Japanese Encephalitis in Australia
Japanese encephalitis virus (JEV), first discovered in Japan in 1871, is an arbovirus (ARthropod-BOrne VIRUS). It is transmitted by mosquitoes, which are arthopods - invertebrate animals that have jointed legs, like spiders, flies and centipedes.
Most of the medically important arboviruses come from mosquito and tick bites. In Australia, these viruses are associated with illnesses such as Ross River virus disease and Murray Valley encephalitis.
Arboviruses can be found in several different virus groups. JEV comes from a group called Flaviviruses. The most important human disease associated with this group of viruses is Dengue fever. Other Flaviviruses are Yellow fever, Zika, West Nile, and Murray Valley encephalitis viruses.
Japanese encephalitis causes high levels of human disease in many Asian countries, including Korea, Japan, Vietnam and India. In our region, JEV related disease occurs in Papua New Guinea.
Humans are ‘dead end hosts’ for JEV. This means we don’t have high enough levels of the virus in our blood to infect a mosquito if they bite us. So, even though the virus can make people very sick, it cannot be transferred from one person to another by a mosquito that has bitten a JEV infected person.
However, other vertebrate animals are ‘amplifying hosts’ for JEV, like pigs and some birds, particularly birds that wade in water, such as Herons, Cranes and Ibises.
Most people who become infected with JEV don’t get sick, with fewer than one in 100 developing symptoms. Still, the virus can cause brain infection, or encephalitis, which causes headache, vomiting and fever. The chances of dying from JEV encephalitis are high, and those who survive may develop disabilities from infection.
Japanese encephalitis has been in Australia before. In 1995, there was an outbreak in the Torres Strait Islands, and in 1998, two people became sick with the disease on Cape York.
In 2022, JEV has resurfaced on mainland Australia with human disease and wide-spread infection at piggeries in four states. The disease has been declared a Communicable Disease Incident of National Significance.
We don’t know why JEV seems to have suddenly reappeared in Australia, or how long it has been here. However, arboviruses, like all viruses, keep surprising us. It’s possible that people fell ill and weren’t diagnosed before it was known that JEV had returned to Australia. Since Australians haven’t been travelling much due to COVID-19, it’s probable that a doctor wouldn’t think of JEV as a cause of viral encephalitis. Another reason for the virus not being detected is that not many people get sick from JEV, so even if a lot of people were infected with the virus the chance of cases being diagnosed is not very high. The Australian health system, including diagnostic virology, has been under a lot of strain with COVID-19, which is another potential reason why JEV cases are less likely to be diagnosed.
Global heating has and will continue to change disease transmission. There are many studies that show climate and weather influence arbovirus transmission, which is why Ross River virus is seasonal in its transmission. When it rains there is water for mosquitoes to lay their eggs in, which leads to more mosquitoes. Temperature influences adult mosquito survival too.
Although the exact cause of the latest JEV outbreak in Australia is unknown, it’s quite likely it was brought here by birds travelling further south than normal.
Since people can only be infected with JEV from a mosquito, it’s best to protect yourself from being bitten. Mosquito repellents and coils, and light-coloured loose-fitting clothing that covers most of your body are some of the best ways to protect yourself. Vaccines against JEV also protect those at greater risk, such as those who work at or live near piggeries or come into contact with feral pigs. There are no animal vaccines for JEV available in Australia right now.
We have seen unexpected arbovirus outbreaks around the globe in recent decades involving West Nile, Zika, and Chikungunya viruses. Australia has good communicable disease surveillance and control systems in place, but a National Centre for Disease Control would further strengthen these efforts, along with strengthened collaboration between animal and human health experts. It’s also time for greater focus on mitigation against global heating and adaption to our changing environment.
By Associate Professor David Harley
UQ Centre for Clinical Research
Dr Harley is an expert in arboviruses, particularly Ross River and dengue viruses. He has also published on Chikungunya, Zika, tuberculosis and invasive meningococcal disease.
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