As the wet weather continues, infectious disease experts have suggested almost 750,000 people could be at risk from the mosquito-borne virus.
Infectious disease experts have warned that ongoing wet weather could make Japanese encephalitis more prevalent as summer approaches.
Up to three-quarters of a million people, particularly those located near piggeries, are potentially at risk this year according to research led by scientists at QIMR Berghofer in Brisbane.
They outlined their modelling in a peer-reviewed article published in the most recent edition of Clinical Infectious Diseases.
‘This initial analysis demonstrates that around 3% of the human population in Australia may be vulnerable to [Japanese encephalitis] infection following the recent expansion in geographic distribution,’ the authors wrote.
‘The interval until the next … outbreak is impossible to predict but a warming climate and extreme flood events are likely to exacerbate transmission and increase the frequency and severity of outbreaks.’
The 3% figure is largely based on the number of people who live within four kilometres of piggeries, which have been a particular focal point for infections this year.
The major carrier of the disease is cited as the Culex annulirostris mosquito species, which according to the authors ‘proliferates under optimal conditions’ and is ‘capable of dispersing more than four kilometres per day’.
According to the Department of Health and Aged Care, there have been 42 cases in humans and seven deaths since the beginning of 2021.
QIMR Berghofer’s Associate Professor Greg Devine, a senior author on the paper, said researchers are ‘extremely concerned’ about the potential for further outbreaks in a 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,’ he said.
‘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.’
Before this year, the disease had only been seen sporadically in Australia, mostly in the far north of the country.
However, in 2022 the disease was detected in more than 70 piggeries in New South Wales, Queensland, South Australia and Victoria.
Fewer than 1% of Japanese encephalitis cases are believed to be symptomatic, leading researchers to estimate the number of overall cases to be in the thousands.
For those developing symptoms, the fatality rate is estimated at around 30%, with around half of survivors said to have long-term neurological damage.
There are two vaccinations available in Australia that protect against the disease, but neither is currently covered under the National Immunisation Program, nor are they funded by states and territories.
The vaccines include Imojev, a live attenuated vaccine, and an inactivated vaccine called JEspect, also known as Ixiaro.
Vaccination is recommended for high-risk individuals, including:
- people working at and/or living close to piggeries which have tested positive for the disease
- animal transport workers
- veterinarians and students working with pigs
- laboratory workers handling Japanese encephalitis
- those involved in outdoor activities near mosquito populations, such as camping and fishing, particularly near waterways.
However, research published in the recent study indicates that none of the recent cases in humans is linked to an occupational hazard.
‘This suggests that it is the dispersal capacity of the mosquito, rather than the occupation of the infected human, that dictates the current risk,’ researchers wrote.
‘This presents a quandary for Australia’s health authorities, who are battling with limited vaccine supplies and the identification of at-risk groups.’
According to a press release highlighting the study, researchers are investigating whether one possible solution to the vaccine shortage could be addressed by reducing the dosage.
The analysis also involved researchers from the University of Queensland School of Public Health, Queensland University of Technology (QUT), and the London School of Hygiene and Tropical Medicine.
Japanese encephalitis is described as the leading cause of viral encephalitis in Asia, causing an estimated 25,000 deaths in the region every year.
For more information about Japanese encephalitis, including vaccination recommendations, see the Australian Immunisation Handbook.
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