One person has died and four people remain in hospital in South Australia, their cases possibly linked to mosquito-borne diseases, prompting a warning from health officials.
SA Health says people should avoid exposure to mosquitoes, following the recent detection of various flaviviruses in southern parts of Australia - spread through the bite of infected insects.
"In South Australia, there have been seven cases of acute encephalitis identified in the past month, which are currently undergoing investigation for flavivirus infection," executive director of health protection Chris Lease said.
"All of these people required hospitalisation with four people currently still in hospital, and one person having sadly passed away."
Dr Lease said the Kokobera virus had been detected in adult mosquitoes in the SA Riverland in recent weeks, while the rare Japanese encephalitis virus had been found in pigs in NSW, Victoria and Queensland and the West Nile virus in horses in NSW.
"Kokobera virus, Japanese encephalitis virus, West Nile virus, and Murray Valley encephalitis virus all belong to the flavivirus group," he said.
"Most people who are infected with these viruses are asymptomatic or develop a mild febrile illness, but a small proportion of infected people - less than one per cent - will develop encephalitis, which may be fatal or cause long-term neurological damage."
Symptoms of encephalitis may include confusion, headaches, neck stiffness, tremors, drowsiness and seizures.
Japanese encephalitis virus is endemic in the Torres Strait and had not previously been detected in NSW or Victoria, and has never been detected in South Australia.
Murray Valley encephalitis virus and West Nile virus are endemic in birds in northern Australia with occasional cases occurring in humans.
Chief Veterinary Officer Mary Carr said the Department of Primary Industries and Regions was undertaking surveillance measures within SA to monitor for Japanese encephalitis virus (JEV).
"The normal life cycle of JEV is between waterbirds and mosquitoes, which may then accidentally spill over to pigs and horses, but there are currently no confirmed livestock detections of JEV in South Australia." Dr Carr said.
Dr Lease said, in addition to the seven cases of acute encephalitis, there had also been an increase in the number of cases of Ross River virus with 77 reported so far this year compared to 48 at the same time last year.
The increase in all mosquito-borne diseases has been linked to the La Nina weather event across southeastern Australia this summer.
