Mapping the risk of Japanese encephalitis in Australia

13 August 2024

In 2022, mainland Australia experienced its first outbreak of Japanese encephalitis virus (JEV). As a result, 45 people were infected and six lost their lives. Researchers at the Australian Institute of Tropical Health and Medicine (AITHM) have identified areas at risk of future outbreaks to inform public health planning.

Japanese encephalitis is a disease caused by JEV. It is spread to humans through mosquito bites, which become infected by biting pigs and waterbirds that are carrying the virus. While most people show no symptoms when infected, some will experience fever and headaches, and severe cases can lead to permanent neurological complications or death.

The research, led by AITHM mathematical modeller Associate Professor Roslyn Hickson and virologist Associate Professor Paul Horwood, focused on understanding the distribution of three mosquito species suspected of transmitting the disease, as well as waterbirds and domestic and feral pig populations in Australia.

“Wild waterbirds, like egrets and herons, are the main reservoirs of JEV. Culex mosquitos bite these birds and then spread the virus among them,” Associate Professor Horwood said. “The mosquitos also bite and infect pigs, who can transmit the virus to each other. So, once JEV gets into piggeries and pig populations, it circulates and amplifies quickly. However, JEV cannot be transmitted from person to person; it’s only spread to humans via mosquito bites.”

To identify areas at highest risk of JEV transmission in Australia, the team — comprised of a veterinary epidemiologist, a wildlife ecologist, an entomologist, a machine learning expert, a veterinary pathologist, a virologist and an infectious diseases modeller — used maths and modelling to create an ecological niche model: a scientific tool used to predict where certain species might live based on environmental conditions.

“Basically, we fed the model environmental parameters that we know these species need to survive. Things like water availability, humidity, temperature and so on, and it shows us where they are likely to be present,” Associate Professor Hickson said.

The findings indicate that the relative risk of transmission of JEV is widespread across Australia. However, areas with high densities of piggeries and feral pigs, particularly along the northern and eastern coastlines of the continent, are at greater risk.

“These regions have wet, humid conditions that support year-round mosquito populations, abundant wetlands, and significant feral pig numbers, creating the perfect environment for JEV to circulate,” Associate Professor Horwood said.

According to Associate Professor Hickson, combating the virus will require ongoing surveillance of the key animals involved in its circulation and transmission.

“Our findings highlight the need for mosquito control and biosecurity measures to prevent contact between feral pigs, domestic pigs, and water birds,” she said. “There is still a lot more research that needs to be done and much to learn about JEV in Australia.”

For a disease as complex as JEV, a holistic approach that considers the different animal species, the environment, and the social situations that can lead to outbreaks is crucial for developing effective strategies to manage and mitigate its impact. This research represents a significant step forward in understanding JEV's behaviour in a unique environment like Australia and underscores the importance of a multifaceted approach.

The estimated risk of JEV transmission in animals in Australia where the estimated risk ranges from zero (white) and low (yellow) to high (red). Source: Furlong et al. Japanese Encephalitis Enzootic and Epidemic Risks across Australia. Viruses. 2023; 15(2):450. Note: colour bar difference from source.