barmah Forest virus have been detected in parts of regional Victoria, South Australia and New South Wales. These pathogens are common to these areas.

But Murray Valley encephalitis cases have also occurred in NSW and Victoria. Despite being named for a 1950 outbreak in this Murray River region, this virus is endemic to northern Australia and rarely occurs in southern regions.

Authorities are also warning of new detections of West Nile/Kunjin virus and Japanese encephalitis, which have been detected along southern river regions despite being usually associated with the tropics.

Although many people will be asymptomatic when stung by a mozzie carrying these diseases, some can be badly affected, with symptoms common to each include fever, headaches and vomiting.

Japanese Encephalitis can cause swelling of the brain, and has resulted in deaths in Australia since its return in February 2022.

Why are these viruses moving in southern states?

A combination of factors has made southern mainland regions suitable for mosquitoes carrying the diseases to grow in number.

It’s easy to assume that viruses typically associated with Australia’s tropics are appearing in southern regions because mosquitoes are bringing them down.

But mosquitoes are merely vectors – they don’t naturally harbour viruses – instead, they’re like flying needles, which pick up viruses when they stick their noses into one species and then transfer it when they prick another.

Why are some people so irresistible to those itch-causing bloodsuckers?

And it’s some of Australia’s favourite icons that carry the virus most of the time. Ross River virus occurs in macropods – kangaroos and wallabies. Likewise Murray Valley encephalitis and West Nile/Kunjin is known to be carried by horses and some birds.

Japanese encephalitis, explains Professor Craig Williams from the University of South Australia’s Sansom Institute for Health Research, likely penetrated Australia’s southern regions thanks to migratory waterbirds flying into the nation’s northern regions from Asia.

“Water birds, we know, are migratory: they move from waterhole to waterhole, and it’s most likely that viruses were introduced into the Northern Territory or adjacent areas in late 2021,” Williams tells Cosmos.

“[Then] local waterbirds in northern territory became infected, and then the virus was able to move steadily across the continent, facilitated by bird flight.

“The increased rainfall provides more mosquito habitat, and so you’ll get more mosquito breeding… But to enable disease transmission, you need more than just mosquito breeding and biting, you need the carriers of the viruses to also enter the area.”

A possible emergence of a drier, hotter El Niño climate event at the end of 2023 could reverse these prime breeding conditions.

Aerial view of two rivers intersecting
Australian river communities are at increased risk of mosquito-borne disease due to recent floods. Credit: Mark Evans / Stringer / Getty Images

Prevention, and monitoring for disease is recommended as surveillance ramps up

In the meantime, locals in flood-affected areas along the Murray-Darling basin are being encouraged to take preventative measures against infection, like using insect repellent and suitable body coverings. People working outdoors or participating in outside activities are also more likely to be infected with these viruses.

Those with severe symptoms are being encouraged by authorities to seek emergency medical care and confirm infection with a blood test.

For Williams, the emergence of Japanese encephalitis in the southern parts of Australia where previously  it had only twice occurred on the northern fringes of the continent, highlights the increasing risk of incoming disease.

He’s hopeful that the recently announced government-funded disease control centre will further boost focus on viral monitoring.

“You’ve had SARS-CoV-2 [the virus that causes COVID-19], which has made a lot of people say ‘Hey, isn’t it time we had a central policy, research and surveillance system for these diseases?’, and then all of a sudden we’ve got Japanese encephalitis on top of that,” he says.

“That’s further illustrated our fragmented surveillance and our general lack of preparedness [for new disease] as a nation.”

Vaccines for Japanese encephalitis are recommended by the Australian health department for people at-risk of infection.

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