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Will Japanese encephalitis return this summer?

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5th December, 2023

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.

A group of pigs in a muddy yard.
Fer­al and domes­tic pigs can be infect­ed with Japan­ese encephali­tis virus and also infect mos­qui­toes that feed on them. Cameron Webb (NSW Health Pathology)

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:

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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