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Murray Valley encephalitis: summer is over but mosquito-borne disease remains a risk in northern Australia

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12th April, 2024

Cooler temperatures are fading our memories of summer and reducing numbers of mosquitoes in southern parts of Australia. But up north, warmer temperatures and plenty of rain will keep mosquitoes active.

While their bites are annoy­ing, more con­cern­ing is the dis­eases mos­qui­toes car­ry. Health author­i­ties have recent­ly warned local com­mu­ni­ties and trav­ellers head­ing to the Kim­ber­ley and Pil­bara regions of West­ern Aus­tralia to be vig­i­lant to the risk of one par­tic­u­lar mos­qui­to-borne infec­tion – Mur­ray Val­ley encephalitis.

Which mosquito-borne diseases are a risk?

Aus­tralia is for­tu­nate to be gen­er­al­ly free of many of the world’s most dan­ger­ous mos­qui­to-borne diseases.

Each year glob­al­ly, malar­ia can cause hun­dreds of thou­sands of deaths and dengue infects hun­dreds of mil­lions of peo­ple. While these two dis­eases aren’t a high risk in Aus­tralia, we do have a num­ber of virus­es spread by mos­qui­toes that can cause severe and poten­tial­ly fatal illness.

Thou­sands of Aus­tralians are infect­ed with Ross Riv­er or Barmah For­est virus each year, and while these dis­eases aren’t fatal, they can be debil­i­tat­ing. Symp­toms can include fever, rash, joint pain and fatigue.

Author­i­ties in Queens­land and New South Wales have recent­ly issued warn­ings about these diseases.

In recent years, we’ve seen increased activ­i­ty of the Mur­ray Val­ley encephali­tis virus and the close­ly relat­ed Kun­jin virus. This is due to explo­sions in mos­qui­to num­bers as a result of per­sis­tent flood­ing.

Mur­ray Val­ley encephali­tis virus cas­es in humans are rare but fatal­i­ties do occur. Kun­jin virus, which has the poten­tial to cause human dis­ease, can also severe­ly affect ani­mals.

New mos­qui­to-borne virus­es have emerged in Aus­tralia, with wide­spread activ­i­ty of Japan­ese encephali­tis virus in south­ern regions of Aus­tralia record­ed for the first time in 2021–22. This had sig­nif­i­cant impacts on human health, as well as eco­nom­ic con­se­quences for the pork indus­try due to the repro­duc­tive loss­es result­ing from infect­ed pigs. The Aus­tralian gov­ern­ment declared a com­mu­ni­ca­ble dis­ease inci­dent of nation­al significance.

A close-up image of a mosquito on a grey surface.
Mos­qui­toes car­ry a vari­ety of dis­eases. Cameron Webb (NSW Health Pathology)

Why is Murray Valley encephalitis so dangerous?

Mur­ray Val­ley encephali­tis virus is one of the most dan­ger­ous pathogens spread by mos­qui­toes in Aus­tralia. The virus belongs to the fla­vivirus fam­i­ly along­side Japan­ese encephali­tis, dengue, yel­low fever and West Nile virus­es; the most impor­tant mos­qui­to-borne virus­es on the planet.

The virus is only spread by mos­qui­to bite (it doesn’t spread from per­son to per­son). Mos­qui­toes, most notably a com­mon Aus­tralian species Culex annulirostris, trans­mit the virus to humans. This species is found in fresh­wa­ter habi­tats and acquires the virus from bit­ing a waterbird.

Most peo­ple infect­ed don’t get sick – per­haps as few as one in 1,000 devel­op symp­toms. For those who do, these can range from fever and headache to paral­y­sis and encephali­tis (swelling of the brain).

Symp­toms are vari­able but fatal­i­ty rates for peo­ple with symp­to­matic dis­ease can be up to 30%, with up to 50% of peo­ple expe­ri­enc­ing per­ma­nent neu­ro­log­i­cal com­pli­ca­tions requir­ing life-long med­ical care.

From Australian X disease to Murray Valley encephalitis

While Mur­ray Val­ley encephali­tis virus can be found in many parts of Aus­tralia, out­breaks in south-east­ern Aus­tralia have caused the most con­cern, espe­cial­ly through­out the Mur­ray Dar­ling Basin region, due to the high human pop­u­la­tion. That said, activ­i­ty in oth­er regions is still a worry.

The virus is thought to have been caus­ing an ill­ness known as “Aus­tralian X dis­ease” since at least the ear­ly 1900s. The most sig­nif­i­cant out­break was in 1974, result­ing in 58 cas­es.

Dur­ing the sum­mer of 2022–23, the virus was detect­ed in mos­qui­to and sen­tinel chick­en sur­veil­lance pro­grams in NSW, Vic­to­ria and South Aus­tralia. A total of 26 human cas­es were report­ed across Aus­tralia in 2023 after only a hand­ful of cas­es since 2011, which saw 16 cases.

There’s been no evi­dence that Mur­ray Val­ley encephali­tis virus is present along the east coast of Aus­tralia. Activ­i­ty of the virus is gen­er­al­ly lim­it­ed to regions west of the Great Divid­ing Range.

A mosquito trap comprised of a black bucket, battery operated motor and a plastic collection container.
Health author­i­ties across Aus­tralia use mos­qui­to traps to help mon­i­tor virus­es such as Mur­ray Val­ley encephali­tis. Cameron Webb (NSW Health Pathology)

What about northern Australia?

Mur­ray Val­ley encephali­tis is con­sid­ered endem­ic in north­ern Aus­tralia. It’s detect­ed almost every year in health sur­veil­lance pro­grams in WA and the North­ern Ter­ri­to­ry.

Human cas­es occur too. Although few­er peo­ple live in these regions, north­ern Aus­tralia (includ­ing tourists vis­it­ing the area) has account­ed for most cas­es of Mur­ray Val­ley encephali­tis over the past 30 years.

Sur­veil­lance is crit­i­cal to pro­vide an ear­ly warn­ing of ele­vat­ed out­break risk. In the north of WA, health author­i­ties have detect­ed Mur­ray Val­ley encephali­tis virus in local mos­qui­to pop­u­la­tions and their sen­tinel chick­en sur­veil­lance pro­gram. This prompt­ed the recent warn­ings for the Kim­ber­ley and Pil­bara regions.

How­ev­er, no cas­es of human infec­tion have been report­ed this year.

How can the community and travellers protect themselves?

While activ­i­ty of Mur­ray Val­ley encephali­tis virus across north­ern Aus­tralia should be expect­ed every year, the recent warn­ings are a reminder of the poten­tial health risk asso­ci­at­ed with mosquitoes.

With no vac­ci­na­tion avail­able for Mur­ray Val­ley encephali­tis – and no cure – the only way to pre­vent becom­ing infect­ed is to avoid mos­qui­to bites. Wear­ing light, loose-fit­ting cloth­ing, avoid­ing peak mos­qui­to activ­i­ty times around dawn or dusk, and using a suit­able insect repel­lent con­tain­ing DEET, picaridin or oil of lemon euca­lyp­tus are effec­tive ways to help pre­vent bites.

Andrew Jar­dine and Jay Nichol­son from the Depart­ment of Health, West­ern Aus­tralia, con­tributed expert advice to this article.The Conversation

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

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


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