A Victorian man is reportedly in a critical condition in hospital after contracting Japanese encephalitis from a mosquito bite. This news comes after both Victoria and New South Wales issued public health alerts in recent weeks warning about the virus.

So what is Japan­ese encephali­tis, and how can you pro­tect your­self and your fam­i­ly if you live, work or are hol­i­day­ing in mos­qui­to-prone regions this summer?

Mosquito-borne diseases in Australia

Rel­a­tive to oth­er parts of the world, Aus­tralia has tra­di­tion­al­ly been very low risk for poten­tial­ly life-threat­en­ing mos­qui­to-borne diseases.

There’s no wide­spread dengue, yel­low fever or malar­ia. But there are still many virus­es that local mos­qui­toes can spread.

About 5,000 cas­es of mos­qui­to-borne dis­ease are report­ed in Aus­tralia each year. The vast major­i­ty of these are due to Ross Riv­er virus. The dis­ease this virus caus­es is not fatal, though it can be severe­ly debilitating.

Dis­ease caused by two oth­er pathogens, Japan­ese encephali­tis virus and Mur­ray Val­ley encephali­tis virus, are much rar­er but poten­tial­ly fatal.

The symp­toms of human dis­ease caused by these two virus­es are similar.

Most peo­ple infect­ed show no symp­toms. In mild cas­es, there may be fever, headache and vom­it­ing. In more seri­ous cas­es, peo­ple may expe­ri­ence neck stiff­ness, dis­ori­en­ta­tion, drowsi­ness and seizures. Seri­ous ill­ness can have life­long neu­ro­log­i­cal com­pli­ca­tions and, in some cas­es, the dis­ease is life-threatening.

There’s no spe­cif­ic treat­ment for either dis­ease, though there is a vac­cine for Japan­ese encephali­tis which may be appro­pri­ate for cer­tain peo­ple at high risk (more on that later).

The influence of weather patterns

Mur­ray Val­ley encephali­tis virus has been known in Aus­tralia for many decades. After a sig­nif­i­cant out­break across the Mur­ray Dar­ling Basin region in 1974, activ­i­ty has gen­er­al­ly been lim­it­ed to north­ern Aus­tralia.

Out­breaks in south­east­ern Aus­tralia often accom­pa­ny flood­ing brought on by La Niña weath­er pat­terns. Floods pro­vide ide­al con­di­tions for mos­qui­toes, as well as the water­birds that har­bour the virus.

Japan­ese encephali­tis virus is close­ly relat­ed to Mur­ray Val­ley encephali­tis virus. Mos­qui­toes pick up both virus­es by bit­ing water­birds. But Japan­ese encephali­tis virus has only recent­ly become wide­spread in Australia.

After flood­ing rains brought on by La Niña in 2020, con­di­tions that per­sist­ed for three years, Mur­ray Val­ley encephali­tis virus returned and Japan­ese encephali­tis virus arrived for the first time.

Japan­ese encephali­tis virus was ini­tial­ly dis­cov­ered in south­east­ern Aus­tralia dur­ing the sum­mer of 2021–22, and the boom in mos­qui­to and water­bird pop­u­la­tions that fol­lowed flood­ing at the time con­tributed to its spread.

There have been around 80 cas­es of dis­ease caused by these two virus­es com­bined over the past four years. This includes sev­en deaths due to Japan­ese encephali­tis across Queens­land, NSW, South Aus­tralia and Vic­to­ria.

Addi­tion­al deaths have been report­ed due to Mur­ray Val­ley encephali­tis in recent years – two each in West­ern Aus­tralia and the North­ern Ter­ri­to­ry.

In the sum­mer of 2023–24, hot and dry sum­mer con­di­tions returned, mos­qui­to num­bers declined, and the num­ber of cas­es of dis­ease caused by Japan­ese encephali­tis virus and Mur­ray Val­ley encephali­tis dropped.

Now both virus­es appear to be back. So what’s going on?

What’s different this summer?

This sum­mer, Japan­ese encephali­tis virus has been detect­ed in mos­qui­toes and fer­al pigs in NSW. The virus has also been detect­ed in envi­ron­men­tal sur­veil­lance in north­ern Vic­to­ria, and we know at least one per­son has been affect­ed there.

Mean­while, Mur­ray Val­ley encephali­tis virus has been detect­ed in sen­tinel chick­en flocks – which health author­i­ties use to test for increased mos­qui­to-borne dis­ease risk – in NSW and in the Kim­ber­ley region of West­ern Aus­tralia.

It’s unusu­al to see activ­i­ty of these virus­es when con­di­tions are rel­a­tive­ly dry and mos­qui­to num­bers rel­a­tive­ly low.

Some regions of Aus­tralia may have expe­ri­enced flood­ing, but for many regions of the coun­try, con­di­tions have been hot and dry. This is bad news for mosquitoes.

There is no evi­dence that mos­qui­to num­bers are boom­ing like they did back when La Niña brought floods to the Mur­ray-Dar­ling Basin.

There also isn’t any evi­dence of more water­bird activ­i­ty. In fact, num­bers have declined in recent years.

So why are Japan­ese encephali­tis virus and Mur­ray Val­ley encephali­tis virus active again when the con­di­tions appear to be less favourable?

Despite pre­dic­tions of a rare mid-sum­mer return of La Niña, there’s still spec­u­la­tion about what this means for tem­per­a­ture and rain­fall. We may not see flood­ing, but there is still like­ly to be enough water around for mosquitoes.

For Japan­ese encephali­tis virus, it may be that fer­al pigs are play­ing a more impor­tant role in its spread. We know num­bers are on the rise and with dri­er con­di­tions, per­haps mos­qui­toes and fer­al pigs, and oth­er wildlife, are gath­er­ing togeth­er where they can find bod­ies of water.

After its unex­pect­ed arrival, it now seems Japan­ese encephali­tis virus is here to stay. But how this virus inter­acts with local mos­qui­toes and wildlife, under the influ­ence of increas­ing unpre­dictable cli­mat­ic con­di­tions, requires more research.

How can you reduce your risk this summer?

The pub­lic health alerts in Vic­to­ria and NSW focus espe­cial­ly on spe­cif­ic regions in north­ern Vic­to­ria and around Grif­fith and Nar­romine in NSW where the virus has been detected.

If you live or work in areas at risk of Japan­ese encephali­tis, seek advice from your local health author­i­ty to see if you are eli­gi­ble for vac­ci­na­tion. Res­i­dents in spec­i­fied local gov­ern­ment areas in affect­ed regions in both states are cur­rent­ly eli­gi­ble for a free vaccine.

But there is no vac­cine avail­able for Mur­ray Val­ley encephali­tis or Ross Riv­er viruses.

Wher­ev­er you live, mos­qui­to bite pre­ven­tion is key. Apply insect repel­lent when out­doors, espe­cial­ly dur­ing dawn and dusk when mos­qui­toes are most active or at any time of the day if you’re in bush­land or wet­land areas where num­bers of mos­qui­toes may be high.

You can get bet­ter pro­tec­tion by also cov­er­ing up with a long-sleeved shirt, long pants, and cov­ered shoes.

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

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