Warming temperatures combined with flood waters could leave almost 750,000 Australians vulnerable to Japanese encephalitis – a disease that until last year was confined to Asia and far-northern Australia.
The mosquito-borne disease was first detected on the Australian mainland in 1998 but its range expanded dramatically this year. Cases were reported in dozens of southern piggeries (pigs are one of the main carriers of the virus) and there were also 31 confirmed cases in humans and six deaths.
A paper published on Monday in the medical journal Clinical Infectious Diseases warned that anyone living within 4km of an infected piggery is potentially at risk. This is because the mosquito that spreads Japanese encephalitis, Culex annulirostris, is a strong flier and can fly several kilometres.
Researchers led by the QIMR Berghofer Medical Research Institute in Brisbane conducted modelling based on piggery distributions and the human population, and concluded that up to 3% of the population, or 740,546 people, could be at risk. The modelling is a worst-case scenario, assuming all mapped piggeries are vulnerable to infection and that all infected mosquitos fly 4.4km over their lifetimes.
Pigs are the other major amplifiers of the virus, developing high enough viral levels in their blood to infect mosquitoes for around four days.
The paper’s senior author and head of the QIMR Berghofer Mosquito Control Laboratory, Associate Prof Greg Devine, said people living close to populations of wading birds may also be at risk.
Waterbirds, particularly wading birds such as herons and egrets, are the main source of Japanese encephalitis as they hold enough virus in their blood to infect mosquitoes for up to seven days.
“We are extremely concerned about further outbreaks of Japanese encephalitis in Australia because of this third consecutive La Niña this year,” Devine said.
“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.
“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.”
Japanese encephalitis is primarily a disease endemic in parts of Asia, where it is the most common vaccine-preventable cause of encephalitis, a serious and potentially fatal swelling of the brain. However, about 99% of infections cause no symptoms, and of those who do get sick, most will experience mild, cold-like symptoms.
Associate professor of medical entomology with New South Wales Health Pathology, Dr Cameron Webb, said the risk of disease spread due to flooding is unlikely to be immediate. This is because mosquitos thrive in shallower, more stagnant bodies of water and in warmer weather than currently being seen on the south-east coast.
But he said there is a risk that in coming weeks as flood waters dissipate and the weather warms, Japanese encephalitis and other mosquito-driven diseases may emerge.
“It is important to remember that lots of mosquitoes also doesn’t necessarily guarantee a major outbreak of disease,” he said. “They don’t hatch out of the flood waters already infected with the virus so they’ve often got to bite an animal first, to pick up the virus before it passes on to us. And so it becomes a very complex puzzle to solve when you try to predict the timing of outbreaks.
“But the flooding provides a lot of water into the environment, which is great for mosquitoes and so we certainly need to be alert. People need to take whatever steps they can to avoid mosquito bites. And they also need to be aware if they are assisting with the clean-up that aside from mosquitoes, there are all kinds of other diseases and waterborne pathogens that they may be at risk of being exposed to.”
