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Explainer: what is Ross River virus?

Ross River virus infection is the most commonly reported mosquito-borne disease in Australia, with more than 4,000 cases of illness are reported every year. Activity has been recorded from every state…

The saltmarsh mosquito, Aedes vigilax, transmits Ross River virus in many coastal regions of Australia. Mr Stephen Doggett (Medical Entomology, Pathology West - ICPMR Westmead)

Ross River virus infection is the most commonly reported mosquito-borne disease in Australia, with more than 4,000 cases of illness are reported every year.

Activity has been recorded from every state and territory in the country. And while Ross River virus generally considered a disease of rural regions, it is increasingly active at our urban fringes.

Symptoms

Ross River virus is not fatal but it can be debilitating. The symptoms typically include “flu-like symptoms” such as fever, chills, headache, muscle and joint pain and a general feeling of fatigue.

The symptoms develop a week or so after being bitten by an infected mosquito. But while they typically last less than two weeks, there are cases where fatigue, muscle and joint pain persist for many months.

Ross River virus is a notifiable disease, so infection can only be confirmed through a blood test. For this reason – and because the severity and duration of symptoms can be highly variable – it is strongly suspected that many cases go unreported and that the official statistics are a dramatic underestimate of the total number of infections each year.

There are no specific treatments or vaccines, so preventing mosquito bites is critical in avoiding disease.

It’s difficult to predict outbreaks and governments don’t have the capacity to undertake broad-scale mosquito control in all regions, so health authorities generally focus their efforts on raising awareness and issuing warnings, based on information from local surveillance programs.

Transmission

There are more than 40 mosquito species that may play a role in transmitting the virus. This is a problem because the mosquitoes transmitting the virus during outbreaks may vary from region to region, making it difficult to track.

It is also a problem because the environmental drivers of mosquito abundance will change from region to region. This means that in some locations, the risks of an outbreak may be triggered by rainfall while in others, it is the tidal flooding of coastal wetlands.

While it is not surprising that outbreaks generally occur when mosquito populations are high, abundant mosquito populations and wetlands don’t guarantee an outbreak will occur.

Mosquitoes generally don’t hatch out of the wetlands infected with Ross River virus, they must bite an infected animal first. The most important reservoirs for Ross River virus are macropods (kangaroos and wallabies).

Kangaroos and wallabies are critical in determining the risks of Ross River virus. Dr Cameron Webb (University of Sydney and Pathology West)

There have been Ross River virus cases from most of our major metropolitan centres. In many urban fringe areas, newly constructed wetlands are increasing the abundance of local mosquito populations, while the control of foxes and feral cats has led to an increase in populations of wallabies.

Wallabies using the bushland corridors along our rivers and estuaries therefore may be increasing the risks of Ross River virus. This potential has been illustrated by the recent public health warnings issued due to the detection of Ross River virus in mosquitoes collected along the Georges River in southern Sydney where wallabies are common within Georges River National Park.

Prevention

It’s important to protect yourself from mosquito bites with protective clothing or topical insect repellents when you’re around wetland and bushland areas, particularly at dawn and dusk when mosquitoes are most active.

Remember, even though mosquito populations are falling, autumn is actually the peak period of virus activity. You may not be swamped by mozzies but you should still take precautions to avoid bites.