Mosquitoes and Murray Valley encephalitis

What the illness is

MVE is caused by the Murray Valley encephalitis virus, a flavivirus. Most people who catch it have no symptoms; fewer than one in a hundred develop illness, and about 1 in 800 develop severe illness with encephalitis or meningoencephalitis.

You cannot catch it from another person, or from birds or other animals: it needs a mosquito to carry it.

Where it comes from, and when

The virus is endemic in northern Australia and Papua New Guinea, where cases or small outbreaks turn up every few years, usually at the end of the wet season. South-eastern Australia has had eight outbreaks, at irregular intervals, since 1917.

In Victoria, in years of high virus activity, its main hosts are waterbirds, and herons (the Nankeen night-heron above all), cormorants and darters are the ones most often infected. The main mosquito carrying it during epidemics is Culex annulirostris, which breeds in fresh water. Anyone can be bitten, but people who live, work or spend time outdoors in northern Victoria, particularly the inland riverine regions running up towards the Murray River, are at greater risk.

For a Melbourne household, the season matters as much as the map. The department watches mosquitoes through Victoria’s breeding season, which inland usually runs from early November to late April; on the coast it tends to start earlier and finish later. Mosquitoes caught in that program are tested for MVE, Ross River, Barmah Forest, Japanese encephalitis and West Nile (Kunjin) viruses. The Victorian Government has run this kind of integrated mosquito program with other agencies, including selected councils, since 1974, and funds councils in high-risk areas to do surveillance.

Keeping the bites off

Avoiding bites is the most important protection against mosquito-borne diseases, and the department pairs it with removing breeding sites around your home. For MVE it names these:

• Use a repellent containing picaridin or DEET on all exposed skin.
• Get rid of stagnant water where mosquitoes can breed, around the house or the campsite.
• For babies, you may need to put repellent on their clothes rather than their skin, and keep it off the hands of babies and young children.

Whose job the mosquitoes are

In Victoria, the Public Health and Wellbeing Regulations 2019 set out who is responsible for mosquito management, and the answer is broad: every landowner or occupier, public or private, residents included. Those regulations replaced the 2009 version on 14 December 2019 and were written to cover emerging and potential vector-borne disease risks too. That includes the householder.

On the medical side, every mosquito-borne disease is notifiable to the department, and MVE is an “urgent” notifiable condition that doctors and pathology services must report straight away, suspected or confirmed.

MVE is the rare one

The mosquito-borne infections people catch most often in Victoria are Ross River virus and Barmah Forest virus. Japanese encephalitis, MVE and West Nile (Kunjin) virus are rare, but can cause severe disease. Ross River virus disease usually shows 3 to 9 days after infection, though it can take up to 21, and can bring joint inflammation and pain, tiredness, muscle aches and a rash. Barmah Forest virus disease usually shows 7 to 10 days after infection, though it can take up to 21, and many people who catch it have no symptoms at all. Japanese encephalitis is likewise rare, and can be a serious brain infection; mosquitoes pass it to people. Of more than 275 mosquito species in Australia, only a few bite people and fewer still carry human disease.

If you are thinking of having a yard treated for mosquitoes, getting ready for a treatment sets out what Victoria’s health advice asks of a household on the day.