Murray Valley Encephalitis

The Murray Valley encephalitis virus was first isolated in an epidemic in 1951 (CDNA 2013).

Earlier epidemics in 1917 (114 cases), 1918 (67 cases), and 1925 (10 cases) were probably also due to the virus. In 1974, the only Australia-wide outbreak occurred, involving 58 cases of encephalitis and resulting in about 20% of cases dying (CDNA 2013;. Since then, there have been very few cases (CDNA 2013).

The virus is endemic in Northern Australia (northern Western Australia and the top of the Northern Territory), but rarely affects humans (SA Health 2022).

The occasional spread to the southern states occurs during times of heavy rainfall (Health.vic 2023).

Transmission of Murray Valley Encephalitis

People contract the virus by being bitten by an infected mosquito. The common banded mosquito, Culex annulirostris, is the usual culprit. You can find this mosquito (or it can find you) throughout Australia (Queensland Government 2017).

It breeds in freshwater in spring, summer and autumn. It likes surface pools, rivers, natural wetlands and irrigation waters, but also enjoys backyards. It is especially common in the Murray-Darling river basin in summer (NSW Health 2022).

Why then does Murray Valley encephalitis only occur at odd intervals in the Murray-Darling basin?

The reason is that the mosquito can only be infected by biting an infected water bird, usually a heron or egret. These birds migrate into flooded areas, often by the thousands (NSW Health 2022).

Other animals including non-water birds, kangaroos and horses can also be infected but cannot pass the infection on to humans (NSW Health 2016).

In New South Wales, advantage is taken of the fact that chickens can be infected by maintaining ‘sentinel flocks’. These are bled periodically, and if the virus is detected, the panic button is pushed (NSW Health 2016).

Epidemics in horses occasionally occur, but it is unknown what exactly this means for humans. The disease in horses usually shows up as depression, incoordination and other neurological signs (CDNA 2013; Oke 2015).

If you have lived in an area affected by Murray Valley encephalitis for a long time, you will probably have at least some immunity to the virus. If you are a visitor or recent arrival and go bushwalking, boating, fishing, bird-watching or camping near swamps, lagoons, dams or temporary pools of water near grassy areas, you must be especially careful. Babies and children are also at increased risk, as they will have not lived for long enough to gain natural immunity (Queensland Government 2017).

The common banded mosquito is nocturnal, feeding mainly in the early evening and pre-dawn. So, if you can, stay indoors from just before sunset and all night. If you must go out, wear loose (they can bite through tight clothing), light-coloured clothing with long sleeves, long trousers, socks and a hat (Queensland Government 2017).

Use a diethyltoluamide (DEET) or picaridin repellant on exposed skin and reapply often (check the directions) (Queensland Government 2017). Don’t use these on a baby less than two months old. If the child is older than two months but under one year, make sure the active ingredient is less than 10% and spray onto clothing rather than skin. If you are using sunscreen, apply this first. If you want to be really sure, use permethrin-treated clothing and gear (Better Health Channel 2023; CDC 2022).

At home, the first thing you should do is get rid of all places where mosquitoes can breed. This could include containers, ponds, depressions, tanks, pot plant drip trays and bromeliads. Make sure screens are in good order (Healthy WA 2018).

If you are camping out, use a mosquito-proof tent or mosquito net. You can also spray residual pyrethroids around the campsite and nearby shrubbery (Better Health Channel 2023; Queensland Government 2017).

Unfortunately, because of the multitude of breeding areas and the fact that the mosquito can fly up to 2 km, the sort of mass control efforts sometimes carried out by councils may not achieve much.

Symptoms of Murray Valley Encephalitis

If, in spite of all your precautions, you do get bitten by an infected mosquito and become infected, the odds are that you will never know it. Your chances of becoming severely ill are estimated to be as little as 1 in 1,000 (SA Health 2022).

If you do happen to get sick, the illness will come on anywhere between 5 and 28 days after you’ve been bitten, but most likely between 7 and 12 days. You may experience mild symptoms including headache, fever, nausea, vomiting, loss of appetite, diarrhoea and muscle aches. Infants may become irritable and ‘floppy’ (SA Health 2022; NSW Health 2022).

In most cases, this will be it. In a few cases, meningitis (infection of the membranes around the brain) or encephalitis (severe brain infection or inflammation) may develop (SA Health 2022).

If you develop meningitis or encephalitis, you may experience a severe headache, neck stiffness, sensitivity to bright lights, drowsiness, confusion and trouble with coordination and speech. Children may experience seizures or fits (NSW DoH 2016; Healthy WA 2018).

In the worst-case scenario, you may lose consciousness, pass into a coma or die. If on the other hand, you survive, you have a 40% chance of permanent neurological complications such as paralysis or brain damage (NSW Health 2022; SA Health 2020).

Your chance of recovering completely is only about 40%, but you will have life-long immunity (SA Health 2022).

The laboratory can prove you have the disease by showing a significant rise in antibodies in blood specimens taken a week apart or by detecting the virus in cerebrospinal fluid by a nucleic acid test (NSW Health 2016).

Unfortunately, there is no cure for the disease. Treatment is supportive only. In the early or mild stages, you can take paracetamol (not aspirin or ibuprofen, which may cause bleeding), have lots of rest and drink plenty of water. In the later stages, you will require hospitalisation, possibly in ICU (CDC 2022; NSW Health 2016).

There is no vaccine available (SA Health 2022) and, given the rarity of the disease, unlikely to ever be one.