The resurgence of Japanese encephalitis (JE) this summer and the detection of Murray Valley encephalitis (MVE) in mosquitoes has health authorities concerned, but the likelihood of death from either remains low.
There have been seven deaths from 45 cases of JE notified in Australia since January 1, 2021, with almost all of the cases from 2022.
Alerts for the mosquito-borne viruses have followed flooding in New South Wales, Victoria, and South Australia, which have created the perfect environment for numbers of the insects to grow rapidly.
No humans have contracted MVE this summer, but it has been detected in mosquitoes during monitoring in northern Victoria and regional parts of NSW, such as Menindee and Griffith.
There have been multiple cases of JE detected in recent months following the floods.
Is severe illness likely?
Dominic Dwyer, a medical virologist with NSW Health Pathology, said MVE was similar to JE, and most people did not know if they had been infected with either.
"They both have encephalitis in the name, meaning brain inflammation," Professor Dwyer said.
"With both of them [brain inflammation] is relatively uncommon. Only one in a couple of hundred people who get infected will get the encephalitis."
But Professor Dwyer said for the small number of people who did develop inflammation, the fatality rate was significant.
"Maybe a quarter or even a third of people who get either MVE or JE will die if they get the encephalitis," he said.
"Of those who survive, often they've got quite pronounced long-term brain damage."
During an outbreak of MVE in 1974, about one-third of people who presented with encephalitis in Victoria died, one-third were left with residual brain damage, and one-third recovered completely.
Differences and similarities
Professor Dwyer said MVE was of Australian origin while JE was the most common cause of viral brain infection in South-East Asia.
"They're in the same family of viruses but genetically they are different," he said.
Cameron Webb is a medical entomologist at the University of Sydney and NSW Health Pathology.
Dr Webb said the type of mosquito most likely to spread both MVE and JE was Culex annulirostris, which loves living in freshwater.
"All of these habitats across the environment have been filled up with rain and, as floodwaters recede, the pools and puddles will be an ideal habitat," he said.
Both viruses also have similar symptoms, including fever, headache, and vomiting.
Symptoms for more severe infections include neck stiffness, coma, and seizures.
Why are the viruses emerging now?
Dr Webb said it was not surprising that mosquito populations had been "massive" this year, given the recent flooding.
"As we head towards the end of summer, mosquito populations are generally expected to be starting to hit their peak," he said.
Dr Webb said mosquitoes typically picked up MVE and JE from waterbirds, which were also thriving due to flooding, before passing the viruses onto humans.
He said if more mosquitoes were biting it increased the likelihood of virus exposure.
"Large populations of mosquitoes doesn't necessarily guarantee an outbreak of disease," Dr Webb said.
"Certainly, it increases the chance that an infected mosquito is going to bite somebody."
Which virus has health authorities more concerned?
Professor Dwyer said MVE had been around for a long time and there had been occasional outbreaks, such as in the 1970s.
There have been detections of JE in Australia in the past but it did not properly emerge until last year.
"With Japanese encephalitis being relatively new, everybody at all age groups is at risk of being infected," Professor Dwyer said.
"It sparks concern because it's new and we don't really know the extent of it and whether it's likely to now be part of our ecology forever.
"It could well be that Japanese encephalitis might persist like Murray Valley has persisted."
Dr Webb said surveillance programs, such as sampling mosquito populations and sentinel chickens, played a crucial role in the early detection of mosquito-borne viruses.
He said mosquitoes did not travel far, so people who lived in or travelled to regions where there was virus activity should be vigilant.
"Some mosquitoes we know can fly maybe 2 to 5 kilometres away from their wetlands," Dr Webb said.
How to protect yourself
There is no treatment available for either virus, and a vaccine only available for JE, making personal protection the best line of defence.
The JE vaccine is widely available across Australia and eligibility requirements are determined by state and territory public health units.
Dr Webb said the most important form of protection was mosquito repellent, particularly products that contained diethyltoluamide [DEET], picaridin, or oil of lemon eucalyptus.
"It doesn't really matter what formulation you choose, it can be roll-on, a cream, gel, or a spray, but choose the product that has some of those active ingredients," he said.
Other ways to prevent mosquito bites:
- Cover up — wear long, loose-fitting, light-coloured clothing
- Limit outdoor activity if lots of mosquitoes are about
- Remove stagnant water where mosquitoes can breed around a home or campsite
- On holidays, make sure accommodation is fitted with mosquito netting or screens
