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Japanese Encephalitis Virus’ Australian arrival

9 Dec 2022

Historically a condition of Asia and the Pacific, the mosquito-borne Japanese Encephalitis Virus (JEV) entered Australia’s consciousness in February 2021 with a fatal, locally acquired infection. In 2022, JEV was declared a Communicable Disease Incident of National Significance; over 42 human cases (and seven fatalities) have been notified at time of writing. With a third consecutive La Niña event underway, we caught up with Dr Paul De Barro, Senior Principal Research Scientist Health & Biosecurity at CSIRO, to contextualise the disease for remote area health professionals.

This inter­view was con­duct­ed in ear­ly Novem­ber 2022. Con­sult the lat­est resources from rel­e­vant health author­i­ties for the most up-to-date information.

JEV nev­er used to be a con­cern in Aus­tralia. How did it get here?

We’ve had events in Cape York [in the 1990s], but last year things changed dramatically.

The out­break that occurred in south-east­ern Aus­tralia was asso­ci­at­ed with detec­tions of Japan­ese encephali­tis in main­land Aus­tralia below Cape York and across the North­ern west of Aus­tralia… That’s been asso­ci­at­ed with this ongo­ing La Niña event. 

The virus has numer­ous dif­fer­ent reser­voirs – includ­ing var­i­ous species of wad­ing birds, which are migra­to­ry in the sense they move to where the water is. 

Many of these species will move between coun­tries to our north, into Aus­tralia, and then fur­ther south… When it starts to get a bit too cold, they’ll move back north again, but as long as there is water in the south, they will fly back to those watery areas to repro­duce over the spring and summer. 

One pos­si­bil­i­ty is that, as the extra rain­fall in south­ern Aus­tralia has led to the replen­ish­ment of water­ways, birds have moved into those areas… That’s pos­si­bly been bring­ing the virus in from the north of Australia. 

[Also], with all the extra rain­fall, fer­al pig num­bers have been increas­ing. Fer­al pigs are an impor­tant ampli­fy­ing host.

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Can you explain the con­cept of an ampli­fy­ing host?

With Japan­ese encephali­tis, there are three sorts of hosts. There are the dead-end hosts, like humans or mar­su­pi­als, where they can get infect­ed, but the virus is unlike­ly to build up, [such that it can] be acquired by a mos­qui­to, that can then spread it further. 

Then there’s reser­voir birds, where the virus will tick over but it doesn’t build up to large amounts; and third­ly, ampli­fy­ing hosts, pigs being the prime exam­ple. These are hosts where the virus builds up to very high titres, and they become impor­tant sources for sub­se­quent transmission. 

The thing that con­nects these togeth­er is mos­qui­tos. In Aus­tralia there’s a mos­qui­to called the Culex annulirostris, known as the com­mon band­ed mos­qui­to. It takes advan­tage of La Niña con­di­tions, because it repro­duces rapid­ly in shal­low stand­ing water.

This mos­qui­to is also a bit dif­fer­ent from mos­qui­tos that trans­mit a virus like dengue. Those mos­qui­tos tend to stay very local… and feed most­ly on humans, so they live in around human habitation. 

Where­as the com­mon band­ed mos­qui­to can move 20, 30, 40, 50 kilo­me­tres or more… and it’s polyphagous, mean­ing the females will seek many dif­fer­ent sorts of species to blood-feed, to mature their eggs. 

[This leads to] a com­plex inter­ac­tion of dif­fer­ent hosts. In Aus­tralia we don’t nec­es­sar­i­ly under­stand the impor­tance of these dif­fer­ent hosts in our sys­tem, but fer­al pigs and domes­tic pigs are going to be a very impor­tant ampli­fy­ing source of the virus… If mos­qui­toes per­sist in a par­tic­u­lar area, there’s also a pos­si­bil­i­ty the virus could cycle through those mos­qui­tos – get passed through the moth­er, through its eggs to its off­spring. That’s less cer­tain but is anoth­er possibility

Could exten­sive flood­ing through­out the nation’s east be cre­at­ing the per­fect storm’ for mos­qui­to num­bers – and there­fore the risk of JEV?

Absolute­ly. As we start to move into the warmer months of spring and sum­mer, we’re already see­ing increas­es in mos­qui­to numbers. 

The ques­tion is whether there is still Japan­ese encephali­tis resid­ing in these south­ern areas, or whether it has to come in again from the north. We don’t know whether the virus will per­sist in the south year on year, or whether it has to be rein­tro­duced each year [by migra­to­ry birds]. 

There’s anoth­er mos­qui­to-borne virus called Mur­ray Val­ley Encephali­tis (MVE) which occurs in the south-east cor­ner of Aus­tralia. Its cycle is large­ly through birds, but with MVE you can have an out­break one year and see noth­ing the year after. So, it’s pos­si­ble we won’t see Japan­ese encephali­tis this year, but the pre­con­di­tions are cer­tain­ly there, such that if the virus has man­aged to per­sist, it will reappear.

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Could JEV ever become as seri­ous here as it is in Asia, in terms of peo­ple impact­ed per capita?

Asia has large num­bers of peo­ple, rice pad­dies, dif­fer­ent mos­qui­tos… The way in which peo­ple live is dif­fer­ent. So, it’s prob­a­bly not going to turn into a sim­i­lar sit­u­a­tion in terms of scale.

But it’s almost cer­tain­ly going to be the case that the virus has estab­lished in Aus­tralia… We’ve got evi­dence that it’s present in pig pop­u­la­tions in the north­ern part of the coun­try now. 

Look­ing across arbovirus­es, health pro­mo­tion mes­sag­ing seems sim­i­lar, which is hope­ful­ly help­ful to nurs­es and clients. 

For any mos­qui­to-borne dis­ease, the mea­sures a human can take to pro­tect them­selves are all pret­ty well the same. It boils down to – don’t let mos­qui­tos bite you. 

Use mos­qui­to repel­lents. Many of the mos­qui­tos which are impor­tant tend to be active from dusk through to dawn, so if you’re out­side at dusk, make sure you’re wear­ing repel­lent. If pos­si­ble, wear longer, lighter coloured cloth­ing; dark colours tend to be more attrac­tive to mos­qui­tos. If you’ve got fly­wire on your win­dows, make sure it is intact, and make sure you keep the win­dows closed. Not hav­ing win­dows open, using air-con­di­tion­ing, helps to reduce mos­qui­to activ­i­ty with­in the house.

Clean­ing up flow­er­pots, dog bowls, or water baths around the house isn’t going to be all that use­ful for pro­tect­ing against Japan­ese encephali­tis, sim­ply because this mos­qui­to is not breed­ing in those areas… But if you are deal­ing with dengue and the like, clean­ing up in and around your house can help.

JEV vac­ci­na­tions have been made avail­able in Aus­tralia to those liv­ing in sta­tis­ti­cal­ly high-risk areas. It’s inter­est­ing to com­pare and con­trast. COVID-19 vac­cines are thought to play a role in reduc­ing trans­mis­sion, but JEV vac­cines can’t be preventative?

That’s right. It’s about stop­ping a rather severe dis­ease, because for that one per cent of indi­vid­u­als who have symp­toms for Japan­ese encephali­tis, the out­comes are not fan­tas­tic. But humans aren’t going to be a source of infec­tion because they’re a dead-end host.

With­in an Aus­tralian con­text, how could cli­mate change influ­ence vec­tor-borne disease?

Gen­er­al­ly speak­ing, it’s dif­fi­cult in Aus­tralia to pre­dict what’s going to hap­pen, because we have one of the most vari­able cli­mates of the planet. 

[How­ev­er], we know that as the air gets warmer it holds more mois­ture, and there­fore you tend to get more intense rain­fall events. 

More intense rain­fall events tend to lead to flood­ing and flood­ing leads to stand­ing water envi­ron­ments that vec­tors thrive in.

Want to learn more about vec­tor-borne dis­eases? Watch our webi­nar Human dis­eases caused by insects’ which cov­ers JEV, dengue fever, and malaria.