Buzz around mozzie testing and Ross River research

There has been a spike in the number of reported cases of the mosquito-borne infection Ross River Fever on the Mid North Coast.

Enough so to trigger a warning from the North Coast Public Health Unit to take the appropriate measures to avoid being bitten.

The Ross River virus is the most commonly reported mosquito-borne disease in Australia and is spread by the bite of a mosquito.

There are about 40 different mosquito species implicated in its transmission and it is classified as a notifiable disease.

It is estimated there are around 5,000 cases of illness across the country and between 500 and 1,500 cases per year in NSW. However, there are likely to be many more people that experience a much milder illness and never get blood tests to confirm infection.

The local Health District has been identified as a mosquito hot spot with transmission rates for Ross River Fever and Barmah Forest Fever doubling from 2019.

Mosquito expert, Dr Cameron Webb works for University of Sydney and NSW Health Pathology and has over 20 years' experience exploring the impacts of urbanisation and waterway health on mosquito populations.

"One of the things unique about the Mid North Coast and other parts coastal NSW is that they have a diverse range of mosquito habitats," Dr Webb said.

"In NSW, we've got over 60 different types of mosquitoes and the majority of them pose little pest or public health significance. But half a dozen are a real concern and are found in specific types of environments.

Dr Cameron Webb servicing a mosquito trap.

Dr Cameron Webb servicing a mosquito trap.

"Around Port Macquarie there are tidally influenced environments such as saltmarsh, coastal swamp forests and tea tree swamps.

"These combined and you will have mosquitoes emerging from several different habitats in the one area.

"This season has been unique. We've had exceptional rain after a long period of drought - that's a perfect storm for mosquitoes and they've persisted longer into autumn than we normally have seen."

Dr Webb works in regions around Byron Bay and Ballina with local and state government authorities to better integrate mosquito management into overall urban development strategies. This includes the construction and rehabilitation of wetlands as well as urban design elements.

The saltmarsh mosquito (Aedes vigilax) is one of the most common mosquitoes of pest and public health importance in coastal regions of NSW.

The saltmarsh mosquito (Aedes vigilax) is one of the most common mosquitoes of pest and public health importance in coastal regions of NSW.

"In Sydney we know we have a high concentration of mosquitoes which is a symptom of the poor health of the wetlands. Rehabilitation of those wetlands can reduce the quantity of mosquitoes produced.

"But as we also open up coastal areas to urban development, people are living closer to wetlands than they were before. Also, people moving to areas like this from the city for example, are not fully understanding the health risks of mosquitoes or taking the appropriate measures to avoid being bitten."

While Ross River Fever is not a life-threatening disease, it can be severely debilitating with some people able to recover within weeks, while others experience painful symptoms for months.

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"There is a role to play for both local councils and the health department about making sure good, clear messages are shared in the community to avoid mosquito bites. And this is done in most parts very well done by state health authorities," Dr Webb said.

"Like any public health message, we need to ensure the community gets the best advice to make those changes and to avoid being bitten.

"And that can be from choosing the right sorts of repellents to reducing the opportunities for mosquitoes to breed in your own back yard."

Local councils across New South Wales trap mosquitoes and send to NSW Health Pathology for weekly testing from the start of spring through until the middle of autumn.

NSW Health coordinates the arbovirus and mosquito monitoring program across the state.

Mosquitoes are lured into the traps, counted and tested to see if they are carrying known pathogens.

Mosquitoes are lured into the traps, counted and tested to see if they are carrying known pathogens.

Mosquitoes are collected using traps baited with carbon dioxide. They trick the mosquitoes into thinking the trap is an animal.

"We look at the types of mosquitoes caught in the traps, count and identify them and test them to see if any are carrying viruses of human health importance," Dr Webb said.

"Those results help shape measures around mosquito management and mosquito-borne disease."

Dr Webb said councils can then use that data in many ways from including it into decision-making for new urban developments to risk management in wetland rehabilitation projects and community awareness campaigns.

What is difficult to predict is the prevalence of mosquito-borne diseases in an area because the combination of risk factors in complex and evolving.

"Mosquito populations are determined by seasonal differences, rainfall, temperature and tidal inundation of local wetlands," Dr Webb explained.

"What we do know with climate change is that we will see more mosquitoes around in early spring and into autumn. But what we won't see necessarily is an introduction of mosquitoes spreading more pathogens such as dengue fever or the zika virus into NSW.

"The management of our coastal water bodies and wetlands will always have the potential to change mosquito populations. A reduction in mosquito populations can be the benefit of improved waterways."

A mosquito will only spread the Ross River or Barmah Forest Fever virus to a human after it has bitten an infected animal such as a kangaroo or wallaby.

"What that means is there's a really complex puzzle to put together when predicting outbreaks. Not only do we have to worry about the weather and wetlands, we also have to worry about wildlife carrying those diseases," Dr Webb said.

"One of the reasons why surveillance programs by councils and NSW Health are important is because it is real time monitoring."

And some people are more likely to be bitten by mozzies than others.

Mosquitoes can sense and are attracted to the carbon dioxide humans breathe out as well as the smell of our skin.

"All of us have hundreds of chemical compounds we sweat out and depending on that cocktail of smells can determine if we get bitten or not," Dr Webb said.

"One of the fascinating things with all the dozens of mosquito species we have, they don't all have the same preference. Some prefer to bite birds, others kangaroos - it's the same with one type of person over another.

"On top of that, when you are bitten by a mosquito, she injects spit into you to get the blood flowing. And everyone will also react differently to mozzie spit."

And if you've seen more dragonflies about lately, then it may have something to do with the mozzies. Dragonflies love to snack on mosquitoes, but certainly not enough to keep numbers manageable.

How to avoid being bitten

  • When outside cover up as much as possible with light-coloured, loose-fitting clothing and covered footwear.
  • Use an effective insect repellent on exposed skin. Re-apply repellent within a few hours, as protection wears off with perspiration. The best mosquito repellents contain Diethyl Toluamide (DEET) or Picaridin.
  • Cover all windows, doors, vents and other entrances with insect screens.
  • Clean up your backyard, mow lawns and remove all water-holding rubbish including tyres and containers. Fill pot plant bases with sand to avoid standing water.

More information about mosquito-borne infections is available on the NSW Health website.