Japanese encephalitis has catapulted seemingly out of nowhere, and with the disease having reached four states and sadly causing two confirmed deaths, Australians are looking to answer questions about this new-to-us viral infection.
What is Japanese encephalitis and what causes it?
As the name suggests, Japanese encephalitis is a type of encephalitis, or inflammation of the brain. Symptoms of the disease include fever, headache, vomiting and seizures.
It's caused by the Japanese encephalitis virus (JEV), an RNA virus in the Flaviviridae family. Other viruses in this family include dengue virus. They are arboviruses - short for arthropod-borne viruses, referring to the fact that they are spread by insects like ticks and mosquitoes.
Specifically, JEV is spread by mosquitoes in the Culex genus. Pigs and wading birds like herons and egrets are "amplifying hosts": that means the virus can infect them and replicate to levels high enough that it can go on to infect a mosquito that bites the larger animal.
Humans and horses can both be infected by the virus via a mosquito bite, but we are "dead-end hosts": the virus can't replicate to high enough levels in our blood to transfer to a mosquito if we're bitten again.
Scientists aren't quite sure of why JEV replicates better in pigs than in humans.
You can't catch the virus from another person or by eating pork products from an infected pig. JEV needs to be spread by mosquito bite and particularly to get into the blood - so it can't do you any harm if you breathe it in or eat food containing it.
The vast majority of JEV infections are asymptomatic - the Australian Immunisation Handbook says that only between 1 in 25 and 1 in 1000 infections actually cause clinical disease.
But for those who develop the disease, it's often very serious. Between 20 per cent and 30 per cent of symptomatic cases are fatal, and 30 to 50 per cent of people who survive the acute illness experience ongoing neurological symptoms.
Like many viral diseases, the more dangerous symptoms like fever or encephalitis are part of our body's response to the viral infection, not directly caused by the virus itself.
Children and older people seem to be more vulnerable.
Why is Japanese encephalitis spreading in Australia now?
Japanese encephalitis has been present in Australia for a while, but largely confined to tropical areas - small corners of the Cape York Peninsula and the Torres Strait. Vaccination programs instituted following the first recorded outbreak in the Torres Strait in 1995 have provided protection for humans, and Queensland health department mosquito control activities helped keep the disease, if not the virus, largely at bay in those areas.
For Nigel Beebe, an associate professor at the University of Queensland and CSIRO working on several mosquito-borne diseases including JEV and malaria, the current spread to the south is unfortunate but not exactly surprising.
"We have been thinking that this was going to happen since around 2000," he says.
"In Australia we have access to wading birds that fly in from Papua New Guinea, we have a large feral pig population - so they're both amplifying hosts - and we have a very good mosquito vector."
That vector is Culex annulirostrus, or the common banded mosquito - a freshwater mosquito found widely across eastern Australia, as well as in Papua New Guinea.
Several experts have suggested that climate change and Australia's recent weather conditions could have tipped the balance in favour of the virus's southwards spread.
"The reports of multiple cases of Japanese encephalitis acquired in Australia occurring at the same time as severe flooding serves as a warning of the significant potential for new human health threats associated with climate change, including the emergence of new pathogens and the appearance of known infections in new localities," says Karin Leder, a professor in the School of Public Health and Preventive Medicine at Monash University.
The current La Nina cycle has likely influenced migration of birds that carry the virus and increased the abundance of mosquitoes. Rain and flooding can create wetland environments in new places, which draw birds and mosquitoes together and potentially bring them into closer contact with both humans and pigs.
"Regardless of La Nina, if our climate is to become warmer, and challenged by flood events, we may find that JEV will circulate widely and continuously," says Gregor Devine, group leader of the Mosquito Control Laboratory at QIMR Berghofer.
What can we do to protect ourselves?
Unfortunately, there are no specific treatments for Japanese encephalitis, but there are two vaccines against JEV currently approved for use in Australia, and they're both highly effective.
One vaccine (Imojev) contains a live attenuated virus, and the other (JEspect) contains an inactivated virus.
The Victorian state government has recently flagged that it wants to make JEV vaccines more broadly accessible, given the current outbreak.
Many strategies to reduce the risk of JEV infection will also help counter other mosquito-borne flaviviruses such as Murray Valley encephalitis and Kunjin virus (a West Nile strain). These are already established in southern Australia; they're also likely to increase with the current favourable mosquito breeding conditions - and we don't have vaccines for them.
Targeting the mosquitoes is a worthwhile strategy. That includes taking extra care to avoid mosquito bites by applying strong insect repellent and eliminating pools of stagnant water that promote mosquito breeding.
Avoiding the outdoors at dusk and dawn, when mosquitoes are most active, and wearing long, light-coloured clothing (mosquitoes are attracted to dark colours) also help.
There are also strategies to control the mosquito population, like insecticides that kill larvae - but they can be difficult and expensive to implement over a broad area, especially when weather conditions favour the mosquitoes.
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