Up to 750,000 people in Australia could be at risk of Japanese encephalitis this summer

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A research team led by QIMR Berghofer is extremely concerned about the potential for a major outbreak of Japanese encephalitis virus (JEV) in Australia this summer, driven in part by a third consecutive La Niña which is already causing widespread flooding.

QIMR Berghofer’s Associate Professor Greg Devine is senior author of an academic paper which has used modelling to conclude that as many as 750,000 people could be at risk of exposure to the virus which until only last year, was confined to Asia and far northern Australia.

JEV is maintained in wild birds, and is spread between these hosts and other animals including pigs, humans and horses, by mosquitoes. The virus dramatically expanded its range in Australia this year with confirmed cases in dozens of piggeries throughout southern Australia. There were 31 confirmed human cases and six deaths in 2022, and it is feared JEV may now be endemic.

The extended La Niña weather event we have been experiencing is thought responsible for the virus expansion because it created new wetlands which attracted infected water birds and provided breeding grounds for the common vector mosquito, Culex annulirostris. Feral pigs may also play a role in transmission and during 2022 these have tested positive for JEV in Queenslandthe and the Northern Territory. The February 2022 outbreak was first detected in domestic piggeries across several states.

The paper published in Oxford Academic’s Clinical Infectious Diseases journal warns that, if piggeries become a focus for infection, anyone living within four kilometres of an infected piggery is potentially at risk. That’s because the mosquito that spreads JEV is a strong flier. Modelling based on piggery distributions and the human population concludes that up to 3% of the population, or 740,546 people, could be at risk.

Senior author and head of the QIMR Berghofer Mosquito Control Laboratory, Associate Professor Greg Devine, said people living close to populations of wading birds may also be at risk.

“We are extremely concerned about further outbreaks of Japanese encephalitis in Australia because of this third consecutive La Niña year. The wet and warm weather creates the right environment for mosquitoes to proliferate and may encourage changes in the distributions of the wild birds that maintain the virus during Australia’s winter months.

“Most Australians have not been exposed to the virus before so they have no immunity. We are urging people to take precautions. The best protection is vaccination, but currently that’s not available to everyone. The next best protective measure is to avoid being bitten by a mosquito,” A/Prof Devine said.

Fact Sheet about Japanese encephalitis

Co-author and infectious diseases epidemiologist from The University of Queensland School of Public Health Professor Colleen Lau said measuring the spread of JEV is difficult as most people are asymptomatic.

“Only around 1% of people who become infected will have any symptoms. However, among those who do develop symptoms the fatality rate is around 30%, and of those who survive, around half will have some sort of long-term neurological damage.”

“Symptoms range from a fever through to severe encephalitis (inflammation of the brain). There is no specific treatment for Japanese encephalitis, so we can only manage symptoms and provide supportive management. There are two highly effective and safe vaccines available in Australia but supplies are limited so health authorities are prioritising vaccination for high risk groups,” Professor Lau said.

UQ, QIMR Berghofer and Dr Deb The Travel Doctor are investigating possible solutions to the JEV vaccine shortage by reducing the dose required.

Infectious diseases epidemiologist Associate Professor Laith Yakob from the London School of Hygiene and Tropical Medicine, collaborated on the research and provided modelling expertise.

“The modelling and mapping we conducted allowed us to estimate the potential population at risk, and therefore the potential number of vaccine doses required, as well as where these vaccines might be targeted. The models also help us identify locations for enhanced surveillance efforts to better protect Australian residents in future mosquito seasons,” A/Prof Yakob said.

The modelling adds to recent blood surveys in five regional NSW towns conducted by NSW Health and the National Centre of Immunisation Research and Surveillance (NCIRS) which found nearly 9% of participants had been infected with JEV, suggesting the virus is very common in those areas.

Research co-author and Director of the Queensland Alliance for One Health Services Associate Professor Ricardo Soares Magalhães from the UQ School of Veterinary Science said JEV is being taken very seriously with a concerted, well-funded and co-ordinated, multi-agency response between public health, veterinary health and environmental authorities.

“In terms of the impact on animals, JEV is devastating economically for the piggeries. The virus spreads rampantly through the herd causing abortions and still births. There is no licenced vaccine for pigs so we’re relying on biosecurity measures such as protecting herds from mosquitoes.

“In horses, JEV is largely asymptomatic but among when symptoms do appear, there is a very high fatality rate, especially among foals, with around half of all symptomatic cases dying. It is very debilitating.

“So far we don’t have any confirmed cases in the equine industry in Australia which is really good news, but that doesn’t mean we won’t see cases in the next few months. Right now UQ in partnership with other institutions are accelerating the development of an equine vaccine against JEV,” A/Prof Soares Magalhães said.