A leading mosquito-borne virus expert says back-to-back La Niña seasons are bringing a heightened risk of Japanese encephalitis to the southern states.
Associate Professor Greg Devine, group leader of mosquito control at QIMR Berghofer, says Japanese encephalitis has become "endemic" in southern Queensland and New South Wales – regions that had only ever seen sporadic cases prior to last year.
Japanese encephalitis is a mosquito-borne virus that can cause swelling of the brain in serious cases.
However, it is generally harmless in humans and less than one per cent of cases become serious, however 30 per cent of those serious cases are fatal.
Most cases go undetected as they produce very few or zero symptoms.
Six people have died with the virus in Australia this year, and there have been about 30 confirmed cases.
"There are also neurological symptoms [and] disabilities that often continue in the survivors, so it's a terrible disease for humans," Dr Devine said.
Wet weather sees the virus moving south
Japanese encephalitis was once confined to northern parts of Australia, but Professor Devine said because the virus is carried by wild birds and pigs, more wet weather means more risk in southern states.
"Given we're seeing exactly the same kind of rain patterns as we had last year, we're expecting further outbreaks," he said.
Humans can only catch the virus via a mosquito, but there are animals that can carry it and be affected by it.
Piggeries and feral pigs are thought to be the most dangerous spreaders of the virus, but wild birds are its "natural reservoir".
The virus can cause pigs to deliver stillborn piglets, but birds are largely asymptomatic.
Horses can catch the virus and suffer similar symptoms to humans.
Vaccination programs lacking
There are vaccines against Japanese encephalitis, but Dr Devine said Australia was on the backfoot due to the southern spread of the virus.
"We've not really had to have much of a contingency plan in terms of big vaccination programs," he said.
"There are only two vaccines which are registered in Australia, so it takes time for the companies that makes those vaccines to ramp up their production.
"If pigs are the source of infection for humans, then really anybody who's within the flying range of a mosquito from these piggeries is potentially at risk.
"That's a very large number of when you look at it."
QMIR Berghofer estimates around 800,000 people would fall in that category.
"The trick for state health and federal health is to figure out what is the population at risk, where are they, and who should get the vaccine," Dr Devine said.
A recent study in areas affected by Japanese encephalitis in New South Wales returned results that are both a cause for worry and calm.
Out of just over 1,100 participants, one in 11 had evidence of an infection.
Dr Devine said people can "take some comfort" in the low number of serious cases, but the lack of deep knowledge of how the virus works is cause for concern.
"We don't know exactly what the likelihood is of exposed people going on to develop disease, he said.
"But the point is that because it's vaccine preventable, it would be a terrible shame if we missed the opportunity to really identify those that were most at risk."
Further research needed
Dr Devine said further research by state and territory government is needed to fully understand the future of the virus in Australia.
Continued surveillance of piggeries will be key, he said, but a better understanding of the role of wild birds will also be essential.
"Whether [wild birds'] proximity to human populations also forms a risk because interestingly, it seems that most of the people who've been infected and have been symptomatic … it would appear that very few of them are actually directly connected with pig farms or with the pig industry," he said.
Dr Devine said Japanese encephalitis likely won't be the last virus to rear its head in the coming years.
"Environmental change and climate change are going to bring everybody new problems which they hadn't necessarily predicted."
