GPs are being urged to look out for fever, headache and vomiting as unusually wet seasons appear to have pushed infections further south.
Five Australians have now died from the mosquito-borne disease Japanese encephalitis, and 42 have been infected.
And while winter reduces the risk, clinicians still need to be on the lookout for possible cases.
The Japanese encephalitis virus can cause disease in humans and pigs, and water birds are a known reservoir. It has been found in wild and domesticated pigs in the islands of the Torres Strait and far north Queensland regularly over the past decade, but infections in humans are rare, with the last two cases reported in 1998.
What’s so unusual about this outbreak is how far south the virus has reached, with cases reported in Queensland, New South Wales, Victoria and South Australia.
“It’s much further south than it has previously been detected, and it’s a very, very large area,” said Dr Victoria Brookes, an infectious diseases epidemiologist at the University of Sydney.
Only around one in 100 human cases will be symptomatic. The most common presentation is typical of encephalitis, with the sudden onset of fever, headache and vomiting. Severe infections can progress to neck stiffness, coma, neurological complications and death.
One of the cases reported – a four-month old boy – was admitted to hospital after a febrile convulsion. He had had a fever for two days, was low in energy and hadn’t been feeding well, but it wasn’t until day 14 after admission that the local alert for Japanese encephalitis prompted specific investigation for the virus.
“This case describes a typical symptomatic presentation (1% of infections) characterised by a 5–15-day incubation period, a 2–4-day non?specific prodrome followed by progression to aseptic meningoencephalitis,” the authors of this case study wrote in the Medical Journal of Australia.
Association Professor Paul Griffin, infectious diseases physician and microbiologist at Mater Health Service in Brisbane said doctors should be aware of the possibility of Japanese encephalitis, “particularly if you do practice in an area that might be rural or semi-rural where there might be piggeries located nearby”.
A definitive diagnosis requires lumbar puncture, but Professor Griffin emphasised the importance of “knowing that you can speak to your local microbiology or pathology service, and public health for advice on where best to get assessed”.
Both pigs and water birds are considered reservoirs for the virus, which is problematic for a country where both are widespread. Two consecutive years of La Nina weather systems have brought record-breaking rainfall and flooding, which have in turn boosted populations of water birds and mosquitos, which may explain why the virus has spread so far beyond its usual distribution.
“The mosquito as the vector species have always been here, so it’s likely due to the increased rainfall and increased temperatures that we’re seeing over this last year,” Dr Brookes said.
The longer-term climate outlook for Australia, with increased likelihood of more extreme weather events such as flooding, suggests Japanese encephalitis and other mosquito-borne diseases are likely to become more widespread, she said.
There are two Japanese encephalitis vaccines available in Australia, one live attenuated and one inactivated, which has been available for those working in high-risk settings such as piggeries, mosquito and Japanese encephalitis research. However, Professor Griffin said there was no current change in vaccination policy for this disease.
“Given the specific risks that are really required to become infected and the low numbers, we’re certainly not contemplating rolling those vaccines out on a on a broad scale,” he said.
