Our mosquito and Japanese encephalitis (JE) experts explain the virus, the role mosquitoes play in spreading it, and whether the virusis here to stay.
Dr David Williams, Japanese encephalitis expert at CSIRO's Australian Centre for Disease Preparedness
What is Japanese encephalitis?
Japanese encephalitis (JE) is one of the most important mosquito-borne viral diseases in Asia where the virus in endemic, causing approximately 100,000 cases each year.
Most infections are asymptomatic or cause a mild flu-like illness, but in approximately 1% of infected people the more severe JE disease can develop. Of these clinical JE cases, approximately 20–30% of are fatal, 30–50% develop long‐term side effects such as movement and speech disorders, and the remaining cases fully recover.
How did this virus reach Australia?
How this virus spread to the southeast of Australia is not known with certainty, but the most likely mechanism was by infected waterbirds that migrated from northern Australia to the wetlands and waterways of southeastern Australia. From there, local mosquito populations are likely to have become infected by feeding on infected waterbirds, before spreading the virus to pigs and people.
We also don’t know how it originally got into northern Australia, but it’s most likely that it took place from either infected migratory waterbirds or windblown mosquitoes.
How does the virus spread?
The virus is believed to spread over large distances by infected migratory waterbirds that follow river courses and waterways. When these birds arrive in a new location, they are fed on by local mosquitoes that then become infected and can transmit the virus to humans and animals.
Migratory waterbirds play a role as a reservoir host for the virus and can introduce the virus into new areas or re-introduce the virus into regions where JE might be found seasonally.
Humans and horses are so-called dead-end hosts in that the virus doesn’t replicate to high enough levels in their blood to infect biting mosquitoes. Pigs on the other hand are amplifying hosts of the virus and play an important role in outbreaks; they produce high amounts of virus, which is transmitted to biting mosquitoes.
Another way that the virus can be introduced into new areas is by long range wind dispersal of mosquitoes. Some studies have shown that mosquitoes can travel 100-200 kms in this way, and wind dispersal of infected mosquitoes has been proposed as a mechanism for the introduction of JE virus into northern Australia from Papua New Guinea.
Could the virus be spread by pigs?
It’s known that pigs can shed the virus in their saliva and nasal secretions to other pigs; and it has also been shown that sows can become infected through artificial insemination using infected semen. We don’t know whether either of these routes of infection have played a role in this outbreak – that’s a very difficult thing to work out when there are so many infected mosquitoes flying around piggeries - but it’s possible that these infection routes have played a minor contribution.
What is the risk to humans?
People can become infected with the JE virus when they are bitten by an infected mosquito. Most people that are infected won’t know they’ve been infected – they might experience a mild flu like illness or have no symptoms at all. In about 1 in every 100 infections, severe brain inflammation can occur. When that happens, about a third of cases will be fatal, another third can will have long-term side effects such as movement and speech disorders, and the remaining cases will fully recover.
Can the virus spread person to person?
The JE virus cannot be transmitted directly from an infected person to an uninfected person.
What is the risk to animals?
The two main animal species affected are horses and pigs. In horses, severe brain disease can occur, which is very similar to human JE, and with similar outcomes in terms of survival.
In pigs, JE causes a reproductive disease. In pregnant sows or gilts, infection before about 60-70 days gestation can lead to aborted or mummified fetuses, or birth of weak piglets with signs of brain disease. In infected boars, infection can lead to infertility. It’s important to note JE is not a food safety concern and commercially produced pork meat or pork products are safe to consume.
What do we know about this new outbreak in Australia?
JE was detected in piggeries in southern Queensland, NSW, Victoria and South Australia in late February and early March this year (2022), which was associated with higher-than-normal levels of abortion, stillborn and weak piglets. Based on what is known of how the JE virus infects and causes disease in pregnant sows, the initial infections are believed to have occurred towards the end of 2021.
Unfortunately, there have also been several human cases of encephalitis in each of the affected states that have been either confirmed by laboratory diagnosis or considered probable cases.
Has Australia had a JE outbreak before?
This is not the first time that JE has caused an outbreak or cases in Australia. In 1995, an outbreak occurred in the Torres Strait Islands, causing three cases, including two that were fatal. Evidence of infection was also found in mosquitoes and pigs on the affected islands. Another human case in the Torres Strait Islands occurred in 1998, along with the first Australian mainland case on the Cape York Peninsula. Since then, surveillance activities have shown that the virus regularly appears in the very northern tip of the Cape York Peninsula, probably from Papua New Guinea, where the virus is believed to endemic, via the Torres Strait.
More recently in early 2021, a fatal human case occurred in the Tiwi Islands off the coast of the Northern Territory. This was the first locally acquired human case in Australia since 1998. The 2021 case was found to be caused by a particular type of the JE virus (genotype 4) that had previously only rarely been found, and mainly in Indonesia, but we also now know that it exists in Papua New Guinea.
Is JE here to stay in Australia? Is it likely to spread much further?
It’s difficult to predict whether there will be further spread of JE virus this year, but this is something that state medical entomology teams and relevant public health units are considering and planning for. In the short-term, this will largely depend on weather conditions and how these influence the interplay between mosquitoes, and pigs and waterbirds.
The La Niña weather event has produced above average rainfall that has created widespread larval habitats for the key JE virus mosquito species Culex annulirostris. The recent flooding may prolong the existence of freshwater habitats for this species in some locations, especially once floodwaters recede.
Will the JE outbreak fizzle out with winter and the end of the mosquito season?
As the colder months approach, we should see a reduction in mosquito populations, and with that, a reduction in transmission within piggeries and waterbird populations. We can also expect the northern migration of birds, further reducing the pool of possible hosts. We’ve historically seen a similar seasonal pattern of circulation of other mosquito-borne viruses in Southeast Australia, such as for Murray Valley encephalitis virus, which is a close relative of the JE virus and shares a similar ecology.
Whether the virus will come back next summer or become endemic are scenarios that authorities are preparing for. It is possible that JE virus may follow the pattern of other important mosquito-borne viruses like Murray Valley encephalitis, which tends to contract into the northern parts of Australia in seasons following outbreaks or activity in Southeast Australia. This will be largely influenced by the prevailing weather systems that we experience. Importantly, sensitive surveillance programs will need to be established for monitoring JE virus and control strategies prepared to limit transmission.
Is there a vaccine? Do I need to get vaccinated?
There are two different JE virus vaccines available in Australia, which are very safe and effective for both adults and children. The Government is working closely with states and territories to support the distribution of vaccines to at-risk population groups.
What is CSIRO doing to help against JE?
CSIRO’s Australian Centre for Disease Preparedness (ACDP) is a nationally recognised reference laboratory for arboviruses. Our scientists have been heavily involved in diagnostic testing for affected states, particularly for testing piggeries suspected of being infected. The diagnosis of JE from samples sent from the index property in southern Queensland was the first detection of JE virus in the current outbreak.
As part of diagnostic support, ACDP scientists have provided information and advice to animal and public health laboratories on laboratory diagnostics. This has included testing guidelines and information, including the first genome sequence of the outbreak strain, to assist accurate diagnostic testing.
CSIRO is continuing to support the JE outbreak response and our experts sit on a number of national committees involved in the response including National Arbovirus and Malaria Advisory Committee (NAMAC) and the Public Health Laboratory Network.
ACDP scientists have also been providing expert advice to various government and animal health working groups involved in outbreak response. These groups are working closely with public health counterparts in a One Health approach to coordinate and manage the outbreak.
ACDP scientists have also been working in Papua New Guinea to establish surveillance for JE virus using a One Health approach.
How many human cases of JE have been reported?
The current status of the JE outbreak and human cases numbers are being tracked by the Department of Health here.
Dr Prasad Paradkar, CSIRO principal research scientist and vector-borne disease expert
Which mosquito species transmit the JE virus?
The JE virus is spread by the bite of infected Culex species mosquitoes, predominantly Culex annulirostris which are found around Australia. Also known as the common banded mosquito, they tend to breed in temporary water puddles, including open drains and flood-plain wetlands, exacerbated by heavy rains and flooding.
The other Culex species are Culex sitiens and Culex quinquefasciatus.
How far can an infected mosquito fly?
Culex annulirostris is highly mobile and capable of flying long distances in excess of 10km.
Mosquito numbers are very high right now. What’s caused this?
Australia has been experiencing heavy rainfall this season, due to La Niña conditions, which have led to floods along our East Coast. The flooding of waterways and lakes in the inland Australia, sometimes temporarily formed by above average rainfall, provide a breeding ground for mosquitoes. This increase in water bodies combined with warm temperatures have fostered an increase in mosquito numbers.
What impact will this increase in mosquitoes have in Australia?
With female mosquitoes needing blood to lay more eggs, most mosquito species are a nuisance, biting unsuspecting people, usually at dawn or dusk. Some species of mosquitoes are also responsible for the transmission of diseases like Murray valley encephalitis, Japanese encephalitis or Ross River fever. So, an increase in the number of mosquitoes can lead to increased biting as well as greater risk of disease transmission.
What diseases can mosquitoes spread?
Some species of mosquitoes can transmit diseases of animal and public health importance. In Australia, mosquitoes are known to transmit viruses like Ross River, Murray valley encephalitis, Barmah Forest as well as dengue, Japanese encephalitis and West Nile (Kunjin).
Are there other mosquito-borne diseases we should be wary of?
It is estimated that more than half of the world’s population is at risk of infection from mosquito-borne diseases. In Australia, mosquitoes can carry Murray Valley encephalitis virus, West Nile virus Ross River virus, Barmah Forest virus, dengue virus and now Japanese encephalitis virus.
Mosquitoes responsible for transmission of these viruses are present in Australia and introductions via migratory birds or travelers can lead to outbreaks. There are also invasive mosquito species on our doorstep such as Aedes albopictus which if established here could lead to increased risk of disease outbreaks.
Do all mosquitoes thrive in the same conditions?
Mosquitoes have adapted to various Australian environments. Certain mosquito species, such as Aedes aegypti which transmits dengue, are ‘container breeders’, and thrive in an urban environment. These mosquitoes rely on their desiccation-resistant eggs, surviving months until favourable conditions occur.
Other mosquitoes, such as the species known to transmit JE virus, breed in temporary water puddles, including open drains and flood-plain wetlands, exacerbated by heavy rains and flooding.
How can I protect myself from JE virus and other mosquito-borne viruses?
The two main ways to prevent Japanese encephalitis are to either receive a vaccination for the virus, or to protect yourself from being bitten by mosquitoes. The Department of Health provides more tips on how to avoid mosquitoes at Japanese encephalitis - prevention.