Rainfall, flooding and summer heat have formed the perfect environment for disease-carrying mosquitoes to boom, health authorities warn.
The potentially fatal Murray Valley encephalitis virus was detected in a mosquito in far west of NSW on Wednesday, prompting a warning from NSW Health to avoid being bitten.
The infected mosquito was detected in the Darling River town of Menindee, most likely related to recent floods and rain in the area, NSW Health said.
Mosquitoes spread the virus to humans from infected animals, such as waterbirds including herons and egrets.
The disease cannot pass from one human to another, and people cannot catch the virus by touching an infected animal or eating animal products.
Many people infected with Murray Valley encephalitis do not experience any symptoms, although some will develop a severe infection and the virus can cause neurological problems in rare cases, Health Protection NSW executive director Richard Broome said.
"Only a small proportion of people infected with the virus will experience symptoms, which include fever, headache, nausea, vomiting, loss of appetite, diarrhoea, and muscle aches," Dr Broome said.
"Among those who get a severe infection, some may die or have lifelong neurological complications."
There are no vaccinations or specific treatments for Murray Valley encephalitis, and the best way to avoid infection is to avoid being bitten.
"Avoiding mosquito bites will also protect against other mosquito-borne infections including Japanese encephalitis, Ross River Fever and Barmah Forest virus," Dr Broome said.
It comes as mosquitoes carrying Ross River Fever and Barmah Forest virus are being detected at five times their usual number in South Australia, SA Health said Wednesday.
The authority has also detected high to extreme numbers of a mosquito species known to carry a number of serious diseases including Japanese encephalitis, Murray Valley encephalitis and Kunjin/West Nile Virus.
SA Health detected the mosquitoes in the Mallee region, west of Adelaide.
Vaccinations are available for Japanese encephalitis, but not for the other infections.
SA Chief Health Officer Nicola Spurrier echoed the comments of Dr Broome, saying it was vital people worked to protect themselves against mosquito bites.
"We have an explosion in mosquito populations at the moment, so it is vital that South Australians protect themselves," she said.
Some 45 people people have contracted Japanese encephalitis in Australia since January 2021, according to the most recent government data.
They include 35 definitive cases, and 10 probable cases, in all states and territories except Tasmania and WA.
The virus is a rare but serious disease, transmitted by mosquitoes, which can also infect animals.
A small number of people infected, about one in 250, will develop inflammation of the brain, which can cause permanent damage to the nervous system or death.