Highlands J Virus

Overview and Taxonomy of Highlands J Virus

Highlands J virus (HJV) occupies a distinct and ecologically significant position within the genus Alphavirus, family Togaviridae. It is a mosquito-borne arbovirus that is endemic to North America, where it circulates primarily in an enzootic cycle involving passerine birds and ornithophilic mosquitoes, most notably Culiseta melanura [1, 2]. Despite sharing a common ancestor and ecological niche with the more infamous Eastern Equine Encephalitis virus (EEEV), HJV is generally considered less pathogenic for equids and humans, though it remains a pathogen of veterinary importance, particularly for avian species such as turkeys and certain wild birds [1, 3, 4, 5]. Its classification is deeply rooted in the molecular evolutionary history of the Alphavirus genus, and it is a key representative of the Western Equine Encephalitis (WEE) antigenic complex.

Taxonomic Classification and Serological Position

The taxonomic framework for HJV is well-established. It is classified as a species within the genus Alphavirus, family Togaviridae. Phylogenetically, HJV is a member of the WEE antigenic complex, a group that also includes the eponymous Western Equine Encephalitis virus (WEEV) and the less-studied Fort Morgan virus (FMV) [6]. This complex is unique in that it originated from a historic recombination event between an ancestral EEEV-like virus and a Sindbis (SIN)-like virus, a chimeric genesis that has shaped its genetic and antigenic properties [6]. This recombination likely occurred in North America, endowing the WEE complex viruses with a mosaic genome where the nonstructural protein genes (nsP1-4) are derived from the EEEV lineage, while the structural protein genes (capsid, E3, E2, 6K, E1) originate from the SIN-like ancestor [6].

Serologically, HJV is distinct from EEEV and WEEV but shares cross-reactive epitopes that can complicate diagnostic interpretation. The hemagglutination-inhibition (HI) test and neutralization assays are used to differentiate these viruses, with HJV eliciting specific antibody responses in infected avian and mammalian hosts [7, 8]. The virus was initially described based on isolates from the 1950s and 1960s, with the prototype strain originally obtained from a crested bird in Florida, and subsequent isolations from New Jersey in 1960 further confirming its geographic range [2]. Its taxonomic stability is underscored by genetic data, which show that all known HJV strains descended from a common ancestor, with no evidence of distinct pathotypes that correlate with specific disease outbreaks, such as the 1990 turkey epizootic in Florida [2].

Genetic Structure and Evolutionary Dynamics

HJV possesses a single-stranded, positive-sense RNA genome of approximately 11.7 kb, a canonical structure for alphaviruses that includes a 5' cap, nonstructural protein open reading frames (ORFs) in the 5' two-thirds, and structural protein ORFs in the 3' one-third, followed by a polyadenylated tail. The genetic conservation of HJV is remarkably high across temporal and spatial scales. Studies analyzing 19 strains isolated between 1952 and 1994, focusing on a 1200-nucleotide segment encompassing the E1 gene and the 3' untranslated region (UTR), revealed a slow evolutionary rate estimated at (0.9) to (1.6 \times 10^{-4}) substitutions per nucleotide per year [2]. This rate is comparable to that of EEEV, with which HJV shares primary mosquito and avian hosts, supporting the hypothesis that a prolonged transmission season and vertebrate host mobility are key factors shaping alphavirus evolutionary rates [2].

Phylogenetic reconstruction of these strains suggests the existence of one dominant lineage in North America, with evidence that two or more minor lineages have circulated simultaneously for periods spanning years to decades [2]. Importantly, strains isolated from a horse with encephalitis and those implicated in the turkey outbreak were not phylogenetically distinct from co-circulating strains, indicating that clinical outcome is likely determined by host factors or stochastic events rather than specific viral genetic markers [2]. The 3' UTR, a region often associated with replication and translation efficiency, also shows high conservation, reinforcing the genetic stability of HJV [2].

Evolutionary Origins and Recombination History

The most defining aspect of HJV taxonomy is its origin via recombination, a rare but impactful event in RNA virus evolution. As detailed by Allison et al. (2014), the ancestral recombination event that gave rise to the WEE antigenic complex is a cornerstone of alphavirus evolution [6]. This event resulted in a chimeric virus that subsequently diversified into the modern WEEV, HJV, and FMV. The evidence for this lies in the discordant phylogenies of the nonstructural and structural protein genes; analyses of nsP4 sequences, for example, clearly partition HJV and other WEE complex viruses with EEEV, while structural protein sequences align them with SIN-like viruses [6].

Understanding this recombination is critical for interpreting HJV's biological properties, including its host range and vector preferences. While HJV primarily uses Culiseta melanura as its enzootic vector, it can also be transmitted by bridge vectors such as Aedes and Coquillettidia species, allowing spillover into accidental hosts [1, 7]. The adaptive significance of the chimeric genome is further illuminated by studies of FMV, which possesses a specific nsP4 polymerase mutation that enables it to replicate in Aedes albopictus cells, a vector typically refractory to WEE complex viruses [6]. This suggests that the recombinant backbone of these viruses is not static but continues to evolve, potentially expanding vector range and altering transmission dynamics.

Geographic Distribution and Ecological Niche

The geographic range of HJV is largely contained within the United States, with isolations reported from the Atlantic and Gulf Coast states, the Midwest, and parts of California [1, 2, 9, 5]. It has also been detected in Canada, though its prevalence there is less well characterized [5]. The distribution is closely tied to the range of its primary enzootic vector, Culiseta melanura, which breeds in freshwater swamps and hardwood bogs [7]. Longitudinal studies in central New York have demonstrated significant variation in HJV activity, with epiornitics (avian epidemics) occurring intermittently, as seen in 1986 [7]. These epiornitics are characterized by high antibody titers in sentinel bird species, such as song sparrows (Melospiza melodia), and can occur independently of EEEV activity, indicating distinct ecological drivers [7].

The virus has been isolated from a wide array of hosts, including passerine birds, gallinaceous birds (e.g., ruffed grouse, turkeys), equids, and inadvertently, humans [1, 3, 4, 5]. A recent survey of hunter-harvested ruffed grouse (Bonasa umbellus) in the Upper Midwest from 2018-2022 found HJV RNA in 0.2% (4/1892) of samples, with infected birds showing histologic cardiac lesions consistent with arboviral infection [4]. This highlights that HJV, while often considered less pathogenic than EEEV, can still induce pathology in susceptible wildlife. An interesting case involving a Mississippi sandhill crane (Grus canadensis pulla) illustrates the utility of modern diagnostics; unbiased next-generation sequencing (NGS) identified HJV as the causative agent in the brain of an emaciated bird found after Hurricane Isaac in 2012, underscoring the virus's role in conservation medicine [10].

Public Health and Veterinary Significance

From a public health perspective, the CDC and the World Health Organization (WHO) classify HJV as a potential zoonotic agent, though human cases are exceptionally rare and typically mild or asymptomatic [5]. The virus is included in the list of arboviruses that can cause encephalitis in horses, and surveillance for HJV is often integrated into programs for EEEV and WEEV, as recommended by the World Organisation for Animal Health (WOAH) [1, 9, 5]. In a 1980 survey of Michigan horses, 2.3% (2/87) had neutralizing antibodies to HJV, indicating past exposure [9]. The economic impact of HJV is most notable in the poultry industry, particularly in turkey flocks where infection can lead to decreased egg production and mild neurologic signs, prompting surveillance and biosecurity measures [1, 2].

At the global level, the European Food Safety Authority (EFSA) has identified HJV as one of eight vector-borne disease agents with a potential risk of introduction into the European Union, recognizing its capacity for transmission through competent vectors and its establishment potential [11]. This assessment highlights the importance of continued surveillance and risk modeling for HJV, particularly in the context of climate change and shifting vector distributions [4]. The virus's classification within the WEE complex, its recombinant origin, and its ecological plasticity make it a valuable model for understanding alphavirus evolution and emergence.

Geographic Distribution and Host Range

Highlands J virus (HJV) is a mosquito-borne alphavirus within the Western equine encephalitis (WEE) antigenic complex, endemic to North America [1, 2, 6]. Its geographic distribution and host range are intricately linked to the ecology of its primary mosquito vectors and the population dynamics of its avian amplifying hosts. Understanding these parameters is critical for assessing the risk of spillover into susceptible mammalian hosts, including horses and, rarely, humans, and for predicting the virus’s response to environmental change.

Geographic Distribution

The known geographic range of HJV is predominantly confined to the eastern, southeastern, and midwestern United States, with evidence of enzootic transmission extending into parts of Canada [1, 5]. This distribution is largely coincident with that of its primary enzootic vector, Culiseta melanura, a mosquito species that breeds in freshwater swamps and hardwood bogs. The virus has been isolated or serologically detected across a broad latitudinal gradient, from Florida in the south to the Great Lakes region and New England in the north [1, 2, 5, 7]. Early isolations from the 1950s and 1960s helped establish the virus’s presence along the Atlantic seaboard, and subsequent surveillance has confirmed its persistent circulation in inland foci.

A seminal longitudinal study conducted in central New York (CNY) from 1986 to 1990 documented a significant epiornitic of HJV in 1986, marking only the second known occurrence of the virus in that region [7]. This study, which involved the capture and serological testing of over 6,000 birds from 99 species, provided clear evidence of active HJV transmission in an inland freshwater swamp focus, far from the coastal salt-marsh habitats often associated with eastern equine encephalitis virus (EEEV). The detection of hemagglutination-inhibition (HI) antibodies at titers indicative of recent infection (≥1:160) in multiple avian species confirmed that HJV was not merely a sporadic visitor but was capable of establishing enzootic cycles in these inland ecosystems [7].

Further west, serological surveys have confirmed HJV activity in the Upper Midwest. A study of hunter-harvested ruffed grouse (Bonasa umbellus) in Michigan, Minnesota, and Wisconsin between 2018 and 2022 detected HJV RNA in 0.2% (4/1892) of sampled birds, demonstrating low-level, contemporary circulation in this region [4]. Similarly, a 1980 survey of horses in southwestern Michigan found a low prevalence of neutralizing antibodies to HJV, indicating sporadic exposure in this mammalian host within the state [9]. These findings collectively suggest that HJV occupies a geographic niche that overlaps considerably with EEEV, a pattern supported by molecular evolutionary studies that show both viruses share primary mosquito and avian hosts and exhibit remarkably similar ecological dynamics [2].

The genetic structure of HJV populations across its range is characterized by a dominant, widespread lineage. Phylogenetic analysis of 19 HJV strains isolated between 1952 and 1994, encompassing portions of the E1 gene and the 3' untranslated region, revealed a relatively slow evolutionary rate (0.9–1.6 × 10⁻⁴ substitutions per nucleotide per year) and indicated that all contemporary viruses descended from a common ancestor [2]. While a single dominant lineage appears to be broadly distributed across North America, the study also suggested the presence of two or more minor lineages that may have circulated simultaneously for periods of years to decades [2]. Importantly, strains isolated from a horse with encephalitis and those implicated in a turkey outbreak were not phylogenetically distinct from other strains circulating during the same time periods, suggesting that spillover events are not associated with specific, hypervirulent viral lineages [2].

The potential for HJV to expand its geographic range is a subject of active concern. A risk assessment conducted by the European Food Safety Authority (EFSA) identified HJV as one of eight vector-borne disease agents with an estimated rate of introduction into the European Union (EU) above 0.001 introductions per year [11]. This assessment, which considered the rate of entry, vector transmission, and establishment, highlights the potential for HJV to be translocated via the movement of infected livestock or pets, or through the introduction of infected vectors. The presence of competent mosquito vectors in Europe, such as Culex and Aedes species, could theoretically support local transmission if the virus were introduced [11].

Host Range

The host range of HJV is defined by a classic enzootic cycle involving ornithophilic mosquito vectors and a diverse array of wild passerine birds, which serve as the primary amplifying hosts. Spillover infections can occur in a variety of incidental hosts, including mammals and gallinaceous birds.

Avian Hosts (Primary Amplifying Hosts): Wild birds are the cornerstone of HJV maintenance and amplification. The virus has been isolated from, or antibodies detected in, a wide range of avian species. The longitudinal study in central New York identified song sparrows (Melospiza melodia) as the primary amplifying avian host, based on high rates of virus isolation and seroconversion during epiornitics [7]. Gray catbirds (Dumetella carolinensis) were also identified as a species likely involved in the yearly reintroduction of the virus to the study site, suggesting a potential role for this species in overwintering or long-distance dispersal [7]. Other species with significant seroprevalence included veerys (Catharus fuscescens), blue jays (Cyanocitta cristata), and Florida scrub-jays (Aphelocoma coerulescens) [7, 8].

In Florida, a study of two corvid species revealed that over 15% of both blue jays and Florida scrub-jays had neutralizing antibodies to HJV, confirming frequent exposure in these populations [8]. The detection of HJV in a Mississippi sandhill crane (Grus canadensis pulla) using unbiased next-generation sequencing underscores the virus’s ability to infect even endangered, non-passerine avian species [10]. This particular case, involving an emaciated bird found after Hurricane Isaac, highlights the potential for environmental stressors to exacerbate the impact of HJV infection in vulnerable wildlife populations [10].

The ruffed grouse, a gallinaceous bird, also serves as a sentinel species for HJV activity. The detection of HJV RNA in 0.2% of hunter-harvested grouse in the Upper Midwest, along with histologic cardiac lesions consistent with arboviral infection in approximately half of the infected birds, suggests that HJV can cause subclinical to mild pathology in this species [4]. The low prevalence in grouse, compared to passerines, likely reflects their different habitat preferences and lower exposure to the primary enzootic vector, Culiseta melanura.

Mammalian Hosts (Incidental/Spillover Hosts): Horses are the most clinically relevant incidental mammalian host for HJV. The virus is known to cause encephalitis in equids, although it is considered less pathogenic than EEEV [1, 5]. Serological surveys have confirmed low-level exposure in horses in endemic areas, such as the 1980 Michigan study where a low prevalence of neutralizing antibodies was detected [9]. A strain of HJV isolated from a horse suffering from encephalitis was found to be phylogenetically indistinguishable from other contemporaneous strains, indicating that equine virulence is not a stable genetic trait of a specific lineage [2]. The risk to horses is primarily driven by the activity of bridge vectors, such as Aedes and Coquillettidia species, that feed on both birds and mammals, facilitating spillover from the enzootic cycle.

Human infection with HJV is considered rare, but the virus is recognized as a zoonotic agent capable of causing febrile illness and, in rare instances, encephalitis [1, 5]. The true incidence of human infection is likely underreported due to the non-specific nature of symptoms and the lack of routine diagnostic testing. The risk to humans is similarly mediated by bridge vectors.

Mosquito Vectors: The geographic distribution and host range of HJV are fundamentally shaped by its mosquito vectors. The primary enzootic vector is Culiseta melanura, a bird-biting mosquito that maintains the virus in an avian cycle within swampy habitats [1, 2]. This vector’s strict ornithophily explains the high seroprevalence in passerine birds and the relative rarity of spillover into mammals. However, several mosquito species are capable of acting as bridge vectors. Aedes canadensis, Aedes sollicitans, Coquillettidia perturbans, and various Culex species have been implicated in transmitting HJV from birds to mammals [1]. Experimental studies have demonstrated that Aedes albopictus and Culex quinquefasciatus can acquire and transmit HJV, with the latter being used as a biological sampling tool to measure viremia in experimentally infected chicks, hamsters, and house sparrows [12]. The competence of these bridge vectors is a critical determinant of the risk of epizootic and epidemic transmission.

The host range of HJV is thus a complex interplay between a highly competent enzootic cycle in passerine birds, a diverse suite of mosquito vectors with varying host preferences, and a range of incidental mammalian hosts that are exposed when bridge vector activity is high. The virus’s broad avian host range and the widespread distribution of its vectors ensure its persistence across a large geographic area, while the potential for introduction into new regions, as highlighted by the EFSA risk assessment, warrants continued surveillance [11].

Vector Ecology and Transmission Dynamics

Highlands J virus (HJV) is an arthropod-borne virus (arbovirus) belonging to the genus Alphavirus, family Togaviridae, and is a member of the western equine encephalitis (WEE) antigenic complex [1, 2, 5]. Its ecology is inextricably linked to a complex interplay between competent mosquito vectors, susceptible avian reservoir hosts, and the environmental conditions that facilitate their interaction. Understanding the vector ecology and transmission dynamics of HJV is fundamental to predicting its geographic distribution, assessing its epizootic potential, and implementing effective surveillance and control strategies. The following analysis provides an exhaustive, deep examination of these critical components.

The Enzootic Cycle: Primary Mosquito Vectors and Avian Reservoirs

The enzootic maintenance and amplification of HJV are predominantly driven by a highly specialized transmission cycle involving mosquitoes of the genus Culiseta and a diverse array of passerine birds. The primary enzootic vector is widely recognized as Culiseta melanura, a mosquito species with a pronounced ornithophilic feeding preference, meaning it feeds almost exclusively on birds [1, 2, 5]. This species thrives in freshwater swamp habitats, particularly those dominated by red maple and white cedar, which provide the shaded, organically rich aquatic environments necessary for larval development. The strong ornithophily of Cs. melanura effectively restricts the virus's amplification to the avian population, creating a highly efficient and stable enzootic cycle.

The role of Cs. melanura in HJV transmission has been extensively documented. This mosquito is not only a competent vector but also serves as the primary bridge between the virus and its avian hosts. The virus replicates within the mosquito midgut following an infectious blood meal, disseminates to the salivary glands, and is subsequently transmitted to a naive bird during a subsequent feeding. This vector-virus-host relationship is so well-established that the presence of HJV activity in an area is often inferred from the abundance and infection rates of Cs. melanura [1, 7]. A longitudinal study in central New York (CNY) from 1986 to 1990, a classic inland focus of arboviral activity, provided critical insights into this dynamic. The study identified a single epiornitic (a widespread outbreak in a non-human animal population) of HJV in 1986, during which song sparrows (Melospiza melodia) were identified as the primary amplifying avian host [7]. The serological data from this study indicated that HJV infection was not epidemiologically linked to eastern equine encephalitis virus (EEEV) activity, suggesting independent, though ecologically parallel, transmission cycles [7].

The avian host range for HJV is broad, reflecting the feeding habits of Cs. melanura. Numerous passerine species, including blue jays (Cyanocitta cristata), gray catbirds (Dumetella carolinensis), and various sparrows, are susceptible to infection and can develop viremias of sufficient magnitude to infect feeding mosquitoes [7, 8]. A study of wild jays in Florida found that over 15% of blue jay and Florida scrub-jay samples had neutralizing antibodies to HJV, confirming frequent natural exposure [8]. Importantly, the viremia profiles in house sparrows (Passer domesticus) experimentally infected with HJV were successfully characterized using a novel "biological syringe" technique, where Culex quinquefasciatus and Aedes albopictus mosquitoes were used to draw blood. This study demonstrated that sparrows with virus titers exceeding 10⁵ plaque-forming units (PFU)/mL were capable of generating robust viremias, a level more than sufficient to infect a high proportion of feeding vectors [12]. The high viremia achieved in these avian hosts is a key determinant of the transmission intensity, as it directly correlates with the probability of vector infection.

Epizootic Transmission and the Role of Bridge Vectors

While the enzootic cycle is maintained by Cs. melanura, epizootic transmission, where the virus spills over into incidental hosts such as horses, turkeys, and potentially humans, requires the involvement of bridge vectors. These are mosquito species that have a broader host range, feeding on both birds and mammals. When the enzootic cycle amplifies HJV to a high prevalence in the bird population, bridge vectors become infected and can subsequently transmit the virus to susceptible mammalian or domestic avian hosts [1, 2, 5]. Several mosquito species within the Aedes, Coquillettidia, and Culex genera are implicated as potential bridge vectors. For instance, Aedes canadensis and Coquillettidia perturbans are known to feed on both birds and mammals and have been found to harbor HJV in nature [1, 5].

The significance of bridge vectors was highlighted by the isolation of HJV from a horse suffering encephalitis, as noted in the genetic study of HJV strains [2]. Phylogenetic analysis of this equine isolate showed it was not distinct from strains circulating in birds during the same period, demonstrating that the spillover event did not require a genetically specialized variant [2]. Similarly, HJV has been implicated in outbreaks in domestic turkeys, causing decreased egg production and other clinical signs [1, 2]. The role of bridge vectors in these epizootics underscores the potential for HJV to impact agricultural operations and, though rarely, public health, as the virus has been listed among the zoonotic, encephalitic alphaviruses capable of causing febrile illness and, in severe cases, encephalitis in humans [5, 13].

Genetic Stability and Transmission Dynamics

One of the most remarkable features of HJV ecology, and one that deeply informs its transmission dynamics, is its profound genetic conservation. A landmark study examining the molecular evolution of 19 HJV strains isolated between 1952 and 1994 revealed a very slow evolutionary rate, estimated at only 0.9–1.6 × 10⁻⁴ substitutions per nucleotide per year [2]. This rate is remarkably similar to that calculated for eastern equine encephalitis virus (EEEV), a virus with which HJV shares its primary mosquito vector (Cs. melanura), avian hosts, and geographic distribution [2]. This genetic stability suggests a highly stable and efficient transmission system that exerts little selective pressure for rapid viral change.

The phylogenetic analysis from this work indicated that all HJV strains descended from a common ancestor and that a single dominant lineage has persisted across North America for decades [2]. While minor co-circulating lineages have been identified, they appear to exist for only a few years to a few decades before being outcompeted. This pattern contrasts markedly with other RNA viruses that exhibit high mutation rates and rapid genetic drift. The authors of the study hypothesized that the duration of the transmission season and the mobility of vertebrate hosts are critical factors influencing this slow evolutionary rate. A prolonged transmission season, typical in the temperate regions where HJV circulates, allows for multiple rounds of amplification without necessitating rapid adaptation, while the high mobility of passerine birds promotes extensive viral mixing and the maintenance of a single, dominant lineage across the continent [2].

Spatial and Temporal Patterns of Transmission

The transmission of HJV is characterized by distinct spatial and temporal patterns that are governed by vector population dynamics, avian behavior, and environmental factors. In temperate North America, transmission is intensely seasonal, peaking during the late summer and early fall (July to September) when vector populations are at their highest and the amplification cycle is fully engaged [1, 7]. This pattern was clearly observed in the CNY study, where the 1990 EEEV epiornitic extended from mid-July to the end of September, and the earlier HJV epiornitic followed a similar temporal trajectory [7].

Geographically, the distribution of HJV is closely tied to the availability of suitable freshwater swamp habitats for Cs. melanura. This creates a patchy distribution of transmission foci, such as those well-documented in central New York (including the Oneida Lake and Cicero Swamp areas), Florida, the Gulf Coast states, and the Upper Midwest [1, 4, 7, 8]. Recently, the detection of HJV infection in hunter-harvested ruffed grouse (Bonasa umbellus) in Michigan, Minnesota, and Wisconsin from 2018–2022 demonstrated that this virus continues to circulate enzootically in the Upper Midwest [4]. Of the 1,892 grouse tested, 0.2% (4/1892) were positive for HJV, and approximately half of all arbovirus-infected birds (including those with WNV and EEEV) had histologic cardiac lesions consistent with arboviral infection, suggesting that even low-prevalence infections can have population-level impacts on susceptible species [4]. This finding underscores the importance of continued surveillance in wildlife, particularly in the context of climate change, which is predicted to alter virus-vector-host dynamics and potentially expand the geographic range of competent vectors [4].

Further evidence of the enzootic presence of HJV on a broader geographic scale comes from serosurveys. A study in southwestern Michigan in 1980 detected low-level plaque-reduction neutralizing antibody prevalence for HJV in horses, confirming past or current exposure in that region [9]. Even more critically, a comprehensive risk assessment by the European Food Safety Authority (EFSA) identified Highlands J virus among the vector-borne disease agents with an estimated rate of introduction into the European Union above 0.001 introductions per year, classifying it as a potential emerging threat for the continent [11]. This assessment underscores that the vector ecology and transmission dynamics of HJV are not merely of academic interest but are of direct relevance to global biosecurity and animal health policy, consistent with the World Organisation for Animal Health (WOAH) guidelines for assessing the risk of transboundary animal diseases.

Overwintering and Persistence Mechanisms

A critical, unresolved question in the ecology of HJV is how the virus persists through the winter months when adult mosquito activity ceases in temperate regions. The genetic evidence for a single, dominant lineage suggests a mechanism for long-term maintenance, but the exact process remains elusive. Several hypotheses have been proposed, including:

1. Vertical transmission: The transovarial transmission of HJV from an infected female mosquito to her offspring. If viable, this would allow the virus to survive in the egg stage through the winter and be amplified by the next generation of larvae and adults. While documented for some other arboviruses, the evidence for vertical transmission of HJV is not definitive.
2. Overwintering in adult mosquitoes: Some mosquito species, particularly Culex mosquitoes, enter a state of reproductive diapause during the winter, sheltering in protected microhabitats. It is possible that a small number of infected, diapausing females could survive the winter and reinitiate transmission in the spring.
3. Reintroduction by migratory birds: Given the mobility of avian hosts and the long-distance migration patterns of many passerine species, it is plausible that HJV is reintroduced annually to northern foci from more southern enzootic areas where transmission occurs year-round. The lack of virus isolation from returning adult birds in the CNY study, however, suggests that this may not be the primary mechanism for all inland foci [7].

The existence of these potential overwintering mechanisms, combined with the virus's demonstrated genetic and ecological resilience, makes HJV a persistent and challenging pathogen to manage. The ongoing interplay between the highly specialized Cs. melanura-bird cycle and the broader, opportunistic bridge vector spillover defines the complex and dynamic nature of HJV transmission, a system whose intricacies continue to be unraveled by researchers and are crucial for informing risk assessment frameworks, such as those used by the CDC for other arboviruses.

Molecular Characterization and Genetic Diversity

Taxonomic Position and Recombinant Origin

Highlands J virus (HJV) is a member of the genus Alphavirus within the family Togaviridae, and it occupies a distinctive position as one of only three North American representatives of the Western equine encephalitis (WEE) antigenic complex, alongside Western equine encephalitis virus (WEEV) and Fort Morgan virus (FMV) [6]. The molecular architecture of HJV is fundamentally shaped by a singular, ancient recombination event that occurred between an ancestral Eastern equine encephalitis virus (EEEV) and a Sindbis (SIN)-like virus, giving rise to a chimeric alphavirus that subsequently diversified into the extant WEE complex viruses [6]. This recombination event, which likely transpired in North America, endowed HJV with a mosaic genome that combines elements from two distinct alphavirus lineages, a feature that continues to influence its genetic identity, host range, and evolutionary trajectory [6]. The virus is classified as a biosafety level 3 (BSL-3) agent by the CDC and is recognized by the World Organisation for Animal Health (WOAH) as a notifiable arboviral pathogen due to its capacity to cause encephalitic disease in equids and gallinaceous birds, underscoring its significance in both veterinary and public health contexts [1, 3, 5].

Genome Organization and Structural Features

The HJV genome comprises a single-stranded, positive-sense RNA molecule of approximately 11.7 kilobases, organized in a canonical alphavirus architecture. The 5' two-thirds of the genome encode four nonstructural proteins (nsP1–nsP4) that constitute the viral replicase complex, while the 3' one-third encodes the structural proteins, capsid (C), E3, E2, 6K, and E1, translated from a subgenomic 26S mRNA [10, 2]. The complete genome sequence of HJV, first elucidated from a 2012 isolate obtained from the brain of an endangered Mississippi sandhill crane (Grus canadensis pulla) using unbiased next-generation sequencing (NGS) on the Illumina MiSeq platform, revealed the canonical alphavirus genome organization with high fidelity [10]. This landmark study demonstrated the power of NGS for rapid pathogen discovery and characterization, as the crane isolate had tested negative for a panel of known arboviruses using conventional molecular assays, yet whole-genome sequencing unequivocally identified it as HJV [10]. The 3' untranslated region (UTR) of HJV, which spans approximately 300 nucleotides, contains conserved sequence elements and secondary structures critical for RNA replication, translation, and packaging, and its sequence has been instrumental in phylogenetic analyses and evolutionary rate estimations [2].

Patterns of Genetic Diversity and Evolutionary Dynamics

The genetic diversity of HJV is remarkably constrained relative to many other RNA viruses, a phenomenon that has been systematically documented through molecular evolutionary studies spanning over four decades. Cilnis et al. (1996) conducted a seminal investigation of 19 HJV strains isolated between 1952 and 1994, sequencing approximately 1200 nucleotides encompassing portions of the E1 glycoprotein gene and the 3' UTR [2]. Their analysis revealed an exceptionally slow evolutionary rate, estimated at 0.9–1.6 × 10⁻⁴ substitutions per nucleotide per year, which is among the lowest documented for any alphavirus [2]. This rate is comparable to that observed for EEEV, with which HJV shares primary mosquito vectors (primarily Culiseta melanura), avian amplifying hosts, and overlapping geographic distribution across North America [2]. The authors hypothesized that the short duration of the arboviral transmission season in temperate regions, typically limited to late summer and early autumn, constrains the number of viral replication cycles per year, thereby reducing the annual accumulation of mutations [2]. Furthermore, the high mobility of avian hosts, which can rapidly disperse viruses across vast distances, may homogenize the viral population and limit the opportunities for geographic isolation and genetic drift [2].

Phylogenetic reconstruction of HJV strains consistently supports a monophyletic origin, with all isolates descending from a common ancestor [2]. The tree topology indicates the presence of a single dominant lineage circulating across North America, but also reveals evidence for the coexistence of two or more minor lineages over periods ranging from years to several decades [2]. Importantly, strains isolated from a horse suffering encephalitis and those implicated in a turkey outbreak were not phylogenetically distinct from contemporaneous isolates obtained from other hosts or locations, suggesting that HJV does not segregate into host-specific or disease-specific clades [2]. This genetic conservation implies that the virus maintains a stable consensus sequence across its enzootic cycle, with no evidence for the emergence of hypervirulent or host-adapted variants, at least within the temporal and geographic scope of the strains analyzed.

Molecular Characterization through Next-Generation Sequencing

The application of unbiased NGS has revolutionized the molecular characterization of HJV, enabling the rapid and definitive identification of the virus from complex clinical specimens without a priori knowledge of the pathogen. The 2012 Mississippi sandhill crane case exemplifies this paradigm: post-mortem examination revealed trichostrongyliasis as a possible cause of death, but the isolation of a togavirus-like agent from brain tissue prompted further investigation [10]. After individual RT-PCR assays for EEEV, WEEV, and Venezuelan equine encephalitis virus (VEEV) returned negative results, total nucleic acid extracted from the viral isolate was subjected to Illumina MiSeq sequencing, yielding whole-genome coverage that confirmed HJV as the etiologic agent [10]. Phylogenetic analysis of the crane isolate placed it firmly within the established HJV clade, with high bootstrap support, and demonstrated that it shared >99% nucleotide identity with reference strains [10]. This approach not only provided a definitive diagnosis but also generated a complete genome sequence that can be used for future comparative studies, molecular epidemiology, and vaccine development.

Genetic Conservation and Implications for Epidemiology

The genetic stability of HJV has profound implications for its epidemiology and for the design of diagnostic and control strategies. The low evolutionary rate and lack of antigenic drift suggest that serological assays based on prototype strains should remain effective for surveillance and diagnosis across broad geographic and temporal scales [2, 7]. Hemagglutination-inhibition (HI) and plaque-reduction neutralization tests (PRNT) have been used extensively to detect HJV-specific antibodies in wild bird populations, with studies in New York, Florida, and the Upper Midwest consistently demonstrating seroprevalence rates that reflect enzootic transmission [7, 8]. For example, a longitudinal study in central New York from 1986 to 1990 detected an epiornitic of HJV in 1986, with infected birds mounting HI antibody titers ≥1:160 indicative of recent infection [7]. Similarly, serosurveys of blue jays (Cyanocitta cristata) and Florida scrub-jays (Aphelocoma coerulescens) at Archbold Biological Station in Florida revealed that >15% of individuals in both species had neutralizing antibodies to HJV, confirming widespread exposure [8]. The genetic conservation of HJV ensures that these serological tools remain reliable over time, a critical feature for long-term surveillance programs that monitor arboviral activity in sentinel avian populations.

Recombination and Host Range Considerations

While HJV itself is a product of an ancient recombination event, contemporary evidence for ongoing recombination within the WEE complex is limited. The recombination that gave rise to HJV, WEEV, and FMV involved the acquisition of the structural protein genes from an EEEV-like ancestor and the nonstructural protein genes from a SIN-like virus, a chimeric configuration that is now fixed in the genomes of all three viruses [6]. This recombination event likely conferred a selective advantage by expanding the host range or altering vector specificity, as evidenced by the ability of FMV to replicate in Aedes albopictus cells, a vector species that is normally refractory to infection, due to a specific mutation in the nsP4 polymerase [6]. For HJV, the primary enzootic vector is Culiseta melanura, a mosquito species that feeds preferentially on birds, but the virus has been isolated from a wide range of mosquito genera, including Culex, Aedes, and Anopheles, suggesting a degree of vector plasticity that may be rooted in its recombinant genome [1, 3, 13]. The genetic conservation of HJV across diverse vectors and hosts indicates that the virus has achieved a stable equilibrium with its transmission cycle, with no evidence for adaptive evolution driving host switching or virulence modulation [2, 6].

Population Structure and Phylogeography

The phylogeographic structure of HJV is characterized by a lack of strong geographic clustering, consistent with the hypothesis that avian hosts facilitate long-distance dispersal and genetic mixing. Strains isolated from the Atlantic Coast, Gulf Coast, and inland foci in the Upper Midwest and New York exhibit high sequence similarity, with no evidence for geographically distinct lineages [2, 4]. A recent study of arboviruses in ruffed grouse (Bonasa umbellus) from Michigan, Minnesota, and Wisconsin detected HJV in 4 of 1,892 hunter-harvested birds (0.2%), and the partial sequences obtained from these isolates clustered with reference strains from other regions, further supporting the notion of a panmictic population [4]. The absence of phylogeographic structure simplifies risk assessment and surveillance, as a single diagnostic assay or vaccine candidate is likely to be effective across the entire North American range of HJV. However, the potential for future emergence of divergent lineages cannot be discounted, particularly in the context of climate change, which may alter vector distribution, transmission season length, and host migration patterns, thereby creating new opportunities for viral evolution [4, 11]. The European Food Safety Authority (EFSA) has identified HJV as one of eight vector-borne disease agents with a non-negligible risk of introduction into the European Union, highlighting the need for continued molecular surveillance to detect any incipient genetic diversification that might accompany geographic expansion [11].

Molecular Pathogenesis and Clinical Manifestations

Molecular Pathogenesis

Highlands J virus (HJV) is a mosquito-borne alphavirus belonging to the Western Equine Encephalitis (WEE) antigenic complex within the family Togaviridae [1, 6]. Its molecular pathogenesis is intrinsically linked to its unique evolutionary origin, genomic organization, and the complex interactions it establishes with both avian reservoir hosts and accidental mammalian hosts, including equids and humans. The virus possesses a single-stranded, positive-sense RNA genome of approximately 11–12 kb, which is organized into two main open reading frames (ORFs). The 5′ two-thirds of the genome encodes four nonstructural proteins (nsP1–nsP4) that form the viral replicase complex, while the 3′ one-third encodes the structural proteins, capsid, E3, E2, 6K, and E1, which are translated from a subgenomic 26S RNA [5, 14]. The replication strategy of HJV follows the canonical alphavirus model: upon receptor-mediated endocytosis, the viral genome is released into the cytoplasm, where the nonstructural proteins are translated directly, facilitating the synthesis of a negative-strand RNA intermediate that serves as a template for both genomic and subgenomic RNA production [14].

A defining feature of HJV molecular pathogenesis is its recombinant ancestry. Phylogenetic and evolutionary analyses have demonstrated that the WEE antigenic complex viruses, including HJV, arose from a recombination event between an eastern equine encephalitis virus (EEEV)-like ancestor and a Sindbis (SIN)-like virus [6]. This chimeric origin has endowed HJV with a distinct genotypic and phenotypic profile compared to its parent viruses. The nsP4 polymerase of HJV, for instance, carries specific mutations that influence vector host range, as seen in the related Fort Morgan virus, where a single nsP4 mutation allowed replication in normally refractory Aedes albopictus cells [6]. Such genetic determinants likely contribute to the relatively broad mosquito vector competence of HJV, which includes both Culiseta melanura and various Culex and Aedes species, facilitating its enzootic transmission cycles [1, 6]. Molecular evolutionary studies have revealed that HJV exhibits a remarkably slow evolutionary rate, estimated at 0.9–1.6 × 10⁻⁴ substitutions per nucleotide per year, a rate comparable to that of EEEV, with which it shares primary mosquito and avian hosts [2]. This genetic conservation suggests that HJV occupies a stable ecological niche and has not undergone recent adaptive radiation, although at least three distinct lineages appear to have circulated simultaneously in North America over the past several decades [2].

Host Range, Cell Tropism, and Viral Entry

The molecular determinants of HJV host range reflect its adaptation to both avian and mammalian systems. In avian hosts, which serve as the primary amplifying reservoir, HJV establishes robust viremia. Experimental infections in house sparrows (Passer domesticus) and chicks (Gallus gallus) have demonstrated that viremia titers readily exceed 5.0 log₁₀ plaque-forming units (pfu)/mL of serum, a threshold considered sufficient for infecting feeding mosquitoes and maintaining enzootic transmission [12]. The ability of HJV to replicate to such high titers in birds without causing overt mortality in most passerine species is a hallmark of its coevolution with these reservoir hosts. However, infection is not always benign in birds; clinical and subclinical pathology has been documented. Among hunter-harvested ruffed grouse (Bonasa umbellus), approximately half of the birds found to be infected with HJV (detected in 4 of 1,892 samples; 0.2%) exhibited histologic cardiac lesions consistent with arboviral infection, including myocarditis and lymphoplasmacytic infiltration [4]. This finding, albeit from a low prevalence, indicates that HJV can cause significant tissue damage in certain avian species, potentially contributing to population-level impacts, especially in birds already stressed by other factors such as helminthiasis or habitat degradation [10, 4]. The identification of HJV in the brain of an emaciated Mississippi sandhill crane (Grus canadensis pulla) using unbiased next-generation sequencing (NGS) further demonstrates the neurotropic potential of the virus in a non-reservoir avian host, suggesting that severe neurological disease can occur in susceptible species [10].

In mammalian hosts, the molecular pathogenesis of HJV is characterized by a marked reduction in replication efficiency compared to avian cells, consistent with its classification as an arbovirus primarily adapted to birds. Nevertheless, HJV is capable of infecting equids and, rarely, humans, causing neurological disease. In horses, HJV has been implicated in cases of encephalitis, though such instances are far less frequent than those caused by EEEV or WEEV [2, 5]. Strains isolated from a horse suffering encephalitis were not phylogenetically distinct from contemporaneous avian or mosquito isolates, indicating that neuroinvasiveness in mammals is likely a general property of the virus rather than a trait encoded by a specific subset of strains [2]. The virus is believed to enter the central nervous system (CNS) via hematogenous spread following amplification in peripheral tissues, a pathway common to other encephalitic alphaviruses [5]. Upon CNS entry, HJV likely infects neurons and glial cells, triggering a potent inflammatory response that contributes to the neuropathology, though detailed studies on HJV-specific neuropathogenesis in mammalian models remain limited. The capsid protein and the E2 glycoprotein are critical for cell attachment and entry; the E2 glycoprotein interacts with cell surface receptors, which for alphaviruses include heparan sulfate, integrins, and the laminin receptor, though the precise receptors for HJV have not been fully characterized [5, 14]. Following entry, the virus subverts host cell machinery for replication and translation, and like other alphaviruses, HJV is known to induce host cell shutoff via inhibition of cellular transcription and translation, facilitating viral protein synthesis and evading innate immune responses [14].

Clinical Manifestations in Avian Hosts

The clinical spectrum of HJV infection in birds ranges from asymptomatic, subclinical infection to fatal neurological and systemic disease, depending on the species and age of the host. In the primary amplifying avian hosts, such as song sparrows, gray catbirds, and various passerines, HJV infection typically produces no obvious clinical signs, allowing the virus to circulate enzootically undetected [7, 8]. Serological surveys have revealed that antibodies to HJV are common in wild bird populations; for example, over 15% of both blue jays (Cyanocitta cristata) and Florida scrub-jays (Aphelocoma coerulescens) sampled at Archbold Biological Station in Florida had neutralizing antibodies to HJV, indicating widespread exposure [8]. Infected birds mount a robust humoral immune response, with hemagglutination-inhibition (HI) antibody titers often ≥1:160, indicative of recent infection [7]. Notably, an epiornitic of HJV was documented in central New York in 1986, during which HI titers in captured birds were significantly elevated compared to non-epiornitic periods, highlighting the ability of the virus to undergo periodic amplification cycles [7].

In contrast, severe clinical disease is observed in certain gallinaceous birds and in endangered species. The case of the Mississippi sandhill crane demonstrated that HJV can cause progressive neurological signs, including ataxia, weakness, and eventually death, with the virus isolated directly from the brain [10]. Similarly, the detection of HJV in ruffed grouse, a species of conservation and sporting interest, and the association with cardiac histopathology suggest that infection can lead to reduced fitness and increased mortality, particularly when birds are concurrently infected with other pathogens or facing environmental stressors such as severe weather events or habitat loss [4, 15]. The pathogenesis in these cases likely involves direct viral cytopathology in target organs (brain, heart) compounded by the host's inflammatory response. Given the recognized impact of alphaviruses on wild bird populations, the World Organisation for Animal Health (WOAH) and the International Committee on Taxonomy of Viruses (ICTV) recognize HJV as a significant avian pathogen, warranting continued surveillance, especially under changing climatic conditions that may alter vector-host dynamics [4, 11, 15].

Clinical Manifestations in Mammalian Hosts (Equids and Humans)

In mammals, HJV is considered an incidental pathogen, causing sporadic cases of febrile illness and neurological disease. The virus is a rare, but recognized, cause of encephalitis in horses, with serosurveys indicating low but consistent exposure rates. In a 1980 study of Michigan horses, antibodies to HJV were detected in a small percentage of animals, confirming that equids are exposed through mosquito bites within enzootic foci [9]. Clinical equine HJV infection typically manifests as a mild febrile illness, but can progress to encephalomyelitis, characterized by fever, lethargy, ataxia, head pressing, seizures, and recumbency [5]. The case fatality rate is believed to be lower than that associated with EEEV; however, definitive clinical differentiation from other alphaviral encephalitides is impossible without laboratory confirmation. The molecular basis for the reduced neurovirulence in horses compared to EEEV likely involves differences in the ability of the virus to replicate in equine endothelial cells and to traverse the blood-brain barrier, though specific mechanisms remain to be elucidated.

Human infection with HJV is exceptionally rare, with only a handful of documented cases. The virus is considered to have zoonotic potential, and it is classified by the World Health Organization (WHO) and the US Centers for Disease Control and Prevention (CDC) among the encephalitic alphaviruses that can cause severe neurological disease in humans, though the risk is far lower than that posed by EEEV or VEEV [5, 11]. When human disease occurs, it typically presents as an acute febrile illness with headache, myalgia, and arthralgia, which may progress to meningitis or encephalitis, especially in immunocompromised individuals or the very young [5]. The lack of suspected or confirmed human cases may reflect a combination of low vector competence for anthropophilic mosquitoes, low viremia in mammalian hosts (limiting mosquito-to-human transmission), and a high degree of underdiagnosis. Indeed, HJV is rarely included in routine arboviral diagnostic panels, and the clinical syndrome is indistinguishable from other causes of viral encephalitis [10, 5]. The European Food Safety Authority (EFSA) has identified HJV as one of eight vector-borne disease agents with an estimated rate of introduction into the European Union above 0.001 introductions per year, highlighting its potential as an emerging zoonotic threat, particularly in the context of climate change and expanding vector ranges [11].

Immune Response and Viral Evasion

The host immune response to HJV infection is critical in determining the outcome of infection. In birds, the rapid production of HI and neutralizing antibodies is associated with clearance of viremia and protection from reinfection [7, 8]. These antibodies are directed primarily against the E1 and E2 envelope glycoproteins, which are the major targets of the humoral immune response. Neutralizing antibodies are thought to prevent viral attachment and entry into target cells, thereby limiting the spread of infection. The presence of pre-existing antibodies in adult birds, likely acquired through prior infection, correlates with lower viremia and reduced mosquito infection rates, underlining the importance of herd immunity in modulating enzootic transmission [7, 8]. In mammals, the innate immune response, particularly the type I interferon (IFN) system, is a major hurdle for alphavirus replication. The viral nonstructural proteins, particularly nsP2, have been shown in other alphaviruses to antagonize IFN signaling by suppressing host transcription and promoting degradation of key signaling molecules [5, 14]. While specific studies on HJV immune evasion are lacking, it is likely that similar mechanisms are operative, allowing the virus to replicate to sufficient titers to cause systemic disease. The long-term humoral immunity appears to be robust, and seropositive birds and mammals are considered immune to subsequent infection, which is a key factor in the dynamics of virus circulation [7, 8]. Given the slow evolutionary rate of HJV, antigenic drift is minimal, suggesting that current serological assays and potential vaccines would remain effective over extended periods [2].

Diagnostic Approaches and Surveillance

The diagnostic landscape for Highlands J virus (HJV) is defined by the pathogen's status as an enzootic alphavirus that circulates primarily within avian reservoir hosts and ornithophilic mosquito vectors, occasionally spilling over into domestic poultry, equids, and other incidental hosts. The diagnostic challenge is compounded by the fact that HJV is antigenically cross-reactive with other members of the Western equine encephalitis (WEE) antigenic complex, particularly Eastern equine encephalitis virus (EEEV), with which it shares ecological niches and clinical presentations in susceptible species. Consequently, diagnostic approaches must be carefully selected and interpreted within the context of host species, geographic location, and temporal dynamics of arbovirus transmission.

Serological Approaches: Hemagglutination Inhibition and Neutralization Assays

Serological surveillance has historically formed the backbone of HJV detection in wild and domestic animal populations. The hemagglutination inhibition (HI) assay has been extensively employed for monitoring antibody prevalence in avian hosts, leveraging the ability of alphavirus structural proteins to agglutinate erythrocytes. In longitudinal studies conducted in central New York, HI titers of ≥1:160 were used as a threshold indicative of recent HJV infection, and these titers demonstrated statistically significant elevations during epiornitic periods compared to inter-epiornitic intervals [7]. This serological approach has been instrumental in identifying primary amplifying hosts, with song sparrows (Melospiza melodia) and gray catbirds (Dumetella carolinensis) emerging as key species in the enzootic cycle based on HI seroprevalence data [7]. However, the HI assay suffers from inherent limitations in specificity due to cross-reactivity between alphaviruses, a problem that is particularly acute in regions where EEEV and HJV circulate sympatrically.

Plaque reduction neutralization tests (PRNT) offer superior specificity for differentiating HJV from other alphaviruses and have been applied in both avian and mammalian surveillance. In Florida scrub-jays (Aphelocoma coerulescens) and blue jays (Cyanocitta cristata), combined use of HI and neutralization assays revealed that 15% of sampled individuals harbored neutralizing antibodies against HJV, whereas EEEV seroprevalence reached 31%, demonstrating the ability of these assays to discriminate between closely related alphaviruses [8]. Similarly, serosurveillance of horses in southwestern Michigan during 1980 employed PRNT to detect low-level HJV antibody prevalence alongside antibodies to Cache Valley, Jamestown Canyon, and other arboviruses [9]. The specificity of neutralization assays makes them the gold standard for confirming HJV exposure in epidemiological studies, though they are labor-intensive and require biosafety level 3 containment for live virus work.

Molecular Diagnostics: RT-PCR and Quantitative Real-Time Approaches

The development of reverse transcription polymerase chain reaction (RT-PCR) assays has revolutionized HJV diagnostics, enabling rapid, sensitive, and specific detection of viral RNA from clinical specimens, vector pools, and environmental samples. Standard RT-PCR targeting conserved regions of the E1 envelope glycoprotein gene and the 3' untranslated region (UTR) has been successfully employed for genotyping HJV strains isolated over a 42-year period, from 1952 to 1994, allowing for phylogenetic analyses that revealed a remarkably slow evolutionary rate of 0.9–1.6 × 10⁻⁴ substitutions per nucleotide per year [2]. The genetic conservation of HJV across decades and geographic regions has important implications for molecular diagnostic design, as primer binding sites in conserved genomic regions are likely to remain stable across diverse isolates.

Quantitative real-time RT-PCR (qRT-PCR) has become the modality of choice for high-throughput surveillance applications, particularly in wildlife disease monitoring programs. In a large-scale surveillance study of ruffed grouse (Bonasa umbellus) across Michigan, Minnesota, and Wisconsin, qRT-PCR detected HJV in 0.2% of hunter-harvested birds (4 out of 1,892), with co-infections with West Nile virus and EEEV also identified [4]. The ability to perform simultaneous multiplex detection of multiple arboviruses from a single tissue sample makes qRT-PCR invaluable for surveillance networks that must contend with overlapping arbovirus activity. Importantly, the study also correlated molecular detection with histopathologic findings; approximately half of the HJV-positive birds exhibited characteristic cardiac lesions, suggesting that even subclinical infections may induce pathologic changes detectable by routine histopathology [4]. This integration of molecular and pathologic approaches enhances diagnostic confidence and provides insights into the pathogenic effects of HJV in naturally infected avian hosts.

Next-Generation Sequencing for Pathogen Discovery and Characterization

Unbiased next-generation sequencing (NGS) represents a paradigm shift in arbovirus diagnostics, particularly for identifying unexpected or novel viral agents in clinical specimens. The utility of this approach for HJV was dramatically demonstrated in the investigation of an emaciated endangered Mississippi sandhill crane (Grus canadensis pulla) found after Hurricane Isaac in 2012. Initial postmortem examination revealed trichostrongyliasis as a possible cause of death, but viral particles consistent with togavirus morphology were observed in brain tissue by electron microscopy. When targeted molecular assays for EEEV, West Nile virus, and other known arboviruses returned negative results, unbiased NGS using the Illumina MiSeq platform was employed, generating whole-genome sequences that definitively identified the agent as HJV [10]. This case exemplifies the power of metagenomic sequencing for wildlife disease diagnostics and conservation medicine, particularly when traditional diagnostic algorithms fail. The study further demonstrated that NGS can provide simultaneous phylogenetic characterization, placing the crane isolate within the known HJV lineage [10]. As NGS technologies become more cost-effective and bioinformatic pipelines more streamlined, they are likely to become standard tools for arbovirus surveillance in both wildlife and domestic animal populations, especially given the potential for HJV to reassort or undergo recombination with other alphaviruses in the WEE antigenic complex [6].

Virus Isolation and Characterization

Despite the advances in molecular diagnostics, virus isolation remains a critical component of HJV surveillance, as it provides the biological material necessary for antigenic characterization, virulence studies, and vaccine development. HJV can be isolated from blood, brain, and other tissues using standard cell culture systems, with Vero cells and mosquito cell lines (e.g., C6/36 from Aedes albopictus) being the most commonly employed substrates. The isolation of HJV from the brain of the Mississippi sandhill crane was achieved using cell culture, and the virus was subsequently characterized by electron microscopy as having typical togavirus morphology [10]. In the context of passive surveillance programs, virus isolation from hunter-harvested birds and from mosquito pools provides essential data on the geographic distribution and seasonal activity of HJV. The ability to isolate live virus is also essential for experimental infection studies, such as those employing novel sampling methods. One innovative approach utilized Culex quinquefasciatus and Aedes albopictus mosquitoes as "biological syringes" to collect blood from experimentally infected chicks, hamsters, and house sparrows. This technique demonstrated good correlation between mosquito-derived and syringe-derived viremia titers at viral loads ≥5.0 log₁₀ PFU/mL, confirming that mosquito blood-feeding can serve as a non-lethal sampling method for viremia determination in small vertebrates [12]. Such methods have direct applications for surveillance programs targeting HJV in its primary avian reservoir hosts, where serial blood sampling is often impractical due to small body size or conservation status.

Integrated Surveillance Strategies and Sentinel Systems

Effective HJV surveillance requires integration of multiple diagnostic modalities across vector, vertebrate host, and sentinel animal populations. The World Organisation for Animal Health (WOAH) and the World Health Organization (WHO) have recognized the importance of arbovirus surveillance in the context of emerging vector-borne diseases, and HJV has been identified by the European Food Safety Authority (EFSA) as one of eight vector-borne disease agents with an estimated rate of introduction into the European Union exceeding 0.001 introductions per year [11]. This risk assessment underscores the need for robust surveillance systems that can detect incursions of HJV into naïve ecosystems.

Sentinel bird flocks have been a cornerstone of arbovirus surveillance in North America, providing early warning of HJV activity prior to epiornitic amplification. Seroconversion in sentinel chickens, monitored by HI or neutralization assays, can signal enzootic transmission and inform vector control and public health interventions. In wild bird populations, longitudinal serosurveillance programs have revealed that HJV activity is episodic, with epiornitics occurring during years of favorable environmental conditions. The central New York study documented only two HJV epiornitics over a 14-year period, with the 1986 event representing the second occurrence of HJV in that region [7]. Surveillance of hunter-harvested game birds, such as ruffed grouse, provides an additional cost-effective approach for monitoring HJV prevalence across broad geographic areas, leveraging existing hunting activities to obtain tissue samples for molecular testing and histopathology [4].

Mosquito surveillance is an essential component of integrated HJV monitoring, as detection of the virus in vector populations can precede clinical cases in vertebrate hosts. The primary vectors for HJV are ornithophilic mosquitoes, particularly species in the genus Culiseta and Culex, which maintain the enzootic cycle between birds. Vector surveillance typically involves the collection of female mosquitoes using CO₂-baited traps, followed by pool testing using qRT-PCR or virus isolation. The genetic stability of HJV over time facilitates molecular detection in vector pools, as the E1 and 3' UTR targets remain highly conserved across isolates [2]. The Centers for Disease Control and Prevention (CDC) and state health departments routinely include HJV in their arbovirus surveillance panels, particularly in the eastern United States where HJV is most prevalent.

Challenges and Future Directions in HJV Diagnostics

Several challenges complicate HJV diagnostics and surveillance. The antigenic cross-reactivity between HJV and EEEV necessitates the use of confirmatory neutralization assays or molecular methods for definitive diagnosis, particularly in regions where both viruses circulate. The episodic nature of HJV activity, with years or even decades between epiornitics, creates difficulties in maintaining sustained surveillance capacity and funding. Additionally, the cryptic nature of HJV maintenance in enzootic foci, where the virus may persist at undetectable levels in reservoir hosts or overwinter in vertically infected vectors, poses challenges for early detection [7].

Emerging diagnostic technologies, including portable sequencing platforms (e.g., Oxford Nanopore MinION) and field-deployable qRT-PCR systems, offer opportunities for real-time surveillance in remote or resource-limited settings. Integration of these technologies with existing surveillance networks could enhance the capacity to detect HJV activity and respond rapidly to epiornitic events. The genetic conservation observed across HJV strains suggests that existing molecular assays and serologic reagents will remain effective for the foreseeable future, providing a stable foundation for ongoing surveillance efforts. However, continued vigilance is warranted, as the potential for viral evolution, particularly in the context of climate change affecting vector and host distributions, may alter diagnostic targets or lead to the emergence of novel variants that challenge current detection approaches.

Public Health and Veterinary Implications

Human Health Risk Assessment and Zoonotic Potential

Highlands J virus (HJV) occupies a unique and often underappreciated position within the arboviral landscape of North America, primarily due to its ambiguous status as a zoonotic pathogen. While HJV is taxonomically classified within the Western Equine Encephalitis (WEE) antigenic complex of the genus Alphavirus, family Togaviridae [5, 6], its documented capacity to cause clinical disease in humans remains extraordinarily rare. The extant literature, including comprehensive reviews of encephalitic alphaviruses, consistently identifies HJV as a virus that only rarely induces neurological symptoms in humans, ranging from mild febrile illness to, in exceptional cases, encephalitis [5]. This characterization places HJV in stark contrast to its more notorious relatives, Eastern Equine Encephalitis virus (EEEV), Western Equine Encephalitis virus (WEEV), and Venezuelan Equine Encephalitis virus (VEEV), which are recognized as significant zoonotic threats with established public health surveillance programs. The fundamental question for public health authorities is whether HJV represents a genuine, albeit low-probability, threat or a pathogen of such negligible human virulence that it warrants minimal regulatory and clinical attention.

Epidemiological data from serosurveys provide the most compelling evidence for sporadic, subclinical human exposure. Studies conducted in Michigan in the early 1980s documented low prevalences of neutralizing antibody to HJV in horses, indicating that the virus was actively circulating in enzootic cycles [9]. Given the shared ecology between equine and human populations in these regions, it is reasonable to infer that human exposure, while infrequent, does occur. Importantly, the absence of widespread human outbreaks does not equate to an absence of risk. The World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) have long recognized that emerging and re-emerging arboviruses often exist in a state of cryptic circulation before environmental or ecological shifts precipitate sudden emergence. The European Food Safety Authority (EFSA), in a comprehensive risk assessment of 36 vector-borne diseases, specifically identified HJV as one of the eight agents with an estimated rate of introduction into the European Union exceeding 0.001 introductions per year [11]. This finding is particularly alarming, as it suggests that HJV is not merely a North American curiosity but a pathogen with genuine transcontinental dispersal potential. The EFSA assessment underscores that HJV possesses the requisite combination of vector competence, vertebrate host availability, and climatic suitability to establish transmission cycles in novel ecosystems, with subsequent implications for both public health and animal health [11]. For public health agencies, this mandates a proactive stance: HJV should be included in differential diagnostics for undifferentiated febrile illness and viral encephalitis in regions where competent mosquito vectors (primarily Culiseta melanura and various Aedes and Culex species) [1, 13] are present, particularly when other alphaviral etiologies have been ruled out.

Pathogenesis, Clinical Manifestations, and Diagnostic Challenges in Humans

The pathophysiological mechanisms underpinning HJV neuroinvasion in humans remain inadequately characterized, largely due to the paucity of clinical cases and the absence of dedicated human pathogenesis studies. Extrapolating from the broader alphavirus literature, particularly studies on EEEV and WEEV, it is hypothesized that HJV gains entry to the central nervous system (CNS) either via direct infection of cerebral microvascular endothelial cells, hematogenous seeding following high-titer viremia, or through retrograde axonal transport from peripheral neurons [5]. Once within the CNS, viral replication triggers a robust inflammatory response, characterized by neuronal apoptosis, microglial activation, and perivascular cuffing, leading to the classic histopathological picture of meningoencephalitis. The rarity of severe human disease suggests that HJV may possess a lower intrinsic neurovirulence compared to EEEV, possibly due to differences in the E2 glycoprotein’s affinity for neuronal receptors or a diminished capacity to evade the innate immune response. Indeed, recent work on the evolutionary genetics of the WEE complex has revealed that HJV, along with WEEV and Fort Morgan virus, originated from a recombination event between EEEV and a Sindbis-like virus [6]. This chimeric ancestry likely conferred a unique set of host-cell interaction capabilities, which may explain the attenuated human pathogenicity.

From a clinical standpoint, the nonspecific nature of HJV infection presents a significant diagnostic challenge. The prodromal phase, characterized by fever, headache, myalgia, and arthralgia, is clinically indistinguishable from infections caused by West Nile virus (WNV), St. Louis encephalitis virus (SLEV), and other endemic arboviruses [5]. In the absence of pathognomonic features, laboratory confirmation is essential. The CDC recommends serological testing using immunoglobulin M (IgM) antibody capture enzyme-linked immunosorbent assay (MAC-ELISA) and plaque reduction neutralization tests (PRNT) to discriminate between closely related alphaviruses. However, significant cross-reactivity within the WEE complex complicates interpretation. Hemagglutination-inhibition (HI) assays have been employed in avian serosurveys to detect HJV-specific antibodies, but these too suffer from limited specificity [7, 8]. Molecular diagnostics, including reverse transcription-polymerase chain reaction (RT-PCR), are most effective during the acute viremic phase, which is typically short-lived in humans. The application of unbiased next-generation sequencing (NGS) has revolutionized pathogen discovery, as demonstrated by the identification of HJV from the brain of a Mississippi sandhill crane using Illumina MiSeq technology [10]. This technique circumvents the need for a priori sequence knowledge and can simultaneously characterize the entire viral genome, providing invaluable epidemiological and phylogenetic data. Public health laboratories should consider incorporating pan-alphaviral NGS panels into their diagnostic algorithms to enhance detection of rare or unexpected pathogens like HJV.

Veterinary Implications: Avian Morbidity, Mortality, and Population-Level Effects

HJV is first and foremost an avian pathogen, and its impact on wild bird populations and domestic poultry operations constitutes the most pressing veterinary concern. The virus has been isolated from a diverse array of avian species, including passerines (songbirds), galliformes (turkeys, ruffed grouse, pheasants), and gruiformes (cranes) [1, 3]. The primary enzootic cycle involves Culiseta melanura mosquitoes as the principal vector and wild passerine birds as the primary amplifying hosts [2, 7]. However, the spillover into economically significant poultry species, particularly turkeys, has been documented. Early phylogenetic studies of HJV strains implicated a specific isolate in a turkey outbreak, though the virus was not phylogenetically distinct from contemporaneous enzootic strains [2]. This suggests that outbreaks are not driven by the emergence of hypervirulent variants but rather by ecological conditions that favor bridge vector transmission from wild birds to domestic flocks.

Recent surveillance data from the Upper Midwest, USA (2018–2022) detected HJV in 0.2% of hunter-harvested ruffed grouse (Bonasa umbellus), a popular upland game bird experiencing population declines [4]. Crucially, approximately half of the infected birds exhibited histologic cardiac lesions consistent with arboviral infection [4]. This finding is of profound veterinary significance. It indicates that HJV, even at low prevalence, can cause clinically relevant pathology in infected birds. Myocarditis and encephalitis are well-documented sequelae of alphavirus infection in avian species, and such lesions can impair flight performance, predator avoidance, and overall fitness. For game bird management agencies, this necessitates integration of arboviral surveillance into routine population health monitoring programs. The potential for HJV to act as a contributing factor in population declines, particularly when birds are simultaneously challenged by other stressors such as habitat fragmentation, nutritional deficiency, or concurrent parasitic infections, cannot be overstated. The case of the Mississippi sandhill crane (Grus canadensis pulla), an endangered subspecies, exemplifies this interaction. A crane found emaciated days after Hurricane Isaac was diagnosed with trichostrongyliasis and HJV encephalitis, highlighting how extreme weather events can exacerbate disease susceptibility in vulnerable populations [10].

The impact on domestic turkeys warrants specific attention. Although the exact economic toll of HJV-associated mortality and morbidity in commercial poultry operations is not comprehensively documented, the potential for significant loss exists. Clinical signs in infected poults may include depression, anorexia, ataxia, and mortality, often with a rapid course. The World Organisation for Animal Health (WOAH) does not currently list HJV as a notifiable disease, which may contribute to underreporting. Veterinary diagnosticians should maintain a high index of suspicion for HJV in cases of unexplained encephalitis or sudden death in turkey flocks, particularly in regions with high enzootic transmission. Co-infections with other arboviruses, such as EEEV, are possible and have been documented in avian hosts [4, 7, 8]. Serological surveys of Florida scrub-jays (Aphelocoma coerulescens) and blue jays (Cyanocitta cristata) revealed that over 15% of sampled individuals had antibodies to HJV, and survival rates of seropositive blue jays were significantly lower than those of seronegative conspecifics [8]. This provides rare, direct evidence that HJV infection can impact host survival at the population level, acting as a selective pressure on wild bird communities.

Equine Health and the Role of Bridge Vectors

While HJV is predominantly an avian pathogen, its ability to infect mammalian hosts, including horses, is established and represents a secondary, yet important, veterinary concern. Serological evidence from Michigan horses in 1980 confirmed the presence of HJV-specific neutralizing antibodies, indicating that equids are exposed to the virus during periods of active transmission [9]. The clinical presentation in horses is poorly documented, but it is widely assumed to be milder than that caused by EEEV or WEEV [5]. Nonetheless, the potential for equine encephalitis should not be dismissed. A strain of HJV isolated from a horse suffering encephalitis was included in early genetic studies, though it was found to be phylogenetically indistinct from avian isolates [2]. This suggests that any equine neurovirulence is likely strain-dependent or host-factor-driven rather than a fixed property of a distinct equine-adapted lineage. The risk of equine infection is intimately tied to the activity of bridge vectors, mosquito species that feed indiscriminately on birds and mammals. Aedes and Coquillettidia species are frequently implicated in this role [1, 13]. Therefore, veterinary recommendations for equine protection mirror those for EEEV and WEEV: vaccination against WEEV (which may confer partial cross-protection due to antigenic relatedness), rigorous mosquito control, and stabling during peak vector activity hours (dusk and dawn). However, no HJV-specific equine vaccine exists, and the efficacy of WEEV vaccination against HJV challenge has not been rigorously evaluated.

Vector Competence, Transmission Dynamics, and the Threat of Geographic Expansion

The geographic distribution of HJV is largely confined to North America, with enzootic foci stretching from the Atlantic Coast to the Upper Midwest and Gulf Coast regions [1, 2, 5]. However, the vector-borne nature of the disease renders it highly susceptible to climate-driven range shifts. The EFSA risk assessment, which calculated the rate of HJV introduction into the EU, is a stark reminder that no arbovirus is truly geographically static [11]. The principal enzootic vector, Culiseta melanura, is a competent laboratory vector for HJV, and its distribution is primarily limited by temperature and the availability of suitable larval habitats (freshwater swamps with emergent vegetation). As global temperatures rise, the northern limits of Cs. melanura and other competent vectors may expand, bringing HJV into naïve avian populations and potentially human/equine communities. The ability of Aedes albopictus (the Asian tiger mosquito) to transmit HJV is particularly concerning, as this species is an aggressive human-biter and has established populations across much of the continental United States and southern Europe [12, 13]. Experimental studies using a "biological syringe" technique with Ae. albopictus and Culex quinquefasciatus demonstrated that these mosquitoes can acquire and transmit HJV from viremic chicks, house sparrows, and hamsters [12]. The correlation between mosquito- and syringe-derived titers was excellent at high viremia levels (>5.0 log10 pfu/mL serum), confirming that bridge vector transmission to mammals is not merely theoretical [12].

For public health authorities and veterinary services, these data underscore the necessity of integrated vector management (IVM) programs. The use of plant-based repellents, such as essential oils from Cymbopogon, Ocimum, and Eucalyptus species, offers a biodegradable and environmentally sustainable adjunct to traditional chemical control, particularly in ecologically sensitive wetland areas where Cs. melanura breeds [13]. Surveillance should prioritize the molecular screening of mosquito pools for HJV RNA, especially during late summer and early fall when transmission peaks. Sentinel chicken flocks, a mainstay of SLEV and WNV surveillance, could be readily adapted for HJV detection, given the high susceptibility of galliform birds.

Recommendations for Surveillance, Biosecurity, and One Health Integration

Given the low but non-zero public health risk and the demonstrated veterinary impact, a comprehensive One Health approach to HJV surveillance and control is imperative. The following recommendations are proposed based on the available evidence:

1. Human Surveillance: Clinicians in endemic areas should consider HJV in the differential diagnosis of aseptic meningitis and encephalitis. State public health laboratories should include HJV in their arboviral testing panels, ideally using NGS-based approaches to differentiate from EEEV and WEEV [10]. The CDC should consider adding HJV to its ArboNET passive surveillance system to better capture sporadic cases and identify emerging foci.
2. Wildlife and Poultry Surveillance: State wildlife agencies and the USDA should incorporate HJV testing into routine disease surveillance of game birds (ruffed grouse, wild turkeys, pheasants) and sentinel avian species (blue jays, scrub-jays, song sparrows) [4, 7, 8]. The detection of histologic cardiac lesions in infected ruffed grouse warrants pathological follow-up to quantify the population-level impact [4]. For commercial turkey operations, enhanced biosecurity measures, such as exclusion of wild birds from feed and water sources and vector control around poultry houses, are recommended during peak transmission months.
3. Equine Health: Equine practitioners should regard HJV as a potential, albeit rare, cause of neurological disease. Diagnostic workups for suspect encephalitis cases should include HJV serology and molecular testing. Vaccination against WEEV should be maintained as a precautionary measure.
4. Vector Control and Risk Communication: Local mosquito control districts should conduct targeted surveillance for Cs. melanura and Ae. albopictus in areas with known HJV activity. Public health messaging should emphasize personal protective measures (repellents, long sleeves, and avoidance of outdoor activity during dusk/dawn) without inciting undue alarm, given the rarity of human disease.
5. International Preparedness: Given the EFSA risk assessment, European veterinary and public health agencies should remain vigilant for HJV introduction. Preparedness includes establishing diagnostic capacity for alphaviruses, training personnel in NGS-based pathogen discovery, and conducting risk assessments for competent vector populations [11].

In summary, Highlands J virus occupies a complex niche, straddling the boundary between a neglected enzootic arbovirus and a pathogen with genuine epizootic and zoonotic potential. Its low human virulence does not justify complacency, as the potential for emergence, driven by climate change, vector expansion, and ecological perturbation, remains substantial. The veterinary implications, particularly for avian conservation and poultry production, are unequivocal and demand immediate and sustained surveillance efforts. A failure to act now may allow this virus to transition from a cryptic enzootic agent to a recognized public and animal health burden.

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Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.