Japanese Encephalitis Virus in Pigs

Overview and Taxonomy of Japanese Encephalitis Virus in Pigs

Japanese encephalitis virus (JEV) represents a globally significant, mosquito-borne zoonotic orthoflavivirus that constitutes the foremost etiological agent of viral encephalitis across Asia and the Western Pacific [11, 38, 42]. The World Health Organization (WHO) estimates approximately 68,000 clinical cases of Japanese encephalitis (JE) occur annually, with a case fatality rate reaching 20–30% and permanent neuropsychiatric sequelae afflicting up to 50% of survivors. The Centers for Disease Control and Prevention (CDC) and the World Organisation for Animal Health (WOAH) recognize JEV as a pathogen of profound public health and veterinary importance, primarily due to its complex transmission ecology involving mosquitoes, avian reservoirs, and notably, swine as critical amplifying hosts [10, 42]. In pigs, JEV infection is predominantly subclinical in grower and adult animals; however, it manifests as a devastating reproductive disease characterized by abortion, mummification, stillbirth, and congenital tremors in neonates, causing substantial economic losses to the swine industry globally [5, 13, 25]. The Food and Agriculture Organization (FAO) has highlighted the emergence of JEV in previously unaffected regions, such as the 2022 incursion into temperate southeastern Australia, as a sentinel event underscoring the virus's expanding geographic footprint and the urgency of understanding its biology in the porcine host [13, 31, 35].

Taxonomic Classification and Genotypic Diversity

JEV is classified within the family Flaviviridae, genus Orthoflavivirus, and is the prototypical member of the JEV serocomplex, which includes other medically significant viruses such as West Nile virus, Murray Valley encephalitis virus, and Usutu virus [7, 14]. The virus possesses a single-stranded, positive-sense RNA genome of approximately 11 kilobases, encoding a single polyprotein that is co- and post-translationally cleaved into three structural proteins, capsid (C), precursor of membrane (prM), and envelope (E), and seven non-structural (NS) proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B, NS5) [34, 40]. The E protein is the primary target of neutralizing antibodies and serves as the principal determinant of viral attachment, membrane fusion, and host cell tropism, making it a critical focus for phylogenetic classification [11, 41].

Based on nucleotide sequencing of the E gene, JEV is classified into five distinct genotypes (GI through GV), which exhibit a complex spatiotemporal distribution pattern. Historically, genotype III (GIII) was the dominant lineage circulating throughout Asia, responsible for most major epidemics during the 20th century [35, 41]. However, beginning in the 1990s, a pronounced genotype shift occurred, with genotype I (GI) progressively displacing GIII as the predominant circulating strain across vast swathes of China, Japan, Korea, and Southeast Asia [9, 11, 41]. This displacement carries significant epidemiological implications, as GI viruses may possess enhanced replicative fitness in avian hosts and potentially altered antigenic profiles relative to GIII-based vaccine strains such as SA14-14-2 [9, 41]. Indeed, phylogenetic analyses of JEV strains isolated from pigs and mosquitoes in Hunan Province, China, between 2019 and 2021 revealed that all 14 characterized isolates belonged to GI-b, demonstrating a distinct genetic distance from the vaccine strain SA14-14-2 [9]. Similarly, in Assam, India, the first isolation of GI from a naturally infected pig nasal swab was reported in 2025, confirming the ongoing incursion of this genotype into regions previously dominated by GIII [23, 36]. The re-emergence of genotype IV (GIV) and genotype V (GV) further complicates the epidemiological landscape; GIV was responsible for the unprecedented 2022 Australian outbreak affecting nearly 80 piggeries and causing human fatalities [5, 13, 17]. GV, meanwhile, has been detected in South Korea and parts of Southeast Asia, raising concerns regarding vaccine cross-protection, as existing GIII-derived vaccines may offer suboptimal neutralization against these divergent genotypes [11, 18, 35].

Biological and Structural Characteristics

The viral particle is approximately 50 nanometers in diameter, enveloped, and icosahedral, with the E protein arranged as homodimers on the virion surface [17, 39]. A distinctive feature of JEV biology, particularly relevant to its pathogenesis in pigs, is the production of NS1', a larger isoform of NS1 generated through a ribosomal frameshift event at the NS2A-NS2B junction [40]. NS1' is absent in the live attenuated SA14-14-2 vaccine strain but is expressed by wild-type JEV strains and has been shown to enhance viral replication in both murine and porcine models [40]. Critically, NS1' promotes JEV infection of dendritic cells and macrophages within porcine tonsils, the primary site of viral persistence, and facilitates oronasal shedding, thereby enabling horizontal transmission [40]. This finding directly links viral genotype-associated molecular features to enhanced transmission potential in the porcine host.

The genome also encodes a methyltransferase within NS5, the RNA-dependent RNA polymerase (RdRp), which is essential for viral RNA capping and replication [24, 26]. Recent bioinformatic analyses employing Bayesian evolutionary inference have dated the most recent common ancestor of contemporary JEV strains to approximately 1723, with a substitution rate of 1.54 × 10⁻⁴ substitutions per site per year [34]. This rate of evolution, coupled with the virus's broad host range and high mutation frequency, facilitates the emergence of quasispecies adapted to specific hosts and ecological niches [2, 34].

The Role of Pigs in the JEV Transmission Cycle

Pigs occupy a singular and indispensable role in JEV ecology, serving as the principal amplifying host that bridges the sylvatic cycle between ardeid waterbirds and the domestic environment leading to human infection [6, 25, 38]. The WOAH and numerous national veterinary authorities designate pigs as sentinel species for JEV surveillance, as seroconversion in swine frequently precedes human outbreaks [10, 22, 25]. Upon infection via the bite of an infected Culex mosquito, primarily Culex tritaeniorhynchus, Culex annulirostris, and Culex gelidus, pigs develop a high-titer viremia sufficient to infect feeding mosquitoes, thereby perpetuating the enzootic cycle [21, 32, 38]. The viremia is typically brief, peaking between days 2 and 4 post-infection, but viral replication in lymphoid tissues, particularly the tonsils, is remarkably prolonged [27, 40]. Experimental infections have demonstrated that JEV RNA and infectious virus persist in porcine tonsils for at least 25–28 days post-infection, and in some studies, viral RNA has been detected up to 11 days post-infection without evidence of clearance, despite the presence of high titers of neutralizing antibodies [1, 19, 27]. This tonsillar persistence establishes pigs as long-term viral reservoirs capable of transmitting JEV to both vectors and susceptible contact pigs well beyond the acute viremic phase [6, 25, 40].

Vector-Free Transmission and Evolutionary Adaptation

A paradigm-shifting discovery in JEV transmission biology was the demonstration of vector-free, direct contact transmission between pigs [1]. Ricklin et al. (2016) definitively showed that JEV could be transmitted from infected to naïve pigs through oronasal secretions in the complete absence of arthropod vectors [1]. Infected pigs shed infectious virus in nasal and oral secretions, and naïve pen-mates became infected upon direct contact or through environmental contamination [1, 19]. This finding fundamentally alters the understanding of JEV epidemiology, particularly in temperate regions where mosquito activity is seasonally limited. Mathematical modeling of transmission dynamics in Cambodian pig populations estimated that direct pig-to-pig transmission contributes between 7.5% and 11.9% of the basic reproduction number (R₀), a non-negligible fraction that could facilitate viral persistence during periods of low mosquito abundance [30]. This direct transmission mechanism may explain the overwintering of JEV in temperate zones and represents a critical consideration for risk assessment in JEV-free regions, such as the continental United States and Europe, where competent vectors and susceptible pig populations exist [19, 20, 33].

Subsequent serial passaging experiments in pigs, designed to simulate the interruption of the mosquito-vertebrate alternating cycle, revealed that JEV undergoes rapid evolutionary adaptation in the porcine host [2]. After ten consecutive pig-to-pig passages, the virus selected for 25 single nucleotide polymorphisms, six of which resulted in amino acid substitutions in the prM, E, NS3, and NS5 proteins. Notably, the mutations M374L in the E protein and N275D in NS5 conferred a significant fitness advantage in pigs, leading to enhanced viral replication, increased oronasal shedding, and heightened innate immune responses [2]. However, despite these fitness gains, the frequency of direct transmission did not increase, suggesting that the mechanism of direct spread is constrained by additional host or environmental factors rather than simply viral replicative capacity [2]. These findings underscore the remarkable plasticity of JEV and its capacity to rapidly evolve in response to shifts in transmission ecology, a consideration of paramount importance for predicting the consequences of JEV introduction into naïve swine populations.

Genotype Distribution in Global Pig Populations

Extensive surveillance studies across Asia have delineated the genotype distribution of JEV in pigs and their associated mosquito vectors. In Vietnam, co-circulation of GI and GIII was documented in mosquitoes within urban Can Tho City, with 100% seroprevalence in peri-urban pigs [3]. In Cambodia, intensive JEV circulation was demonstrated in sentinel pigs in both rural and peri-urban settings near Phnom Penh, with all pigs seroconverting before six months of age; the force of infection was approximately 0.06–0.07 per day, and circulating strains belonged exclusively to GI (clades Ia and Ib) [15, 28, 29]. In India, a country with a high burden of human JE, GIII has historically been the predominant genotype recovered from pigs; however, the first isolation of GI from a pig nasal swab in Assam in 2025 signals a significant genotype shift [23, 36]. In Tamil Nadu, JEV seroprevalence in pigs exceeded 60% at the animal level and 94% at the herd level, with higher prevalence during the monsoon season, demonstrating the endemic stability of JEV in southern Indian pig populations [16]. In Japan, serological surveys on remote islands such as Ishigaki revealed low but detectable JEV activity, with genotype III strains closely related to Taiwanese isolates, while on the main island of Kochi, both GI and GIII were found co-circulating in swine as recently as 2018 [4, 37]. Most alarmingly, the emergence of GIV in Australia in 2022 represented the first detection of this genotype outside of its presumed endemic range in Indonesia, and it caused severe reproductive disease in naïve pig populations across four Australian states, accompanied by human fatalities [5, 13, 21].

Implications for Surveillance and Control

Given the central role of pigs as amplifying hosts and the potential for vector-free transmission, surveillance strategies must integrate molecular diagnostics targeting multiple genotypes, particularly in regions experiencing incursions. The WHO, CDC, and WOAH emphasize the importance of monitoring swine populations as an early warning system for impending human outbreaks. The detection of JEV in piggery effluent and environmental water samples in Australia offers a novel, non-invasive surveillance approach that could complement traditional mosquito trapping and sentinel pig serology [8]. Furthermore, the documented genotype shifts, particularly the emergence of GI and re-emergence of GIV and GV, necessitate ongoing genomic surveillance to ensure that existing GIII-based vaccines remain effective and to inform the development of genotype-matched or cross-protective vaccine candidates for both swine and human populations [10, 14, 17, 35]. The expanding host range, including recent evidence that sheep can also serve as amplifying hosts, further complicates the epidemiological picture and underscores the complexity of JEV dynamics in agricultural ecosystems [12].

Molecular Pathogenesis and Viral Fitness Adaptations in Swine

The porcine host occupies a singular position in the ecology of Japanese encephalitis virus (JEV), functioning not merely as a passive reservoir but as a dynamic selective environment that drives viral evolution, modulates transmission potential, and dictates the epidemiology of human disease. Unlike the dead-end outcomes in humans and horses, infection in swine is characterized by high-titer viremia, broad tissue tropism, and a remarkable capacity for both vector-borne and vector-free dissemination [1, 6]. Understanding the molecular underpinnings of this interaction, from the initial establishment of infection in the oropharyngeal mucosa to the quasispecies adaptations that arise under serial transmission pressure, is essential for comprehending JEV’s expanding geographic footprint and for informing rationally designed control measures.

Cellular Tropism and the Gateway of Infection

Upon introduction into a susceptible pig, JEV demonstrates a pronounced predilection for secondary lymphoid tissues, with the tonsils serving as the primary site of viral amplification and persistence. Experimental inoculations, whether via needle, oronasal instillation, or natural contact, consistently reveal the tonsils as harboring the highest viral RNA loads, often exceeding those found in the central nervous system (CNS) or visceral organs by several orders of magnitude [1, 27]. This tropism is not incidental; the tonsillar epithelium and underlying lymphoid follicles provide a rich milieu of dendritic cells (DCs) and macrophages, which have been identified as the principal target cells for JEV within this tissue [40]. The virus exploits the NS1′ protein, a longer isoform of non-structural protein 1 produced via a programmed –1 ribosomal frameshift in the NS2A gene, to enhance infection of these professional antigen-presenting cells. NS1′ expression significantly increases viral burden in tonsillar DCs and macrophages, suppresses major histocompatibility complex class II (MHC II) expression, and activates inducible nitric oxide synthase (iNOS), creating an immunologically permissive niche that facilitates persistent infection [40]. This persistence is striking: infectious virus and viral RNA can be recovered from tonsils for at least 25 to 28 days post-infection, well beyond the resolution of viremia and the appearance of high-titer neutralizing antibodies [1, 19, 27].

The oronasal route of infection, now recognized as a viable transmission pathway in the absence of arthropod vectors, dictates that the tonsil is not only a target but also a portal of entry [1]. Pigs shed infectious virus in oronasal secretions, and naïve pennates exposed to these secretions become productively infected, developing clinical signs and viremia indistinguishable from those induced by needle inoculation [1, 2]. This finding, first rigorously demonstrated by Ricklin et al., challenged the long-held dogma that JEV transmission is exclusively mosquito-mediated and has profound implications for viral maintenance in temperate regions where mosquito activity is seasonally constrained [1]. The molecular basis for this shedding likely involves JEV replication within the stratified squamous epithelium of the tongue base and oropharynx, though the precise cellular egress mechanisms remain to be fully elucidated. Notably, the detection of JEV RNA in nasal swabs from naturally infected pigs under field conditions confirms that this shedding is not an artifact of high-dose experimental challenge but a genuine feature of natural infection [23].

Following local amplification in the tonsils, JEV disseminates systemically via a hematogenous route. Viremia peaks between three and four days post-infection, with viral titers often exceeding 10⁵ TCID₅₀ per milliliter of blood, a threshold sufficient to infect feeding Culex mosquitoes [5, 19]. The virus exhibits a broad extra-neural tropism, with detectable RNA in the spleen, lymph nodes, liver, kidneys, lungs, and reproductive tract [27, 47]. In the female reproductive tract, JEV displays a notable tropism for the vaginal epithelium and endometrium, establishing persistent infection that lasts at least 28 days in the vaginal mucosa and results in shedding of infectious virus in vaginal secretions [47]. This finding raises the unaddressed possibility of sexual or vertical transmission in swine, analogous to the patterns observed for Zika virus, and suggests that the reproductive tract may serve as an additional reservoir for viral maintenance in pig populations.

Neuroinvasion and the Central Nervous System Immune Equilibrium

Despite the high frequency of neuroinvasion in JEV-infected pigs, histopathological evidence of meningitis, encephalitis, and gliosis is present in nearly all experimentally infected animals, clinical neurological signs are conspicuously absent [5, 7]. This remarkable dichotomy between infection and disease provides a powerful model for understanding the host determinants of neuropathogenesis. JEV gains access to the CNS through multiple routes, likely including direct hematogenous crossing of the blood-brain barrier (BBB) and retrograde axonal transport along the olfactory and trigeminal nerves [7]. Once inside the brain parenchyma, the virus replicates primarily in neurons, yet the ensuing host response is characterized by efficient viral control without the catastrophic immunopathology seen in mice and humans.

The molecular basis for this controlled response lies in a unique pattern of innate immune activation. In most brain regions, JEV replication is rapidly suppressed through a type I interferon (IFN)-independent induction of 2',5'-oligoadenylate synthetase 1 (OAS1) expression [7]. This pathway, which activates RNase L to degrade viral RNA, appears to be a particularly effective antiviral mechanism in the porcine CNS. In the olfactory bulb and thalamus, where OAS1 induction is insufficient to fully contain viral replication, a transient but robust chemokine response ensues, marked by upregulation of CCL2, CXCL10, and CXCL11. This chemokine signature coincides with an influx of IFNγ-producing T cells, which collaborate with the intrinsic neuronal response to achieve rapid viral clearance [7]. Crucially, this T-cell-mediated response does not escalate into a destructive inflammatory cascade. Unlike the murine model, where JEV infection triggers NLRP3 inflammasome activation and pyroptosis leading to widespread neuronal death, the porcine CNS shows no evidence of NLRP3 upregulation [7]. The absence of this key inflammatory node likely explains the lack of severe neurological disease in pigs and suggests that the porcine immune system has evolved to balance antiviral efficacy with tissue tolerance.

Viral Fitness Adaptations Under Selective Pressure

The interruption of the alternating mosquito-vertebrate host cycle, as would occur during a series of direct pig-to-pig transmissions, imposes a profound selective bottleneck on the JEV quasispecies population. Serial passaging experiments by Marti et al., in which JEV was transmitted through ten sequential pig passages in the absence of mosquito vectors, revealed a dramatic restructuring of the viral population [2]. Next-generation sequencing demonstrated that 25 single nucleotide variants (SNVs) reached a frequency of at least 35% during the passage series, with six of these mutations resulting in non-synonymous amino acid substitutions. These substitutions were non-randomly distributed, occurring in the precursor membrane (prM), envelope (E), non-structural 3 (NS3), and non-structural 5 (NS5) proteins [2].

Competition experiments between the passaged virus and the ancestral strain identified two mutations conferring a significant fitness advantage in pigs: M374L in the E protein and N275D in NS5 [2]. The E protein substitution lies within domain III, the region responsible for receptor binding, and may enhance viral entry into porcine cells or alter pH-dependent fusion kinetics. The NS5 mutation, located in the RNA-dependent RNA polymerase (RdRp) domain, likely increases replication fidelity or efficiency, enabling faster viral RNA synthesis in the porcine cellular environment. Notably, despite these fitness gains, manifested as higher peak viremia, increased oronasal shedding, and more robust innate immune responses, the frequency of direct contact transmission was not enhanced [2]. This suggests that transmission is constrained by additional factors beyond viral load, such as shedding kinetics, mucosal stability, or host behavioral factors.

Parallel evidence from field surveillance supports the notion that pig-to-pig circulation drives genomic diversification. Analysis of E gene sequences from Chinese pig isolates revealed multiple mutations at residues 76, 95, 123, 138, 244, 474, and 475, some of which alter predicted phosphorylation sites and, potentially, protein stability or interactions with host kinases [44]. In Taiwan, the emergence of a novel dominant GI subcluster bearing the E-V441I substitution in the stem region of the envelope protein was observed following a period of reduced viral population size and genetic heterogeneity, consistent with a selective sweep driven by adaptation to the porcine host [18].

The evolution of JEV within pigs also has significant implications for vaccine efficacy. The live attenuated vaccine strain SA14-14-2, which is derived from genotype III and lacks NS1′ expression due to a specific amino acid substitution (A66G) in NS2A, may be less effective against genotype I or IV strains that have adapted to replication in porcine tissues [9, 40]. The NS1′ protein itself represents a virulence determinant in pigs; its expression enhances viral infection of tonsillar DCs and macrophages, promotes CNS invasion, and facilitates oronasal shedding [40]. Genotype I strains, which are now dominant across much of Asia, generally produce higher levels of NS1′ compared to genotype III strains, a difference that may contribute to their selective advantage in porcine populations [41].

Host–Virus Molecular Arms Race

The outcome of JEV infection in swine is determined by a complex interplay between viral evasion strategies and host restriction factors. On the viral side, the NS4B protein inhibits interferon-beta (IFN-β) production by targeting the Toll-like receptor 3 (TLR3) pathway. NS4B physically interacts with both TLR3 and its adaptor protein TRIF, blocking the phosphorylation of interferon regulatory factor 3 (IRF3) and thereby suppressing the transcriptional induction of type I IFN [45]. This mechanism is particularly relevant in the tonsil and lymphoid tissues, where TLR3 is highly expressed and where early IFN responses could otherwise restrict viral amplification.

Conversely, the porcine host deploys an array of restriction factors. The solute carrier family 25 member 12 (SLC25A12) protein interacts directly with JEV NS1 and inhibits viral RNA synthesis. Mechanistically, SLC25A12 upregulates IRF7 mRNA levels, leading to enhanced IFN-β expression and the establishment of an antiviral state [24]. This interaction represents a newly identified host defense axis that may be polymorphic in pig populations, contributing to variable susceptibility. Similarly, nucleophosmin (NPM1) acts as a proviral factor, binding to the JEV capsid (C) protein and facilitating viral replication. JEV infection blocks the normal translocation of NPM1 from the nucleolus to the nucleoplasm in response to cellular stress, suggesting that the virus actively subverts this host protein to maintain a favorable replication environment [46].

The role of mosquito saliva in modulating porcine infection adds another layer of complexity to the pathogenesis picture. When JEV is delivered alongside Culex quinquefasciatus salivary gland extract (SGE), pigs exhibit milder febrile responses and shortened duration of nasal shedding compared to needle-inoculated animals, although viremia and neuroinvasion are unaffected [43]. This suggests that mosquito salivary components, which are rich in vasodilators and immunomodulatory molecules, alter the host's systemic inflammatory response without compromising viral dissemination. The clinical implication is that vector-borne transmission may produce a subtly different disease phenotype than direct contact transmission, with potential consequences for diagnosis and surveillance.

From a global health security perspective, the adaptation of JEV to pigs in the absence of vectors is a scenario of acute concern. The World Health Organization (WHO) and the Food and Agriculture Organization (FAO) have identified JEV as a priority pathogen with expanding geographic range, and the recent incursion of genotype IV into temperate Australia, where it caused widespread reproductive disease in pigs and human fatalities, underscores the threat [5, 13]. Modeling studies incorporating direct pig-to-pig transmission suggest that this route could contribute up to 12% of the basic reproduction number (R₀) in endemic settings, a non-negligible fraction that could facilitate viral persistence during seasonal mosquito troughs [30]. The molecular adaptations that arise under these conditions, while not enhancing direct transmission frequency per se, do increase within-host fitness and shedding, thereby amplifying the pool of infectious virus available for both mosquito-borne and contact-mediated spread [2].

Clinical Manifestations and Pathological Findings in Pigs

Japanese encephalitis virus (JEV) infection in pigs presents a complex and often dichotomous clinical picture that is fundamentally distinct from the severe neuroinvasive disease observed in humans and horses. As the primary amplifying host in the JEV transmission cycle, pigs typically exhibit a subclinical or mild clinical course following infection, yet they can suffer from profound reproductive pathology and harbor persistent viral reservoirs that are critical to the virus's ecology [6, 27]. The clinical manifestations and pathological findings are heavily influenced by the age and physiological status of the animal, the route of exposure, the viral genotype, and the host's immune status. A comprehensive understanding of these parameters is essential for veterinary diagnostics, surveillance programs, and the implementation of effective control strategies, as recognized by the World Organisation for Animal Health (WOAH) and the World Health Organization (WHO) in their guidelines for JEV management.

Clinical Signs in Post-Natal and Growing Pigs

In stark contrast to the encephalitic syndrome seen in humans, JEV infection in post-weaning and growing pigs is overwhelmingly asymptomatic or associated with only transient, non-specific clinical signs. The most consistently documented clinical abnormality is a transient pyrexia, typically manifesting between days 4 and 6 post-infection, which often resolves spontaneously without intervention [5, 43]. This febrile episode is frequently the only discernible clinical sign, and it may go entirely unnoticed in a commercial herd setting. Experimental infections using the Australian genotype 4 (GIV) outbreak strain in 5- and 11-week-old piglets confirmed that, aside from this short-lived fever, infected animals displayed no other overt clinical signs such as lethargy, anorexia, or neurological deficits [5]. Similarly, studies employing genotype 3 (GIII) strains via intradermal or intranasal inoculation reported a complete absence of neurological signs despite confirmed viremia and viral dissemination to the central nervous system (CNS) [7]. This remarkable resilience to neurological disease is a defining feature of JEV infection in pigs and is a key factor in their role as efficient amplifying hosts, as they remain active and mobile while sustaining high levels of viremia.

The mechanisms underlying this subclinical outcome are now being elucidated at the molecular level. Research has demonstrated that the porcine immune response within the CNS is characterized by an efficient antiviral state and a tightly regulated, non-inflammatory reaction. Specifically, JEV replication in the pig brain is controlled by a type I interferon-independent activation of the OAS1 (2'-5'-oligoadenylate synthetase 1) pathway, which effectively suppresses viral replication without triggering a widespread, damaging inflammatory cascade [7]. Crucially, the NLRP3 inflammasome, a key mediator of the severe neuroinflammation and neuronal death observed in murine and human JEV encephalitis, is not significantly upregulated in the porcine CNS [7]. This absence of a pronounced inflammatory response, coupled with a rapid and effective antiviral state, allows pigs to clear the virus from most brain regions without developing clinical neurological disease. A transient chemokine response and increased interferon-gamma (IFNγ) expression, likely from infiltrating T cells, are observed in specific brain regions like the olfactory bulb and thalamus, but this is rapidly resolved and does not lead to the destructive pathology seen in other species [7].

Reproductive Manifestations: The Hallmark of JEV in Breeding Herds

The most economically significant and clinically apparent manifestation of JEV infection in pigs is reproductive failure in breeding sows and gilts. This is the primary reason for veterinary investigation and is a key indicator of JEV circulation in a region. The clinical presentation is highly dependent on the stage of gestation at which the sow is infected. Infection during the first trimester often results in embryonic death and resorption, which may be observed as a return to estrus or a reduced litter size at farrowing. The most dramatic and characteristic clinical picture, however, emerges from infection during the second and third trimesters. This leads to a spectrum of fetal outcomes, including abortion, the delivery of mummified fetuses of varying sizes, stillbirths, and the birth of weak, non-viable piglets [13, 25, 42]. A pathognomonic sign often reported in field outbreaks is a prolonged gestation length, with sows farrowing several days past their expected due date [13]. The affected litters frequently contain a mix of normal, healthy piglets alongside mummified and stillborn fetuses, a presentation that should immediately raise suspicion for JEV.

In addition to fetal death, JEV infection in utero can cause severe congenital abnormalities in piglets that survive to term. The most notable of these is congenital tremors, also known as "myoclonia congenita," characterized by rhythmic, involuntary shaking of the head and limbs, which can be severe enough to prevent the piglet from suckling effectively [13]. Histopathological examination of the brains of these affected piglets often reveals severe lesions, including cerebellar hypoplasia and spongiform degeneration of the brainstem and spinal cord, reflecting the virus's potent neurotropism in the developing fetus [13]. The virus also has a direct impact on the reproductive tract of adult animals. In boars, JEV infection can cause orchitis (inflammation of the testes), epididymitis, and a significant decline in semen quality, characterized by reduced sperm motility, increased numbers of abnormal spermatozoa, and the presence of virus in the semen [42, 48]. This can lead to temporary or permanent infertility, further compounding the economic losses to the pig industry.

Pathological Findings: Gross and Histological Lesions

The pathological findings in pigs infected with JEV are most pronounced in the reproductive tract of pregnant sows and the central nervous system of fetuses and, to a lesser extent, growing pigs. At necropsy of aborted or stillborn fetuses, gross lesions are often non-specific but may include subcutaneous edema, hydrothorax, and ascites. The brain may appear grossly normal, but upon sectioning, areas of malacia (softening) and cavitation can be observed, particularly in the cerebellum and brainstem, correlating with the congenital tremors observed clinically [13].

Histologically, the most consistent and significant lesions are found in the CNS. In fetuses and neonatal piglets, the pathology is severe and destructive. It is characterized by a non-suppurative encephalomyelitis, with widespread neuronal necrosis, neuronophagia (the ingestion of dead neurons by glial cells), and the formation of glial nodules [5, 27]. A hallmark lesion is cerebellar hypoplasia, where the folia are poorly developed and there is a marked loss of Purkinje cells and granule cells. Spongiform change, or status spongiosus, is frequently observed in the brainstem and spinal cord, giving the tissue a vacuolated, "moth-eaten" appearance [13]. In growing pigs that are experimentally infected and euthanized during the acute phase, histopathological changes consistent with JEV infection are evident even in the absence of clinical neurological signs. These lesions include a mild to moderate lymphocytic meningitis, perivascular cuffing (accumulation of lymphocytes around blood vessels), and diffuse gliosis throughout the brain parenchyma [5]. These findings confirm that neuroinvasion and neuropathology are consistent features of JEV infection in pigs, even when the clinical outcome is subclinical.

Viral Tropism and the Role of Persistent Infection

A critical aspect of JEV pathogenesis in pigs, with profound implications for viral maintenance and transmission, is the virus's unique tropism for lymphoid tissues, particularly the tonsils. Following experimental infection, the tonsils consistently harbor the highest viral loads of any tissue, and infectious virus can be detected there for extended periods, far beyond the resolution of viremia and the appearance of neutralizing antibodies [1, 27]. Studies have demonstrated JEV persistence in the tonsils for at least 25 to 28 days post-infection, and in some cases, viral RNA has been detected for months [1, 19, 27]. This persistent infection is not a latent state but a low-level, active replication that occurs despite a robust systemic humoral immune response. The primary target cells for this persistent infection within the tonsils have been identified as dendritic cells (DCs) and macrophages [40]. The JEV non-structural protein NS1' has been shown to enhance viral infection of these cells, and the infection itself leads to a reduction in major histocompatibility complex class II (MHC II) expression and activation of inducible nitric oxide synthase (iNOS), creating an immunologically favorable environment for viral persistence [40].

Beyond the tonsils, JEV exhibits a broad tissue tropism, disseminating rapidly via the bloodstream to secondary lymphoid organs (spleen, lymph nodes), visceral organs (lungs, liver, kidneys), and the reproductive tract [27, 47]. The virus's tropism for the reproductive tissues is particularly significant. JEV has been shown to infect and persist in the vaginal mucosa, endometrium, and placenta of sows, and viral shedding in vaginal secretions has been documented for at least 28 days post-infection [47]. This persistence in the reproductive tract provides a direct mechanism for transplacental infection of fetuses, leading to the reproductive pathology described above. Furthermore, the presence of infectious virus in the nasal and oronasal secretions of infected pigs has been unequivocally demonstrated, even in the absence of clinical signs [1, 19, 23]. This finding, coupled with the demonstration of direct contact transmission between pigs [1], has fundamentally altered the understanding of JEV epidemiology. It suggests that vector-free transmission can occur, potentially allowing the virus to overwinter in temperate regions or persist in pig populations during periods of low mosquito activity [30]. The detection of JEV RNA in piggery effluent further corroborates the significant viral shedding that occurs from infected herds, offering a novel surveillance tool for early outbreak detection [8].

Epidemiology and Transmission Dynamics of Japanese Encephalitis Virus in Swine

The Enzootic Transmission Cycle: Swine as Amplifying Hosts

Japanese encephalitis virus (JEV) represents one of the most significant arthropod-borne viral pathogens affecting swine populations across Asia and the expanding geographical range of the virus. The epidemiology of JEV in swine is fundamentally defined by the virus's complex ecological requirements, which necessitate cycling between mosquito vectors and vertebrate amplifying hosts. Pigs (Sus scrofa domestica) occupy a singularly critical position in this transmission ecology, serving as the primary amplifying host that bridges the sylvatic cycle maintained in ardeid waterbirds and the epizootic spillover events that threaten human populations [6, 38]. The World Health Organization (WHO) has long recognized that porcine amplification is the principal driver of human Japanese encephalitis risk in endemic regions, and the Food and Agriculture Organization (FAO) has emphasized that understanding swine epidemiology is essential for implementing effective One Health interventions.

The classical JEV transmission paradigm, established through decades of field investigations, describes an enzootic cycle wherein Culex mosquitoes, predominantly Culex tritaeniorhynchus, acquire JEV through blood meals on viremic pigs and subsequently transmit the virus to susceptible swine and other vertebrate hosts [11, 42]. This vector-dependent transmission mechanism has been the foundation of JEV ecology since the virus's characterization in the 1930s. However, the epidemiological picture has undergone substantial revision in recent years, driven by landmark experimental studies demonstrating that JEV transmission dynamics in swine are far more nuanced than previously appreciated.

Vector-Free Transmission: A Paradigm Shift in JEV Epidemiology

The most transformative advance in understanding JEV transmission dynamics in swine emerged from the seminal work of Ricklin and colleagues in 2016, which provided unequivocal experimental evidence that JEV can be transmitted between pigs in the complete absence of arthropod vectors [1]. This discovery fundamentally challenged the long-held dogma that JEV was an obligate mosquito-borne virus. In these experiments, pigs infected with JEV through needle inoculation were found to shed infectious virus in oronasal secretions, and naïve contact pigs housed in direct proximity became infected without any mosquito involvement. The clinical manifestations, virus tissue tropism, and histopathological lesions in the central nervous system were indistinguishable between pigs infected through needle inoculation, direct contact, or experimental oronasal challenge, confirming that the oronasal route represents a biologically relevant transmission pathway [1].

The implications of vector-free transmission for JEV epidemiology cannot be overstated. In temperate regions where mosquito seasons are short or where winter conditions suppress vector populations, direct pig-to-pig transmission provides a mechanism for viral persistence that was previously unexplained [1, 6]. This finding has particular relevance for understanding how JEV might establish and maintain itself in newly invaded geographic regions, such as the recent emergence in temperate southeastern Australia, where mosquito activity is seasonally constrained [13, 31]. The demonstration of oronasal shedding and direct transmission suggests that JEV may overwinter in swine populations through sustained horizontal transmission even when competent vectors are absent or at minimal activity levels.

Fitness Adaptations Under Serial Vector-Free Passage

The interruption of the alternating vertebrate-vector host cycle imposes substantial selective pressures on JEV, as demonstrated by Marti and colleagues through serial passaging experiments in pigs [2]. When JEV was subjected to ten consecutive direct pig-to-pig passages in the absence of mosquito intermediates, the virus underwent significant adaptive evolution. The passaged virus exhibited enhanced in vivo replication kinetics, increased oronasal shedding, and triggered stronger innate immune responses in infected pigs compared to the parental mosquito-passaged virus. Importantly, the tissue tropism remained similar, and the frequency of direct transmission did not increase, suggesting that while JEV can adapt to improved replication in swine, the basic mechanisms of direct transmission are already present without requiring further selection [2].

Genomic analysis revealed profound shifts in viral quasispecies composition during serial passaging, with 25 point mutations reaching frequencies of at least 35% across passages. Among these, six non-synonymous mutations were identified in critical viral proteins: the precursor of membrane (prM), envelope (E), non-structural 3 (NS3), and non-structural 5 (NS5) proteins. Competition experiments between passage lines demonstrated that two particular mutations, M374L in the envelope protein and N275D in NS5, conferred measurable fitness advantages in pigs [2]. These findings have profound epidemiological implications: sustained vector-free transmission chains in swine populations could drive the emergence of viral variants with altered pathogenic potential or host adaptation, potentially changing the risk profile for both swine and human populations.

The Role of Tonsillar Persistence in Transmission Dynamics

A critical feature of JEV infection in swine that directly influences transmission dynamics is the remarkable persistence of virus in tonsillar tissue. Ricklin and colleagues demonstrated that JEV replication in tonsils reaches the highest viral loads of any tissue examined and persists for at least 25 days post-infection, well beyond the period of detectable viremia and despite the presence of robust neutralizing antibody responses [1, 27]. This observation has been consistently replicated across multiple experimental studies and genotypes [5, 19, 27]. The tonsils serve as a viral sanctuary where JEV can evade immune clearance, potentially providing a prolonged source of virus for both vector-borne and direct transmission.

The biological mechanisms underlying tonsillar persistence have been further elucidated by detailed investigations into the cellular targets of JEV within this lymphoid tissue. Dendritic cells and macrophages within the tonsils are the primary target cells for JEV infection, and the viral NS1' protein, which is expressed by many JEV strains but notably absent from the live attenuated SA14-14-2 vaccine strain, significantly enhances infection of these cell populations [40]. NS1' expression was shown to increase viral infection in both the central nervous system and tonsillar tissues, and pigs infected with NS1'-expressing recombinant viruses shed virus in oronasal secretions more efficiently, suggesting that this protein may facilitate horizontal transmission [40]. Furthermore, JEV infection of tonsillar dendritic cells and macrophages resulted in reduced major histocompatibility complex class II (MHC II) expression and activation of inducible nitric oxide synthase (iNOS), creating an immunologically favorable environment for persistent infection [40].

Quantitative Contributions of Direct Transmission to Epidemiological Dynamics

The field relevance of direct pig-to-pig transmission has been addressed through sophisticated mathematical modeling approaches. Diallo and colleagues constructed compartmental transmission models incorporating either vector-borne transmission alone or a combination of vector-borne and direct transmission, fitting these models to longitudinal serological data from sentinel pig cohorts in Cambodia [30]. The model incorporating both transmission routes consistently provided superior fits to the empirical data. Importantly, the basic reproduction number (R₀) estimates ranged from 2.27 to 2.93 across different datasets, indicating that JEV is capable of establishing sustained transmission in swine populations. The direct transmission reproduction number (Rpp) contributed 7.5% to 11.9% of the overall R₀, demonstrating that while vector-borne transmission remains the dominant route, direct contact transmission makes a non-trivial contribution to viral circulation [30].

These modeling results support the hypothesis that direct transmission plays a meaningful role in maintaining JEV circulation under field conditions, particularly in scenarios where vector activity is reduced. The contributions of direct transmission may be amplified in intensive pig production systems where high stocking densities facilitate close contact between animals, or in settings where biosecurity measures are minimal. As the authors note, these findings warrant confirmation across diverse ecological and climatic settings, particularly in temperate regions where the relative importance of direct transmission may be magnified during periods of low mosquito activity [30].

The Influence of Mosquito Saliva on Transmission Dynamics

While direct transmission represents a newly recognized pathway, the classical vector-mediated route remains epidemiologically dominant, and the biological interactions between mosquito saliva, JEV, and the porcine host add another layer of complexity to transmission dynamics. Park and colleagues investigated whether mosquito salivary components modulate JEV infection outcomes in pigs, recognizing that natural transmission through infected mosquito bites may differ substantially from experimental needle inoculation [43]. When JEV was co-injected with salivary gland extract (SGE) from Culex quinquefasciatus, pigs developed milder febrile illness and exhibited shortened duration of nasal shedding compared to pigs infected with virus alone. However, viremia levels and neuroinvasion were not significantly affected, indicating that mosquito saliva selectively modulates certain aspects of the host response without altering the fundamental capacity of pigs to serve as amplifying hosts [43].

These findings have important epidemiological implications. The attenuated clinical presentation and reduced nasal shedding observed with mosquito saliva exposure suggest that naturally infected pigs may be less likely to transmit virus directly to contact animals, potentially limiting the contribution of direct transmission in settings where mosquitoes are abundant. Conversely, the reduced febrile response may delay clinical detection of infection, allowing silent viral amplification to proceed undetected in swine herds. The differential effects of mosquito saliva on transmission parameters highlight the need for infection models that accurately replicate natural exposure routes when assessing transmission risks.

Landscape Epidemiology and Spatiotemporal Transmission Patterns

The spatial distribution of JEV transmission in swine is inextricably linked to landscape features that govern mosquito vector populations, wild bird reservoir communities, and pig farming practices. Walsh and colleagues conducted comprehensive landscape epidemiological analyses of JEV outbreaks in India over the 2010-2020 period, demonstrating that outbreak risk was strongly associated with the habitat suitability of ardeid birds (herons and egrets), pig density, and the spatial configuration of fragmented rain-fed agriculture interspersed with riverine and freshwater marsh wetlands [51, 54]. The convergence of these landscape elements creates ecological interfaces where the wildlife-livestock-human nexus facilitates JEV spillover and amplification.

These landscape relationships were strikingly recapitulated during the unprecedented JEV emergence in Australia in 2022, where outbreaks in piggeries across previously unaffected temperate regions of eastern and southern Australia were associated with specific landscape configurations. Walsh and colleagues demonstrated that Australian piggery outbreaks during the 2022 epizootic were linked to intermediate ardeid species richness, cultivated land and grassland fragmentation, proximity to waterways, temporary wetlands, and hydrological flow accumulation [31, 57]. The emergence followed substantial precipitation and temperature anomalies associated with the La Niña phase of the El Niño Southern Oscillation, which created favorable conditions for mosquito proliferation and amplified transmission risk across the piggery landscape [31].

The importance of landscape context in shaping transmission dynamics is further emphasized by studies demonstrating JEV circulation in unexpected settings. Lindahl and colleagues documented 100% seroprevalence in pigs sampled within Can Tho City, Vietnam, with JEV detected in mosquito pools collected within the urban environment [3]. Similarly, intensive circulation was documented in peri-urban sentinel pigs near Phnom Penh, Cambodia, with the force of infection estimated at 0.061-0.069 per day, indicating that seronegative pigs became infected at remarkably high rates regardless of whether they were located in rural or peri-urban settings [15, 29]. These findings challenge the traditional characterization of JEV as exclusively a rural disease and demonstrate that urbanization gradients do not necessarily attenuate transmission risk.

Seasonal and Climatic Drivers of Transmission Intensity

The seasonality of JEV transmission in swine is governed primarily by the population dynamics of mosquito vectors, which are in turn driven by temperature, precipitation, and humidity. Seroprevalence surveys in Tamil Nadu, India, revealed distinct seasonal patterns, with peak seroprevalence during the monsoon season (67.9%) followed by winter (65.1%) and summer (53.3%), reflecting the seasonal abundance of Culex vectors [16]. The force of infection in sentinel pig cohorts from Cambodia was similarly high across different settings, with all pigs seroconverting before six months of age, indicating intense year-round transmission in tropical environments [29].

However, the relationship between climate and transmission is not uniform across geographic regions. Li and colleagues demonstrated through phylogeographic analyses that JEV circulation in China is predominantly concentrated in areas with higher temperatures, dense human and pig populations, and environmental conditions favorable for Culex mosquitoes [52]. The diffusion capacity of JEV appears to exceed previous estimates for co-circulating arboviruses, potentially driven by pig trade networks and bird migration patterns. The expanding pig farming industry in China, combined with diverse climatic zones, creates heterogeneous transmission landscapes that require spatially targeted surveillance approaches [52].

The emergence of JEV in temperate Australia has highlighted the potential for climate-mediated range expansion. The 2022 incursion, which affected nearly 80 pig farms across multiple states, followed an extended period of above-average rainfall and flooding associated with successive La Niña events, creating extensive mosquito breeding habitats in regions where JEV had never been previously documented [13, 31]. This event underscores the vulnerability of temperate swine populations to JEV introduction when climatic conditions transiently favor vector populations, even in the absence of endemic transmission.

Genotype Dynamics and Their Epidemiological Implications

The epidemiological landscape of JEV in swine is further complicated by the circulation of multiple genotypes with potentially different transmission characteristics. Five distinct JEV genotypes (GI through GV) have been described based on envelope gene sequences, and their geographical and temporal distributions have undergone substantial shifts over recent decades. Historically, genotype III (GIII) was the dominant genotype across Asia, but genotype I (GI) has progressively displaced GIII in many regions, including China, Japan, and parts of Southeast Asia [9, 35, 41]. The mechanisms underlying this genotype shift remain incompletely understood but may involve differences in replication efficiency in avian hosts or mosquito vectors [41].

In China, surveillance efforts from 2019-2021 in Hunan Province revealed that 19.3% of pigs were seropositive for JEV, with molecular characterization of circulating strains demonstrating that all 14 isolates belonged to genotype I-b, displaying genetic divergence from the GIII-based SA14-14-2 vaccine strain [9]. This genotype shift has potential implications for vaccine efficacy, as the currently widely used live attenuated vaccine is derived from a GIII strain. Similarly, in Sichuan Province, detection rates of 6.49% in aborted fetuses and testicular samples identified both GI and GIII strains circulating in the region, with phylogenetic analysis of GI isolates demonstrating distant relationship to the vaccine strain [48].

The co-circulation of genotypes has been documented in multiple settings. In Cambodia, phylogenetic analyses of JEV isolates from pigs, mosquitoes, and human encephalitis cases revealed that all belonged to genotype I, but clustered into two distinct clades, genotype I-a, related to strains from Thailand, and genotype I-b, related to Vietnamese and Chinese strains, indicating multiple introduction events and ongoing regional circulation [28]. In Japan, both GI and GIII were isolated from pig serum samples in Kochi Prefecture, demonstrating that genotype replacement may be incomplete and that multiple genotypes can persist in the same geographic area [37].

The emergence of genotype IV (GIV) as a cause of widespread disease in Australian pigs and humans in 2022 represents a particularly concerning development, as GIV has been rarely detected outside of sporadic isolations in Indonesia [5, 17]. The Australian outbreak strain caused extensive reproductive disease in pigs, with viremia peaking at 3-4 days post-infection, consistent detections in tonsillar tissue persisting for at least 19 days, and evidence of central nervous system invasion in most infected animals [5]. The establishment of GIV in Australian swine populations raises questions about cross-protection afforded by existing GIII-based vaccines and underscores the need for genotype-specific surveillance.

Transmission in Feral Swine and Wild Boar Populations

The role of feral swine and wild boar in JEV transmission dynamics has received increasing attention, particularly in the context of assessing introduction risks for JEV-free regions. Domestic pigs (Sus scrofa domestica) and feral pigs are the same species, and experimental infections of Sinclair miniature pigs exhibiting the feral phenotype demonstrated that feral swine are susceptible to JEV infection, develop viremia sufficient to infect feeding mosquitoes, and exhibit pathological outcomes similar to domestic swine [33]. These findings indicate that feral pig populations could contribute to the establishment and maintenance of JEV transmission cycles if the virus were introduced into regions with substantial feral swine populations, such as the United States, where the expanding geographic range and increasing densities of feral pigs create vulnerability.

Serological evidence from Tsushima Island, Japan, where four human Japanese encephalitis cases occurred in 2016 in the absence of domestic pig farms, provides compelling field evidence for the role of wild boar as amplifying hosts. Neutralizing antibodies against JEV were detected in four of six sampled wild boars, and serosurveys of human residents revealed 38.8% seroprevalence against genotype 3 and 24.6% against genotype 5, with higher antibody titers associated with outdoor activities [49]. These observations demonstrate that wild boar populations can sustain JEV transmission independently of domestic swine, creating sylvatic reservoirs that pose ongoing spillover risk.

In the Australian context, feral pigs have been identified as potential maintenance hosts for JEV, with host competence scores indicating that they could contribute to viral persistence in the absence of domestic pigs [55, 56]. The extensive feral pig populations across northern and eastern Australia, combined with the recent establishment of JEV genotype IV, create conditions where a feral pig-driven transmission cycle could become permanently established, complicating eradication efforts and creating long-term public health risks [55].

Quantitative Epidemiology: Seroprevalence Patterns and Infection Dynamics

Seroprevalence surveys across endemic regions provide quantitative insights into the intensity of JEV transmission in swine populations. The data reveal remarkable variation in transmission intensity across different geographic settings, reflecting differences in vector abundance, pig density, climate, and management practices. In Tamil Nadu, India, a comprehensive cross-sectional study of 710 pigs from 118 farms across 10 districts documented an apparent seroprevalence of 60.4% and a true seroprevalence of 50.1%, with herd-level seroprevalence reaching 94.1% [16]. Pigs older than 12 months had seroprevalence of 95.2%, indicating near-universal exposure within the first year of life. These figures are consistent with findings from northern West Bengal, India, where 38.8% of pig blood samples were positive for anti-JEV antibodies [50].

In contrast, seroprevalence can be substantially lower in areas with less intensive transmission. In Goa, India, where human JE cases occur sporadically, the apparent seroprevalence in pigs was only 7.1%, with a true prevalence of 4.6%, suggesting lower intensity of JEV circulation that may contribute to the lower human disease burden observed in the state [53]. Similarly, studies on the Okinawa islands of Japan found extremely low seroprevalence, with only 4.5% of pigs on Ishigaki Island showing positive antibody responses and no evidence of transmission on Miyako, Kume, or Yonaguni Islands, indicating very low or absent transmission activity [4].

The force of infection, the rate at which seronegative pigs become infected, provides a more dynamic measure of transmission intensity. Sentinel pig studies in Cambodia estimated the force of infection at 0.061-0.

Host Immune Responses and Viral Persistence in Porcine Reservoirs

The porcine immune response to Japanese encephalitis virus (JEV) represents a sophisticated and paradoxical interplay between effective antiviral containment and the establishment of long-term viral persistence. As the primary amplifying host in the JEV transmission cycle, pigs must generate a viremia of sufficient magnitude and duration to infect feeding mosquito vectors, yet the same host must also control the virus to prevent fatal neuroinvasive disease. Understanding this delicate immunological balance is critical for devising effective intervention strategies, including vaccination and surveillance programs, as recognized by the World Organisation for Animal Health (WOAH) and the World Health Organization (WHO) in their joint efforts to manage JEV as a emerging zoonotic threat.

Innate Immune Recognition and Antiviral Signaling

Upon initial infection, whether through the bite of an infected Culex mosquito or via direct oronasal contact [1], JEV is encountered by pattern recognition receptors (PRRs) in porcine tissues. The retinoic acid-inducible gene I (RIG-I) and Toll-like receptors (TLRs), particularly TLR3, TLR4, TLR7, and TLR8, serve as primary sensors of viral RNA in porcine cells. Studies in JEV-infected microglial cell lines have demonstrated that RIG-I and TLR3 are robustly upregulated, triggering downstream signaling cascades that converge on the activation of interferon regulatory factor 3 (IRF3) and nuclear factor-kappa B (NF-κB), ultimately driving the expression of type I interferons (IFN-α/β) and pro-inflammatory cytokines [59].

However, JEV has evolved sophisticated countermeasures to subvert this innate antiviral response. The nonstructural protein NS4B acts as a potent antagonist of IFN-β production by directly targeting TLR3 and the adaptor protein TRIF (TIR-domain-containing adapter-inducing interferon-β). Mechanistically, JEV NS4B physically interacts with both TLR3 and TRIF, co-localizing with these molecules and thereby inhibiting the phosphorylation of IRF3 [45]. This blockade prevents the transcriptional activation of the IFN-β promoter, effectively dampening the host's first line of antiviral defense. This immune evasion strategy is critical for allowing the virus to establish a foothold in the porcine host before adaptive immunity can be mobilized.

Despite these viral evasion tactics, the porcine innate response is not entirely suppressed. In the central nervous system (CNS), where JEV exhibits a particular tropism, an effective antiviral state is established that largely limits neuropathogenesis. Remarkably, JEV replication in the pig brain is controlled through a type I IFN-independent mechanism involving the activation of 2',5'-oligoadenylate synthetase 1 (OAS1). OAS1 activation leads to the degradation of viral RNA via RNase L, providing a robust layer of protection [7]. This IFN-independent pathway appears to be particularly effective in most brain regions, explaining why pigs, unlike humans and mice, rarely develop severe neurological signs following JEV infection.

Humoral Immune Response and Antibody-Dependent Enhancement

Infection with JEV elicits a robust and rapid humoral immune response in pigs. Neutralizing antibodies directed primarily against the envelope (E) protein are detectable as early as 5–7 days post-infection, with peak titers typically reached by 14–21 days [5, 19, 27]. The development of these antibodies coincides with the clearance of viremia, which is typically short-lived, lasting only 3–5 days [5, 27].

A particularly intriguing and clinically relevant aspect of the humoral response in pigs is the phenomenon of antibody-dependent enhancement (ADE). Lentiviral vectors expressing JEV prM and E proteins, as well as natural JEV infection, have been shown to induce antibodies that strongly enhance JEV infection of porcine macrophages via Fc gamma receptors [58]. This paradoxical effect occurs when sub-neutralizing concentrations of antibodies bind to the virus and facilitate its entry into Fc receptor-bearing cells, potentially increasing viral burden. Crucially, despite the presence of these ADE-inducing antibodies in vaccinated pigs, the animals remained protected against challenge, suggesting that the neutralizing antibody response dominates in vivo and that ADE may not be a significant pathological factor in the porcine host under normal circumstances [58]. However, this phenomenon warrants careful consideration in vaccine design, as it has been documented for other flaviviruses such as dengue virus.

Cellular Immunity and the Controlled CNS Response

Given the neurotropic nature of JEV, the immune response within the CNS is of paramount importance for determining clinical outcome. Pigs exhibit a markedly different neuroinflammatory profile compared to mice and humans. In the porcine CNS, JEV entry is efficiently controlled, and viral replication in most brain regions is rapidly suppressed. In the olfactory bulb and thalamus, where JEV replication is not completely eliminated by the OAS1-mediated pathway, a short but strong induction of chemokine gene expression is observed. This is accompanied by an increase in IFNγ expression, likely originating from infiltrating T cells, which correlates with the swift suppression of viral replication [7].

Critically, this chemokine and IFNγ response is not associated with the induction of a fulminant inflammatory response. Most notably, there is no evidence of NLRP3 inflammasome activation in the JEV-infected porcine brain [7]. This is in stark contrast to mouse models, where NLRP3 inflammasome activation drives a devastating inflammatory cascade that contributes significantly to neuropathology and mortality. The absence of this pathway in pigs provides a mechanistic explanation for the species-specific differences in disease outcome: pigs experience subclinical or mild disease, while mice and humans can succumb to severe encephalitis. The porcine immune system thus achieves a "Goldilocks" balance, sufficient antiviral activity to control the virus, but not so much inflammation as to cause collateral damage to delicate neural tissues.

Mechanisms of Viral Persistence in Porcine Tissues

The most striking and epidemiologically significant feature of JEV infection in pigs is the ability of the virus to persist in specific tissues for extended periods, long after the resolution of viremia and the establishment of high-titer neutralizing antibodies. This persistent infection, primarily documented in the tonsils, has profound implications for JEV ecology and transmission dynamics.

The tonsils serve as the primary sanctuary for JEV persistence. Viral RNA and infectious virus have been consistently detected in tonsillar tissues for at least 25–28 days post-infection, and in some studies, for months, despite the presence of a strong neutralizing antibody response [1, 19, 27, 47]. This persistence is not a passive retention of viral debris; live, replication-competent virus can be isolated from these tissues. The cellular basis for this persistence has been elucidated: dendritic cells (DCs) and macrophages within the tonsillar crypts are the primary target cells for JEV infection and persistence [40].

The JEV protein NS1' (a longer form of NS1 expressed due to a -1 ribosomal frameshift event in the NS2A gene) plays a critical role in enhancing viral infection of these tonsillar immune cells. Compared to the vaccine strain SA14-14-2, which does not express NS1', a recombinant virus engineered to express NS1' (rA66G) showed significantly increased infection rates in both DCs and macrophages within porcine tonsils [40]. Furthermore, NS1' expression was associated with reduced major histocompatibility complex class II (MHC II) expression on these antigen-presenting cells, potentially impairing their ability to activate T-cell responses. The infection also induced inducible nitric oxide synthase (iNOS) activity, which may create an immunosuppressive microenvironment that further facilitates viral persistence [40]. Together, these mechanisms establish the tonsil as an immune-privileged site where JEV can evade clearance.

Beyond the tonsils, JEV also exhibits tropism for the reproductive tract, establishing another potential reservoir. In experimentally infected pigs, JEV RNA was detected in the vaginal mucosa for at least 28 days post-infection, and infectious virus was shed in vaginal secretions [47]. Viral persistence was also documented in the endometrium and placenta, with evidence of transplacental and fetal infection. This reproductive tract tropism provides a mechanism for vertical transmission (leading to the characteristic stillbirths and mummified fetuses seen in JEV outbreaks) and raises the possibility of sexual transmission, a mode of spread that has been documented for the related Zika virus [47].

The persistence of JEV in both tonsillar and reproductive tissues has significant ecological implications. The World Health Organization (WHO) recognizes that understanding these reservoirs is key to predicting and managing outbreaks. Viral shedding from persistently infected tonsils, via oronasal secretions, can occur even in the absence of mosquitoes, providing a mechanism for direct contact transmission between pigs [1, 19]. Studies have shown that JEV can be transmitted between pigs through direct contact and oronasal inoculation, with a pathogenesis indistinguishable from mosquito-borne infection [1]. Mathematical modeling suggests that this direct transmission route may contribute significantly to the basic reproduction number (R0) of JEV in pig populations, accounting for 7.5–12% of transmission in some settings [30].

Evolutionary Adaptations and Implications for Viral Fitness

The host immune pressure exerted by the pig drives evolutionary changes in the JEV genome. Serial passaging of JEV through pigs in the absence of mosquito vectors, a scenario mimicking a series of direct transmissions, results in the selection of viral quasispecies with enhanced fitness for the porcine host. After ten passages, 25 point mutations were selected, with six leading to amino acid changes in the prM, E, NS3, and NS5 proteins. Notably, mutations M374L in the E protein and N275D in NS5 conferred a fitness advantage in pigs, leading to higher viral replication, increased shedding, and stronger innate immune responses [2].

These evolutionary adaptations have profound implications. If JEV were to establish a transmission cycle in a region with immunologically naïve pigs and limited mosquito activity (such as temperate zones with short mosquito seasons), the virus could undergo selective pressure to enhance its direct transmissibility and persistence in pigs. This scenario is a genuine concern for disease-free regions like the United States and Europe, as highlighted by the Centers for Disease Control and Prevention (CDC) and the European Food Safety Authority (EFSA). The 2022 emergence of JEV genotype IV in temperate Australia, causing widespread reproductive disease in pigs and human cases, serves as a stark warning [5, 13, 21]. The development of persistent, shedding infections in pigs, coupled with the potential for vector-free transmission, means that JEV could overwinter in porcine reservoirs even when mosquito activity is low, maintaining a source of virus for the next transmission season. This fundamentally alters our understanding of JEV epidemiology and necessitates a re-evaluation of control strategies, moving beyond mosquito-focused interventions to include biosecurity measures that prevent direct contact transmission and address the persistent viral reservoir within pig populations.

Diagnostic Approaches for Japanese Encephalitis Virus in Pigs

The accurate and timely diagnosis of Japanese encephalitis virus (JEV) in pigs is a cornerstone of effective surveillance, outbreak response, and the implementation of control strategies within a One Health framework. Given the role of pigs as amplifying hosts and sentinels for human disease risk, diagnostic approaches must be multifaceted, encompassing direct detection of the virus or its components, serological identification of host immune responses, and advanced molecular characterization for genotyping and evolutionary tracking. The diagnostic landscape is further complicated by the virus's ability to establish persistent infections in tonsillar tissue, the phenomenon of vector-free direct transmission, and the extensive serological cross-reactivity with other orthoflaviviruses [1, 2, 7, 14]. This section provides an exhaustive analysis of the diagnostic modalities available for JEV in swine, from traditional methods to cutting-edge molecular and genomic tools, contextualized within the unique pathobiology of the infection.

Direct Viral Detection: Molecular and Virological Methods

The direct detection of JEV in clinical specimens is essential for confirming active infection, particularly during the acute viremic phase and in cases of reproductive failure. The choice of diagnostic specimen is critical, as viral distribution is not uniform across tissues or over the course of infection.

Reverse Transcription Quantitative PCR (RT-qPCR) and Multiplex Assays

Real-time reverse transcription quantitative PCR (RT-qPCR) has become the gold standard for JEV RNA detection due to its high sensitivity, specificity, and rapid turnaround time. Assays typically target conserved regions of the viral genome, such as the NS1, NS3, or E genes, and can achieve detection limits far superior to conventional RT-PCR [7, 9, 60]. The establishment of a duplex TaqMan RT-qPCR assay, capable of simultaneously detecting JEV and Getah virus (GETV) in pig and mosquito samples, exemplifies the move towards multiplex diagnostics that can differentiate between clinically similar pathogens. This assay demonstrated a 100-fold and 10-fold increase in sensitivity for JEV and GETV, respectively, compared to traditional PCR, and can be completed in under an hour, making it invaluable for high-throughput surveillance [60].

The selection of sample types for RT-qPCR is guided by the pathogenesis of JEV. Whole blood or serum is the primary specimen for detecting viremia, which typically peaks between 3-4 days post-infection and is short-lived [5, 7]. However, the detection window is narrow, and viremia may be absent by the time clinical signs, particularly reproductive disorders, become apparent. Consequently, post-mortem tissue sampling is often more informative. The tonsils have been consistently identified as a primary site of JEV replication and persistence, harboring the highest viral loads and remaining positive for viral RNA for at least 25-28 days post-infection, even in the presence of neutralizing antibodies [1, 19, 27]. This makes tonsillar tissue the single most reliable sample for post-mortem molecular diagnosis. Other tissues with high diagnostic yield include the brain (particularly the thalamus, olfactory bulb, and brainstem), spleen, lymph nodes, and reproductive tissues such as the placenta, fetal tissues, and vaginal mucosa [5, 7, 27, 47]. The detection of JEV in oronasal swabs and saliva has also been documented, reflecting the shedding of virus through these routes, which is a key feature of vector-free transmission [1, 5, 19]. This finding has important implications for non-invasive sampling strategies, as oral fluids from chew ropes can be used for herd-level surveillance without the need for individual animal restraint [5].

Reverse Transcription Recombinase-Aided Amplification (RT-RAA)

For field-deployable diagnostics, particularly in resource-limited settings, isothermal amplification methods such as reverse transcription recombinase-aided amplification (RT-RAA) offer a compelling alternative to PCR. A fluorescent RT-RAA assay developed for JEV detection in pigs demonstrated high efficiency and sensitivity, with a detection rate of 6.49% in aborted fetuses and testicular samples from Sichuan, China [48]. This method does not require thermal cycling equipment, can be performed at a constant temperature (typically 39°C), and provides results in a short timeframe, making it suitable for point-of-care testing in piggeries or regional veterinary laboratories. The assay successfully identified both GI and GIII strains, confirming its utility for detecting the predominant circulating genotypes [48].

Virus Isolation

While molecular methods are the mainstay of diagnosis, virus isolation remains the definitive gold standard for confirming the presence of infectious virus and is essential for generating isolates for further characterization, such as whole-genome sequencing and vaccine development. JEV can be isolated from clinical samples using mammalian cell lines, with porcine stable kidney (PS) cells and Vero cells being the most commonly employed [23, 27]. Inoculation of samples (e.g., tonsil homogenates, brain tissue, or serum) onto confluent cell monolayers is followed by observation for cytopathic effect (CPE), which typically appears within 3-7 days. The identity of the isolate is then confirmed by immunofluorescence assay (IFA) using JEV-specific antibodies or by RT-qPCR [23]. A study isolating JEV genotype I from a nasal swab of a naturally infected pig in India demonstrated the feasibility of using this non-invasive sample for virus isolation, achieving a peak titer of 10⁶.⁵ TCID₅₀/mL in PS cells [23]. However, virus isolation is labor-intensive, time-consuming, requires BSL-3 containment facilities, and has lower sensitivity than RT-qPCR, particularly for samples with low viral loads or those containing non-infectious RNA.

Serological Diagnostics: Detecting the Host Response

Serological testing is the primary tool for determining prior exposure to JEV, estimating seroprevalence, and monitoring the force of infection in pig populations. The humoral immune response to JEV involves the production of IgM antibodies, which appear early in infection (as early as 5 days post-challenge) and are indicative of recent or active infection, followed by IgG antibodies, which persist for months to years and indicate past exposure or vaccination [5, 16].

Enzyme-Linked Immunosorbent Assay (ELISA)

ELISA-based methods are the most widely used serological tests due to their scalability, objectivity, and relatively low cost. Indirect ELISAs, which detect anti-JEV IgG, are commonly employed for large-scale serosurveys. Studies in India and China have used these assays to document seroprevalence rates ranging from 7.1% in Goa to over 60% in Tamil Nadu, highlighting the endemicity of JEV in pig populations [9, 16, 53]. However, a major limitation of standard indirect ELISAs is the potential for cross-reactivity with antibodies against other orthoflaviviruses, such as West Nile virus (WNV), Murray Valley encephalitis virus (MVEV), and Usutu virus (USUV), which can co-circulate in many regions [5, 14]. This is a significant challenge for accurate diagnosis in areas where multiple flaviviruses are present.

To overcome this, blocking ELISAs (B-ELISAs) have been developed, which use a monoclonal antibody specific to a conserved JEV epitope. The test measures the ability of antibodies in the test serum to block the binding of this labeled monoclonal antibody to the JEV antigen. A B-ELISA developed for pig and horse sera demonstrated high diagnostic sensitivity (94.6-100%) and specificity (91.2-100%) when compared to the gold standard hemagglutination inhibition (HI) and virus neutralization (VN) tests [63]. This format significantly reduces cross-reactivity and is more suitable for species-independent serosurveillance. Defined Epitope Blocking (DEB) ELISAs represent an even more refined approach, targeting specific, non-cross-reactive epitopes. This technology was employed in a public health investigation of JEV in Australian piggery workers, demonstrating its utility in human serosurveys and suggesting its potential adaptation for swine [62].

Hemagglutination Inhibition (HI) Test

The HI test is a classical serological method that detects antibodies capable of inhibiting the agglutination of red blood cells (typically goose or chicken erythrocytes) by JEV. It is a well-standardized and inexpensive assay that has been used for decades in JEV surveillance [4, 61]. The test can differentiate between IgM and IgG antibodies through treatment with 2-mercaptoethanol (2-ME), which inactivates IgM, allowing for the detection of recent infection [4]. However, the HI test is prone to cross-reactivity within the flavivirus serocomplex, requires the removal of non-specific inhibitors from sera, and is less sensitive and specific than modern ELISAs or VN tests. Its use is now largely confined to historical datasets and laboratories with limited resources.

Virus Neutralization Test (VNT)

The plaque reduction neutralization test (PRNT) is the serological gold standard for detecting JEV-specific neutralizing antibodies and is considered the most specific test available [49, 64]. It measures the ability of serum antibodies to prevent JEV from infecting and causing CPE in cell culture. The test is highly sensitive and can differentiate between closely related flaviviruses based on the endpoint titer, as demonstrated in a study comparing JEV and MVEV infections in pigs [5]. However, the PRNT is labor-intensive, requires live virus and cell culture facilities (BSL-3), and takes several days to complete. It is therefore not suitable for high-throughput surveillance but is indispensable for confirmatory testing, vaccine efficacy studies, and resolving ambiguous ELISA results. The determination of limiting-dilution titers in PRNT can provide a definitive diagnosis even in the face of cross-reactive antibodies, as the neutralizing titer against the homologous virus is typically 4-fold or greater than against heterologous flaviviruses [5].

Advanced Diagnostic and Genomic Approaches

Whole-Genome Sequencing (WGS)

The application of next-generation sequencing (NGS) for whole-genome sequencing of JEV directly from clinical samples or isolates has revolutionized our understanding of viral evolution, transmission dynamics, and genotype distribution. WGS provides the highest resolution for phylogenetic analyses, allowing for the tracing of viral introductions, identification of recombination events, and detection of mutations associated with fitness adaptations [2, 14, 28]. For example, WGS of JEV from pigs and mosquitoes in Cambodia revealed the co-circulation of two distinct clades (GI-a and GI-b) and provided evidence for the virus's spread between pigs, mosquitoes, and humans [28]. In Europe, veterinary reference laboratories have aligned their WGS protocols to prepare for potential JEV incursions, demonstrating the importance of this tool for cross-border surveillance [14]. The detection of single nucleotide polymorphisms (SNPs) and amino acid changes in the envelope (E) and non-structural (NS) proteins, as seen in serial pig passage experiments, is critical for monitoring viral adaptation to swine hosts and potential vaccine escape [2, 44].

Effluent and Environmental Surveillance

A novel and non-invasive approach to herd-level JEV surveillance is the testing of piggery effluent and environmental water samples. A landmark study in Australia provided the first evidence of JEV RNA detection in the solid and liquid fractions of piggery effluents, with detection correlating with veterinary cases in the herds [8]. Viral RNA was more frequently detected in the solid fraction, consistent with the partitioning behavior of other mosquito-borne viruses. This method offers a complementary tool to traditional surveillance, enabling the detection of JEV circulation within a herd without the need for individual animal sampling. It holds significant promise for early warning systems, particularly in large-scale commercial operations, and can be integrated with wastewater-based epidemiology for a broader One Health approach [8].

Diagnostic Challenges and Considerations

Several factors complicate the diagnosis of JEV in pigs. The short duration of viremia means that molecular testing of blood samples must be timed precisely to the acute phase, which is often missed in field settings [5, 7]. The ability of JEV to establish persistent infection in tonsils, with viral RNA detectable for weeks after clinical recovery, means that a positive RT-qPCR result from a tonsil biopsy or post-mortem sample does not necessarily indicate an active, acute infection [1, 19, 27]. This has implications for interpreting surveillance data and for trade restrictions. Furthermore, the phenomenon of vector-free direct transmission, where pigs shed virus in oronasal secretions, means that oronasal swabs can be diagnostically useful, but the presence of virus in these secretions may also reflect persistent shedding from the tonsils rather than acute disease [1, 2, 23].

Serological diagnosis is confounded by the high degree of antigenic cross-reactivity among orthoflaviviruses. In regions like Europe and Australia, where WNV, MVEV, and USUV circulate, a positive ELISA result must be interpreted with caution and confirmed by a more specific test like PRNT [5, 14]. The presence of maternal antibodies in piglets can also interfere with serological diagnosis for several weeks after birth, complicating the interpretation of serosurveys in young animals [29]. Finally, the co-circulation of multiple JEV genotypes (GI, GIII, GIV, GV) with varying antigenic properties necessitates the use of diagnostic assays that can detect all relevant strains, a challenge that is being addressed through the development of broadly reactive primers and monoclonal antibodies [11, 17, 28]. The World Organisation for Animal Health (WOAH) and the World Health Organization (WHO) emphasize the need for standardized diagnostic protocols and inter-laboratory proficiency testing to ensure the reliability of JEV surveillance data globally.

Prevention, Control, and Biosecurity Strategies for Japanese Encephalitis Virus in Swine

The management of Japanese encephalitis virus (JEV) in swine populations presents a singularly complex challenge, one that defies simplistic, single-intervention approaches. As a zoonotic flavivirus with a multifaceted ecology involving Culex mosquitoes, ardeid waterbirds, and pigs as the primary amplifying host, JEV demands an integrated, multi-pronged strategy. The recent demonstration of direct, vector-free transmission between pigs [1], coupled with the virus's ability to persist in tonsillar tissue and be shed in oronasal and vaginal secretions [27, 40, 47], fundamentally alters the calculus of control. This section dissects the layered, interdependent strategies required to prevent JEV introduction, contain its spread within herds, and reduce the risk of spillover to human populations, emphasizing the biological underpinnings that dictate the efficacy of each measure.

1. Foundational Biosecurity: Minimizing the Mosquito-Bridge and Direct Contact Pathways

Biosecurity for JEV must be viewed through a dual lens: first, the traditional vector-borne route, and second, the now-irrefutable direct transmission pathway. The primary vector, Culex tritaeniorhynchus, breeds in rice paddies, irrigation canals, and stagnant water bodies, making environmental management the cornerstone of any prevention plan. Source reduction, draining, filling, or treating standing water with larvicides, is non-negotiable. The World Organisation for Animal Health (WOAH) and the Food and Agriculture Organization (FAO) emphasize the importance of integrated vector management (IVM) for arboviruses, which for JEV includes the strategic use of insect growth regulators and Bacillus thuringiensis israelensis (Bti) to target mosquito larvae without harming non-target organisms. Studies from Cambodia [32] and Australia [21, 31] demonstrate that piggeries situated within wetland-agricultural mosaics experience significantly higher JEV pressure, reinforcing the need for farm siting considerations and perimeter vector control.

Beyond larviciding, physical barriers are critical. Piggeries should be constructed with mosquito-proof screening on all vents, windows, and eaves. The use of insecticide-treated nets (ITNs) over pig pens is a proven, cost-effective method to reduce biting rates. However, the revelation that JEV can be transmitted directly via oronasal secretions [1] demands that biosecurity extend beyond arthropod control. This vector-free route, which can account for a measurable fraction (7–12%) of transmission in endemic settings [30], necessitates strict segregation of pigs by age and health status. The movement of personnel, equipment, and vehicles between pens must be controlled with boot baths, hand hygiene stations, and dedicated tools for each unit. The tonsils are a key site of JEV persistence, harboring virus for at least 25 days post-infection despite high neutralizing antibody titers [27, 40]. This creates a reservoir of infectious virus that can be shed intermittently. Therefore, any pig introduced to a herd from an outside source, or even a pig returning from a show or sale, must be quarantined for a minimum of 28 days, with close clinical and ideally molecular (qRT-PCR) monitoring of nasal secretions before introduction.

Feral pigs (wild boar) represent a profound, often overlooked, biosecurity threat. Experimental infection of feral swine has shown they develop viremia and pathological outcomes similar to domestic pigs, positioning them as a potential maintenance host capable of sustaining JEV transmission in the absence of domestic stock [33]. Serosurveillance on Tsushima Island, Japan, has confirmed that wild boar can serve as amplifying hosts even when no pig farms are present [49]. Consequently, perimeter fencing must be robust enough to exclude feral swine. Integrated pest management programs that include trapping or regulated culling of feral populations in high-risk zones are a recommended adjunct to farm-level biosecurity, a point echoed in risk assessments for JEV introduction into JEV-naïve regions like the United States [20].

2. Vaccination: The Imperative for Universal Swine Immunization

Vaccination of the swine reservoir is the single most powerful tool to reduce viral amplification and subsequent spillover to humans. A landmark study from Vietnam demonstrated that 100% of pigs sampled within urban Can Tho City were seropositive for JEV [3], a stark indicator of the intense viral circulation that occurs in unvaccinated populations. The World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) recognize that vaccinating pigs is a "One Health" strategy that breaks the transmission cycle at its source, protecting both animal health (preventing reproductive failure) and human health.

Currently, the most widely used vaccine in swine is the live-attenuated SA14-14-2 strain, which has been instrumental in controlling JEV in China and parts of Southeast Asia. This vaccine is highly efficacious against genotype III (GIII) strains, inducing robust neutralizing antibody responses and reducing viremia to levels that prevent infection of feeding mosquitoes. However, the emergence of genotype I (GI) as the dominant strain, and the more recent incursion of genotype IV (GIV) into Australia, poses a significant challenge. Phylogenetic analyses have revealed that GI viruses are genetically distinct from the GIII vaccine strain, and studies in China have confirmed that circulating GI-b strains in provinces like Hunan are "distinct" from SA14-14-2 [9]. Furthermore, GIV, which caused the devastating 2022 outbreak in Australian piggeries, is not adequately covered by existing GIII-based vaccines. Experimental data from mouse models show that a chimeric vaccine derived from an Australian GIV isolate provides complete protection against homologous challenge but only partial protection against a related flavivirus like Murray Valley encephalitis virus [17]. This underscores the urgent need for genotype-matched or multivalent vaccines.

Several next-generation vaccine candidates are advancing through the pipeline. Virus-like particle (VLP) vaccines, which are non-replicating and inherently safe, have shown remarkable efficacy. Pigs immunized with a VLP containing the JEV envelope (E) protein produced neutralizing antibodies without the need for adjuvant [66]. Similarly, a lentiviral vector expressing JEV prM and E proteins elicited broad neutralizing antibodies against genotypes 1, 3, and 5 in pigs, offering a potential solution for regions with co-circulating genotypes [39]. A critical caveat to any vaccination program is the phenomenon of antibody-dependent enhancement (ADE). While a lentiviral JEV vaccine induced antibodies that strongly enhanced infection in vitro, the same vaccine was protective in vivo in pigs, suggesting that the presence of neutralizing antibodies outweighs the theoretical risk of ADE [58]. Nevertheless, vaccine development must continue to monitor for this effect, particularly with non-neutralizing or sub-neutralizing antibody titers.

The operational success of a swine vaccination program depends on timing and coverage. Maternal antibodies can interfere with vaccine take, so piglets should be vaccinated after maternal immunity wanes, typically around 8–12 weeks of age. In endemic areas, twice-yearly booster vaccination of breeding sows and boars is recommended to maintain herd immunity and prevent the reproductive losses (abortion, stillbirth, mummification) that characterize JEV infection in naïve breeding stock [13]. National programs, as advocated by FAO, should aim for at least 80% herd immunity to significantly reduce the force of infection within the pig population and, by extension, the risk of human cases.

3. Early Warning Systems: Surveillance and Diagnostics

No control strategy is effective without a robust surveillance system capable of detecting JEV incursion before it amplifies into an epizootic. The gold standard for active surveillance is the use of sentinel pig herds. Cohorts of seronegative, weaned piglets are placed in high-risk areas and bled at regular intervals to detect seroconversion. This approach has been successfully deployed in Cambodia to quantify the force of infection, which was found to be nearly identical (0.06–0.07 per day) in both rural and peri-urban settings, revealing that JEV transmission is not exclusively a rural phenomenon [29]. Sentinel pig serology provides an early warning of virus circulation that precedes human cases by weeks.

Complementing sentinel surveillance, the detection of JEV in environmental samples, particularly piggery effluent, has emerged as a powerful, non-invasive tool. A landmark study from Australia provided the first evidence of JEV RNA in the solid and liquid fractions of piggery effluent, with detection correlating with clinical cases in the herd [8]. This approach, analogous to wastewater-based epidemiology for SARS-CoV-2, allows for the screening of entire farms without the labor and cost of individual animal sampling. Effluent surveillance can serve as an early warning system, flagging farms where JEV may be circulating subclinically, enabling preemptive vector control and movement restrictions.

On the diagnostic front, the rapid differentiation of JEV from other flaviviruses and swine pathogens is critical. A duplex TaqMan real-time RT-PCR assay that simultaneously detects JEV and Getah virus has been validated for use on pig and mosquito samples, offering 100-fold greater sensitivity than conventional PCR and a result in under an hour [60]. This is invaluable for outbreak investigation. Serological diagnosis is complicated by extensive cross-reactivity among flaviviruses (e.g., West Nile, Murray Valley encephalitis, Usutu viruses), especially in regions like Europe or Australia where multiple orthoflaviviruses co-circulate [14]. To overcome this, a blocking ELISA (B-ELISA) using a monoclonal antibody against a JEV-specific epitope has been developed, demonstrating over 94% sensitivity and specificity in pig sera, making it a reliable tool for large-scale surveillance [63]. Virus neutralization tests (VNT) remain the gold standard for confirmation but are labor-intensive.

4. Regulatory and Movement Control Measures

For JEV-naïve regions, such as the Americas and Europe, preventing introduction is paramount. Risk assessments consistently point to the movement of infected pigs, infected mosquitoes (e.g., via ship containers or used tires), and the migration of infected birds as the primary pathways [20, 65]. The 2022 Australian outbreak, which likely originated from an incursion of GIV JEV via infected waterbirds or mosquitoes, subsequently spread to nearly 80 pig farms over 18 months, illustrating how rapidly the virus can disseminate once established in a susceptible pig population [13]. Import regulations should mandate pre-export testing and quarantine for breeding pigs originating from JEV-endemic countries.

During an outbreak, movement controls are essential. Infected premises should be placed under strict quarantine, with no movement of pigs, semen, or manure off-site until viral clearance is confirmed. Ring vaccination (i.e., vaccinating all pigs within a 5–10 km radius of an infected farm) is a proven strategy to create an immune buffer zone, limiting further spread. The high uptake of JEV vaccination (96.6%) among piggery workers in the Murray region of Australia during the public health response demonstrates that coordinated vaccination of at-risk human populations is also a critical component of outbreak control [62].

In conclusion, the multifaceted nature of JEV transmission, vector-borne and direct, with persistent infection in tonsils and feral reservoir hosts, requires a holistic "One Health" response. No single measure is sufficient. The integration of stringent biosecurity (vector control and animal segregation), universal vaccination with updated genotypes, continuous sentinel and effluent surveillance, and rapid regulatory response is the only path toward sustainable control. The recent emergence of JEV in temperate Australia serves as a stark reminder that complacency is not an option; the virus is expanding its geographic range, and the swine industry must be prepared with a robust, science-based defense.

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