A brief overview of a report describing the 2022 discovery of Japanese encephalitis virus (JEV) in Australia. This was the first local outbreak in southern Australia and set the stage for an ongoing virus presence.
Some JEV backstory
JEV is a virus belonging to the species Orthoflavivirus japonicum.[18] Infection was rare in Australia but is a leading cause of childhood viral encephalitis in Asia.[15]
Infection results from the bite of a member of the Culex genus of mosquitoes but can’t be transmitted from human to human.
The mosquitoes are infected after feeding on infected birds (perhaps bats [23]), creating a maintenance cycle of the virus. Pigs act as amplifying hosts from which mosquitoes can also acquire JEV. Maintenance and amplification don’t rely on humans, cattle, water buffalo, snakes, dogs, pheasant, wallaby or kangaroo species tested so far, and the jury is still considering horses [25], except as dead-end hosts due to limited virus production in their blood.[20]
Where these cycles occur, JEV movement through animals is described as enzootic transmission because the disease or pathogen is always present, regardless of severity or sequelae, in a non-human animal population in a specific area (the human equivalent is the endemic transmission).
Recently, a sixth genotype of the species, Culex tritaeniorhynchus, was identified in Australia, where it was not previously thought to have spread.[16] The authors recommended surveillance of this species because it was a well-known competent JEV vector, but little was known about its distribution in Australia.
JEV genetic variation
JEV exists in five Genotypes: GI to GV. Traditionally, these have been defined using the JEV envelope (E) gene, but the entire genome sequence or that of any structural gene (C, prM, or E) aligns in forming five distinct genotypes.[31, 34]
The 2022 outbreak involved JEV GIV and Cx. annulorostris was recently found to be a highly competent JEV GIV vector that can travel >4 kilometres per day.[17, 21] At the time of writing, this species is considered the primary vector of JEV in Australia.[19, 35]
JEV GIV virus was also by the Queensland Public Health Virology laboratory team after screening 4,010 wild-caught mosquitoes collected in 34 traps, divided into 90 pools. Nine of 15 JEV real-time RT-PCR-positive pools were put up for virus culture, and one yielded an infectious JEV isolate (QLD_S46716_M2022).[33]
The exact mosquito species harbouring this virus was not identified, but the trap yielding the isolate was dominated by Aedes vitiger.[33]
JEV disease
Infection commonly results in mild or no symptoms. Approximately 1 per 250 cases are symptomatic.[15] Therefore, for every case definition that relies solely on symptomatic case identification, it can be presumed that many more cases have not been identified.
Symptomatic disease can include severe inflammatory central nervous system disease (encephalitis) with lifelong complications and sometimes death.[5] In other words, it is a rare but high-consequence infection.
Among pigs bitten by infected mosquitoes, JEV replication can lead to weak or stillborn piglets or central nervous system disease [1] and thus has a significant impact on the animals and the piggery.[4]
Absent the use of sentinel animals or mosquito trapping and specific testing, JEV spread is challenging to detect early because severe disease is rare in humans and because it is most evident among pigs when during their birthing seasons. Therefore, prevention in the form of a vaccine for the pigs—there is already a safe human vaccine [32]—would be preferable to reacting to the harm caused once the infection is established in a mosquito-bird-pig cycle.
Horses can also be infected and may show no signs or symptoms, develop a temperature, become weak or uncoordinated or show behavioural changes.
Detecting JEV infection requires expertise
RT-PCR to detect this enveloped single-stranded RNA virus in humans is challenging as the human viraemic period can be fleeting.
Antibody tests are therefore essential, specifically seeking IgM (recent infection) in the cerebrospinal fluid, finding a switch from IgM to IgG antibodies (seroconversion), or a suitable rise in IgG levels between two separate blood sample collections. However, antibodies to JEV cross-react with other arboviruses, so careful test design is essential, along with good travel and exposure history.[20] Furthermore, previous vaccinations will produce false positive results as they generate antibodies to the vaccine components, which include key genes or the total virus (based on JEV GIII [29]) in the attenuated or inactivated versions [28, 35]
The lab testing in the new study described below sought antibodies in the patient’s blood to indicate past infection.
The new study about part of the 2022 outbreak
In Australia, a JEV outbreak was declared a Communicable Disease Incident of National Significance (CDINS) on 4 March 2022, just after Queensland reported a human case and because of the appearance of death and abnormalities in piggeries.[1, 35]
JEV was to be detected at more than 70 pig farms in Queensland, New South Wales (NSW), Victoria and South Australia and in 45 humans, of whom seven died.[3] It can be expected that many more true infections went under the radar or were much less severe.[30]
This study looked at data up to February 2022, specifically in the Murray region of NSW, where eight cases were confirmed. Of 322 piggery workers and/or residents who attended a NSW-arranged Health Clinic, 311 (96.6%) received one or other JEV vaccine, 302 (93.8%) completed a questionnaire, and more than half were male (204/302; 67.5%).[3]
Testing and findings
Blood was collected from 178 (55.3%) clinic attendees, but only 165 returned results; the others were ineligible or lost. Of 153 born in Australia, 129 (84.3%) answered the questionnaire and were tested. Of those, 10 (6.5%) were positive using a complicated method to block the detection of antibodies from different but related flaviviruses. These defined epitope-blocking enzyme-linked immunosorbent assay (DEB ELISA) findings were among a population not vaccinated or travelled overseas, suggesting locally acquired infection. The authors noted that they couldn’t be sure the infection hadn’t occurred before the identified outbreak but suggested it was unlikely.
The questionnaire identified elevated – but not statistically significant – odds of testing positive among those who worked on a farm or who didn’t work on a farm but visited a farm (I think I’ve got that right😬) and who were male. The authors couldn’t definitively nail down where the seropositive humans acquired their cases. There was limited evidence to point to commercial piggeries. The authors pointed to a later serosurvey, which found that 80 (8.7%) of 917 volunteers (no mention of their past or present symptoms) from regional NSW towns at risk of JEV infection after the 2022 outbreak had JEV antibodies.[31]
The authors suggested that monitoring pig seroconversion and the presence and number of JEV-infected mosquitoes would be useful.
What might have fuelled the 2022 outbreak in Australia?
A variety of drivers for the widespread 2022 Australian outbreak have been proposed.[22]
Extra high summer rainfalls due to La Niña weather patterns created new wetlands in the south of Australia that may have received JEV-infected wading birds from the north. From there, new infections and maintenance cycles were created whenever opportunities arose.
The rains may also have helped the proliferation and spread of competent Culex spp. vectors.
Where these conditions overlapped with existing piggeries, the stage was set for spillover into amplifying pig hosts. An additional and more mobile amplifying host population is feral pigs, which can be found across Australia.[26]
Risk to humans from JEV infection during the 2022 outbreak
In areas near the 2022 outbreak, around one in eleven people had evidence of JEV infection, but studies couldn’t clarify the riskiest activities that lead to infection. However, we already know the importance of protecting ourselves from mosquito-borne virus infections and how to protect ourselves to reduce their risk.
Reducing the chances of getting bitten
Generally, people who work with pigs should control mosquito populations at and around their location and take precautions to prevent mosquito bites by wearing long-sleeved shirts, long pants, and covered shoes and by using an effective insect repellent.[7,11] It’s worth checking the active ingredients in your repellent against these.[12,13]
A poster from the Australian Department of Health & Aged Care outlining the importance of applying repellent to avoid mosquito-borne diseases.[10]
There are also multiple areas in several States where residence enables eligibility for JEV vaccination. The National Centre for Immunisation Research and Surveillance website has a specific section in its FAQs [32], or Australians can check their local Department of Health by Googling “JEV vaccination” plus their jurisdiction’s name.
How do things stand in Australia post-2022?
Not great.
On 23 June 2023, the CDINS was stood down as there had been no new cases since December 2022.[3]
Apart from the 45 human cases and deaths and administration of 125,000 doses of a JEV vaccine, JEV was also detected in samples collected from piggeries and feral pigs in the Northern Territory, South Australia, North Queensland and northern Western Australia.[3, 24]
Australia is currently working on two JEV vaccines for pigs [4]; however, infected mosquitoes and\or symptomatic human cases have continued to be detected in late 2024 in Victoria [6] and 2025 in New South Wales [8] and Queensland [9].
Surveillance of trapped mosquitoes is ongoing and yields results, as does detecting JEV in serious human illness. But because noticeable JEV disease is rare in humans and most notable during the birthing season in pigs, the virus seems to have kept hidden from existing surveillance efforts before 2022 and thoroughly established itself near Australia’s highest population densities.
Japanese encephalitis virus (JEV) in Australia is no longer one of those exotic things we associate with the far north; it now has a good foothold in southern Australia, and a high risk of enzootic transmission exists within reach of all major population centres. It is unlikely that JEV will be eliminated from Australia now that its cycles are so well established.[19]
Surveillance, mosquito control and bite-prevention measures, and consideration of JEV testing in acute central nervous system human disease will be necessary in the future.
All of this serves as yet another example of how quickly things can change when a new pathogen meets unsuspecting humans. And how costly the impact of that change can be in terms of health, healthcare, death, concern and public health responses. An ounce of prevention and all that….
This raises one particular question for me. Why didn’t we pick this up through sentinel animals and laboratory testing? If we had, could we have caught this southerly much sooner and interrupted the establishment of enzootic transmission before humans died?
References
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