Explainers

Murray Valley Encephalitis


The Murray Valley encephalitis virus was first isolated in an epidemic in 1951.

Earlier epidemics in 1917 (114 cases), 1918 (67 cases) and 1925 (10 cases) were probably also due to the virus. The next epidemic was in 1956. In 1974, there was the only Australia-wide outbreak, involving 58 cases of encephalitis and resulting in about 20% of cases dying. The next epidemic was in 2001. In between epidemics, there were no or very few cases.

Things are rather different in the Top End of the Northern Territory and in the Kimberley region of North-West Western Australia. Here, there are one or two cases every year. These occur mainly in The Wet and following months, especially March to May.

The occasional spread to the southern states occurs during times of heavy rainfall during the summer monsoon season via seasonal flooding of the Murray-Darling river system.

Transmission of Murray Valley Encephalitis

People get 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, except in Tasmania.

It breeds in fresh water 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.

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

Other animals, including other birds, kangaroos and horses, can be infected but cannot pass the infection on to other mosquitoes. This is also true of humans.

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.

Epidemics in horses have paralleled those in humans, at least in 1974 and 2001. The disease in horses usually shows up as depression, incoordination and other neurological signs.

If you live in an area affected by Murray Valley encephalitis, you will probably have at least some immunity. At least, you probably will if you are not a baby or toddler or have only recently moved in to the area. 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.

The 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 hat.

Use a diethyltoluamide (DEET) or picaridin preparation and use it often enough (check the directions). Don’t use these on a baby less than 2 months old. If the child is older than this but less than 1 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.

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.

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.

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

Symptoms

If, in spite of all your precautions, you do get bitten by an infected mosquito and become infected, the odds are you will never know it. Researchers have estimated your chances of becoming sick as little as 1 in 1,000.

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 about 14 days later. For the next week or two, you will have headache, fever, nausea and vomiting. You won’t feel like eating and you may have diarrhoea and muscle aches. Infants will probably be irritable and ‘floppy’.

In most cases, this will be it. Gradually or suddenly, you’ll feel like your old self again. In a few cases (up to 1 in 20 in children), meningitis (infection of the membranes around the brain) or encephalitis (severe brain infection or inflammation) may develop.

Symptoms are much the same as for these conditions caused by other agents. You will have a severe headache, neck stiffness, sensitivity to bright lights, drowsiness, confusion and trouble with coordination and speech. Little kids are likely to have seizures or fits.

In the worst case (15-30% of cases), you will become weaker, tremble, become delirious, lose consciousness, pass into a coma and die. If on the other hand, you survive, you have a 30-50% chance of being deaf, having epilepsy or paralysis, or some other brain injury.

Your chance of recovering completely is only about 40%. At least you will have life-long immunity.

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.

Unfortunately, there is no pill to cure the disease. 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. In the later stages, you will be in hospital, probably in ICU.

There is no vaccine available and, given the rarity of the disease, unlikely to ever be one.

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