What is Japanese encephalitis?

Japanese encephalitis (JE) is a disease caused by the Japanese encephalitis virus (JEV), which is transmitted by mosquitoes. It is common in the tropical regions of Asia and was first detected in NSW in late February 2022. It exists in a natural cycle with some birds being the main host. The virus can also infect other animals, such as pigs.

Why did Japanese encephalitis appear in NSW?

NSW Health is working to know more about how the virus arrived in NSW, but it's likely warm and wet conditions contributed to increased mosquito breeding and transmission.

The mosquito that spreads JEV has been in NSW a long time, especially western and regional NSW. We know this because this mosquito spreads other similar mosquito-borne diseases, such as Murray Valley encephalitis.

Why did NSW Health do a serosurvey?

A serosurvey is a survey of serological (blood) samples.

Most people infected with JEV will show no or only minor symptoms. For this reason, there was very little information on how widespread the virus was, how many people were infected and what activities increased the chance of getting bitten by an infected mosquito.

Testing the blood of people who lived in areas where JEV may have been, combined with a questionnaire, could help answer many of these questions. The answers could help us understand the spread of JEV in NSW and can assist in ensuring people can protect themselves, like getting the vaccine to people who need it most.

The serosurvey was conducted in five towns in regional NSW in June and July 2022. The locations were chosen because they were areas where infected mosquitoes were most likely to be found when JE first emerged in NSW. The aim was to use the information to build a response for the next mosquito season (usually late spring to early autumn).

The overall serosurvey findings will be made available to the public once all participants have been provided with their individual results.

What kind of test is done for Japanese encephalitis?

A blood test is done, which looks for antibodies to JEV. Your body makes antibodies when it fights an infection or when you are vaccinated, just like your skin makes a scar when it gets an injury.

Blood samples that did not have antibodies against JEV are ‘negative’ for the virus. These samples didn't require any further testing.

Blood samples that did have antibodies for JEV are ’positive’ for the virus. Positive samples were also tested for antibodies against closely related viruses that can be spread by mosquitoes, Murray Valley encephalitis Virus and Kunjin Virus (further information below). This was to check that the antibodies found were due to JEV.

Where was the test done?

The test was performed at the Institute of Clinical Pathology and Medical Research at Westmead, part of NSW Health Pathology.

I received a positive result from the serosurvey – what does it mean?

A ’positive’ result for JEV means that antibodies for the virus were detected in your blood. This means that you have either been infected with the virus by a mosquito or have been vaccinated against JEV. You may not have known that you were infected, as very few people (less than 1 in 250) show symptoms.

People who have antibodies to JEV will have protection from disease. The length of time you are protected will depend on how long ago you were infected, whether you get bitten by an infected mosquito (or vaccinated) again, and the strength of your immune system. Most people stay protected for many years.

People who test positive during the serosurvey are not recommended to receive the JE vaccine at this time as they already have protective antibodies.

People who test positive during the serosurvey will not be ‘counted’ in NSW or national case numbers, because a single positive blood test does not meet the definition of an outbreak case. However, the result will be used for response planning in NSW.

I received a negative result from the serosurvey – what does it mean?

A ’negative’ result for JEV means that antibodies for the virus were not detected in your blood. This means that your blood does not show any sign of previous infection. People who are negative do not have protection against the disease. You may be able to get the free JE vaccine; please check the eligibility criteria for NSW.   

If you have a negative result but have previously been vaccinated against JEV, please speak to your doctor. You may have tested negative for many reasons, including how long it has been since your vaccination, what type of vaccine you got, and the strength of your immune system.

What are Murray Valley encephalitis virus and Kunjin virus?

Murray Valley encephalitis Virus and Kunjin Virus are also mosquito-borne viruses. Like with JEV, most people with antibodies to these viruses may not realise they have been previously infected, since very few people develop symptoms. However, similar to JEV, a small proportion of those infected can develop severe disease.

There is no vaccine for either of these diseases, which makes it so important to prevent mosquito bites.

How can I prevent myself, my family and my community from getting bitten by infected mosquitos?

  1. Stop mosquito bites:
    • Cover up while outside. Wear loose, light-coloured clothing and covered footwear.
    • Use insect repellent on all areas of exposed skin. Use brands that contain DEET, picaridin or oil of lemon eucalyptus, and follow the directions on the label. Higher concentrations will provide longer protection.
    • Avoid being outdoors at dawn and dusk, or in areas where mosquitoes are common like swamps and marshlands.
  2. Kill mosquitos and stop them breeding:
    • Get rid of mosquitoes inside the house by using plug-in insecticide products, flying insect knock-down sprays and fly screens.
    • Cover openings such as windows and doors with fly screens
    • Remove water-holding containers and clean up outside your house, including rubbish, tires and open bins where mosquitoes can breed. Keeping lawns mowed and gutters and drains clean will also help prevent breeding.

Where can I find more information?

We have lots of useful factsheets available:

Current as at: Monday 12 September 2022
Contact page owner: Health Protection NSW