Mosquito virus threat increased by intense wet weather
Australian mosquitoes carry a range of diseases that can be potentially fatal to humans.
Eastern Australia welcomed a deluge of rain, easing the bushfire threat and filling dams, but the wet weather could also boost mosquito numbers as the pernicious insects flock to floodwaters to breed.
The latest mosquito monitoring report from NSW says "very high" numbers are concentrated in Sydney's western suburbs, including Parramatta and in the Georges River at Bankstown and Illawong.
Large numbers were also recorded on the coast at Port Macquarie and on the Queensland border while in inland areas, populations are low.
However, recent heavy rainfall could trigger dormant mosquito eggs to hatch and swarm flooded regions.
Medical entomologist Cameron Webb, of NSW Health Pathology, says more mosquitoes are to be expected as the rain fills up wetlands and flows into bushland.
"Australia is home to dozens of different kinds of mosquitoes which either breed in salt water wetlands and briny mangroves or in fresh-water habitats, irrigated areas and bushland," Dr Webb told AAP.
The most concerning are the aedes vigilax and culex annulisrostris mosquitoes, salt and fresh-water breeders respectively, which can carry diseases such as Ross River Virus.
Only female mosquitoes bite as they need the nutrients in blood for their larvae.
But if one bites a wallaby or kangaroo infected with the virus, they can transfer the illness to their next human victim.
And while it isn't fatal, Ross River can cause a fever, severe joint pain, swelling and fatigue lasting weeks or months.
Dr Webb said an increase in mosquito numbers doesn't always lead to an increase in infections.
He warned climate change could also increase mosquito numbers.
"It's difficult to predict but on balance, more mosquitoes are likely as there's an extension of the season into spring and into autumn," he said.
And wiping them out isn't easy, as Dr Webb discovered when bushfires destroyed one of his mosquito monitoring stations last month.
When he visited the site a few days later he found the mosquitoes had returned.
"They moved back in very quickly so we know the eggs must have survived the fires," he said.
By lying dormant in the soil, it was likely they were protected from the intense heat, he added.
"They're pretty hardy, they survive through winters and drought as eggs and as soon as those areas flood, they hatch."
However, the bushfires also wiped out most of the mosquitoes' food source which could have long-term effects.
"It could be that as the surviving animals move nearer to people where there's food then the mosquitoes may follow," he said, leading more opportunities for diseases to be transferred.
"We just don't know, we've never had an extreme event like this before."
Diseases carried Australian carried by Australian mosquitoes
Ross River Virus
The most common mosquito-borne disease in Australia, it induces flu-like symptoms including muscle and joint pain, fever, swelling and fatigue which can last weeks or months. It can be contracted Australia-wide.
Ross River virus is transmitted to humans by more than 40 species of mosquitoes from infected kangaroos and wallabies.
Barmah Forest Virus
Similar to Ross River virus, it causes joint pain and flu-like symptoms.
It can be contracted Australia-wide and is transmitted through kangaroos and wallabies.
Murray Valley Encephalitis / Kunjin Virus
A rare but potentially fatal disease, in most people it causes no illness (or a mild one) but can lead to a severe brain infection with symptoms including seizures, delirium and coma.
Kunjin Virus is a milder form of the disease.
It can be contracted in northern parts of Queensland, the Northern Territory and Western Australia, particularly during the wet season.
It is transmitted by mosquitoes from water birds such as herons.
A fever from three to seven days, intense headache and After infected, a person becomes immune to that type of Dengue.
Occasional outbreaks occur in Far North Queensland and cases are usually diagnosed in people who have recently visited parts of Asia.
It is carried by the aedes aegypti mosquito which breeds in or near homes and is transmitted when the mosquito bites another person with Dengue fever.
It is caused by five types of plasmodium (single-celled parasites), flu-like symptoms can reappear months or years later and infection by Falciparum malaria is potentially life-threatening.
Australia was declared malaria-free in 1981 however several hundred cases are reported each year among travellers to regions endemic with the disease.
It is carried by a Anopheles mosquito which has bitten another person with malaria.