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Japanese Encephalitis Virus’ Australian arrival
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.
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.
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.