The last two summers have been swarming with mosquitoes thanks to near constant rain and flooding brought on by La Niña.

With the return of El Niño, and a hot, dry sum­mer in store, what’s the out­look for Japan­ese encephali­tis and oth­er mos­qui­to-borne diseases?

First, let’s look back at the last two summers

The boom in mos­qui­toes over the last two springs and sum­mers didn’t just bring an increased annoy­ance of buzzing and bites but also out­breaks of poten­tial­ly fatal mos­qui­to-borne disease.

The first out­break of Japan­ese encephali­tis virus was first detect­ed in south­ern regions of main­land Aus­tralia in Feb­ru­ary 2022.

This was fol­lowed by the return of Mur­ray Val­ley encephali­tis in ear­ly 2023, which turned out to be the biggest out­break in the south­ern states since 1974.

These out­breaks were the result of more than just more mos­qui­toes. Flood­wa­ters pro­vid­ed ide­al breed­ing con­di­tions for water­birds, the key “reser­voirs” of these viruses.

Mos­qui­toes pick up the infec­tion after feed­ing on the birds and then sub­se­quent­ly spread the virus­es to peo­ple when they bite.

What’s different about Japanese encephalitis?

Out­breaks of Japan­ese encephali­tis virus in tem­per­ate regions of Aus­tralia in 2022 came as a sur­prise. There had been activ­i­ty in north­ern Aus­tralia and the Tor­res Strait, but it was gen­er­al­ly only con­sid­ered a risk to over­seas travellers.

In India, South­east Asia, and the West­ern Pacif­ic, Japan­ese encephali­tis is con­sid­ered one of the most dan­ger­ous mos­qui­to-borne dis­eases, with tens of thou­sands of cas­es of severe infec­tion each year.

While the major­i­ty of peo­ple infect­ed suf­fer no or very mild symp­toms, some will expe­ri­ence neck stiff­ness, fever, headache and, in the most severe cas­es, per­ma­nent neu­ro­log­i­cal com­pli­ca­tions or death.

How­ev­er, a vac­cine is avail­able that can sig­nif­i­cant­ly lim­it seri­ous illness.

The dis­cov­ery of Japan­ese encephali­tis virus in Australia’s south­ern states trig­gered a dec­la­ra­tion of a “com­mu­ni­ca­ble dis­ease inci­dent of nation­al sig­nif­i­cance”. This was in place from March 2022 through June 2023. A total of 45 peo­ple were infect­ed, sev­en of whom sad­ly died.

It wasn’t just peo­ple who were at risk. The impact on com­mer­cial pig­geries, which farm pigs for pork pro­duc­tion, was dev­as­tat­ing and required urgent strate­gies to con­trol mos­qui­toes.

Pig­geries weren’t the source of the out­break, they were the “canaries in the coalmine” – sig­nalling the spread of the virus ear­ly on and the need to pro­tect the broad­er community.

What caused outbreaks in piggeries?

Our research inves­ti­gat­ed how dif­fer­ent land­scapes and weath­er pat­terns influ­ence inter­ac­tions between wildlife, mos­qui­toes, and out­breaks of Japan­ese encephali­tis virus.

We looked at 62 pig­geries where the virus had been detect­ed and some loca­tions where the virus had also been detect­ed in mos­qui­toes, along with water­bird and fer­al pig habi­tats, rain­fall and temperature.

Some of the results were unex­pect­ed. Pig­geries were at high­est risk of an out­break when the num­ber of dif­fer­ent water­bird species in their loca­tion was “just right”. If there were too few or too many, the risk of an out­break was reduced.

High rain­fall and flood­ing pro­vid­ed excel­lent con­di­tions for mos­qui­toes, with tem­po­rary wet­lands and flood­ed areas pos­ing a greater risk than per­ma­nent wetlands.

Tem­po­rary wet­lands may have pro­vid­ed habi­tat for water­birds whose nor­mal habi­tat and move­ment pat­terns were dis­rupt­ed due to the exten­sive La Niña flooding.

Or per­haps per­ma­nent wet­lands sup­port a greater diver­si­ty of aquat­ic life (includ­ing ani­mals that eat mos­qui­toes) that helped keep mos­qui­to num­bers low­er than tem­po­rary waterbodies.

So what might happen this summer?

The return of El Niño is expect­ed to bring below aver­age rain­fall and above aver­age tem­per­a­tures. But that can be unpre­dictable. Wet­lands are already dry­ing up. Bush­fires have replaced floods.

Mos­qui­to pop­u­la­tions are expect­ed to decline sharply. Sur­veil­lance pro­grams of state and ter­ri­to­ry health author­i­ties, such as New South Wales and Vic­to­ria, are already report­ing mos­qui­to pop­u­la­tions far low­er than pre­vi­ous seasons.

So we may not see as much Japan­ese encephali­tis this sea­son. But that doesn’t mean it will dis­ap­pear completely.

It doesn’t mat­ter how hot and dry it gets, mos­qui­toes are resilient and will per­sist. They’ll seek out the same envi­ron­ments where water remains. So too will water­birds and fer­al pigs.

Author­i­ties are also on alert for the return of Ross Riv­er virus along the coast. Despite the low­er rain­fall, the mos­qui­toes that live in salt­wa­ter wet­lands will thrive fol­low­ing flood­ing by high tides, espe­cial­ly “king tides”.

Com­bined with extreme weath­er, even dur­ing hot and dry sum­mers, out­breaks of Ross Riv­er virus can occur.

How can you reduce your chance of getting these viruses?

To pro­tect your­self and fam­i­ly from mos­qui­to bites and mos­qui­to-borne disease:

• keep fol­low­ing the advice of health authorities
• try to avoid bush­land and wet­land areas in late after­noon and ear­ly evening
• cov­er up with long-sleeved shirts, long pants and cov­ered shoes
• apply a rec­om­mend­ed insect repellent
• check your eli­gi­bil­i­ty for a Japan­ese encephali­tis vac­ci­na­tion.

Cameron Webb, Clin­i­cal Asso­ciate Pro­fes­sor and Prin­ci­pal Hos­pi­tal Sci­en­tist, Uni­ver­si­ty of Syd­ney; Michael Walsh, Senior Lec­tur­er of Infec­tious Dis­ease Epi­demi­ol­o­gy, Uni­ver­si­ty of Syd­ney, and Vic­to­ria Brookes, Lec­tur­er in Epi­demi­ol­o­gy and One Health, Uni­ver­si­ty of Sydney

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

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