Australia’s Japanese encephalitis outbreak blamed on climate change

A Culex mosquito, the kind of insect that can spread Japanese encephalitis

Konstantin Nechaev / Alamy

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

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

The migration of mosquitoes carrying Japanese encephalitis from Asia to Australia has taken many experts by surprise. “The Japanese encephalitis virus was completely off the radar for us,” says Roy Hall of the University of Queensland in Brisbane.

The virus is transmitted through Culex mosquitoes that have already bitten an infected animal, such as pigs or waterfowl.

Experts believe the infection may have entered Australia after recent flooding along the east coast created additional wetlands. These may have attracted migrating waterfowl from Asia, carrying the virus. “We know that these birds often follow flooded streams,” says Hall.

Local mosquitoes may have bitten these birds as they traveled along the waterways. The mosquito population in Australia is higher than normal due to recent hot and humid weather which has encouraged insect breeding.

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

The virus has already been detected in pigs on 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 of the QIMR Berghofer Medical Research Institute in Brisbane.

Record rainfall, flooding and hot temperatures may have created a ‘perfect storm’ that allowed the Japanese encephalitis virus to gain a foothold in Australia, with climate change potentially to blame, says Karin Leder of the Monash University of Melbourne.

“We are seeing changes in rainfall and temperature that affect the behaviors of birds that harbor the virus, as well as an increase in the breeding of mosquitoes that spread it,” she says.

The Japanese encephalitis virus has already 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 those infected develop no symptoms or experience mild flu-like discomfort. The virus spreads to the brain in about one in 250 cases, leading to complications such as seizures, tremors and paralysis. Up to one in three people who develop these severe symptoms die from the infection. In the regions of Asia and the Western Pacific, an estimated 13,600 to 20,400 people die each year from Japanese encephalitis.

There is no cure, but administering fluids and oxygen can support the body as it fights the virus. Vaccines can prevent infection, however, Australia only has 15,000 doses in stock. The government says it is importing an additional 130,000 doses, which will be available from the end of March.

Vaccines will initially be prioritized for high-risk groups, such as piggery workers and veterinarians. “When we know more about the magnitude of the risk in various geographic areas, we will be able to make informed decisions about who else to vaccinate,” says Leder.

In the meantime, people who live in areas with high mosquito density should wear long-sleeved clothing, apply mosquito repellent and eliminate standing water around their homes, she says.

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

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

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

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