Potential for emergence of Japanese encephalitis in the European Union

No autochthonous human cases of Japanese encephalitis (JE) have been reported to date in the European Union (EU). In this study, we assess the likelihood of Japanese encephalitis virus (JEV) introduction and transmission within the EU and propose outbreak response measures.

likelihood of JEV introduction and transmission within the EU and to provide options for response to the potential emergence of the disease.

| Travellers and imported equids
Humans and equids can be infected; however, as they develop a lowlevel and short viraemia (Mohsin et al., 2022), infected travellers and infected imported equids would not contribute to the subsequent transmission of the virus into the EU.

| Infected mosquitoes
Theoretically, infected adult mosquitoes could be inadvertently transported via aircraft and transmit the virus upon arrival in the EU.This phenomenon is rare and primarily described for airport or luggage malaria transmitted by imported Anopheles mosquitoes (Isaacson, 1989); similar evidence exists for airport dengue (Whelan et al., 2012).The mosquitoes responsible for the enzootic transmission of JEV among birds have a preference for biting birds (ornithophilic) rather than humans (anthropophilic), unlike malaria and dengue vectors.As a result, the likelihood of these mosquitoes stowing away in luggage or aircraft is significantly lower, but the possibility should not be discounted.

Impacts
• The probability of Japanese encephalitis virus introduction into the European Union is currently very low, with viremic bird migration being the most plausible pathway of introduction.
• Environmental conditions in the European Union are expected to be suitable for autochthonous Japanese encephalitis virus circulation, therefore there is a high likelihood of virus transmission in the region after virus introduction in environmentally adequate areas.
• The European Union should strengthen preparedness for early detection of human and animal cases to ensure the containment of a possible emergence of Japanese encephalitis.

F I G U R E 1
Regions and countries where Japanese encephalitis virus has been detected and bird migration pathways.Sources for the map: (Centers for Disease Control and Prevention, 2022;Preziuso et al., 2018;Ravanini et al., 2012;Simon-Loriere et al., 2017; The East Asian-Australasian Flyway Partnership, 2023).Introduction of infected eggs and adults through cargo ship transporting plants or used tires is also plausible, especially considering that vertical transmission of JEV from the adult mosquitoes to its larvae has been demonstrated for several Aedes species (Rosen et al., 1980).
The passive entry of JEV-infected mosquitoes via air currents from JE-endemic areas into the EU is improbable considering the lack of prevailing winds from Asia to Europe and the large distances mosquitoes would have to travel.

| Viremic migratory birds
Waterbirds belonging to the Ardeidae family (egrets and herons) serve as the natural reservoir of JEV in Asia (Mulvey et al., 2021).
These birds can become highly viremic without showing any clinical symptoms, allowing for the efficient dissemination of JEV over long distances.Currently, there are no documented waterbird flyways that extend across both JEV-endemic regions and the EU (The East Asian-Australasian Flyway Partnership, 2023).However, some flyways, such as the Central Asia flyway (covering the Indian subcontinent, Eastern Middle East and Siberia), the West Asian-East African flyway (covering Southern Africa, Middle East and Siberia) and the Black Sea-Mediterranean flyway (covering West and Central Africa, Southern Europe, Northern Middle East and Northern Russia), have partial overlaps (The East Asian-Australasian Flyway Partnership, 2023) (Figure 1).These pathways traverse the Middle East, Caucasus and Central Asia, where mosquito species like Cx. (Cux.)tritaeniorhynchus, the primary JEV vector in Asia and other competent JEV vectors like Cx. pipiens are present.The viremic period of JEV in waterbirds is estimated to be 3 to 7 days (Boyle et al., 1983); considering that herons can fly about 4000 km (~2485 miles) in 5 to 7 days (Winden et al., 2010), the viremic period should be sufficient for waterbirds to travel from JEV-endemic areas to resting and nesting places along these pathways.For instance, there are 2500 km (~1553 miles) between the north-western fringe of the JE-endemic area and the Caspian Sea.This suggests that the introduction of JEV via migratory birds and the subsequent establishment of a transmission cycle in these regions cannot be excluded.Our hypothesis is supported by an ecological niche modelling study that concluded that parts of the Arabian Peninsula and West Türkiye are suitable for JEV establishment (Samy et al., 2018).The establishment of a JEV transmission cycle in the Middle East, Caucasus and Central Asia regions would be facilitating the introduction of the virus into the EU, either directly or following an introduction and establishment in Africa.This process would likely take several migratory seasons.
Despite the unique identification of a person co-infected with yellow fever virus and JEV in Angola in 2016, there is to date no evidence of JEV circulation on the African continent (Simon-Loriere et al., 2017) (Figure 1).Similarly, there is no evidence of JEV circulation in the Middle East, Caucasus and Central Asia regions.
Eventually, for the waterbirds on the Black Sea/Mediterranean flyway to efficiently transport the virus to the EU, we must assume that they are susceptible to the virus, do not suffer from severe disease and that they also maintain high-level viraemia to infect mosquitoes upon their arrival.

| Imported pigs and pig semen
The demonstrated role of pigs as amplifiers makes the trade of infected pigs a plausible pathway for virus dissemination.Additionally, pig semen can be considered a potential introduction pathway as JEV is shed in the semen of infected boars, and the insemination of sows with infectious semen could lead to their infection (Guerin & Pozzi, 2005).However, currently, there is no import of live pigs or pig semen from Asia or the Western Pacific into the EU, mostly because of ongoing restrictions applied due to other diseases such as foot and mouth disease and African swine fever.

| Other introduction pathways
The environmental persistence of flaviviruses, including JEV, is limited.
The introduction of JEV into the EU through matrices such as animal feed and fomites is considered negligible.Furthermore, the probability of JEV introduction via legal importation of infected birds (e.g.pets, zoos, collections and participation in events) is very low, provided that proper adherence to veterinary rules and quarantine regulations is followed.Importation through other animal species is unlikely as well; although some zoo species such as water buffalo or raccoons could potentially be infected by JEV, they are considered dead-end hosts with limited ability to spread the virus (Ohno et al., 2009).

| LIKELIHOOD OF TR ANS MISS ION OF THE VIRUS IN THE EU
Japanese encephalitis virus is found in areas exhibiting considerable diversity in terms of climate and environment, ranging from Southern Australia, through Northern Japan to India (Centers for Disease Control and Prevention, 2022).Despite some differences in climate between JE endemic countries and the EU countries, we could assume that the climatic and environmental conditions in southern Europe are suitable for JEV establishment.These species include both ornithophilic species that could serve as enzootic vectors and generalist species that could act as bridge vectors.Four confirmed vector species are present in the EU: Aedes (Stegomyia) albopictus, Ae. (Aedimorphus) vexans, Cx. pipiens and tritaeniorhynchus, with the two Culex spp.considered the most competent (Schaffner et al., 2013;Van den Eynde et al., 2022).
While Ae. albopictus and Cx.pipiens are widely distributed in Europe, the other mentioned species have a more limited range.
Aedes albopictus is primarily attracted to mammals, suggesting it may not naturally contribute to bird-to-human transmission.Culex tritaeniorhynchus, the main vector of JEV in Asia, is found in Albania, Azerbaijan, Georgia, Greece and in Türkiye (Robert et al., 2019).

| Potential reservoir bird species in the EU
Waterbirds of the Ardeidae family are common in Europe (European Union for Bird Ringing (Euring), 2022) and some, for example, the black-crowned night heron (Nycticorax nycticorax) and the little egret (Egretta garzetta), can have high viraemia (EFSA Panel on Animal Health et al., 2017).It is expected that waterbirds, passerines and gulls have the ability to disperse and sustain the virus within the EU (Nemeth et al., 2012).
Serological cross-protection between JEV and WNV, which is part of the JEV serocomplex, has been demonstrated experimentally in red-winged blackbird (Agelaius phoeniceus) (Nemeth et al., 2009).
Theoretically, this could limit JEV circulation in the EU; however, the rapid spread of JEV in Australia, where closely related viruses like Kunjin virus and Murray Valley encephalitis virus (also a member of the JEV serocomplex) are present, suggests that cross-protection among birds may not be sufficient to prevent the emergence of JEV in a new area.There is currently no information available regarding similar cross-protection among equids and humans.

| Swine as potential amplifier host in the EU
We can assume that the European pig and wild boar populations would be efficient amplifier hosts of JEV in the EU.The pig industry is the largest meat industry in the EU with 142 million pigs registered in 2021 (Eurostat, 2021).Almost 60% of the pigs in the EU are kept in Spain, Germany, Denmark and France.It should be noted that, among the countries with the highest number of pigs, WNV circulation has been reported in Spain, Germany and France (European Centre for Disease Prevention and Control, 2023;Eurostat, 2021).
In 2018, most of the pigs (75%) were kept in large commercial holdings, backyard farming of pigs representing only 3% of the industry, of which the majority was in Romania (European Parliament, 2020).
Considering the high density of pigs on large commercial farms, JEV outbreaks in those farms would result in intense/numerous virus transmission among pigs, either via mosquitoes or through direct pig-to-pig transmission (via the oronasal route) (Ricklin et al., 2016).
Wild boar could also play an important role in the amplification and spread of the virus within the EU.A recent study in Ihikawa, Japan, demonstrated a high prevalence of JEV antibodies in wild boars and active circulation of the virus in winter (Komiya et al., 2019).The high density and the free movement of these animals in the EU could become a concern in the event of a JEV outbreak in the EU.
The substantial population of pigs and wild boar in the EU, coupled with their rapid birth rate and turnover, could contribute to the establishment and maintenance of JEV transmission.The primary JEV vector in Asia, Cx. tritaeniorhynchus, exhibits a strong feeding preference for pigs.This mosquito species is already established in the EU and its distribution is expanding (Patsoula et al., 2017).In particular, it was found to be abundant in rice fields (Lytra & Emmanouel, 2014).Consequently, this species could potentially drive an outbreak in the Mediterranean region, although other mosquito species might also be involved in that area as well as in other regions.The role of pigs and wild boar as amplifiers of the virus and the capacity of the virus to overwinter in these animals need to be assessed.

| Mosquito bite prevention and mosquito control
In the event of a JEV outbreak in the EU, the first and immediate response measure should focus on mitigating the exposure of humans and susceptible animals to potentially infected mosquitoes.
Individuals residing in or visiting affected areas should adhere to personal protective measures, including the use of mosquito repellent, wearing protective clothing and ensuring that mosquitoes are unable to enter houses by utilizing mosquito screens, preferably insecticide-treated ones.Equids in affected areas should also be protected against mosquito bites.
Avoiding mosquito bites of pigs is expected to be extremely challenging, even more so for birds.Dutta et al. (2011) reported a reduction of seroconversion both in pigs and humans after the use of insecticide-treated mosquito nets in India on pig stables and human beds in smallholder farms (Dutta et al., 2011).However, the use of mosquito screens may reduce airflow and potentially compromise animal welfare, thus requiring further research in the European context.The use of insect repellent on pigs is not expected to be costefficient, and it may render their meat unsuitable for consumption.
Ultra-low-volume adulticiding has been shown to be able to reduce WNV circulation in the United States, especially if applied intensively and early in the season (Lothrop et al., 2008).However, there is, more generally, a critical lack of evidence linking preventive interventions that reduce mosquito populations, such as larviciding, to effective reductions in viral circulation (European Centre for Disease Prevention and Control, 2020).

| Vaccination and treatment
Several JE vaccines humans are available commercially, but only one vaccine, the Ixiaro vaccine, has been granted marketing authorization by the European Medicines Agency for use in the EU (European Medicines Agency, 2021).In terms of prioritization during an outbreak situation, vaccination should be considered for individuals working or residing at pig farms, working in abattoirs, working directly with mosquitoes (e.g.vector surveillance and control), and those involved in laboratory handling of JEV-infected samples.
Vaccination of the general population, particularly high-risk groups, would depend on vaccine availability and the epidemiological situation.In outbreak localities, ring vaccination strategies could be implemented.
Vaccines for equids and pigs are available, although not approved for use in the EU.Scientific literature provides limited information on the vaccine's efficacy in reducing viraemia and clinical signs in pigs or horses in endemic areas.An exception to this is found in reports from South Korea, where a vaccination program utilizing a live-attenuated strain [Anyang300] has been successfully conducted for over 40 years in swine and horses (Kim et al., 2020;Nah et al., 2015).Experiments showed no development of viraemia in pigs when challenged with wild virus.
Nevertheless, antigenic differences between virus genotypes seem to diminish the efficacy of vaccines in use in pigs in enzootic areas (Nah et al., 2015).Therefore, approaches are ongoing for the development of new vaccines incorporating the currently circulating JEV strains.Should such efficient vaccines against JEV strains emerging in Europe gain approval for use in the EU, the prospect of vaccinating of high-value animals like racehorses could be considered.However, the cost-efficiency of pig vaccination and its potential impact on trade would need to be evaluated carefully.
Currently, there are no licensed therapeutics against JE for humans.However, clinical trials for promising drug candidates are ongoing.

| Safety of substances of human origin
Transmission of the virus via blood and solid organs (e.g.liver) donations has been seldom reported (Cheng et al., 2018;Qi et al., 2020).
In the case of JE emergence in the EU, the need for deferral and/ or testing of substances of human origin would have to be assessed.

| Control of animal movement
In Australia, where JEV is now considered to be established, the (European Commission, 2016, 2018, 2020) Animal Health authorities recommend pig tracing and surveillance in domestic and wild animals, and mosquitoes with the aim to determine the extent of JEV spread and to establish control areas around infected premises (Animal Health Australia, 2020; Australian Government, 2023).In these control areas, pig movement should be restricted; specifically, movement of pigs for restocking or breeding purposes should be permitted only under specified conditions, such as obtaining permits, or during seasons when mosquitoes are not active (Animal Health Australia, 2020;EFSA Panel on Animal Health et al., 2017).
In the EU, where the disease is absent, JE is classified as a 'Category E disease.' Specific control measures, including surveillance, notification, and reporting, are prescribed (European Commission, 2016, 2018, 2020).[Correction added on 10 February 2024, after first online publication: The preceding paragraph was corrected.]

| IMPAC T OF CLIMATE CHANG E
Climate change can contribute to the emergence and spread of new communicable diseases or the re-emergence of previously known ones.Changes in climatic conditions, such as unusually high levels of rainfall and warmth, can create favourable environments for mosquitoes to thrive.Additionally, climate change can influence the migration patterns of birds, including their routes and timing (Diehl, 2020;Liang et al., 2021).However, the relationship between climate change and disease dynamics is complex and not yet fully comprehended.Further research is needed to better understand the intricate interplay between climate change and disease transmission.

| CON CLUS ION
The current global distribution of JEV makes the likelihood of its introduction into the EU very low, with viremic bird migration being the most plausible pathway.However, the likelihood of introduction would significantly increase if the virus were to become established in regions such as the Middle East, Caucasus, Central Asia or the African continent.
Environmental conditions in the EU are expected to be suitable for virus circulation and therefore the likelihood of virus transmission in the EU after introduction in a receptive area, meaning in an area with adequate mosquito vectorial capacity to sustain transmission, is considered to be high.Viral spread within the EU would likely take place via bird and pig movements as well as the relocation of vectors transported by wind (windblown mosquitoes).
To mitigate or potentially contain the emergence of JE in the EU, early detection of human and animal cases is crucial.Given the low likelihood of introduction, establishing a dedicated JE surveillance in the EU is not justified.However, it remains important to ensure the early detection of JE emergence in the EU; this could be done through the adaptation of existing bird and mosquito surveillance activities for WNV but also through the timely etiological and differential diagnosis of viral encephalitis autochthonous cases.Diagnostic laboratories, in the EU, both in the human and animal health sector, shall develop and maintain capabilities for the rapid, specific and sensitive detection of JEV infections.
Japanese encephalitis risk areas Japanese encephalitis virus detection in birds in 1997-2000 and mosquitoes in 2010, within the EU One human case of Japanese encephalitis virus and yellow fever virus co-infection in 2016 Central Asian flyway West Asian-East African flyway Black Sea-Mediterranean flyway Birds flyways: Numerous mosquito species in Europe have shown competence as JEV vectors in laboratory experiments or have been found infected with the virus in nature (Van den Eynde et al., 2022).