Equine Arteritis Virus

Overview and Taxonomy of Equine Arteritis Virus

Taxonomic Position and Phylogenetic Classification

Equine arteritis virus (EAV) is the prototypical member and the namesake of the family Arteriviridae, a taxon of enveloped, positive-sense, single-stranded RNA viruses that belongs to the order Nidovirales [18, 29]. This order also encompasses the families Coronaviridae, Mesoniviridae, and Roniviridae, all of which share a characteristic nested set of subgenomic mRNAs, the defining feature from which the order derives its name ("nido" from the Latin for "nest") [11, 18]. Within the Arteriviridae family, EAV has been taxonomically grouped into the genus Equarterivirus, a classification that reflects its unique evolutionary trajectory and host specificity among the arteriviruses [18]. Historically, the Arteriviridae were classified as a subfamily within the Togaviridae, but advances in molecular phylogenetics and genomic characterization have firmly established their distinct position within the nidovirus supergroup, underscoring their profound differences in genome organization, replication strategy, and structural complexity compared to alphaviruses and flaviviruses [29].

The taxonomic hierarchy of EAV is as follows: Realm Riboviria, Kingdom Orthornavirae, Phylum Pisuviricota, Class Pisoniviricetes, Order Nidovirales, Family Arteriviridae, Genus Equarterivirus, and Species Alphaarterivirus equid [29]. This recent reclassification under the International Committee on Taxonomy of Viruses (ICTV) framework distinguishes EAV from other arteriviruses such as porcine reproductive and respiratory syndrome virus (PRRSV), which occupies a separate genus (Betaarterivirus), and simian hemorrhagic fever virus (SHFV), which belongs to the genus Gammaarterivirus [5, 29]. The phylogenetic divergence between these viruses is substantial, with estimates suggesting that the equine and porcine arteriviruses separated from a common ancestor many millennia ago, likely co-evolving with their respective mammalian hosts [17, 29]. Notably, EAV is the only arterivirus known to infect equids, and it does not cause disease in non-equid species under natural conditions, a specificity that is intimately linked to its receptor usage and host-cell factor dependencies [1].

Genomic Architecture and Virion Structure

The EAV genome is a single-stranded, positive-sense RNA molecule of approximately 12.7 kilobases (kb) in length, making it one of the smallest genomes among the nidoviruses [18, 29]. The 5′ end of the genome is capped, and the 3′ end is polyadenylated, a classical hallmark of positive-strand RNA viruses that allows the genomic RNA to function directly as an mRNA upon entry into the host cell cytoplasm [29]. The genome encodes ten functional open reading frames (ORFs), which are organized into two distinct genetic modules: the replicase region and the structural protein region [18, 25]. ORF1a and ORF1b occupy the 5′-proximal two-thirds of the genome and are translated directly from the genomic RNA to produce two large polyproteins, pp1a and pp1ab, the latter arising from a −1 ribosomal frameshift event at the junction of ORF1a and ORF1b [29]. These polyproteins are subsequently cleaved by viral proteases, including the papain-like protease 2 (PLP2) and the 3C-like serine protease (nsp4), into at least 13 nonstructural proteins (nsps) designated nsp1 through nsp12, with nsp7 undergoing further processing into nsp7α and nsp7β [5, 21, 25]. These nsps assemble into the viral replication and transcription complex (RTC), which is membrane-associated and responsible for genome replication and the synthesis of a nested set of subgenomic mRNAs [14].

The structural protein region, encompassing ORF2a, ORF2b, ORF3, ORF4, ORF5, ORF6, and ORF7, is expressed from subgenomic mRNA templates [22, 29]. These ORFs encode the seven structural proteins of the virion: the minor envelope glycoproteins GP2 (ORF2a), GP3 (ORF3), and GP4 (ORF4); the major envelope glycoprotein GP5 (ORF5); the unglycosylated membrane protein M (ORF6); and the nucleocapsid protein N (ORF7) [7, 22, 29]. ORF2b encodes a small, non-glycosylated envelope protein known as E, which is also incorporated into the virion [22]. The mature EAV particle is spherical, approximately 50–60 nanometers in diameter, with a relatively smooth surface that lacks the prominent spike proteins characteristic of coronaviruses [29]. The nucleocapsid, composed of multiple copies of the N protein complexed with genomic RNA, exhibits a helical, tubular morphology of approximately 15 nanometers in diameter, distinguishing it from the icosahedral capsids of many other RNA viruses [29]. The viral envelope is a lipid bilayer derived from the host cell membrane, into which the glycoprotein complexes are embedded. The primary building block of the envelope is the disulfide-linked GP5/M heterodimer, which is the most abundant structural protein complex and is essential for virus budding and the induction of neutralizing antibodies [7]. In addition, a trimeric complex of GP2, GP3, and GP4 forms a distinct spike that is critical for virus entry into susceptible cells, particularly through its role in receptor recognition and membrane fusion [4, 26].

Functional and Structural Features of Viral Proteins

The structural and nonstructural proteins of EAV exhibit remarkable functional diversity, each contributing to various aspects of the viral life cycle, including entry, replication, immune evasion, and assembly. The major envelope protein GP5 is a type I transmembrane glycoprotein with an N-terminal ectodomain that contains the principal neutralizing epitopes of the virus [7, 27]. Structural modeling of the GP5/M dimer using advanced computational techniques, such as AlphaFold3, has revealed a conserved architecture comprising a short ectodomain, three helical transmembrane segments, and a β-sheet-rich endodomain [7]. The EAV GP5 ectodomain is notably longer than its PRRSV counterpart, featuring four α-helices and a disulfide-linked β-sheet that forms the most variable and surface-exposed region of the dimer [7]. This region is the target of neutralizing antibodies and is subject to immune evasion through antigenic drift and glycan shielding, as multiple conserved and variable N-glycosylation sites are present on GP5 [7]. The M protein, in contrast, is a non-glycosylated triple-spanning transmembrane protein that serves as the scaffold for virion assembly and budding, and its interaction with GP5 is stabilized by hydrophilic interactions within the lipid bilayer [7, 20].

The minor glycoproteins GP2, GP3, and GP4 form a heterotrimeric complex that is essential for viral tropism and entry [4, 22]. GP3 exhibits a particularly unusual biosynthetic pathway. When expressed alone in transfected cells, GP3 adopts a dual membrane topology, with a fraction of the protein inserting into the membrane in a type I orientation and another fraction retaining a type II orientation [4]. This dual topology is mediated by an N-terminal signal peptide that is not cleaved under normal conditions due to the presence of overlapping N-glycosylation sequons (NNTT) at the signal peptide cleavage site [30]. Glycosylation at these adjacent sites physically inhibits signal peptidase activity, resulting in a mature GP3 that retains its signal peptide as a non-cleaved membrane anchor [26, 30]. However, during authentic EAV infection, GP3 exhibits a single type I topology, suggesting that the viral replication environment or interactions with other viral proteins enforce a uniform orientation [4]. The C-terminal hydrophobic domain of GP3 is essential for its peripheral membrane attachment and its association with GP2 and GP4, whereas the RXR motif in the C-terminus functions as an endoplasmic reticulum (ER) retention signal, ensuring proper assembly of the heterotrimeric complex [4, 26].

The nonstructural proteins of EAV play critical roles in viral replication and host immune modulation. Nsp1, the most N-terminal cleavage product of the replicase polyprotein, has been identified as a potent antagonist of the type I interferon (IFN) response [24]. Nsp1 inhibits IFN-β promoter activation and downstream signaling, thereby suppressing the innate antiviral response in equine endothelial cells, which are primary targets of EAV infection [24]. Nsp10, a bifunctional protein containing a helicase domain and a zinc-binding domain, has recently been shown to degrade the mitochondrial antiviral-signaling protein (MAVS) through a mechanism involving the recruitment of E3 ubiquitin ligases Smurf1 and MARCH5 [5]. This targeted degradation of MAVS disrupts the RIG-I/MDA5 signaling pathway, effectively blunting the host’s ability to mount an IFN response against the virus [5]. The zinc finger motifs of nsp10, specifically residues D249, S287, and the S1/F39/N41 sites, are critical for its interactions with MAVS and its E3 ligase partners, highlighting a sophisticated viral strategy for innate immune evasion [5]. Additionally, nsp2 and nsp5, two transmembrane subunits of the replicase, have been implicated in the sensitivity of EAV to cyclophilin inhibitors, with adaptive mutations in these proteins conferring resistance to cyclosporine and alisporivir [14].

Receptor Usage, Host Cell Tropism, and Entry Mechanisms

One of the most defining biological features of EAV is its broad host cell tropism, which contrasts sharply with the restricted macrophage-specific tropism of other arteriviruses such as PRRSV and SHFV [1]. While most arteriviruses utilize the macrophage-specific scavenger receptor CD163 as their primary entry receptor, EAV has evolved to employ a distinct set of host cell factors, enabling it to infect a wide range of cell types, including endothelial cells, epithelial cells, fibroblasts, smooth muscle cells, monocytes, and T lymphocytes [1, 6, 23]. This expanded tropism is directly linked to the virus’s ability to cause systemic vasculitis, reproductive failure, and respiratory disease in infected equids [29].

A landmark study employing a genome-wide CRISPR knockout screen identified the plasma membrane tetraspanin CD81 as an essential entry receptor for EAV [1]. CD81 is a ubiquitously expressed protein involved in cell adhesion, migration, and signal transduction, and its broad tissue distribution provides a molecular basis for the wide tropism of EAV [1]. Knockout of CD81 rendered cells completely resistant to EAV infection, while susceptibility was restored upon reconstitution with CD81 expression [1]. Importantly, CD81 was not required for infection with other arteriviruses, underscoring the concept of receptor switching within the Arteriviridae family [1]. Mapping studies using horse/possum CD81 chimeras localized the critical EAV-interacting domain to alpha helix "D" of the large extracellular loop (LEL) of CD81, establishing that specific structural features of this tetraspanin are necessary for viral entry [1].

In addition to CD81, EAV also utilizes the chemokine receptor CXCL16 as an attachment and entry factor, particularly in the context of CD3+ T lymphocyte infection and the establishment of long-term persistent infection in stallions [9, 12, 19]. The equine ortholog of CXCL16 (EqCXCL16) exists in two major allelic variants: EqCXCL16S (susceptible) and EqCXCL16R (resistant) [19]. These alleles differ by four non-synonymous nucleotide substitutions that result in four amino acid changes in the extracellular domain of the protein [19]. The EqCXCL16S isoform functions as an efficient EAV receptor, mediating viral entry and infection of CD3+ T cells, whereas EqCXCL16R is non-permissive for infection [9, 19]. The presence of at least one copy of the EqCXCL16S allele is strongly associated with the establishment of long-term persistent infection in stallions, making genotyping of CXCL16 a valuable tool for identifying animals at risk of becoming viral carriers [9, 10, 13]. Furthermore, the CXCL16/CXCR6 axis has been implicated in the maintenance of viral persistence, as it modulates local inflammatory and immune responses in the male reproductive tract, orchestrating a dysfunctional CD8+ T lymphocyte response that fails to clear the virus [12, 15].

Cell surface vimentin has also been identified as an attachment factor that facilitates EAV infection [6]. Virus overlay protein-binding assays and Far-Western blotting revealed that a 57 kDa protein, subsequently identified as vimentin, binds to EAV particles and is present on the surface of susceptible cells [6]. Pre-treatment of equine cells with anti-vimentin antibodies or with Withaferin A, a compound that disrupts vimentin filaments, partially inhibited EAV infection [6]. Overexpression of equine vimentin in non-permissive HEK-293 cells increased their susceptibility to EAV infection, further supporting the role of this intermediate filament protein as a co-factor in the entry process [6]. The current working model proposes that EAV entry involves a multi-step process: initial attachment to cell surface vimentin, followed by engagement of the primary receptor CD81, and subsequent internalization via clathrin-mediated endocytosis [2]. The low pH of the endosomal compartment triggers conformational changes in the viral glycoproteins, leading to membrane fusion and release of the nucleocapsid into the cytoplasm [2]. Inhibitors of endosomal acidification, such as ammonium chloride, effectively block EAV infection, confirming the importance of the endocytic pathway for productive entry [2].

Genetic Diversity, Evolution, and Molecular Epidemiology

EAV exhibits a moderate degree of genetic diversity, with viral strains classified into several phylogenetic groups based on sequence analysis of the ORF5 (GP5) gene, the most variable region of the genome [3, 28, 31]. Historically, EAV strains have been divided into two major clades: the North American group (group I) and the European group (group II), which are further subdivided into multiple subgroups [31]. For example, the prototype Bucyrus strain, isolated in Ohio in 1953, belongs to the North American group, whereas strains circulating in Europe, such as those from the 2019 UK outbreaks, fall into phylogroup D within the European lineage [3, 29]. A harmonized phylogenetic classification scheme has been proposed to standardize the naming of EAV clades, with groups designated A through G, encompassing both equine and asinine strains [31].

The major glycoprotein GP5 is the most immunodominant and variable structural protein, and its sequence diversity is driven by immune selection pressure, particularly at neutralizing epitopes [7, 8]. Peptide microarray analyses have identified multiple immunodominant epitopes within GP5, but no single epitope is recognized by all EAV-positive sera, highlighting the antigenic diversity among circulating strains [8]. Non-synonymous mutations in GP5 can lead to antigenic drift, allowing the virus to evade pre-existing neutralizing antibodies and establish infection in vaccinated or previously exposed animals [7]. In addition to GP5, significant genetic variation is observed in GP3, GP2, and nsp2, with the highest variability in GP3 during persistent infection [3, 16, 22]. The intrahost evolution of EAV during persistent infection in the stallion reproductive tract is characterized by genome-wide purifying

Molecular Pathogenesis: Receptor Usage and Entry Mechanisms

The entry of equine arteritis virus (EAV) into host cells represents a paradigm of molecular complexity and evolutionary adaptation within the Arteriviridae family. Unlike its close relatives, such as porcine reproductive and respiratory syndrome virus (PRRSV), which strictly utilize the macrophage-specific scavenger receptor CD163, EAV has evolved a multifaceted entry strategy that engages a distinct repertoire of host cell surface molecules. This divergence is not merely a biochemical curiosity; it underpins the remarkably broad cellular and tissue tropism of EAV, enabling infection of endothelial cells, epithelial cells, monocytes, and lymphocytes, and directly influences the pathogenesis of equine viral arteritis (EVA). The molecular choreography of EAV entry is a multi-step process involving initial attachment, engagement of specific entry receptors, and subsequent internalization via endocytic pathways, culminating in the release of the viral genome into the cytoplasm.

The Primary Entry Receptor: CD81 and the Paradigm of Receptor Switching

For decades, the identity of the primary receptor for EAV remained elusive, a critical gap in understanding its unique pathogenesis. The seminal discovery that EAV employs the tetraspanin CD81 as a requisite entry receptor fundamentally altered our understanding of arterivirus biology [1]. This finding represents the first documented example of receptor switching within the Arteriviridae family, a molecular event with profound implications for viral tropism and disease ecology. CD81 is a ubiquitously expressed plasma membrane protein belonging to the tetraspanin superfamily, characterized by four transmembrane domains and two extracellular loops, a small extracellular loop (SEL) and a large extracellular loop (LEL) that contains the functional binding interface for viral glycoproteins. The broad tissue distribution of CD81, in stark contrast to the macrophage-restricted expression of CD163, provides a compelling molecular explanation for EAV’s ability to infect a diverse array of cell types beyond the monocyte/macrophage lineage [1, 29].

The experimental evidence establishing CD81 as an essential entry factor is robust and multi-faceted. Genome-wide CRISPR knockout screens identified CD81 as a non-redundant host factor; genetic ablation of CD81 renders cells completely resistant to EAV infection, while having no effect on the infectivity of other arteriviruses that utilize CD163 [1]. This specificity was further confirmed by demonstrating that pre-incubation of virions with soluble recombinant CD81 protein neutralizes infectivity, likely by saturating the viral attachment sites. Crucially, bypassing the entry step via direct transfection of the EAV genomic RNA into CD81-knockout cells yields robust viral replication and production of infectious progeny, unequivocally demonstrating that CD81 is required specifically at the entry stage and is dispensable for downstream replication [1].

The structural basis of the EAV-CD81 interaction has been mapped with remarkable precision using a combination of cross-species screening and chimeric protein analysis. Screening of CD81 orthologs from various mammalian hosts revealed that the brushtail possum (Trichosurus vulpecula) CD81 is non-permissive for EAV entry, providing a natural template for structure-function analysis. By constructing a panel of horse/possum CD81 chimeras, the critical determinant for EAV entry was localized to alpha helix “D” within the large extracellular loop (LEL) of CD81 [1]. This specific structural element, a conserved amphipathic helix, is predicted to form a critical binding interface for the viral glycoprotein complex. The identification of this discrete molecular determinant not only explains species-specific barriers to infection but also suggests that subtle conformational changes in this helix could modulate receptor function and potentially influence viral fitness and host range.

Attachment Factors and the CXCL16 Axis: A Dual Role in Entry and Persistence

While CD81 functions as the primary entry receptor, the initial tethering of EAV to the host cell surface is facilitated by attachment factors that concentrate virions at the plasma membrane, enhancing the efficiency of subsequent receptor engagement. Cell surface vimentin, an intermediate filament protein traditionally considered a cytoskeletal component, has been identified as a critical attachment factor for EAV [6]. Virus overlay protein-binding assays and far-Western blotting using membrane fractions from EAV-susceptible equine pulmonary artery endothelial cells (EECs) and equine dermal fibroblasts (E. Derm) identified a 57 kDa protein as a prominent EAV-binding partner, subsequently confirmed as vimentin by liquid chromatography-tandem mass spectrometry (LC-MS/MS) [6]. The functional relevance of this interaction is supported by several lines of evidence: (i) susceptibility to EAV infection across a panel of mammalian cell lines correlates with vimentin expression; (ii) pre-treatment of cells with anti-vimentin polyclonal antibodies or the vimentin-disrupting agent Withaferin A partially inhibits EAV infection; and (iii) ectopic overexpression of equine vimentin in non-permissive HEK-293 cells significantly enhances their susceptibility to EAV [6]. These data position vimentin as a bona fide attachment factor that facilitates viral binding but is not absolutely required for entry, as its inhibition only partially blocks infection.

A second, and arguably more complex, host factor involved in EAV entry is the chemokine CXCL16. The role of CXCL16 in EAV pathogenesis is uniquely dichotomous, functioning both as an entry receptor on specific cell subpopulations and as a critical determinant of the long-term carrier state in stallions. The equine orthologue of CXCL16 (EqCXCL16) exists as two major allelic variants, EqCXCL16^S (susceptible) and EqCXCL16^R (resistant), which differ by four non-synonymous nucleotide substitutions resulting in four amino acid changes at positions 40, 49, 50, and 52 of the mature protein [19]. The EqCXCL16^S isoform functions as an efficient entry receptor for EAV, particularly on a subpopulation of CD3^+ T lymphocytes, whereas the EqCXCL16^R isoform is significantly less supportive of viral entry [19]. This functional difference is not attributable to altered chemotactic activity, as both isoforms exhibit equivalent chemoattractant potential; rather, the EqCXCL16^S isoform possesses significantly higher scavenger receptor and adhesion properties, which likely enhance its ability to capture and internalize virions [19].

The clinical and epidemiological significance of this receptor polymorphism cannot be overstated. Stallions carrying at least one copy of the EqCXCL16^S allele (genotypes CXCL16^S/S or CXCL16^S/R) are at a dramatically increased risk of becoming long-term persistent carriers following natural infection, continuously shedding virus in their semen for months to years [9, 10, 13, 19]. In contrast, stallions homozygous for the EqCXCL16^R allele are highly resistant to the establishment of persistent infection [9, 10, 19]. This genetic association is so robust that a TaqMan® allelic discrimination qPCR assay has been developed for rapid genotyping of stallions, enabling identification of high-risk individuals that should be prioritized for vaccination [9]. The mechanistic link between CXCL16-mediated entry and persistence is further elucidated by transcriptomic analyses of the ampullae, the primary site of EAV persistence in the stallion reproductive tract. Persistent infection is associated with enhanced expression of both CXCL16 and its cognate receptor CXCR6 on infiltrating lymphocytes, suggesting that the CXCL16/CXCR6 axis is exploited by EAV to modulate the local immune environment, inducing a dysfunctional CD8^+ T lymphocyte response characterized by upregulation of inhibitory receptors and transcription factors such as eomesodermin (EOMES) and nuclear factor of activated T-cells cytoplasmic 2 (NFATC2) [12].

Internalization Pathways: Endocytic Route and the Role of Proteolytic Processing

Following receptor engagement, EAV is internalized into host cells via a dynamin-dependent, clathrin-mediated endocytic pathway. High-resolution imaging flow cytometry, which combines the statistical power of flow cytometry with the spatial resolution of microscopy, has been instrumental in delineating the kinetics and mechanism of EAV internalization [2]. This approach has revealed that EAV entry into Vero cells proceeds through the endosomal trafficking route, with statistically significant alterations in the rates of internalization and uncoating occurring within the first hours of infection [2]. The dependence on endosomal acidification is demonstrated by the potent inhibitory effect of ammonium chloride, a lysosomotropic agent that raises endosomal pH and blocks pH-dependent fusion events [2].

The low pH environment of the maturing endosome is not sufficient for viral fusion; it also serves to activate host cell proteases that are essential for priming the viral glycoproteins. The serine protease inhibitor camostat mesylate, which inhibits the activity of transmembrane serine proteases such as TMPRSS2, has been shown to effectively block EAV entry, indicating that proteolytic cleavage of one or more viral surface proteins is a prerequisite for membrane fusion [2]. This requirement for proteolytic processing is a common feature among enveloped viruses, including influenza virus and SARS-CoV-2, and represents a conserved vulnerability that can be targeted therapeutically.

The Viral Fusion Machinery: Glycoprotein Complex Architecture and Membrane Fusion

The actual membrane fusion event is mediated by the viral glycoprotein spikes, which are composed of a heterotrimeric complex of GP2, GP3, and GP4, along with the GP5/M dimer that forms the primary structural scaffold of the viral envelope [7, 29]. The GP5/M dimer, predicted by AlphaFold3 to adopt a conserved architecture comprising a short ectodomain, three helical transmembrane regions, and a β-sheet-rich endodomain, is the principal target of neutralizing antibodies [7]. The GP5 ectodomain is notably longer than its PRRSV counterpart, containing four α-helices and a disulfide-linked β-sheet that forms the most variable and surface-exposed region of the spike, housing neutralizing epitopes [7]. Adjacent conserved and variable N-glycosylation sites on GP5 suggest a sophisticated immune evasion strategy involving both antigenic drift and glycan shielding, where the dense carbohydrate coat masks conserved epitopes from antibody recognition [7].

The GP3 protein, a component of the minor spike complex, exhibits a unique and controversial topology. When expressed in isolation, GP3 displays a dual topology, with a fraction of molecules adopting a type I membrane orientation (N-terminus in the lumen, C-terminus in the cytoplasm) and another fraction adopting a type II orientation [4]. This dual topology is attributed to the presence of an N-terminal signal peptide that is not cleaved due to the inhibitory effect of adjacent N-glycosylation sites at the overlapping sequon NNTT [30]. However, in the context of a replicating virus, GP3 appears to adopt a single type I membrane topology, suggesting that the presence of the other glycoproteins (GP2 and GP4) in the heterotrimeric complex imposes a uniform orientation [4]. The hydrophobic C-terminus of GP3 is essential for membrane attachment and virus viability, functioning as a peripheral membrane anchor rather than a transmembrane domain [26, 30]. The RXR motif in the C-terminus of GP3 serves as a co-factor for endoplasmic reticulum (ER) retention, ensuring proper assembly of the glycoprotein complex, though the primary retention signal remains unidentified [4].

Cell Tropism and the Determinants of Target Cell Susceptibility

The molecular determinants of EAV cell tropism are a direct reflection of the expression patterns of its entry receptors and attachment factors. CD81 is expressed on virtually all nucleated cells, providing a broad baseline of potential susceptibility. However, the actual permissiveness of a given cell type is modulated by the expression of CXCL16, vimentin, and potentially other, as-yet-unidentified cofactors. In vivo, EAV exhibits a pronounced tropism for endothelial cells, particularly those of the small arteries and capillaries, which is the pathological basis of the characteristic vasculitis [29]. Equine pulmonary artery endothelial cells (EECs) are highly permissive and express high levels of CD81 and vimentin [6].

Monocytes and macrophages are also important targets, but with a notable heterogeneity. Flow cytometric analysis using a recombinant EAV expressing mCherry has revealed that a minor population of CD14^hi monocytes are the preferential initial targets for EAV infection [23]. Within 36 hours post-infection, the infected cell population shifts to a CD14^lo phenotype that co-expresses CXCL16, suggesting that EAV infection itself may drive monocyte differentiation or that CXCL16-expressing cells are selectively enriched during viral replication [23]. This is particularly relevant given that CXCL16 expression on CD3^+ T lymphocytes is the genetic determinant of the carrier state in stallions [19]. Dendritic cells (DCs), both immature and mature, are also susceptible to EAV infection, with replication being most efficient in mature DCs [32]. Infection of DCs leads to a significant downregulation of co-stimulatory molecules such as CD83, inhibition of endocytic and phagocytic capacity, and a reduced ability to stimulate T cell proliferation, representing a key mechanism of immune evasion [32].

The upper respiratory tract mucosa, the primary portal of entry for EAV, contains a complex mixture of target cells. Using a polarized equine nasal and nasopharyngeal mucosal explant system, it has been demonstrated that CD172a^+ myeloid cells (a marker for macrophages and dendritic cells) and CD3^+ T lymphocytes are the primary targets of EAV infection in the respiratory mucosa [33]. This early infection of immune cells within the mucosa likely facilitates rapid dissemination of the virus to distant organs, including the reproductive tract, where it can establish persistence in stallions. The ability of EAV to infect and modulate the function of antigen-presenting cells and T lymphocytes at the site of entry is a sophisticated strategy that simultaneously promotes viral spread and subverts the development of an effective adaptive immune response.

Epidemiology: Transmission, Host Range, and Global Distribution

Equine arteritis virus (EAV) exemplifies a pathogen whose epidemiological profile is inextricably linked to its biological idiosyncrasies, most notably its capacity to establish a long-term persistent infection in the reproductive tract of stallions, its deployment of a non-canonical receptor (CD81) that broadens cellular tropism, and its circulation as diverse phylogenetic lineages across disparate equid populations. Understanding the transmission dynamics, host range, and global distribution of EAV requires a synthesis of molecular virology, population genetics, and field-based serosurveillance, drawing on decades of investigation into this prototypical arterivirus.

Modes of Transmission: Venereal, Respiratory, and Vertical Pathways

The primary and epidemiologically most consequential route of EAV transmission is venereal, mediated through the semen of persistently infected stallions. Following acute infection, a variable proportion of stallions, ranging from 10% to 70% depending on host genetic factors, become long-term carriers, shedding virus continuously in their semen for months to life [9, 19]. This carrier state is testosterone-dependent and localized to the accessory sex glands, particularly the ampullae of the vas deferens, where EAV persists despite robust local inflammatory and mucosal antibody responses [12, 15, 16]. The seminal plasma of carrier stallions contains high viral loads, often exceeding 10⁹ RNA copies per milliliter, and the virus remains infectious in cryopreserved semen used for artificial insemination, thereby enabling international dissemination of strains [28, 37, 41]. Epidemiological investigations have repeatedly demonstrated that the introduction of a seronegative mare to a carrier stallion, or the use of contaminated semen, is a potent driver of outbreak initiation [10, 36].

Respiratory transmission constitutes a secondary but important route, particularly during acute outbreaks or when susceptible animals are congregated. The virus is shed in nasal secretions and exhaled aerosols during the febrile phase of infection, and horizontal spread via the respiratory tract has been documented in settings where seronegative animals are introduced into groups harboring persistently infected individuals [36]. In a landmark outbreak among a show stallion population, acute clinical signs, detection of EAV RNA in nasal swabs and blood, and seroconversion among contact animals provided unequivocal evidence that seminal fluids could serve as a source for respiratory infection, likely through contamination of shared facilities or direct aerosolization [36]. Experimental inoculation models using polarized equine respiratory mucosal explants have confirmed that CD172a⁺ myeloid cells and CD3⁺ T lymphocytes in the nasopharyngeal epithelium are early targets of EAV, establishing the respiratory mucosa as a portal of entry [33].

Vertical transmission occurs during gestation, with pregnant mares susceptible to abortion, particularly following infection with virulent strains during the third trimester. The virus crosses the placenta, infecting fetal tissues and causing necrotizing arteritis of the fetal vasculature, leading to fetal death and expulsion [18, 29]. Transplacental infection is a hallmark of EAV pathogenesis, but its contribution to population-level maintenance is minimal compared to the venereal carrier state, as aborted fetuses and placental membranes are unlikely to perpetuate transmission cycles in most management systems.

Host Range and Cellular Tropism: Beyond the Horse

The natural host range of EAV is restricted to members of the family Equidae, encompassing horses (Equus ferus caballus), donkeys (Equus asinus), mules, and hinnies [34, 40]. Historically, the virus was considered primarily a pathogen of domestic horses, but serosurveillance and viral isolation studies have increasingly revealed infection in other equids. A seminal investigation of feral donkeys in Chile identified a divergent EAV genotype that phylogenetic analysis suggests diverged from equine strains at least 100 years ago, establishing that asinine strains constitute a distinct genetic clade with potentially different pathogenic and transmission characteristics [17]. This finding has profound epidemiological implications: it indicates that EAV has circulated in donkey populations for extended periods, likely with independent evolutionary trajectories, and that cross-species transmission between horses and donkeys may be constrained by host-specific barriers.

The molecular basis of EAV tropism has been illuminated by recent discoveries regarding viral entry mechanisms. Unlike other arteriviruses that utilize the macrophage-specific receptor CD163, EAV employs the ubiquitously expressed tetraspanin CD81 as a primary receptor [1]. This receptor switching event has expanded the cellular tropism of EAV beyond macrophages to include epithelial cells, endothelial cells, fibroblasts, and lymphocytes, a critical factor underpinning its systemic pathogenesis and its ability to establish persistence in the male reproductive tract [1, 6]. CD81 is expressed on virtually all nucleated cells, and its engagement by EAV explains the broad permissiveness of diverse cell types observed both in vitro and in vivo. Importantly, cross-species transmission barriers can arise from incompatibility between viral glycoproteins and host CD81 orthologs: screening of CD81 from brushtail possums demonstrated that this ortholog fails to support EAV entry, identifying a molecular constraint that may restrict EAV to equid hosts [1]. Further complexity in host range determinants comes from the identification of cell-surface vimentin as an attachment factor, cells lacking vimentin are refractory to EAV infection, and overexpression of equine vimentin increases susceptibility in otherwise nonpermissive human cell lines [6]. Thus, the host range of EAV is governed by a multi-step entry process requiring both CD81 and vimentin, with additional contributions from the CXCL16/CXCR6 axis in specific lymphocyte subpopulations.

Donkeys and mules are susceptible to EAV infection, but seroprevalence rates are generally lower than in horses. A large-scale serosurvey across western Europe, encompassing 1,425 equids, reported anti-EAV antibody prevalence of 10.2% in horses, 7.7% in donkeys, and 6.4% in mules and hinnies, marking the first detection of seropositivity in mules in Europe [40]. Whether these differences reflect reduced exposure, lower susceptibility, or differential immune clearance remains unresolved, but the presence of a distinct asinine genotype in South America suggests that the virus may be adapted to donkey hosts [17]. No evidence supports EAV infection in non-equid species, including humans, and the virus is not considered zoonotic. Its listing as a notifiable pathogen by the World Organisation for Animal Health (WOAH) underscores its economic impact on the equine industry rather than any public health risk.

The Carrier Stallion: A Genetic and Evolutionary Nexus

The ability of EAV to persist in the male reproductive tract is the cornerstone of its epidemiology, and host genetics dictate the probability of carrier state establishment. A landmark genome-wide association study localized the determinant to the equine ortholog of CXCL16 on chromosome 11 [19]. Two major allelic variants were identified: CXCL16S (susceptible) and CXCL16R (resistant), which differ by four non-synonymous nucleotide substitutions resulting in amino acid changes at positions 40, 49, 50, and 52 of the CXCL16 protein. The CXCL16S allele encodes a protein that functions as an EAV entry receptor on CD3⁺ T lymphocytes, while CXCL16R does not support viral entry [19]. Stallions homozygous for CXCL16S or heterozygous (CXCL16S/R) are at significantly elevated risk of becoming long-term carriers, whereas those homozygous for CXCL16R are highly resistant to persistence [9, 10, 13, 19]. A TaqMan allelic discrimination qPCR assay has been developed to genotype stallions for CXCL16 alleles, enabling pre-pubertal screening to identify high-risk individuals who can be vaccinated to prevent carrier state establishment [9].

The molecular mechanisms underlying CXCL16-mediated persistence are now being elucidated at the transcriptional level. Transcriptomic analysis of ampullae from carrier stallions reveals that EAV persistence is associated with enhanced expression of both CXCL16 and its receptor CXCR6 on infiltrating CD8⁺ T lymphocytes, implicating this chemokine axis in modulating local immune responses [12]. The CXCL16/CXCR6 axis appears to drive a dysfunctional CD8⁺ T lymphocyte response characterized by expression of the transcription factors eomesodermin (EOMES) and NFATC2, along with upregulation of inhibitory receptors, leading to a state of immune exhaustion that permits viral persistence despite ongoing inflammation [12]. This represents a sophisticated viral strategy to exploit host genetic variation and immune regulatory pathways for long-term maintenance.

Intrahost evolution during persistence is non-stochastic, driven by genome-wide purifying selection with specific regions, particularly ORF1a (nsp2), ORF3, and ORF5, accumulating the majority of nucleotide substitutions [16]. Deep sequencing of sequential semen isolates from carrier stallions reveals that acute infection imposes a selective bottleneck on viral populations entering the reproductive tract, followed by diversifying selection during the establishment of persistence [16]. The ORF5 gene, encoding the major neutralization determinant GP5, exhibits the highest intrahost evolutionary rates, suggesting ongoing immune pressure that drives antigenic variation [7, 16]. Furthermore, polymorphisms in GP4 have been identified, including a unique truncation at position 149 in a UK outbreak strain that had not previously been described in any arterivirus [3]. These evolutionary dynamics ensure that viral populations within a single carrier stallion are genetically heterogeneous, comprising quasispecies that may differ in fitness, antigenicity, and transmissibility.

Global Distribution and Molecular Epidemiology

EAV has a worldwide distribution, with serological evidence of infection reported from every continent where equids are raised, although prevalence varies dramatically by region, management system, and host species. The virus is considered endemic in many parts of Europe, the Americas, and Asia, while some regions, including Iceland, Japan, and New Zealand, have historically maintained very low prevalence or freedom from infection through rigorous import restrictions [34, 39]. The global distribution is shaped primarily by the international movement of horses and semen, with persistently infected carrier stallions serving as the primary vectors for transboundary spread.

Phylogenetic analyses based on ORF5 sequences have classified EAV strains into several major phylogroups, designated A through E, with additional subclusters reflecting geographic and temporal patterns [3, 31]. European strains predominantly cluster within phylogroups D and E, while North American strains include representatives from groups A, B, and C. A comprehensive analysis of UK outbreak strains from 2019 demonstrated that all isolates belonged to phylogroup D and were closely related to viruses circulating in Europe between 2004 and 2011, indicating sustained circulation of a stable lineage within the region [3]. Bayesian phylogenetic reconstruction of these outbreaks allowed inference of transmission direction, confirming the importance of carrier stallion movement in propagating infection across farms [3].

The asinine genotype identified in South American donkeys represents a deeply divergent lineage that may warrant classification as a separate genotype. Phylogenetic dating suggests that these strains diverged from equine EAV strains at least 100 years ago, indicating a long-standing independent evolutionary history in donkey populations [17]. The practical implications of this divergence are significant: diagnostic assays based on equine-derived sequences may underestimate infection prevalence in donkeys, and cross-protection from existing equine vaccines is uncertain.

Seroprevalence surveys provide valuable snapshots of regional exposure. In Turkey, a cross-sectional study of 262 horses in the inner Aegean and Central Anatolia regions reported 8.4% seropositivity, with significantly higher rates in animals over four years of age (11.05%) compared to younger horses (2.47%), consistent with age-related accumulation of exposure [35]. Participation in races and festivals was identified as a significant risk factor, likely reflecting increased contact with animals from diverse origins. In Serbia, a survey of 340 horses revealed 15.88% seropositivity overall, with markedly higher rates on stud farms (21.8%) compared to horses kept on private properties (7.2%), underscoring the role of breeding operations as hubs of transmission [38]. Many of these seropositive stallions were found to shed EAV in their semen, confirming that asymptomatic carriage sustains circulation [38, 41]. A recent large-scale European survey encompassing Spain and the UK reported overall seroprevalence of 9.7% in unvaccinated equids, with Catalonia showing the highest rate (15.6%), followed by Andalusia (8.1%), and southeast UK (3.3%), demonstrating significant geographic heterogeneity even within relatively contiguous regions [40].

The absence of systematic surveillance in many regions, particularly Africa, Asia, and parts of South America, likely leads to substantial underreporting. The virus can circulate silently in populations with minimal clinical disease, as the majority of infections are subclinical, and diagnostic capacity is limited in many endemic areas [29]. Outbreaks of clinical disease are often episodic and may follow the introduction of a carrier stallion into a seronegative herd, as documented in outbreaks in Argentina, the UK, and the United States [3, 28, 36]. The re-emergence of previously circulating strains after periods of quiescence, as reported with a genetic outlier strain in Europe, suggests that EAV can persist in cryptic reservoirs and re-emerge when susceptible populations are reintroduced [31].

The epidemiological landscape of EAV is thus characterized by a delicate interplay between host genetics (CXCL16 polymorphism), viral evolution (intrahost quasispecies dynamics and antigenic drift), management practices (use of carrier stallions for breeding, international movement), and ecological factors (co-circulation of equine and asinine strains). A comprehensive understanding of these interconnected factors is essential for designing effective control strategies, including targeted vaccination of high-risk genotypes, screening of semen for international trade, and surveillance of both clinical and subclinical infections across all equid populations.

Clinical Signs and Pathological Features of Equine Viral Arteritis

Equine viral arteritis (EVA) is a multifaceted, economically significant disease of equids, with a clinical spectrum that ranges from subclinical infection to a severe, systemic, and occasionally fatal illness. The disease is caused by the equine arteritis virus (EAV), a positive-sense, single-stranded RNA virus belonging to the family Arteriviridae, order Nidovirales [18, 29]. The clinical presentation is profoundly influenced by a triad of interacting factors: the intrinsic virulence of the infecting viral strain, the immunological and genetic susceptibility of the host, and a range of environmental and management stressors [29]. A hallmark of EAV pathogenesis is its pronounced tropism for vascular endothelial cells, particularly those of small arteries, and specific populations of mononuclear cells, including macrophages and a subset of CD3+ T lymphocytes [23, 32, 33]. This dual tropism underpins the two principal categories of clinical disease: the acute influenza-like illness driven by systemic vasculitis and the chronic, persistent infection localized to the reproductive tract of stallions [12, 29].

Acute Clinical Signs: The Vasculitic Syndrome

Following an incubation period of typically 3 to 14 days after natural exposure via the respiratory or venereal route, the onset of clinical signs is often sudden [18, 36]. The cardinal sign of acute EVA is a biphasic or remittent fever, often exceeding 105°F (40.5°C), which may persist for 2 to 5 days [18]. This pyrexia is a direct consequence of the systemic inflammatory response triggered by widespread endothelial damage.

The most pathognomonic clinical sign is the development of dependent edema, resulting from increased vascular permeability due to endothelial cell necrosis and inflammation. This edema is most commonly observed in the supraorbital fossae, eyelids, and the distal limbs, but can also involve the prepuce, scrotum, and ventral abdomen [18, 29]. Affected horses may exhibit a characteristic "sawhorse" stance due to pain and stiffness from myositis and laminitis, which, while rare, is one of the most severe complications. Ocular involvement is a frequent feature, presenting as conjunctivitis, serous to mucopurulent ocular discharge, chemosis, and periorbital edema, often giving the animal a distressed appearance [18, 29].

A prominent respiratory component is characterized by serous rhinitis, progressing to a mucopurulent nasal discharge, a frequent dry, hacking cough (tracheitis), and tachypnea. In severe cases, particularly in young foals, a fulminating interstitial pneumonia can develop, which is often rapidly fatal [18]. An urticarial-type skin reaction is also a classic manifestation, presenting as raised, erythematous wheals, particularly over the neck, trunk, and flanks [18]. Other non-specific signs include profound depression, anorexia, and colic-like abdominal pain. Hematological abnormalities during the acute phase include leukopenia, particularly lymphopenia, followed later by a leukocytosis [29].

Reproductive Pathology: Abortion and Fetal Infection

EAV is a highly significant cause of reproductive loss in pregnant mares. Following infection of a seronegative pregnant mare, especially one in the later stages of gestation, typically between 3 and 10 months, the virus can cross the placenta, leading to placentitis, fetal infection, and abortion [18, 29]. Abortions can occur either during the acute phase of the disease or as a sequelae weeks later, sometimes without the mare showing any prodromal clinical signs. The aborted fetus is often fresh and autolytic, but may show characteristic pathological lesions consistent with a systemic viral infection. Fetuses may exhibit a severe, diffuse, necrotizing vasculitis and arteritis, affecting most organs, with a particularly pronounced interstitial pneumonia and myocarditis [29]. While some infected foals may be born alive, they are typically weak, viremic, and often succumb to a severe, progressive interstitial pneumonia within days [18]. In these cases, the pathological findings in the lung are diagnostic, characterized by extensive thickening of the alveolar septa due to mononuclear cell infiltration, type II pneumocyte hyperplasia, and hyaline membrane formation [29]. It is critical to distinguish EAV-induced abortion from other causes such as equine herpesvirus-1 (EHV-1), necessitating laboratory confirmation. The OIE (World Organisation for Animal Health) recognizes EVA as a notifiable disease, underscoring its international regulatory importance for the equine breeding industry.

The Carrier State: Persistent Infection in the Stallion

Perhaps the most unique and epizootiologically critical aspect of EAV infection is its ability to establish a long-term persistent infection (LTPI) in the reproductive tract of a subset of stallions [9, 12]. While the acute clinical signs in the adult stallion are typically mild to moderate and self-limiting, the virus does not clear from the accessory sex glands, particularly the ampullae of the vas deferens [16]. This results in the continuous shedding of infectious virus in the semen, often for months or even for life, making the carrier stallion the sole natural reservoir for the virus and the primary mechanism for its perpetuation within the equine population [9, 10, 13].

This carrier state is a consequence of a complex interplay between the virus and the host's immune system. A seminal discovery in this field was the identification of the host genetic determinant for this persistence: allelic variation in the equine CXCL16 gene [19]. Specifically, stallions carrying at least one copy of the susceptible allele (EqCXCL16^S) are at a significantly elevated risk of becoming long-term carriers following natural infection, whereas those homozygous for the resistant allele (EqCXCL16^R) are highly resistant to this state [9, 10, 19]. The CXCL16 protein functions not only as a potent chemokine but also as a critical entry receptor for EAV on a specific subset of CD3+ T lymphocytes. The susceptible allele produces a protein isoform (EqCXCL16S) that acts as an efficient viral receptor on these cells, facilitating infection and the subsequent establishment of persistence [19]. Carossino et al. (2019) further elucidated the molecular mechanisms of this persistence, demonstrating that the ampullae of carrier stallions exhibit a unique immunological milieu. This environment is characterized by a dysfunctional CD8+ T lymphocyte response, orchestrated by the transcription factors eomesodermin (EOMES) and NFATC2, and modulated by the upregulation of inhibitory receptors, a state suggestive of T-cell exhaustion [12]. The CXCL16/CXCR6 axis plays a central role, acting as a "hub" gene that drives a specific transcriptional network, effectively enabling the virus to evade the local inflammatory and humoral immune responses [12, 15].

Pathological Correlates and Histopathology

The fundamental pathological lesion underlying the clinical signs of acute EVA is a panvasculitis and perivasculitis affecting small- to medium-sized arteries and veins throughout the body [29]. Histologically, this is characterized by necrosis of the tunica media, with infiltration of the vessel wall and surrounding adventitia by neutrophils, lymphocytes, and macrophages. The resulting endothelial swelling, sloughing, and thrombosis lead to the characteristic hemorrhage, edema, and ischemia seen in target organs. In the lung, vasculitis leads to severe interstitial pneumonia, with alveolar edema and fibrin exudation. In the brain, arteritis of the meningeal vessels can lead to neurological signs, including ataxia and depression. The edema in the limbs and scrotum is a direct reflection of increased vascular permeability. The epithelial and endothelial tropism of EAV is further explained by recent discoveries regarding viral entry. While the macrophage-specific receptor CD163 is used by other arteriviruses, EAV has undergone a "receptor switch," utilizing the ubiquitously expressed tetraspanin CD81 as a primary entry receptor [1]. Additionally, cell surface vimentin acts as an important attachment factor, facilitating infection in a wide range of cell types [6].

Pathogenesis of Immune Evasion and Cell Death

EAV deploys sophisticated strategies to subvert the host's innate and adaptive immune responses, facilitating both acute replication and long-term persistence. A key mechanism is the inhibition of type I interferon (IFN) production. Go et al. (2014) demonstrated that EAV nonstructural protein 1 (nsp1) is a potent IFN antagonist, effectively suppressing the activation of the IFN-β promoter in infected equine endothelial cells [24]. Furthermore, the virus directly targets the mitochondrial antiviral-signaling protein (MAVS), a central adaptor in the RIG-I-like receptor pathway. The EAV helicase nsp10 recruits the E3 ubiquitin ligases Smurf1 and MARCH5 to mediate the proteasomal degradation of MAVS, thereby crippling the cell's ability to mount an effective antiviral state [5].

The functional impairment of dendritic cells (DCs) represents another critical evasion strategy. Moyo et al. (2023) showed that EAV infection of monocyte-derived dendritic cells significantly downregulates expression of co-stimulatory molecules like CD83 and inhibits their endocytic and phagocytic capacity. Critically, EAV-infected DCs have a reduced ability to stimulate T cell proliferation, thereby blunting the adaptive immune response. The culmination of this interaction is the killing of infected DCs via apoptosis, effectively eliminating key antigen-presenting cells early in the infection process [32]. The induction of apoptosis is a double-edged sword in EAV pathogenesis. While it contributes to the clinical signs and tissue damage, it may also be a mechanism to limit cellular metabolism and spread. EAV induces apoptosis in a cell-type-dependent manner, activating both the intrinsic (mitochondrial) and extrinsic (death receptor) pathways, as well as the endoplasmic reticulum (ER) stress pathway, with the activation of caspases 3, 8, 9, and 12 [42, 43]. The viral GP5 protein has been specifically implicated as a potent inducer of apoptosis [44]. This complex interplay of direct viral cytopathology, immune evasion, and host cell death orchestrates the full spectrum of clinical and pathological features seen in equine viral arteritis.

Diagnostic Approaches: Virological, Serological, and Molecular Methods

The accurate and timely diagnosis of equine arteritis virus (EAV) infection is paramount for effective disease surveillance, outbreak control, and the management of the carrier state in stallions, which represents the primary viral reservoir. The diagnostic landscape for EAV is multifaceted, encompassing classical virological techniques, serological assays for antibody detection, and a sophisticated array of molecular methods that have revolutionized our understanding of viral evolution, host genetics, and pathogenesis. Given that EAV is a notifiable pathogen to the World Organisation for Animal Health (WOAH) in many jurisdictions, the diagnostic approach must be both robust and standardized to ensure international trade and movement of equids are not compromised. The following sections provide an exhaustive analysis of these methodologies, integrating the latest research findings to present a comprehensive overview of the current state-of-the-art.

Virological Methods: Virus Isolation and Phenotypic Characterization

Virus isolation remains a cornerstone of EAV diagnostics, particularly for the confirmation of active infection in clinical cases and for the identification of persistently infected carrier stallions. The gold standard for isolation involves the inoculation of cell cultures, most commonly rabbit kidney-13 (RK-13) cells, with clinical specimens such as nasal swabs, EDTA-blood (buffy coat), tissues from aborted fetuses, or, most critically, semen [3, 36, 38]. The characteristic cytopathic effect (CPE), which manifests as cell rounding, detachment, and eventual lysis, is typically observed within 2–7 days post-inoculation. However, the sensitivity of virus isolation can be variable and is highly dependent on the quality of the sample, the viral load, and the specific cell line used. While RK-13 cells are the standard, other lines such as Vero, BHK-21, and equine dermal (E. Derm) cells have also been employed, each exhibiting differential susceptibility and apoptotic responses to EAV infection [42, 43]. For instance, the Bucyrus reference strain induces a distinctive apoptotic pathway depending on the cell line, with caspase-8 activation being absent in BHK-21 cells, highlighting the importance of cell line selection for optimal viral recovery [42].

The biological significance of virus isolation extends beyond mere detection. Isolates are essential for subsequent antigenic and genetic characterization, including the determination of neutralization profiles and the study of viral quasispecies. The process of isolation itself can act as a selective bottleneck, potentially enriching for variants that are more fit for growth in cell culture. This is particularly relevant when studying the intrahost evolution of EAV during persistent infection in the stallion reproductive tract. Deep sequencing of sequential viral isolates from semen has revealed that the initial virus populations undergo a selective bottleneck during acute infection, followed by extensive genome-wide purifying selection during persistence [16]. This underscores that while virus isolation provides a tangible viral stock for study, it may not fully represent the entire mutant spectrum present in the original clinical sample. Furthermore, the use of primary cell cultures, such as equine pulmonary artery endothelial cells (EECs) or monocyte-derived dendritic cells (MoDC), offers a more physiologically relevant system for studying viral tropism and pathogenesis, as EAV has been shown to replicate efficiently in these cells, with virulent strains causing significant downregulation of surface markers like CD14 and CD163 [24, 32].

Serological Approaches: From Virus Neutralization to Advanced Immunoassays

Serological testing is indispensable for determining the seroprevalence of EAV in a population, confirming prior exposure or vaccination, and monitoring the immune response in individual animals. The WOAH-prescribed gold standard for serodiagnosis is the virus neutralization test (VNT), which measures the titer of neutralizing antibodies in serum or seminal plasma [27, 38, 41]. The VNT is highly specific and provides a functional measure of antibody activity, but it is labor-intensive, requires cell culture facilities, and takes several days to complete. Moreover, it is subject to inter-laboratory variability, which has driven the development of alternative, more standardized assays.

Enzyme-linked immunosorbent assays (ELISAs) have emerged as powerful tools for high-throughput screening. Several commercial ELISAs are available, including those from Nisseiken Co., Ltd. and VMRD Inc. Comparative evaluations have demonstrated that while both assays exhibit high sensitivity (99.4%), the VMRD-ELISA boasts significantly higher specificity (98.1%) compared to the Nisseiken-ELISA (85.4%), making it the superior choice for diagnostic applications where false positives must be minimized [39]. The development of a competitive ELISA (cELISA) has further provided a well-validated alternative to the VNT, offering the advantages of speed, reproducibility, and the ability to test a large number of samples simultaneously [46]. These assays typically utilize whole virus lysates or recombinant structural proteins as antigens.

A more refined approach involves the use of peptide-based ELISAs. By employing synthetic peptides representing immunodominant epitopes, such as those from the GP5 and M proteins, researchers have developed assays that can be easily standardized between laboratories [27]. A comprehensive peptide microarray study identified seven immunodominant peptides with diagnostic potential, five of which were located within GP5 and two within the replicase polyprotein (nsp2 and nsp10) [8]. Critically, no single peptide was recognized by all positive sera, indicating that a cocktail of multiple peptides is necessary to achieve high diagnostic sensitivity (90%) and specificity (100%) across diverse EAV strains and individual horse immune responses [8]. This peptide-based approach is particularly valuable for distinguishing vaccinated from naturally infected animals (DIVA) if the vaccine lacks specific epitopes.

The characterization of the mucosal antibody response in the reproductive tract of persistently infected stallions has added another layer of complexity to serological interpretation. EAV elicits a local mucosal antibody response in seminal plasma, including IgA, IgG1, IgG3/5, and IgG4/7 isotypes. Interestingly, while IgG1 and IgG4/7 possess virus-neutralizing activity, they are ineffective at clearing the infection, suggesting that EAV employs sophisticated immune evasion mechanisms at the mucosal surface [15]. This finding has profound implications for the interpretation of serological data from carrier stallions, as the presence of neutralizing antibodies in serum or seminal plasma does not equate to viral clearance.

Molecular Methods: The Vanguard of EAV Detection and Characterization

Molecular diagnostic techniques, particularly reverse transcription-polymerase chain reaction (RT-PCR) and its quantitative variants (RT-qPCR), have become the methods of choice for the rapid, sensitive, and specific detection of EAV RNA. These assays are indispensable for screening semen from breeding stallions, as they can detect the viral genome even in the presence of neutralizing antibodies and in samples with low viral loads that may be negative by virus isolation [28, 41]. Real-time RT-PCR assays targeting conserved regions of the EAV genome, such as the ORF1b or ORF7 genes, are widely used for routine diagnosis and have been instrumental in confirming outbreaks, such as the 2019 UK outbreaks, where they facilitated the early detection of silent infections [3, 36].

A landmark advancement in molecular diagnostics is the development of a TaqMan® allelic discrimination qPCR assay for genotyping the equine CXCL16 gene [9]. This assay is not for detecting the virus itself, but for identifying the host genetic determinant of long-term persistent infection (LTPI) in stallions. The CXCL16 gene exists in two major allelic variants: CXCL16^S (susceptible) and CXCL16^R (resistant). The CXCL16^S allele encodes a protein that functions as an entry receptor for EAV on CD3+ T lymphocytes, and stallions carrying at least one copy of this allele are at significantly higher risk of becoming long-term carriers [10, 13, 19]. The TaqMan® assay, which targets a single nucleotide polymorphism (SNP) in exon 2, shows perfect agreement with Sanger sequencing and flow cytometric phenotyping of CD3+ T cell susceptibility [9]. This test is a powerful tool for risk assessment, allowing breeders to identify prepubertal colts that carry the susceptible genotype and prioritize them for vaccination to prevent the establishment of the carrier state.

Beyond detection and genotyping, molecular methods are essential for genetic characterization and phylogenetic analysis. Sanger sequencing of specific genomic regions, particularly ORF5 (encoding GP5), is routinely used for molecular epidemiology. However, the advent of next-generation sequencing (NGS) has provided an unprecedented resolution of EAV evolution. Full-length genome sequencing of EAV from clinical samples has revealed the presence of extensive intrahost viral quasispecies, with single nucleotide variants (SNVs) numbering from 41 to 310 within individual persistently infected stallions [10, 16]. NGS has been used to map the direction of transmission during outbreaks and to identify key adaptive mutations associated with persistence. For example, during persistent infection in the stallion reproductive tract, positive selection pressure drives non-synonymous substitutions predominantly in ORF1a (nsp2), ORF3 (GP3), and ORF5 (GP5) [16]. The identification of a unique truncation in GP4 at position 149 in a 2019 UK strain, a feature not previously described in arteriviruses, underscores the power of NGS for discovering novel genetic determinants of viral fitness [3].

The integration of molecular diagnostics with functional studies has also been transformative. For instance, the identification of CD81 as a required host factor for EAV entry was achieved through a genome-wide CRISPR knockout screen, a sophisticated molecular approach that bypasses traditional virological methods [1]. Similarly, computational modeling using AlphaFold3 has predicted the three-dimensional structure of the GP5/M dimer, revealing conserved and variable regions that are targets for neutralizing antibodies and immune evasion [7]. These structural insights, derived from molecular sequence data, are now informing the rational design of next-generation vaccines.

Finally, the development of novel biosensor technologies represents the cutting edge of EAV diagnostics. Electrochemical immunosensors, which utilize specific antibodies immobilized on gold electrodes to detect viral proteins, offer the potential for rapid, point-of-care testing. A recent study demonstrated the feasibility of this approach by screening seven different receptors (antibodies) to select the optimal one (F6 10G) for detecting EAV protein with high sensitivity and specificity [45]. While still in the research phase, such technologies promise to deliver rapid, field-deployable diagnostic tools that could significantly enhance outbreak response capabilities.

Immune Response and Vaccination Strategies

The interplay between equine arteritis virus (EAV) and the host immune system is a complex, dynamic battleground that ultimately dictates the outcome of infection, ranging from rapid viral clearance to the establishment of a lifelong, sexually transmitted carrier state in stallions. A comprehensive understanding of both the virological strategies for immune evasion and the host’s multifaceted defense mechanisms is paramount for the rational design of next-generation vaccines and immunotherapeutics. This section dissects the innate and adaptive immune responses to EAV, delineates the sophisticated countermeasures employed by the virus, and critically evaluates current and emerging vaccination strategies against this economically significant pathogen.

Innate Immune Sensing and Viral Countermeasures

The initial host defense against EAV relies upon the rapid detection of pathogen-associated molecular patterns (PAMPs) by pattern recognition receptors (PRRs), leading to the induction of type I interferons (IFNs) and pro-inflammatory cytokines. EAV infection is sensed by host cells, triggering an IFN response through the mitochondrial antiviral-signaling protein (MAVS)-mediated signaling pathway [5]. However, EAV has evolved potent mechanisms to dismantle this first line of defense. A critical viral antagonist is nonstructural protein 1 (nsp1), which has been identified as the main suppressor of type I IFN production in equine endothelial cells (EECs). Infection with EAV did not elevate IFN-β mRNA levels, and nsp1 was shown to potently inhibit IFN-β promoter activation [24]. Beyond nsp1, the virus employs a multi-pronged strategy to cripple innate immunity.

The most profound evasion mechanism recently elucidated involves the viral helicase nsp10. EAV nsp10 directly targets MAVS, the central signaling hub on the mitochondrial membrane, for proteasomal degradation. This is achieved by recruiting the cellular E3 ubiquitin ligases Smurf1 and MARCH5 to polyubiquitinate MAVS, thereby eliminating its ability to propagate antiviral signals [5]. This targeted degradation of MAVS represents a novel function for an arteriviral helicase and underscores the virus’s commitment to neutralizing the innate response. Furthermore, this degradation is dependent on the dimerization of nsp10 through its zinc finger motifs, with specific residues on both nsp10 (e.g., D249, S287) and MAVS being critical for this interaction [5]. This sophisticated, multi-faceted suppression of the IFN system, involving both nsp1 and nsp10, ensures that initial viral replication can proceed relatively unchecked.

At the cellular entry level, EAV interacts with a growing repertoire of host factors, which influences not only tropism but also the subsequent immune response. While the tetraspanin CD81 has been identified as a critical receptor, facilitating entry into a broad range of cell types, its engagement may not directly trigger classic antiviral signaling cascades [1]. In contrast, the interaction with cell surface vimentin, which acts as an attachment factor, suggests a more complex interplay that may modulate cellular processes [6]. The utilization of these diverse receptors and attachment factors, coupled with the evasion of MAVS signaling, creates a scenario where early innate activation is severely blunted, allowing the virus to establish a foothold in the host.

The Adaptive Immune Response: A Double-Edged Sword

The adaptive immune response to EAV is characterized by a potent humoral response, a complex cellular response, and a unique capacity for viral persistence that defies immunological clearance, particularly in the reproductive tract of stallions.

Humoral Immunity and Neutralizing Antibodies: Following infection, horses mount a robust antibody response. The primary target for neutralizing antibodies is the major envelope protein GP5, specifically epitopes located within its long, surface-exposed ectodomain [7]. The GP5/M dimer is the principal component of the viral envelope and a key target for neutralization, with neutralizing antibodies directed against GP5 being critical for protection against reinfection [7, 25]. Serological surveys consistently demonstrate a high prevalence of neutralizing antibodies in infected populations, and these are a cornerstone of serodiagnosis [10, 35, 38-40]. Despite this, the virus persists. This paradox is partly explained by the nature of the antibody response in the reproductive tract. In persistently infected stallions, EAV elicits a local mucosal antibody response in the seminal plasma, characterized by IgA, IgG1, IgG3/5, and IgG4/7 isotypes. Notably, while IgG1 and IgG4/7 possess virus-neutralizing activity in vitro, they are ineffective at clearing the virus in vivo, suggesting the presence of potent local immune evasion mechanisms that shield the virus from antibody-mediated neutralization [15].

Cellular Immunity and the Carrier State: Cellular immunity is profoundly modulated by EAV infection, a critical factor in the establishment of the carrier state. The virus specifically targets key antigen-presenting cells. It infects monocytes and monocyte-derived dendritic cells (MoDC), with replication being most efficient in mature MoDC [32]. Critically, infection with a virulent EAV strain leads to downregulation of CD83 on mature MoDC and inhibits their endocytic and phagocytic capacity. This results in a reduced ability of infected MoDC to stimulate T cells, effectively crippling the initiation of an effective adaptive cellular response [32]. This is a powerful immunoevasion strategy that induces a state of functional paralysis in the very cells required to prime antiviral T cells.

The most defining immunological feature of EAV pathogenesis is the establishment of long-term persistent infection (LTPI) in the stallion. This phenomenon is genetically determined, with a specific allele of the equine CXCL16 gene (EqCXCL16S) acting as the primary host determinant. The EqCXCL16S protein isoform functions as a cellular receptor for EAV on CD3+ T lymphocytes, rendering these cells susceptible to infection [9, 19]. This creates a unique viral reservoir within a lymphocyte subset, which is protected from immune-mediated clearance. Furthermore, the persistence is orchestrated by a dysfunctional local CD8+ T lymphocyte response within the reproductive tract. Transcriptomic analysis of the ampullae, the primary site of persistence, has revealed that these CD8+ T cells are driven by transcription factors like eomesodermin (EOMES) and exhibit an exhausted phenotype due to upregulation of inhibitory receptors [12]. The CXCL16/CXCR6 chemokine axis is central to this process, acting as a "hub" gene that drives a specific transcriptional network, leading to unique lymphocyte homing and a non-protective, dysfunctional inflammatory state [12]. This intrahost selection pressure drives viral evolution within the reproductive tract, with key adaptive mutations emerging in the genes encoding nsp2, GP3, and GP5, further refining the virus's ability to persist [16]. Ultimately, EAV replication in dendritic cells and other targets leads to apoptosis-mediated cell death, a final act of immune subversion that eliminates the cellular machinery responsible for propagating an effective immune response [32, 42, 43].

Vaccination Strategies: Current Practices and Future Horizons

Given the economic impact of EAV and the unique challenge of the carrier stallion, vaccination strategies are a global priority. Current modified live virus (MLV) and inactivated vaccines are available and can induce protective neutralizing antibody responses, reducing clinical disease and abortion [11, 25]. However, they have limitations, including the inability to consistently prevent the establishment of LTPI in genetically susceptible stallions.

Targeting the Genetic Basis of Persistence: A revolutionary advancement is the ability to identify stallions at risk of becoming long-term carriers. The development of a TaqMan® allelic discrimination qPCR assay for the fast genotyping of the EqCXCL16 gene allows for the identification of colts carrying the susceptible genotype (EqCXCL16S/S or EqCXCL16S/r) [9]. This has profound implications for vaccination protocols. It is now highly recommended that these susceptible colts be vaccinated against EAV after six months of age to prevent the establishment of the LTPI carrier state following potential natural infection [9]. This represents a shift from a one-size-fits-all approach to a precision vaccination strategy based on host genetics.

Rational Vaccine Design and Immune Enhancement: Research is actively focused on improving vaccine efficacy and safety. Reverse genetic systems using infectious cDNA clones have been invaluable for identifying virulence determinants and attenuation markers, paving the way for safer MLV strains [25]. One promising approach involves the rational modification of the virus to enhance immunogenicity while crippling its immune evasion capacity. For instance, disabling the deubiquitinase (DUB) activity of the viral papain-like protease 2 (PLP2) was investigated as a strategy to create a more potent vaccine. While an EAV mutant lacking PLP2 DUB activity was replication-competent in vivo and could protect against challenge, it did not induce a demonstrably enhanced immune response in the experimental setting used [21]. Nonetheless, this approach highlights the potential of targeting specific viral immune antagonists to unlock a stronger immunostimulatory effect.

Structural insights into key immunogens are also guiding vaccine design. The high-resolution AlphaFold3 model of the GP5/M dimer has identified the GP5 ectodomain as the most surface-exposed region comprising four α-helices and a disulfide-linked β-sheet containing the neutralizing epitopes [7]. Furthermore, the presence of both conserved and variable N-glycosylation sites on GP5 suggests a mechanism of glycan shielding, which could be manipulated in a vaccine antigen to reveal conserved, essential epitopes while masking decoy or variable ones [7].

Alternative Therapeutic Strategies for Carrier Stallions: For managing existing carriers, innovative therapeutic approaches are under investigation. The carrier state is testosterone-dependent, and anti-GnRH vaccination (e.g., with the Equity™ vaccine) has been shown to reduce testosterone secretion and clear EAV shedding in a significant proportion of stallions under field conditions. While effective, this approach can lead to long-term reproductive effects in some animals [37]. Another promising development is the use of recombinant VHH nanobodies, which are antigen-binding fragments derived from camelid heavy-chain-only antibodies. These small, stable molecules have been explored for their potent virus-neutralizing capacity, offering a potential passive immunotherapy for the clearance of EAV in carrier stallions [47].

Finally, in the absence of a foolproof vaccine for all scenarios, antiviral drug development provides a first line of defense for outbreak control. Inhibitors of purine and pyrimidine biosynthesis, including ribavirin and DHODH inhibitors, have been shown to potently suppress EAV replication in vitro [11]. The World Organisation for Animal Health (WOAH) recognizes the importance of controlling EAV, and these diverse strategies, from precision vaccination to immunomodulation of carriers, are critical for global eradication efforts. The path forward requires integrating host genomics with advanced virology to outpace this highly adapted arterivirus.

Control, Prevention, and Biosecurity Measures

Molecular Foundations of Antiviral Intervention

The control of equine arteritis virus (EAV) infection requires a multifaceted approach that is now being revolutionized by our deepening understanding of the virus’s entry mechanisms and host interactions. The recent identification of CD81 as a critical entry receptor for EAV, distinct from the CD163 receptor used by other arteriviruses, has profound implications for targeted intervention strategies [1]. This receptor switching event, where EAV adopted the ubiquitously expressed tetraspanin CD81, explains the virus’s broad tropism beyond macrophages and provides a new molecular target for prophylactic and therapeutic blockade. The demonstration that soluble CD81 can protect cells from infection in vitro, and that specific alpha helix “D” of the CD81 large extracellular loop is essential for viral entry, immediately suggests that biologics designed to occlude this binding interface could serve as potent entry inhibitors [1]. Furthermore, the identification of cell surface vimentin as an attachment factor that facilitates EAV infection complements this picture, indicating that viral entry is a multi-step process involving both primary attachment and receptor-mediated internalization [6]. Consequently, strategies that combine the blockade of CD81 engagement with the inhibition of vimentin-mediated attachment could offer a synergistic antiviral approach, potentially reducing the effective dose and minimizing the risk of resistance emergence. This is particularly relevant given that EAV, like many RNA viruses, exhibits significant genomic plasticity, as evidenced by the non-stochastic evolutionary patterns observed during persistent infection in the stallion reproductive tract, where selective pressure drives mutations in ORF1a (nsp2), ORF3, and ORF5 [16]. Understanding this evolutionary landscape is essential for designing durable control measures that can withstand viral adaptation.

Advanced Vaccination Strategies and Immune Evasion

Despite the availability of both modified-live virus (MLV) and killed virus vaccines, coverage remains insufficient to prevent global outbreaks, as highlighted by the persistent seroprevalence observed across diverse regions [11, 40]. The structural characterization of the GP5/M dimer, the primary target for neutralizing antibodies, has revealed sophisticated immune evasion mechanisms that complicate vaccine development [7]. The GP5 ectodomain features a long, disulfide-linked β-sheet that harbors the most variable neutralizing epitopes, and this region is flanked by both conserved and variable N-glycosylation sites [7]. This arrangement strongly suggests that antigenic drift and glycan shielding are employed by the virus to escape neutralizing antibody responses, a challenge that must be overcome in next-generation vaccine design. The computational modeling of the GP5/M dimer, with its tilted and kinked transmembrane domains stabilized by hydrophilic interactions, provides a structural roadmap for engineering immunogens that present conserved, vulnerable epitopes while masking less effective ones [7]. Moreover, EAV employs a potent arsenal of innate immune antagonists that subvert the host interferon (IFN) response. The nonstructural protein nsp10 has been shown to promote the proteasomal degradation of the mitochondrial antiviral-signaling protein (MAVS) by recruiting the E3 ubiquitin ligases Smurf1 and MARCH5, thereby dampening IFN-β production [5]. Similarly, nsp1 has been identified as the main IFN antagonist in equine endothelial cells, effectively suppressing the type I IFN response [24]. Any effective vaccination strategy must, therefore, account for these viral countermeasures. One promising avenue explored experimentally involves disabling the deubiquitinase (DUB) activity of the papain-like protease 2 (PLP2) within the viral replicase. In vivo studies demonstrated that an EAV mutant lacking PLP2 DUB activity remained replication-competent but was able to protect horses against challenge with a virulent strain, suggesting that such rationally attenuated viruses could serve as safer and more immunogenic vaccine candidates [21].

Management of the Persistent Carrier State in Stallions

The most critical aspect of EAV control centers on the management of persistently infected stallions, which constitute the sole natural reservoir of the virus in equine populations. The discovery that a specific allelic variant of the equine CXCL16 gene (CXCL16S) is strongly correlated with the establishment of long-term persistent infection (LTPI) has provided a powerful genetic tool for risk assessment and targeted prophylaxis [9, 19]. Stallions carrying the CXCL16S allele (either homozygous or heterozygous) are at significantly higher risk of becoming long-term shedders following natural infection, and this trait demonstrates a dominant mode of inheritance [9, 19]. In stark contrast, stallions homozygous for the CXCL16R allele are largely resistant to establishing LTPI. The recent development of a TaqMan® allelic discrimination qPCR assay now enables rapid, routine genotyping of prepubertal colts, allowing veterinarians and breeders to identify those at greatest risk [9]. The recommendation that CXCL16S-positive colts be vaccinated after six months of age is a landmark shift toward personalized preventive medicine in equine virology [9]. Furthermore, the mechanistic link between the CXCL16 allele and viral persistence has been elucidated at the transcriptomic level. In the ampullae, the primary site of EAV persistence, the presence of the CXCL16S allele is associated with enhanced expression of CXCL16 and its receptor CXCR6 on infiltrating lymphocytes [12]. This chemokine axis appears to orchestrate a dysfunctional CD8+ T lymphocyte response driven by transcription factors such as eomesodermin (EOMES) and NFATC2, along with the upregulation of inhibitory receptors, effectively allowing the virus to maintain a foothold despite a robust local inflammatory response and the presence of neutralizing antibodies in seminal plasma [12, 15]. For stallions that have already established LTPI, hormonal intervention offers a non-antiviral therapeutic route. Anti-gonadotropin-releasing hormone (GnRH) vaccination has been shown to reduce testosterone secretion, leading to a significant decrease in viral load in semen and, ultimately, the cessation of shedding in a majority of treated stallions over a period of several months [37]. While this approach can effectively clear the carrier state, it carries the risk of prolonged testosterone suppression and associated reproductive deficits in a subset of animals, necessitating careful case-by-case evaluation [37].

Biosecurity Protocols and Quarantine Strategies

Comprehensive biosecurity measures are the bedrock of EAV prevention in breeding populations and performance horse environments. The epidemiology of EAV highlights that transmission occurs both venereally, through the semen of persistently infected stallions, and horizontally, via the respiratory route following direct or indirect contact with aerosolized virus [29, 36]. The seroprevalence data collected from diverse geographical regions, including Catalonia (15.6%), Andalusia (8.1%), the UK (3.3%), Turkey (8.4%), and Serbia (15.9%), underscore that EAV is widely but heterogeneously distributed, and that imported animals represent a significant risk vector [35, 38, 40]. Quarantine protocols for newly introduced horses should mandate a minimum of 21–28 days of isolation, the standard period for virus excretion and seroconversion, coupled with sequential serological testing using validated enzyme-linked immunosorbent assays (ELISAs) such as the VMRD-ELISA, which has demonstrated superior specificity (98.1%) compared to other commercial kits [39, 40]. It is critical that stallions intended for breeding be tested for the presence of EAV in their semen using reverse-transcription quantitative PCR (RT-qPCR) prior to the breeding season or before the importation or exportation of semen straws [28, 41]. The identification of divergent EAV genotypes in South American donkeys and the re-emergence of genetic outlier strains in the UK emphasize that diagnostic surveillance must employ broadly reactive assays capable of detecting newly emerging variants [3, 17, 31]. The recent 2019 UK outbreaks, where silent infections occurred in horses carrying the CXCL16 heterozygous genotype, highlight the danger of relying solely on clinical signs for detection [3]. Farms and studs should implement a color-coded segregation system based on serostatus, where seronegative animals are housed separately from seropositive animals, and all personnel must adhere to strict hygiene protocols, including the use of dedicated equipment, footbaths, and disposable gloves when handling reproductive fluids [29, 36].

Diagnostic Surveillance and Genomic Monitoring

Surveillance must be underpinned by a toolkit of robust, complementary diagnostic methodologies. While the World Organisation for Animal Health (WOAH)-prescribed virus neutralization test (VNT) remains the gold standard, it is labor-intensive, requires cell culture facilities, and takes 3–5 days to produce results [27]. Competitor ELISAs, such as the peptide-based ELISA targeting the GP5 epitope C, offer a standardized, rapid, and high-throughput alternative that is particularly useful for screening large populations [27]. For antigens, the development of an antigen-capture ELISA using combined polyclonal and monoclonal antibodies (such as the F6 10G receptor) has shown promise for direct detection of viral protein in clinical samples, potentially complementing nucleic acid-based methods [45, 50]. At the molecular level, genomic monitoring through full-length sequencing of EAV isolates from outbreaks and from persistently infected stallions is essential for tracking viral evolution and informing vaccine strain selection. The Bayesian phylogenetic analysis of the 2019 UK outbreak strains revealed directional transmission patterns and identified maximum variability in GP3, GP2, nsp2, GP4, and GP5, with one strain exhibiting a unique truncation in GP4 at position 149, a feature not previously observed in any arterivirus [3]. This level of genetic granularity is critical for understanding how the virus adapts to host selective pressures, especially within the microenvironment of the stallion reproductive tract, where intrahost evolution is driven by purifying selection and where resistance to antiviral interventions (such as the adaptive mutations in nsp2 and nsp5 that confer resistance to cyclophilin inhibitors) can emerge [14, 16]. Monitoring for such escape mutations should be an integral component of any antiviral stewardship program, ensuring that therapeutic interventions remain effective.

Emerging Therapeutic Frontiers and Future Directions

Beyond vaccination and biosecurity, the therapeutic landscape for EAV is expanding rapidly, driven by the repurposing of broad-spectrum antiviral compounds and the discovery of novel biologics. The high-throughput in vitro screening of nucleoside analogs and metabolic inhibitors has identified ribavirin and two inhibitors of dihydroorotate dehydrogenase (DHODH), the fourth enzyme in the de novo pyrimidine biosynthesis pathway, as potent suppressors of EAV replication in equine dermal cells [11]. These compounds effectively inhibit cytopathic effects and reduce infectious virus particle production, and given that both ribavirin and several DHODH inhibitors are either already approved or in advanced clinical trials for human use, their repurposing for emergency use in equine outbreaks is a tangible possibility [11]. The vulnerability of EAV to inhibitors of nucleotide biosynthesis underscores the virus’s reliance on a high flux of metabolic precursors to sustain its rapid replication cycle. Another promising class of inhibitors targets the viral replicase complex directly. Cyclophilin inhibitors such as cyclosporine A (CsA) and alisporivir (ALV) have been shown to inhibit nidovirus replication, with resistance mapping to adaptive mutations in the transmembrane replicase subunits nsp2 and nsp5 [14]. This finding implicates cyclophilins as essential host factors for arteriviral RNA synthesis and suggests that a combination therapy targeting both the host cyclophilin pathway and a viral target could raise the barrier to resistance. Natural products also continue to yield active compounds. The ethanolic extract of Origanum vulgare, rich in caffeic acid, p-coumaric acid, quercetin, carnosic acid, and kaempferol, exhibits significant virucidal and post-infection inhibitory activity against EAV [48]. Similarly, the antimicrobial peptide P34, produced by Bacillus sp., reduces viral titers through a time- and temperature-dependent mechanism likely targeting the viral envelope [49]. These natural compounds could serve as low-cost, locally available options for outbreak containment in resource-limited settings, but their in vivo efficacy and safety profiles require rigorous evaluation. At the forefront of biologic therapy, the development of recombinant VHH nanobodies, single-domain antibodies derived from camelid heavy-chain-only antibodies, offers a highly specific and thermostable tool for clearance of EAV from carrier stallions [47]. These nanobodies can be engineered to neutralize the virus by blocking receptor binding or by inducing aggregation, and their small size facilitates penetration into the tissues of the reproductive tract where the virus sequesters. When combined with CXCL16 genotyping to identify high-risk animals, nanobody therapy could provide a targeted, non-hormonal method to eliminate the carrier state, thereby breaking the chain of transmission at its most critical link.

References

[1] Maloney SM, Shaw TM, Nennig K, Larsen MS, Shah AP, Kumar A, et al.. CD81 is a receptor for equine arteritis virus (family: Arteriviridae). mBio. 2025. DOI: https://doi.org/10.1128/mbio.00623-25

[2] Kublicka A, Lorek D, Mikołajczyk-Martinez A, Chodaczek G, Chwirot A, Bażanów B, et al.. Imaging flow cytometry reveals the mechanism of equine arteritis virus entry and internalization. Scientific Reports. 2025. DOI: https://doi.org/10.1038/s41598-025-87080-x

[3] Bhat S, Karunakaran S, Frossard J, Choudhury B, Steinbach F. Genetic characterization of equine arteritis virus associated with outbreaks in the UK, 2019. Journal of General Virology. 2025. DOI: https://doi.org/10.1099/jgv.0.002181

[4] Matczuk A, Kublicka A, Chodaczek G, Siedlecka M. Dual topology of equine arteritis virus GP3 protein and the role of arginine motif RXR in GP3 ER retention.. Virology. 2024. DOI: https://doi.org/10.1016/j.virol.2024.110122

[5] Zhou B, Chen K, Liang H, Qi T, Guo X, Wen Y, et al.. Equine arteritis virus Nsp10 promotes MAVS proteasomal degradation via E3 ligases Smurf1/MARCH5. Journal of Virology. 2026. DOI: https://doi.org/10.1128/jvi.02061-25

[6] Thieulent CJ, Sarkar S, Carossino M, Bhowmik M, Zhu H, Balasuriya UBR. Cell Surface Vimentin Is an Attachment Factor That Facilitates Equine Arteritis Virus Infection In Vitro. Viruses. 2026. DOI: https://doi.org/10.3390/v18010113

[7] Veit M, Matczuk A. Computational modelling of the equine arteritis virus GP5/M Dimer: Implications for immune evasion and virulence. PLoS ONE. 2026. DOI: https://doi.org/10.1371/journal.pone.0344287

[8] Mayers J, Westcott D, Steinbach F. Identification of Equine Arteritis Virus Immunodominant Epitopes Using a Peptide Microarray. Viruses. 2022. DOI: https://doi.org/10.3390/v14091880

[9] Thieulent CJ, Carossino M, Balasuriya UBR, Graves K, Bailey E, Eberth J, et al.. Development of a TaqMan® Allelic Discrimination qPCR Assay for Rapid Detection of Equine CXCL16 Allelic Variants Associated With the Establishment of Long-Term Equine Arteritis Virus Carrier State in Stallions. Frontiers in Genetics. 2022. DOI: https://doi.org/10.3389/fgene.2022.871875

[10] Socha W, Sztromwasser P, Dunowska M, Jaklińska B, Rola J. Spread of equine arteritis virus among Hucul horses with different EqCXCL16 genotypes and analysis of viral quasispecies from semen of selected stallions. Scientific Reports. 2020. DOI: https://doi.org/10.1038/s41598-020-59870-y

[11] Valle-Casuso J, Gaudaire D, Martin-Faivre L, Madeline A, Dallemagne P, Pronost S, et al.. Replication of Equine arteritis virus is efficiently suppressed by purine and pyrimidine biosynthesis inhibitors. bioRxiv. 2020. DOI: https://doi.org/10.1038/s41598-020-66944-4

[12] Carossino M, Dini P, Dini P, Kalbfleisch T, Loynachan A, Canisso I, et al.. Equine arteritis virus long-term persistence is orchestrated by CD8+ T lymphocyte transcription factors, inhibitory receptors, and the CXCL16/CXCR6 axis. PLoS Pathogens. 2019. DOI: https://doi.org/10.1371/journal.ppat.1007950

[13] Socha W, Larska M, Rola J. Molecular investigation of allelic variants of EqCXCL16 gene in equine arteritis virus infected stallions of selected horse breeds in Poland.. Infection, Genetics and Evolution. 2020. DOI: https://doi.org/10.1016/j.meegid.2020.104455

[14] Wilde AHd, Zanden ALBd, Jong Ad, Bárcena M, Snijder E, Posthuma C. Adaptive Mutations in Replicase Transmembrane Subunits Can Counteract Inhibition of Equine Arteritis Virus RNA Synthesis by Cyclophilin Inhibitors. Journal of Virology. 2019. DOI: https://doi.org/10.1128/JVI.00490-19

[15] Carossino M, Wagner B, Loynachan A, Cook R, Canisso I, Chelvarajan L, et al.. Equine Arteritis Virus Elicits a Mucosal Antibody Response in the Reproductive Tract of Persistently Infected Stallions. Clinical and Vaccine Immunology. 2017. DOI: https://doi.org/10.1128/CVI.00215-17

[16] Nam B, Mekuria ZT, Carossino M, Li G, Zheng Y, Zhang J, et al.. Intrahost Selection Pressure Drives Equine Arteritis Virus Evolution during Persistent Infection in the Stallion Reproductive Tract. Journal of Virology. 2019. DOI: https://doi.org/10.1128/JVI.00045-19

[17] Rivas J, Neira V, Mena J, Brito B, Garcia A, Gutiérrez C, et al.. Identification of a divergent genotype of equine arteritis virus from South American donkeys. Transboundary and Emerging Diseases. 2017. DOI: https://doi.org/10.1111/tbed.12703

[18] Ge M, Mm A, R FC, I C, Fa O, Mg E. Equine arteritis virus cytopathic effect: caspase-dependent cell death as the major consequence observed. Journal of Microbiology & Experimentation. 2018. DOI: https://doi.org/10.15406/JMEN.2018.06.00210

[19] Sarkar S, Bailey E, Go Y, Cook R, Kalbfleisch T, Eberth J, et al.. Allelic Variation in CXCL16 Determines CD3+ T Lymphocyte Susceptibility to Equine Arteritis Virus Infection and Establishment of Long-Term Carrier State in the Stallion. PLoS Genetics. 2016. DOI: https://doi.org/10.1371/journal.pgen.1006467

[20] Metz GE, Abeyá M, Serena M, Panei CJ, Díaz S, Echeverría MG. Expression of M Protein from LP02/C Equine Arteritis Virus Inhibits Growth of Escherichia Coli M15-pQE30 System. . 2017. DOI: https://doi.org/10.15761/VRR.1000111

[21] Kasteren PBv, Knaap RC, Elzen Pvd, Snijder E, Balasuriya UBR, Born Evd, et al.. In vivo assessment of equine arteritis virus vaccine improvement by disabling the deubiquitinase activity of papain-like protease 2. Veterinary Microbiology. 2015. DOI: https://doi.org/10.1016/j.vetmic.2015.04.018

[22] Rola J, Socha W, Żmudziński J. Sequence analysis of minor protein genes of equine arteritis virus during persistent infection. Bulletin of the Veterinary Institute in Pulawy. 2015. DOI: https://doi.org/10.1515/BVIP-2015-0027

[23] Chelvarajan R, Mondal S, Cook R, Go Y, Sarkar S, Henney P, et al.. CD14hi equine monocytes are preferential initial targets for infection with equine arteritis virus (IRC4P.615). Journal of Immunology. 2015. DOI: https://doi.org/10.4049/jimmunol.194.supp.57.32

[24] Go Y, Li Y, Chen Z, Han M, Yoo D, Fang Y, et al.. Equine Arteritis Virus Does Not Induce Interferon Production in Equine Endothelial Cells: Identification of Nonstructural Protein 1 as a Main Interferon Antagonist. BioMed Research International. 2014. DOI: https://doi.org/10.1155/2014/420658

[25] Balasuriya U, Zhang J, Go Y, Maclachlan N. Experiences with infectious cDNA clones of equine arteritis virus: Lessons learned and insights gained. Virology. 2014. DOI: https://doi.org/10.1016/j.virol.2014.04.029

[26] Matczuk A, Veit M. Signal peptide cleavage from GP3 enabled by removal of adjacent glycosylation sites does not impair replication of equine arteritis virus in cell culture, but the hydrophobic C-terminus is essential.. Virus Research. 2014. DOI: https://doi.org/10.1016/j.virusres.2014.02.005

[27] Metz GE, Lorenzón E, Serena M, Corva S, Panei CJ, Díaz S, et al.. Development of a peptide ELISA for the diagnosis of Equine arteritis virus.. Journal of Virological Methods. 2014. DOI: https://doi.org/10.1016/j.jviromet.2014.04.018

[28] Metz GE, Serena M, Panei CJ, Nosetto E, Echeverría MG. The equine arteritis virus isolate from the 2010 Argentinian outbreak.. Revue scientifique et technique. 2014. DOI: https://doi.org/10.20506/RST.33.3.2331

[29] Balasuriya U, Go Y, Maclachlan N. Equine arteritis virus. Veterinary Microbiology. 2013. DOI: https://doi.org/10.1016/j.vetmic.2013.06.015

[30] Matczuk A, Kunec D, Veit M. Co-translational Processing of Glycoprotein 3 from Equine Arteritis Virus. Journal of Biological Chemistry. 2013. DOI: https://doi.org/10.1074/jbc.M113.505420

[31] Steinbach F, Westcott D, McGowan S, Grierson S, Frossard J, Choudhury B. Re-emergence of a genetic outlier strain of equine arteritis virus: Impact on phylogeny. Virus Research. 2014. DOI: https://doi.org/10.1016/j.virusres.2014.12.009

[32] Moyo N, Westcott D, Simmonds R, Steinbach F. Equine Arteritis Virus in Monocytic Cells Suppresses Differentiation and Function of Dendritic Cells. Viruses. 2023. DOI: https://doi.org/10.3390/v15010255

[33] Vairo S, Broeck WVd, Favoreel H, Scagliarini A, Nauwynck H. Development and use of a polarized equine upper respiratory tract mucosal explant system to study the early phase of pathogenesis of a European strain of equine arteritis virus. Veterinary Research. 2013. DOI: https://doi.org/10.1186/1297-9716-44-22

[34] Glaser A, Chirnside ED, Horzinek MC, Vries ADd. equine arteritis virus. CABI Compendium. 2022. DOI: https://doi.org/10.1201/b19719-33

[35] Ince O, Şevik M. Equine arteritis virus seroprevalence and risk factors in horses in the inner Aegean and Central Anatolia regions of Turkey. Journal of the Hellenic Veterinary Medical Society. 2022. DOI: https://doi.org/10.12681/jhvms.26849

[36] Otzdorff C, Beckmann J, Goehring L. Equine Arteritis Virus (EAV) Outbreak in a Show Stallion Population. Viruses. 2021. DOI: https://doi.org/10.3390/v13112142

[37] Miszczak F, Duncombe LF, Burger D, Ferry B, Legrand L, Fortier G, et al.. Anti-GnRH vaccination of stallions shedding equine arteritis virus in their semen: a field study. Veterinarski arhiv (Tisak). 2020. DOI: https://doi.org/10.24099/vet.arhiv.0790

[38] Lazić S, Lupulović D, Gaudaire D, Petrović T, Lazić G, Hans A. Serological evidence of equine arteritis virus infection and phylogenetic analysis of viral isolates in semen of stallions from Serbia. BMC Veterinary Research. 2017. DOI: https://doi.org/10.1186/s12917-017-1226-x

[39] Bannai H, Nemoto M, Tsujimura K, Yamanaka T, Kokado H, Kondo T. Evaluation of two enzyme-linked immunosorbent assays for the detection of antibodies against equine arteritis virus. Journal of Equine Science. 2018. DOI: https://doi.org/10.1294/jes.29.111

[40] Franco JJ, Gonzálvez M, Cano‐Terriza D, Barbero-Moyano J, Jose-Cunilleras E, Alguacil E, et al.. Equine viral arteritis: Seroprevalence patterns and risk factors in equids from western Europe.. Research in Veterinary Science. 2025. DOI: https://doi.org/10.1016/j.rvsc.2025.105701

[41] Lazić S, Lupulović D, Polaček V, Valcic M, Lazić G, Pašagić E, et al.. Detection Of Equine Arteritis Virus In The Semen Of Stallions In The Republic Of Serbia. Acta Veterinaria. 2015. DOI: https://doi.org/10.1515/acve-2015-0047

[42] Metz G, Abeyá M, Serena M, Panei C, Echeverría M. Evaluation of apoptosis markers in different cell lines infected with equine arteritis virus. Biotechnic & Histochemistry. 2018. DOI: https://doi.org/10.1080/10520295.2018.1521989

[43] Metz GE, Galindo I, Abeyá M, Echeverría MG, Alonso C. Intrinsic, extrinsic and endoplasmic reticulum stress-induced apoptosis in RK13 cells infected with equine arteritis virus.. Virus Research. 2016. DOI: https://doi.org/10.1016/j.virusres.2015.12.010

[44] Metz GE, Serena M, Abeyá M, Dulbecco A, Massone A, Díaz S, et al.. Equine arteritis virus gP5 protein induces apoptosis in cultured insect cells.. Virus Research. 2014. DOI: https://doi.org/10.1016/j.virusres.2014.01.026

[45] Brodowski M, Kowalski M, Białobrzeska W, Pałka K, Walkusz R, Roguszczak J, et al.. Methodology of Selecting the Optimal Receptor to Create an Electrochemical Immunosensor for Equine Arteritis Virus Protein Detection. Chemosensors. 2021. DOI: https://doi.org/10.3390/chemosensors9090265

[46] Chung C, Balasuriya U, Timoney P, Wilson C, Grimm A, Pfahl K, et al.. Equine arteritis virus antibody cELISA, a well-validated alternative to the World Organization for Animal Health (OIE)-prescribed virus neutralization test. Journal of Equine Veterinary Science. 2016. DOI: https://doi.org/10.1016/J.JEVS.2016.02.017

[47] Guerrero M, Carossino M, Go Y, Vincke C, Balasuriya U, Muyldermans S, et al.. Development of recombinant VHH nanobodies as an alternative for clearance of equine arteritis virus in carrier stallions. Journal of Equine Veterinary Science. 2016. DOI: https://doi.org/10.1016/J.JEVS.2016.02.123

[48] Blank D, Côrrea RA, Freitag R, Cleff M, Hübner S. Anti-equine arteritis virus activity of ethanolic extract and compounds from Origanum vulgare. Semina-ciencias Agrarias. 2017. DOI: https://doi.org/10.5433/1679-0359.2017V38N2P759

[49] Silva DEN, Castro CCD, Silva FS, Costa G, Soares MP, Vargas G, et al.. Inhibition of equine arteritis virus by an antimicrobial peptide produced by Bacillus sp. P34. Arquivo Brasileiro De Medicina Veterinaria E Zootecnia. 2017. DOI: https://doi.org/10.1590/1678-4162-9140

[50] Liang H, Zhou B, Hu Z, Chu X, Wang X, Du C, et al.. Development of a Broad-Spectrum Antigen-Capture ELISA Using Combined Anti-p26 Polyclonal and Monoclonal Antibodies for Detection of Equine Infectious Anemia Virus. Microorganisms. 2025. DOI: https://doi.org/10.3390/microorganisms13071500