Australia’s Japanese encephalitis outbreak blamed on climate change

Australia is grappling with its first major outbreak of Japanese encephalitis, a viral disease that has already killed two people. The mosquito-transmitted infection is typically found in rural regions of Asia, but climate change is thought to have driven it further south – and other diseases could follow suit.

Nineteen people have tested positive for the infection across four Australian states: Victoria, New South Wales, South Australia and Queensland. A man in his 60s from Victoria and another man in his 70s from New South Wales have died from the virus.

The outbreak has taken many experts by surprise. “Japanese encephalitis virus was completely off the radar for us,” says Roy Hall at the University of Queensland in Brisbane.

The virus is transmitted via Culex mosquitoes that have previously bitten an infected animal, such as a pig or waterbird.

Experts believe the infection may have entered Australia after recent floods along the east coast created additional wetlands. These may have attracted migratory waterbirds from Asia, carrying the virus over. “We know these birds often follow flooded watercourses,” says Hall.

Local mosquitoes may have bitten these birds as they travelled along the waterways. Australia’s mosquito population is higher than normal due to its recent warm, wet weather assisting the insects’ breeding.

Once mosquitoes are infected, they can pass the virus to dense populations of pigs in commercial farms, causing an “amplifying effect”, says Hall. Mosquitoes that bite infected pigs can spread the virus to people who work with or live near the animals. Japanese encephalitis cannot spread from person to person.

The virus has already been detected in pigs at more than 20 Australian farms, with some fearing the infection could spread to the country’s millions of feral pigs. “These pigs move over very wide ranges,” says Gregor Devine at QIMR Berghofer Medical Research Institute in Brisbane.

Record-breaking rainfall, flooding and warm temperatures may have created a “perfect storm” that allowed the Japanese encephalitis virus to gain a foothold in Australia, with climate change potentially being to blame, says Karin Leder at Monash University in Melbourne.

“We’re seeing changes in rainfall and temperature that are affecting the behaviours of the birds that host the virus, as well as increased breeding of the mosquitoes that spread it,” she says.

The Japanese encephalitis virus has previously been detected in a handful of people in Australia’s Torres Strait Islands, north of the mainland. The ongoing outbreak is a first for mainland Australia.

The vast majority of people who become infected develop no symptoms or experience mild, flu-like discomfort. The virus spreads to the brain in about 1 in 250 cases, causing complications like seizures, tremors and paralysis. Up to 1 in 3 people who develop these severe symptoms die as a result of the infection. In Asia and Western Pacific regions, an estimated 13,600 to 20,400 people die from Japanese encephalitis every year. Children are most likely to be affected.

There is no cure, but administering fluids and oxygen can support the body while it fights off the virus. Vaccines can ward off infection. However, Australia only has 15,000 doses in its stockpile. The government says it is importing another 130,000 doses, which will be available from late March.

Vaccines will initially be prioritised for high-risk groups, like piggery workers and veterinarians. “When we know more about the magnitude of risk in various geographic areas, we’ll be able to make informed decisions about who else should be vaccinated,” says Leder.

In the meantime, people who live in mosquito-dense areas should wear long-sleeved clothing, apply mosquito repellent and remove stagnant water from around their house, she says.

Now the Japanese encephalitis virus has been found animal hosts in Australia, it is “here to stay”, says Devine. “Sometimes it will be unseen and sometimes it will spill over [into humans], but it’s not going to disappear,” he says.

Climate change is also expected to increase the prevalence of other mosquito-borne diseases in Australia, says Devine. For example, dengue-carrying mosquitoes that reside in northern Australia could migrate south as it gets warmer, he says.

Australia should be better prepared for a rise in mosquito-transmitted viruses by ensuring appropriate tests are ready and building the capacity to make vaccines locally, says Hall. “We’re going to see more mosquito-borne diseases,” he says. “Exactly where, exactly when, we don’t know, but it will happen.”