While some parts of southeast Queensland and northern New South Wales are still on alert for flooding, others are starting the difficult clean-up process as flood waters recede.

Stag­nant water after floods pro­vides the per­fect breed­ing ground for mos­qui­toes. So as you clean up, remem­ber to emp­ty con­tain­ers of water and oth­er reser­voirs around your house and yard such as water-filled boats, trail­ers and oth­er large objects. Get rid of debris that may be col­lect­ing water too.

This year, mozzies are car­ry­ing the usu­al virus­es we want to avoid, such as Ross Riv­er virus, but the poten­tial­ly dead­ly Japan­ese encephali­tis virus has also been detect­ed in parts of New South Wales and Queensland.

Will more mozzies mean more disease?

In Feb­ru­ary 2020, floods in north­ern NSW boost­ed mos­qui­to activ­i­ty and increased cas­es of mos­qui­to-borne dis­eases caused by Ross Riv­er and Barmah For­est virus­es in sub­se­quent months.

But while flood waters may boost mos­qui­to num­bers, out­breaks of dis­ease don’t always fol­low. Hur­ri­canes in North Amer­i­ca have been asso­ci­at­ed with increased mos­qui­to pop­u­la­tions but few out­breaks of dis­ease.

In Aus­tralia too, there are few exam­ples of mos­qui­to-borne dis­ease out­breaks after cyclones – with a notable excep­tion. After Trop­i­cal Cyclone Zoe made land­fall in 1974, we had one of the one of the biggest out­breaks of Mur­ray Val­ley encephali­tis virus lat­er in the year.

Warmer weath­er helps dri­ve big mos­qui­to pop­u­la­tions, and the fore­cast pre­dicts a warmer-than-nor­mal autumn. So health author­i­ties are on the look­out for out­breaks of disease.

Japanese encephalitis is already active

Some­what dry con­di­tions in the sum­mer of 2024–25 have meant mos­qui­to pop­u­la­tions in many regions of east­ern Aus­tralia have remained well below average.

But cas­es of Japan­ese encephali­tis virus have been wide­ly detect­ed in Vic­to­ria, NSW and Queens­land – in mos­qui­toes, pig­geries and fer­al pigs.

Humans have also been infect­ed. Cas­es are rare but the dis­ease can be seri­ous, with symp­toms rang­ing from fever, headache, and vom­it­ing through to dis­ori­en­ta­tion, coma, seizure and brain swelling. One per­son has died of the virus this year.

Japan­ese encephali­tis virus first arrived in south­east­ern Aus­tralia over the sum­mer of 2021–2022. That fol­lowed exten­sive flood­ing across the Mur­ray Dar­ling Basin thanks to the arrival of La Niña. At the time, there were phe­nom­e­nal num­bers of mos­qui­toes that con­tin­ued over sub­se­quent years as the above aver­age rain­fall continued.

In the sum­mer of 2022–23, a sig­nif­i­cant out­break of Mur­ray Val­ley encephali­tis was also linked to ongo­ing flood­ing. This dis­ease has sim­i­lar symp­toms to Japan­ese encephali­tis and can also be fatal.

Mos­qui­to num­bers this sum­mer have only been a frac­tion of what was record­ed dur­ing those sea­sons influ­enced by La Niña. The activ­i­ty of Japan­ese encephali­tis in 2024–25 has sci­en­tists scratch­ing their heads, as it goes against the com­mon­ly held the­o­ries that greater mos­qui­to num­bers com­bined with increased water­bird activ­i­ty (typ­i­cal­ly fol­low­ing flood­ing) dri­ve greater trans­mis­sion of virus­es such as Japan­ese and Mur­ray Val­ley encephalitis.

For­tu­nate­ly, there is no evi­dence of these virus­es along the coast of south­east Queens­land through to north­ern NSW.

But regions where the virus has already been active, such as Dar­ling Downs in Queens­land and Moree in NSW, may see sub­stan­tial rain­fall as a result of ex-Trop­i­cal Cyclone Alfred.

Pre­dict­ing out­breaks is incred­i­bly dif­fi­cult and relies not just on mos­qui­to activ­i­ty but also on on the dynam­ics of the the wildlife hosts that car­ry these viruses

But unpre­dictable mos­qui­to-borne dis­ease com­bined with extreme weath­er is not a good mix.

Don’t forget about Ross River virus

While not life threat­en­ing, dis­ease caused by Ross Riv­er virus can be severe­ly debil­i­tat­ing with joint pain and fatigue last­ing many weeks or months.

Thou­sands of cas­es of infec­tion are report­ed across the coun­try each year, includ­ing in urban areas of south­east Queens­land and north­ern NSW.

Con­cerns about Ross Riv­er dis­eases were already raised with heavy rain and flood­ing in north­ern Aus­tralia this sum­mer. Case num­bers often peak at the end or sum­mer and ear­ly autumn. So there is poten­tial for greater activ­i­ty in the com­ing months.

Oth­er mos­qui­to-borne pathogens, such as Barmah For­est virus, may also be cir­cu­lat­ing and may cause cas­es of mild dis­ease but these occur far less com­mon­ly than those due to Ross Riv­er virus infection.

Protect yourself while cleaning up

If you’re out clean­ing up after the storms, try to avoid mos­qui­to bites.

Cov­er up with long-sleeved shirts, long pants and cov­ered shoes for a phys­i­cal bar­ri­er against mos­qui­to bites.

Use top­i­cal insect repel­lents con­tain­ing DEET, picaridin, or oil of lemon euca­lyp­tus. Be sure to apply an even coat on all exposed areas of skin for the longest-last­ing pro­tec­tion.

For those liv­ing or work­ing in regions of Queens­land, NSW and Vic­to­ria at risk of Japan­ese encephali­tis, a safe and effec­tive vac­cine is available.

Cameron Webb, Clin­i­cal Asso­ciate Pro­fes­sor and Prin­ci­pal Hos­pi­tal Sci­en­tist NSW Health Pathol­o­gy and Uni­ver­si­ty of Sydney

This arti­cle is repub­lished from The Con­ver­sa­tion under a Cre­ative Com­mons license. Read the orig­i­nal arti­cle.