Equine Hepacivirus

Overview and Taxonomic Classification of Equine Hepacivirus

Discovery and Taxonomic Position within the Flaviviridae Family

Equine hepacivirus (EqHV), also referred to historically as non-primate hepacivirus (NPHV) or Hepacivirus A, represents a seminal discovery in veterinary virology, fundamentally reshaping our understanding of the Hepacivirus genus and its evolutionary trajectory. First identified in 2011 and 2012 through metagenomic investigations of respiratory secretions and subsequent serological screening, EqHV was immediately recognized as the closest known genetic homologue of human hepatitis C virus (HCV) [4, 24, 26]. This phylogenetic proximity is not merely a matter of sequence similarity but extends to profound parallels in genome organization, hepatotropism, replication kinetics, and the capacity to establish both acute and persistent infections [1, 5, 7]. The virus is classified within the family Flaviviridae, genus Hepacivirus, a genus that has undergone explosive expansion in known diversity over the past decade, now encompassing 14 recognized species designated Hepacivirus A through N [29]. EqHV occupies the species Hepacivirus A, a designation that underscores its position as the archetypal non-human hepacivirus and its critical role as a surrogate model for understanding HCV pathogenesis and immune evasion [5, 19, 24, 29].

The taxonomic framework for EqHV has been refined through comprehensive phylogenetic analyses of conserved genomic regions, particularly the NS3, NS5B, and 5' untranslated region (UTR) sequences [3, 4, 10, 29]. These analyses consistently demonstrate that EqHV, along with its close relative bovine hepacivirus (BovHepV), forms a distinct clade within the Hepacivirus genus, with EqHV exhibiting the greatest degree of genetic relatedness to HCV [24, 27]. The International Committee on Taxonomy of Viruses (ICTV) has formally recognized this relationship, and the proposed taxonomic update by Smith et al. (2016) solidified the assignment of EqHV to Hepacivirus A, while HCV was renamed Hepacivirus C to reflect its unique historical position and to minimize confusion within the expanding genus [29]. This classification is supported by robust phylogenetic bootstrapping and genetic distance calculations that clearly demarcate EqHV from other hepaciviruses infecting rodents, bats, and primates [24, 29].

Genomic Organization and Phylogenetic Subtypes

The EqHV genome is a single-stranded, positive-sense RNA molecule of approximately 9.5 to 10.0 kilobases, exhibiting a genomic architecture that is strikingly similar to HCV [4, 15, 26]. The genome encodes a single polyprotein that is co- and post-translationally cleaved by host and viral proteases into at least ten mature proteins: core (C), envelope glycoproteins E1 and E2, p7, and the nonstructural proteins NS2, NS3, NS4A, NS4B, NS5A, and NS5B [4, 25]. A defining feature of hepaciviruses, including EqHV, is the presence of a highly structured 5' UTR that harbors an internal ribosome entry site (IRES) essential for cap-independent translation initiation [15, 20]. Detailed structural and functional analyses using selective 2' hydroxyl acylation analyzed by primer extension (SHAPE) have elucidated the secondary structure of the EqHV 5' UTR, revealing four stem-loops (SLI, SLIA, SLII, and SLIII) analogous to those in HCV [15]. Mutational analysis demonstrated that SLIII, particularly subdomains SLIIIb and SLIIId containing a conserved GGG motif, is absolutely essential for IRES function, mediating interactions with the 40S ribosomal subunit and eukaryotic initiation factor 3 (eIF3) [15]. Furthermore, the EqHV 5' UTR contains a single, conserved binding site for the liver-specific microRNA, miR-122, located between SLIA and SLII [15, 20]. This miR-122 dependence is a hallmark of hepaciviruses and is thought to be critical for maintaining liver tropism and promoting chronic infection by exploiting the tolerogenic hepatic environment [20]. Chimeric virus studies have confirmed that miR-122 enhances EqHV translation and replication, although variants that become independent of this miRNA can arise under selective pressure, suggesting a dynamic evolutionary interplay [20].

Phylogenetic analyses of EqHV strains collected from diverse geographical regions have consistently revealed a single genotype with at least three, and potentially four, distinct subtypes [2-4, 13]. The three well-established subtypes are designated Subtype 1, Subtype 2, and Subtype 3, with Subtype 1 being the most globally prevalent and extensively characterized [2, 3, 10]. Nardini et al. (2024), in a large-scale national survey of Italian horses, provided compelling evidence for a possible fourth subtype candidate, identified through phylogenetic analysis of NS3 sequences, although this requires further confirmation through full-genome sequencing and additional epidemiological sampling [3]. The classification into subtypes is based on nucleotide sequence divergence thresholds, typically employing cut-off values analogous to those used for HCV genotyping [13]. For instance, Lu et al. (2019) utilized these established cut-offs to classify global EqHV strains into one genotype and three subtypes, with the recombination events they identified occurring exclusively within Subtype 1 strains [13]. This recombination, detected within the NS5A and NS5B genes of American strains, highlights the ongoing evolutionary processes shaping EqHV genetic diversity and has implications for understanding viral fitness and potential immune escape [13].

The geographic distribution of these subtypes is not random. Subtype 1 appears to be ubiquitous, having been identified in North America, Europe, Asia, and Africa [2, 3, 10, 11]. Subtype 2 and Subtype 3 show more regional patterns, with Subtype 2 being particularly prevalent in certain European and Asian populations, while Subtype 3 has been reported in Japan and other parts of Asia [4, 9, 10]. Intriguingly, phylogenetic analyses of South African EqHV strains from Thoroughbred foals did not show deep clustering with isolates from specific continents, suggesting a complex history of viral dissemination that may predate modern horse transport [11]. In Mongolia, where EqHV infection is hyperendemic with a prevalence of approximately 40%, molecular phylogenetic analyses of core, NS3, and NS5B sequences, as well as full-genome comparisons, have revealed the circulation of two distinct subgenotypes, further illustrating the genetic complexity within a single geographic region [9]. The identification of a specific mutation in the 5' UTR of Mongolian strains suggests that local evolutionary pressures are driving sequence divergence [9].

Host Range, Natural History, and Global Distribution

EqHV exhibits a narrow host tropism, primarily infecting equids, with Equus caballus (domestic horses) being the principal reservoir [4, 6, 26]. Experimental cross-species infection studies have demonstrated that donkeys (Equus asinus) are also susceptible to EqHV infection, with nearly identical infection kinetics, including a rapid rise in viremia by day three post-inoculation, followed by seroconversion and viral clearance by week 12 [6]. However, transcriptomic analysis revealed distinct immune signatures in donkeys compared to horses, suggesting species-specific differences in the host response to hepaciviral infection [6]. This finding is clinically relevant, as natural EqHV infection in donkeys appears to be less common, with a lower rate of RNA-positive animals despite a similar seroprevalence, indicating either a lack of full adaptation to this host or a predominantly acute, self-limiting course of infection [6]. Sporadic detection of EqHV RNA has also been reported in dogs, but this is considered a spillover event rather than evidence of sustained transmission within canine populations [12, 24]. Importantly, extensive serosurveys have found no evidence of EqHV viremia in humans, even in populations with high occupational exposure to horses, confirming that EqHV is not a zoonotic pathogen and poses no direct threat to human health [9, 21]. The World Organisation for Animal Health (WOAH) does not currently list EqHV as a notifiable disease, but its presence in commercial equine biological products has significant implications for biosecurity and the safety of veterinary biologics, a concern that aligns with WOAH's standards for the quality of veterinary biologicals [2, 16, 23, 28].

The global distribution of EqHV is now well-documented, with molecular and serological evidence of infection reported from every continent where significant horse populations exist, including North America, South America, Europe, Asia, Africa, and Australia [4, 17]. A comprehensive systematic review and meta-analysis by Bezerra et al. (2022), encompassing 23 studies from four continents, estimated the combined global RNA prevalence of EqHV at 7.88% (95% CI: 5.23–11.69%) [17]. However, this prevalence is highly variable across regions, with the highest proportions reported in Asia (16.13%), followed by South America (12.03%), Africa (8.69%), and Europe (3.63%) [17]. These figures must be interpreted with caution, as they represent the stratification of published epidemiological studies and may not accurately reflect true continental prevalence due to sampling biases and differences in study design [17]. For instance, studies from Mongolia have reported RNA prevalence rates as high as 40%, likely reflecting the unique management practices of large, free-ranging horse herds and the potential for efficient horizontal transmission [9]. In contrast, prevalence in European countries such as France (1.9–6.2%) and Austria (4.15%) tends to be lower, while seroprevalence rates are considerably higher, often exceeding 40–60%, indicating that a large proportion of the equine population has been exposed to the virus and subsequently cleared the infection [8, 10, 11, 17]. The highest seroprevalence reported to date is from South Africa, where 83.7% of Thoroughbred foals tested positive for anti-NS3 antibodies, suggesting that EqHV is hyperendemic in that population [11]. This high seroprevalence, coupled with a lower RNA prevalence (7.93%), is consistent with a pattern of widespread, early-life exposure followed by viral clearance in most individuals [11].

The natural history of EqHV infection is characterized by a spectrum of outcomes, ranging from subclinical, transient viremia to persistent infection with chronic hepatitis [1, 4, 7, 14]. Following experimental inoculation, horses typically become viremic within one to two weeks, with peak viral loads occurring around weeks two to four [5, 18, 19]. A significant proportion of infected horses (approximately 60–80%) clear the virus within several weeks to months, coinciding with the development of a robust adaptive immune response, including the production of neutralizing antibodies and the activation of virus-specific T cells [5, 19, 22]. However, a subset of horses fails to clear the virus and develops a persistent infection, defined as the detection of EqHV RNA in serum for more than six months [1, 7, 14]. This persistent infection is associated with chronic hepatitis, characterized by persistently elevated serum liver biomarkers (e.g., GLDH, GGT, AST, bile acids) and histopathological lesions that closely mirror those seen in chronic HCV infection in humans, including portal and lobular lymphocytic infiltration, lymphoid aggregate formation, piecemeal necrosis, and progressive hepatic fibrosis [1, 7, 14]. Jager et al. (2025) documented a cohort of 19 horses with persistent EqHV infection and chronic hepatitis, with a median duration of documented hepatitis of 18.4 months (range: 5.2–120 months) and viremia of 14.8 months (range: 6.9–55.6 months) [1]. Histopathological findings in these horses included fibrosis, lymphocytic infiltrates, lymphoid aggregates, and individual hepatocyte necrosis, a pattern strikingly reminiscent of HCV-associated liver disease in humans [1]. These observations underscore the potential for EqHV to cause significant, progressive liver pathology in a subset of infected horses, challenging the earlier notion that EqHV infection is invariably benign [1, 7, 14]. The mechanisms that determine the outcome of infection, clearance versus persistence, are not fully understood but are likely influenced by a complex interplay of viral factors (e.g., infectious dose, genetic diversity, quasispecies evolution) and host factors (e.g., age, immune competence, genetic background) [5, 18, 19, 25]. Notably, experimental studies have demonstrated that the infectious dose of EqHV plays a critical role in shaping the host immune response, with high-dose infections eliciting a more robust innate and adaptive immune response that may paradoxically lead to more rapid clearance, while low-dose infections may promote prolonged viremia by evading early immune detection [18].

Molecular Virology and Genomic Organization

Equine hepacivirus (EqHV), formally classified as Hepacivirus A within the genus Hepacivirus of the family Flaviviridae, represents the closest known genetic homologue of the human hepatitis C virus (HCV) [1, 4, 5, 24]. This phylogenetic relationship is not merely a taxonomic curiosity; it underpins the profound structural, functional, and replicative parallels between the two viruses, making EqHV a uniquely valuable outbred animal model for understanding HCV pathogenesis and for the preclinical evaluation of antiviral strategies [5, 19, 26, 30]. The molecular architecture of EqHV, from its virion structure to the intricate regulatory elements within its untranslated regions (UTRs), dictates its hepatotropism, its capacity for both acute and persistent infection, and its interactions with the equine host's cellular machinery.

Virion Morphology and Physicochemical Properties

EqHV is a small, enveloped, positive-sense, single-stranded RNA virus. Based on its classification within the Flaviviridae family and its ultrastructural similarity to HCV, the virion is predicted to be spherical, approximately 40–60 nm in diameter, and comprised of a lipid bilayer envelope studded with glycoprotein spikes [4]. These envelope proteins, E1 and E2, are essential for host cell attachment and entry, a process that is believed to involve a multi-receptor complex analogous to that used by HCV. Studies have demonstrated that equine cells express the canonical HCV entry factors, CD81, occludin (OCLN), claudin-1 (CLDN-1), and the scavenger receptor class B type I (SCAR-B1), and that antibodies against these proteins cross-react with equine homologs, suggesting a conserved entry mechanism [31]. The viral envelope encloses a nucleocapsid composed of the core protein, which encapsidates the genomic RNA. The virus is highly sensitive to organic solvents and detergents, a vulnerability conferred by its lipid envelope, and is generally considered to have a moderate environmental stability, though rigorous disinfection protocols are advised for biosecurity.

Genome Organization and Architecture

The EqHV genome is a single-stranded RNA molecule of approximately 9,200 to 9,500 nucleotides in length, organized in a manner quintessential to the Hepacivirus genus [4, 10, 13]. Like HCV, the genome comprises a single, long open reading frame (ORF) flanked by highly structured 5' and 3' untranslated regions (UTRs) that are critical for translation and replication, respectively [15, 20, 29]. The ORF encodes a single polyprotein precursor of roughly 3,000 amino acids, which is co- and post-translationally cleaved by host and viral proteases into at least ten mature structural and non-structural proteins. The canonical gene order, proceeding from the N-terminus to the C-terminus of the polyprotein, is: NH₂-Core-E1-E2-p7-NS2-NS3-NS4A-NS4B-NS5A-NS5B-COOH [4, 13]. The structural proteins (Core, E1, E2) are located in the N-terminal region, followed by the viroporin p7 and the non-structural (NS) proteins that assemble into the membrane-associated replication complex.

The 5′ Untranslated Region (UTR) and Internal Ribosome Entry Site (IRES)

A hallmark of hepaciviruses is their use of a highly structured 5' UTR to mediate cap-independent translation initiation via an internal ribosome entry site (IRES). The EqHV 5' UTR, while sharing a common four-stem-loop architecture (SLI, SLIA, SLII, SLIII) with HCV, exhibits distinct sequence and structural features [15]. Selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) has elucidated the secondary structure of the EqHV IRES, revealing that it is essential for the recruitment of the 40S ribosomal subunit and the translation initiation factor eIF3. Mutational analysis demonstrated that stem-loop III (SLIII), particularly its sub-domains SLIIIb and SLIIId and a conserved GGG motif, is indispensable for IRES activity. The GGG motif mediates a critical interaction with the 40S ribosomal subunit, while a CUU sequence in the apical loop of SLIIIb interacts with eIF3 [15]. In a fascinating demonstration of functional conservation and divergence, a miR-122 binding site is present between SLIA and SLII in the EqHV 5' UTR. This interaction with liver-specific microRNA-122 (miR-122) enhances translation in the context of a subgenomic replicon, mirroring the miR-122 dependence of HCV replication [15, 20]. Intriguingly, experiments with NPHV/HCV chimeras have shown that while miR-122 enhances EqHV IRES function, the virus can, under selective pressure, evolve to replicate independently of this miRNA, suggesting a strong evolutionary pressure to maintain this interaction for optimal liver tropism and persistence [20].

The Polyprotein: Structural and Non-Structural Proteins

The proteolytic processing of the EqHV polyprotein is a carefully orchestrated event that yields the functional repertoire of the virus.

Structural Proteins (Core, E1, E2): The Core protein is a highly basic, RNA-binding protein that forms the viral nucleocapsid. It is released from the polyprotein by signal peptidase cleavages. Unlike pegiviruses, hepaciviruses possess a basic Core protein, a distinguishing feature of the genus [29]. The E1 and E2 envelope glycoproteins are type I transmembrane proteins responsible for receptor binding and membrane fusion. They are heavily N-glycosylated, and comparative sequence analysis has revealed that the hypervariable region 1 (HVR1), a critical immune evasion domain in HCV that is targeted by neutralizing antibodies, is absent in EqHV E2 [25]. Analysis of intra-host glycoprotein evolution in horses shows that while EqHV does not have an HVR1, its E2 sequence does evolve under selective pressure, and patterns of diversity differ between acutely and chronically infected animals, hinting at distinct immune pressures [25].

p7 and Non-Structural (NS) Proteins: The p7 protein is a small viroporin that forms ion channels in cellular membranes and is critical for the assembly and release of infectious virions in HCV, a function likely conserved in EqHV. The NS2 protein is a cysteine auto-protease that cleaves the NS2-NS3 junction. NS3 is a multifunctional protein with a serine protease domain in its N-terminal third, responsible for processing the downstream NS3-to-NS5B junction, and a C-terminal helicase/NTPase domain that is essential for viral RNA replication [32]. Phylogenetic analyses of the NS3 gene have been instrumental in defining EqHV subtypes globally [2, 3, 10]. NS4A acts as a cofactor for the NS3 protease. NS4B is an integral membrane protein that induces the formation of the membranous web, the scaffold for the viral replication complex. NS5A is a RNA-binding phosphoprotein with no enzymatic activity but with critical roles in both replication and assembly. Intriguingly, a natural recombination event within the NS5A gene has been identified in EqHV subtype 1 strains, highlighting a mechanism for generating genetic diversity [13]. Finally, NS5B is the RNA-dependent RNA polymerase (RdRp), the central catalytic engine of the viral replication complex. This enzyme synthesizes new RNA strands via a de novo initiation mechanism [32]. Molecular modeling studies of the EqHV NS5B polymerase have shown a high degree of three-dimensional structural conservation with the HCV NS5B, particularly in the palm, finger, and thumb subdomains. In silico docking experiments have demonstrated that the HCV nucleoside inhibitor sofosbuvir can bind to the catalytic site of EqHV NS5B with a similar interaction pattern, suggesting a potential therapeutic avenue for treating EqHV infection in horses [32].

The 3′ Untranslated Region (UTR) and Replication Signals

The 3' UTR of the EqHV genome, while less characterized than its 5' counterpart, is equally critical for viral replication. It typically contains a variable region, a poly(U) or polypyrimidine tract, and a highly conserved 3' terminal stem-loop structure often referred to as the 3' X-tail. This terminal structure is essential for initiating negative-strand RNA synthesis by the NS5B polymerase [29]. The presence of a poly(U) tract is a distinguishing feature that can vary between hepaciviruses and pegiviruses [29].

Genetic Diversity, Subtypes, and Recombination

EqHV, despite being a relatively conserved RNA virus compared to HCV, exhibits a clear and globally distributed phylogenetic structure. Phylodynamic analyses based on partial NS3, NS5B, and 5' UTR sequences consistently classify EqHV into a single major genotype, which is further subdivided into three well-supported subtypes: Subtype 1, Subtype 2, and Subtype 3 [2, 3, 10, 13]. A fourth putative subtype (Subtype 4) has been proposed based on phylogenetic analysis of strains circulating in Italy, though this requires further confirmation through complete genome sequencing [3]. These subtypes show distinct geographic distributions and, in some studies, have been associated with differences in viral load; for instance, horses infected with Subtype 1 have been shown to exhibit, on average, four-fold higher viral loads than those infected with Subtype 2, a phenomenon potentially attributable to amino acid substitutions in the NS5B palm domain [10].

The evolutionary history of EqHV is dynamic and includes recombination. A landmark study provided strong phylogenetic and bootscanning evidence for a natural recombination event within the NS5A and NS5B genes of EqHV subtype 1 strains, specifically those circulating in North America [13]. This finding underscores an additional, and previously unappreciated, mechanism for generating genetic diversity beyond the high error rate of the NS5B RdRp. The estimated time to the most recent common ancestor of EqHV is approximately 1,100 CE (95% CI: 291–1,640 CE), a date which argues against the notion that the virus's global dissemination was primarily driven by modern veterinary practices [27]. Instead, these deep evolutionary roots suggest that EqHV has been a long-standing pathogen of equids, with its current global prevalence reflecting a combination of historical host migrations, ancient ecological factors, and more contemporary iatrogenic transmission routes.

Molecular Pathogenesis of EqHV Infection

The molecular pathogenesis of Equine Hepacivirus (EqHV) represents a paradigm of host–virus co-evolution that mirrors, yet fundamentally diverges from, human hepatitis C virus (HCV) infection. As the closest known genetic homologue of HCV, sharing genome organization, hepatotropism, and the capacity for both transient and persistent infection, EqHV provides an unparalleled window into the molecular determinants of hepaciviral disease [5, 24]. The pathogenesis of EqHV infection is orchestrated through a sequence of molecular events beginning with viral entry, continuing through cap-independent translation and replication, and culminating in immune-mediated hepatocyte injury. Critically, the viral lifecycle is intimately linked to host liver-specific factors, particularly microRNA-122 (miR-122), which is exploited by the virus to maintain hepatotropism and promote chronicity [20, 29]. Understanding these molecular interactions is essential not only for managing EqHV-associated disease in horses but also for informing vaccine and therapeutic strategies against HCV, a pathogen responsible for an estimated 58 million chronic infections worldwide, according to the World Health Organization.

Viral Attachment and Entry: The Molecular Basis of Hepatotropism

EqHV exhibits a pronounced hepatotropic phenotype, a tropism that is established at the level of viral entry into hepatocytes. The molecular machinery governing EqHV entry appears to be conserved with that of HCV, exploiting a suite of host cell surface receptors that mediate attachment, internalization, and membrane fusion. Studies utilizing cross-reactive antibodies against the established HCV entry factors have demonstrated the expression of Cluster of Differentiation-81 (CD-81), occludin (OCLN), claudin-1 (CLDN-1), and Scavenger Receptor Class B Member 1 (SCAR-B1) on equine cells, including hepatocytes and even fetal horse kidney cells and equine dermal fibroblasts [31]. Flow cytometric analysis revealed that 37.2% of fetal horse kidney cells expressed CD-81, while OCLN and CLDN-1 were detected on 16.0% and 7.0% of these cells, respectively. Remarkably, equine dermal cells exhibited near-ubiquitous CD-81 expression (96.0%), suggesting a broad potential for viral adsorption, though productive infection is restricted to hepatocytes. Immunohistochemical analysis confirmed the presence of CD-81, OCLN, and CLDN-1 within equine liver tissue and notably on the allantochorionic region of the Thoroughbred placenta, raising the possibility of a molecular conduit for vertical transmission [12, 31].

The functional significance of these receptors in EqHV entry, while inferred from HCV biology, is supported by evolutionary analyses. The closest relatives of HCV, including EqHV, are found in horses and donkeys, and evolutionary studies indicate that only CD81 and OCLN among the entry factors determine species-specificity of infection [27]. This suggests that the molecular compatibility between EqHV envelope glycoproteins and equine CD81/OCLN is a critical determinant of host range. Interestingly, signals of positive selection at CD81 have been observed primarily in bats, not equids, implying that the equine entry machinery may be a relatively permissive environment for hepaciviral glycoprotein interaction [27].

The Internal Ribosome Entry Site and Cap-Independent Translation

Upon internalization and uncoating, the positive-sense RNA genome of EqHV must be translated in a cap-independent manner, a process governed by the 5′ untranslated region (UTR). The EqHV 5′UTR harbours an internal ribosome entry site (IRES) that directs the assembly of the translational machinery, a mechanism that is structurally and functionally conserved across the hepaciviruses [15, 29]. Detailed structural analysis using selective 2′ hydroxyl acylation analysed by primer extension (SHAPE) has defined the secondary structure of the EqHV 5′UTR, revealing four stem–loops designated SLI, SLIA, SLII, and SLIII, by analogy to HCV [15]. Functional mutational analysis demonstrated that SLI is dispensable for IRES-mediated translation, whereas SLIII is absolutely essential. Within SLIII, specific subdomains, SLIIIb, SLIIId, and a conserved GGG motif, are critical. SHAPE analysis provided evidence that this GGG motif mediates interaction with the 40S ribosomal subunit, while a CUU sequence in the apical loop of SLIIIb mediates interaction with eukaryotic initiation factor 3 (eIF3) [15].

This molecular architecture is exquisitely tuned to the hepatic microenvironment. A miR-122 target sequence is located within the EqHV 5′UTR, positioned between SLIA and SLII. The liver-specific microRNA miR-122 binds to this site, enhancing translation in the context of a subgenomic replicon [15]. This reliance on miR-122 is not merely a translational enhancer but is central to the viral lifecycle. Using Argonaute cross-linking immunoprecipitation (AGO-CLIP) in vivo, it was confirmed that AGO binds to the single predicted miR-122 site in the EqHV 5′UTR, sequestering the miRNA and preventing its interaction with cellular mRNAs [20]. This molecular piracy serves a dual purpose: it promotes viral translation and replication while simultaneously exploiting the tolerogenic liver environment, where miR-122 is abundant, to establish a niche for chronic infection. Remarkably, through random mutagenesis of EqHV/HCV chimeric viruses, robustly replicating variants have been isolated that are completely independent of any miRNAs; these variants can even replicate in non-hepatic cells when provided with exogenous apolipoprotein E (ApoE) [20]. The fact that miR-122 dependence has been evolutionarily retained suggests that this mechanism provides a selective advantage by ensuring strict hepatotropism, thereby avoiding detection and clearance by systemic immune surveillance.

The NS5B RNA-Dependent RNA Polymerase and Replication Fidelity

The replication of the EqHV genome is catalyzed by the NS5B protein, an RNA-dependent RNA polymerase (RdRp) that initiates RNA synthesis via a de novo mechanism. Structural modeling of the EqHV NS5B, based on crystallographic templates of HCV NS5B in both open and closed conformations, has revealed well-conserved three-dimensional structures shared with other hepaciviral RdRps, suggesting functional conservation across the genus [32]. This structural homology has direct therapeutic implications: in silico docking studies demonstrate that the nucleotide analogue inhibitor sofosbuvir, a cornerstone of HCV therapy, interacts with EqHV NS5B in a manner similar to its interaction with HCV NS5B, with comparable molecular interaction patterns. In contrast, the non-nucleoside inhibitor dasabuvir shows less conserved binding to EqHV NS5B [32]. These findings suggest that sofosbuvir, or structurally optimized derivatives, could potentially be repurposed for treating EqHV infections, contributing to equine welfare.

Phylogenetic and mutational analyses of the NS3 and NS5B genes have identified mutational profiles that may affect NS3 binding affinity to viral RNA. For instance, analysis of Italian EqHV strains revealed amino acid substitutions in the palm domain of NS5B that correlate with subtype-specific differences in viral load; strains belonging to the main subtype exhibited, on average, four-fold higher serum viral loads compared to those infected with a second subtype [10]. This suggests that subtle structural variations in the replication complex can profoundly influence viral fitness and pathogenesis. Furthermore, natural recombination events have been documented within the NS5A and NS5B genes of EqHV subtype 1 strains, particularly among American isolates, indicating that recombination contributes to genetic diversity and may facilitate immune evasion or altered replication kinetics [13].

Intra-Host Evolution and Immune Evasion

The molecular pathogenesis of EqHV is profoundly shaped by its error-prone replication, which generates extensive intra-host genetic diversity. Amplicon-based deep-sequencing of the glycoprotein-encoding regions E1 and E2 has revealed that intra-host virus diversity is significantly higher in chronically infected horses compared to those that clear the infection acutely, a pattern that mirrors observations in HCV-infected patients [25]. However, overall glycoprotein variability is lower in EqHV compared to HCV, and the selection pressure, particularly within the N-terminal region of E2, is markedly reduced. This is attributable to the absence of a hypervariable region 1 (HVR1) in EqHV, a region unique to HCV that is a primary target for neutralizing antibodies and a key driver of immune escape [25]. The lack of HVR1 in EqHV suggests that the equine virus may employ alternative, perhaps less dynamic, mechanisms for immune evasion. The molecular basis for this difference likely lies in structural constraints of the envelope glycoproteins, which may limit the ability to tolerate extensive amino acid substitutions without compromising receptor binding or membrane fusion.

Longitudinal analysis of intra-host nucleotide variation over a seven-month period in a chronically infected horse revealed 26 nucleotide changes resulting in 11 nonsynonymous substitutions, demonstrating ongoing viral evolution in the face of host immune pressure [33]. The pattern of substitutional evolution, when compared between horses and humans, indicates that EqHV infection of horses represents a powerful surrogate model for studying hepaciviral evolution and the molecular basis of persistence versus clearance [25].

Innate and Adaptive Immune Signatures and the Determinants of Chronicity

The outcome of EqHV infection, whether it results in rapid clearance or progression to chronic hepatitis, is determined at the molecular level by the interplay between viral dose, innate immune activation, and the quality of the adaptive T-cell response. Experimental inoculation with precisely defined viral doses has revealed that the minimal infectious dose is approximately 13 RNA copies, while inoculation with 6–7 copies fails to establish productive infection [18]. Critically, transcriptomic analysis demonstrated a nearly dose-dependent effect: high-dose infections (up to 1.3 × 10⁶ copies) elicited robust upregulation of innate and adaptive immune pathways, as well as inflammatory responses, and were associated with early viral clearance. In contrast, low-dose infections induced weaker immune responses and resulted in prolonged viremia [18]. This suggests that the initial virus–host encounter calibrates the immune response; low-dose inocula may enable EqHV to fly under the immunological radar, evading strong innate activation and thereby establishing a persistent foothold. Paradoxically, inoculation with 6–7 copies that did not result in productive infection was associated with a strong immune response similar to that observed in high-dose infections, indicating that non-productive exposure can prime the immune system without supporting viral replication [18].

At the adaptive immune level, viral clearance is associated with the activation of CD3+ T cells, which are sequestered in the liver during acute infection. In horses that clear EqHV, immunohistochemical analysis has revealed a mean 15-fold increase in intrahepatic CD3+ T cell infiltration at the peak of liver enzyme activity, coinciding with increased mRNA levels of CD4, CD8, and proinflammatory cytokines [22]. This suggests that acute hepatitis, the histopathological hallmark of infection, is mediated by CD3+ T cells targeting infected hepatocytes, and that the anti-viral T cell response is largely compartmentalized within the liver rather than detectable in peripheral blood [22]. The induction of protective immunity is further demonstrated by the observation that horses infected with EqHV are protected against rechallenge with both homologous and heterologous isolates, with only minute amounts of circulating virus detectable upon secondary exposure [19]. This protection correlates with the activation of equine immune responses, including seroconversion and the generation of neutralizing antibodies. The role of B cell responses is underscored by vaccination studies using recombinant E2 glycoprotein: although vaccination did not confer sterilizing immunity, the majority of vaccinated ponies cleared serum EqHV RNA earlier than control animals and demonstrated accelerated recovery from liver insult [30].

Molecular Pathways to Chronic Hepatitis and Fibrosis

In a subset of infected horses, the failure to clear the virus leads to persistent viremia and the development of chronic hepatitis, with histopathological features that closely resemble those seen in human HCV infection. Examination of liver biopsies from horses with documented persistent EqHV infection (median viremia duration of 14.8 months) has revealed fibrosis, lymphocytic infiltration, lymphoid aggregates, and individual hepatocyte necrosis [1]. The molecular basis of this chronic injury involves ongoing immune-mediated hepatocyte destruction, accompanied by cycles of regeneration and fibrogenesis. Transcriptomic analysis of liver biopsies from EqHV-infected horses has identified differentially expressed genes, including viral host factors and immune genes, that distinguish acute from chronic infection [6]. Notably, pathways associated with oxidative stress and cholestasis are activated, as evidenced by metabolomic profiling that reveals increased abundance of pyroglutamic acid and taurine-conjugated bile acids in horses with hepatic dysfunction [34].

The establishment of chronicity is also linked to the ability of EqHV to modulate the host interferon response. Transcriptomic studies of experimentally infected donkeys, which clear the virus relatively rapidly, revealed a distinct set of differentially expressed genes compared to horses, suggesting that hosts with more robust or more appropriately timed interferon responses are more likely to achieve viral clearance [6]. The regulatory role of miR-122 extends beyond translation; by sequestering this liver-specific miRNA, EqHV may disrupt the regulation of host genes involved in lipid metabolism and cell survival, creating a cellular environment conducive to persistent replication [20].

In summary, the molecular pathogenesis of EqHV infection is a multifaceted process governed by the conservation of hepaciviral translation and replication mechanisms, the exploitation of host factors such as miR-122, and the quality of the innate and adaptive immune response. The dose-dependent nature of immune activation, the compartmentalization of T cell responses within the liver, and the structural constraints on glycoprotein evolution collectively determine whether infection resolves or progresses to chronic hepatitis. These insights firmly establish EqHV as not only a significant pathogen of equids but as a critical model for understanding the molecular underpinnings of hepaciviral disease and for developing strategies to combat HCV, a major global health priority recognized by the WHO.

Epidemiology and Global Distribution

Equine hepacivirus (EqHV), taxonomically classified as Hepacivirus A within the family Flaviviridae, represents the closest known genetic homologue of human hepatitis C virus (HCV) and has emerged as a globally distributed pathogen of considerable veterinary and comparative medical significance [4, 24, 29]. Since its initial discovery in 2011, EqHV has been documented across six continents, infecting a substantial proportion of the world’s equine population, with prevalence rates varying dramatically by geographic region, diagnostic methodology, and host demographics [4, 17]. The virus exhibits a remarkable capacity for both acute and persistent infection, with chronic viremia documented for periods exceeding five years in some naturally infected horses, mirroring the clinical trajectory observed in human HCV infection [1, 7, 14]. Understanding the nuanced epidemiological landscape of EqHV is critical not only for equine health management but also for its potential utility as a surrogate model for HCV vaccine development and pathogenesis studies [5, 19, 30].

Global Prevalence and Meta-Analytic Estimates

Comprehensive meta-analyses synthesizing data from 23 studies across four continents have yielded a pooled global RNA prevalence of 7.88% (95% CI: 5.23–11.69%) among horses, though this figure conceals substantial regional heterogeneity [17]. A separate systematic review and meta-analysis adhering to PRISMA guidelines reported a global seroprevalence ranging from 30% to over 80% in certain populations, indicating that a vast majority of horses experience exposure to the virus, with many successfully clearing the infection [4]. The disparity between seroprevalence and RNA prevalence underscores the dynamic nature of EqHV infection: most infected horses mount a detectable antibody response and clear the virus within weeks to months, while a subset develop persistent viremia that can last for years [1, 5, 19]. Importantly, the meta-analysis found no significant influence of sex on infection risk (OR: 0.98; 95% CI: 0.69–1.39), suggesting that male and female horses are equally susceptible to EqHV acquisition [17].

Regional stratification reveals striking differences in RNA prevalence. Asia exhibits the highest pooled proportion at 16.13% (95% CI: 7.79–30.43%), followed by South America at 12.03% (95% CI: 9.58–15.01%), Africa at 8.69% (95% CI: 6.71–11.20%), and Europe at 3.63% (95% CI: 2.10–6.22%) [17]. These estimates, however, must be interpreted with caution as they reflect the geographic distribution of published epidemiological studies rather than true continental prevalence, and many regions remain severely under-sampled [4, 17]. The World Organisation for Animal Health (WOAH) has not yet established formal surveillance guidelines for EqHV, and the virus is not currently listed as a notifiable equine pathogen, contributing to substantial gaps in global epidemiological data.

Continental and Regional Distribution Patterns

Europe has been the subject of extensive EqHV surveillance, with prevalence estimates ranging from 1.9% to 6.2% across various national cohorts. In France, a large-scale study of 1,033 horses from stud farms nationwide detected EqHV RNA in 6.2% of samples, with significantly higher prevalence in Thoroughbreds (8.3%) compared to other breeds [10]. A subsequent survey of 256 French horses yielded a lower RNA prevalence of 1.9%, though this discrepancy likely reflects differences in sampling strategy and population demographics [36, 39]. In Austria, a comprehensive investigation of 386 horses revealed an RNA prevalence of 4.15% and a seroprevalence of 45.9%, indicating widespread exposure [8]. Italian studies have documented a national RNA prevalence of 4.27% among 1,801 horses, with no statistical differences observed across production categories (equestrian, competition, work/meat, reproduction) or macro-regions (North, Central, South, Islands) [3]. Notably, a cluster investigation in a southern Italian stable documented an extraordinary 46.2% RNA prevalence among 13 horses, providing compelling evidence for horizontal transmission within confined populations [33]. Germany has also contributed to the European epidemiological picture, with studies documenting chronic EqHV infection associated with severe hepatopathy and wasting [7, 14]. The European College of Equine Internal Medicine has issued a consensus statement acknowledging EqHV as a cause of mild acute hepatitis and potentially chronic hepatitis in European horses, though the natural route of transmission remains speculative [41].

Asia harbors some of the highest EqHV prevalence rates documented globally. In Mongolia, a study of horses from six geographic regions reported an infection rate of approximately 40%, with 17 of 19 horses remaining viremic after a seven-month interval, suggesting a remarkably high rate of persistent infection [9]. Phylogenetic analysis of Mongolian strains revealed two distinct clusters based on geographic origin, with unique mutations identified in the 5' untranslated region [9]. In China, prevalence estimates have ranged from 3.4% to 8.3% across various provinces. An early study of 177 horses from Guangdong Province, Heilongjiang Province, and Hong Kong detected EqHV RNA in 3.4% of samples [42]. A subsequent investigation of 143 racehorses from five farms in China reported a higher prevalence of 11.9% for EqPV-H, with EqHV co-infection observed in a subset of animals [44]. A more recent study of commercial equine serum products in China confirmed the presence of EqHV, equine pegivirus (EPgV), and Theiler's disease-associated virus (TDAV) in biological products, raising significant biosecurity concerns [16]. In Iran, the first molecular survey of EqHV in Khuzestan Province documented an average prevalence of 4.66%, with phylogenetic analysis placing the Iranian strain (IR1-Ahvaz-2024) within EqHV subtype 1 [2]. Korean studies have reported EqHV RNA prevalence of 8.1% in serum samples, with a notable co-infection rate of 35.3% with equine parvovirus-hepatitis (EqPV-H) [37].

Africa remained a blank spot on the EqHV map until 2018, when a landmark study of 454 Thoroughbred foals in South Africa's Western Cape Province documented a seroprevalence of 83.70% and an RNA prevalence of 7.93% [11]. These figures represent some of the highest prevalence rates reported globally, suggesting that EqHV is hyperendemic in South African Thoroughbred populations. Increasing foal age was associated with decreasing antibody prevalence and increasing RNA prevalence, consistent with waning maternal antibody and active viral acquisition [11]. Phylogenetic analysis of South African strains did not reveal in-depth clustering with isolates from particular continents, suggesting a complex evolutionary history and potential long-standing circulation on the African continent [11].

The Americas have contributed substantially to the epidemiological understanding of EqHV. In the United States, a large-scale molecular survey of 1,195 equine serum samples from Alabama, Georgia, and Texas detected EqHV RNA in 5.6% of samples, with significantly higher prevalence in Thoroughbreds (OR: 9.64) and Quarter Horses (OR: 4.16) compared to other breeds [38]. This study also identified age and sex as risk factors for EqPV-H infection, though these associations were less pronounced for EqHV [38]. In Brazil, a cross-sectional study of 500 horses documented EqHV co-infection with EPgV in 18.3% of samples, with horses aged five years or younger having 4.9 times higher odds of EPgV infection compared to older animals [40]. The first description of EqPV-H in South America was reported in Brazilian horses, with 12.5% of 96 horses testing positive for EqPV-H DNA, and co-infection with EPgV being the most frequent combination [43].

Oceania has been the subject of targeted epidemiological investigations. In Australia, a study of 188 horses detected EqHV RNA in 11.2% of samples, representing the first molecular confirmation of EqHV circulation in Australian horses [36, 39]. Phylogenetic analysis revealed that Australian EqHV strains were genomically clustered, suggesting a relatively recent introduction or limited viral diversity within the Australian equine population, in contrast to French strains which were more broadly distributed [36, 39].

Transmission Routes and Risk Factors

The mechanisms of EqHV transmission have been the subject of intense investigation, yet the natural route(s) of infection remain incompletely defined. Vertical transmission has been definitively documented, with detection of EqHV RNA in fetal tissues and allantochorion samples from mares, providing strong evidence for in utero transmission [12]. Iatrogenic transmission via contaminated blood products is well-established, with EqHV RNA detected in commercial equine serum products, tetanus antitoxin, and mesenchymal stem cell preparations [16, 23, 28, 47]. Theiler's disease, a fulminant hepatitis historically associated with administration of equine-origin biologicals, has been linked to both EqHV and EqPV-H, though recent evidence suggests EqPV-H is the primary etiological agent in most cases [23, 45, 47].

Horizontal transmission independent of iatrogenic routes is strongly supported by epidemiological observations. A cluster investigation in an Italian stable documented 46.2% RNA prevalence among horses introduced into the herd over a one-year period, with near-identical viral sequences suggesting direct horse-to-horse transmission [33]. The detection of EqHV RNA in fecal samples from viremic horses provides further evidence for fecal-oral transmission as a potential natural route [37]. Arthropod-borne transmission has been investigated but remains unconfirmed. A comprehensive survey of over 5,000 mosquitoes across Austria failed to detect EqHV RNA, making mosquito vectors unlikely [8]. However, a recent study in eastern Austria detected EqHV RNA in the heads, thoraxes, and abdomens of Stomoxys calcitrans (stable flies), with minimum infection rates of 1.2% in 2021 and 3.9% in 2022, suggesting that mechanical transmission by hematophagous flies may contribute to field infection rates [35].

Risk factor analyses have identified several variables associated with increased EqHV infection probability. Breed predisposition is evident, with Thoroughbreds consistently demonstrating higher prevalence across multiple geographic regions [10, 11, 38]. In the United States, Thoroughbreds had 9.64 times higher odds of EqHV positivity compared to other breeds, while Quarter Horses had 4.16 times higher odds [38]. Age appears to influence infection dynamics, with younger horses more likely to be RNA-positive in some studies, potentially reflecting recent viral acquisition following waning maternal immunity [11, 40]. Management practices, including intensive production systems and frequent horse transport, have been associated with increased infection risk, likely reflecting enhanced opportunities for horizontal transmission [17, 40]. The infectious dose required to establish productive EqHV infection has been experimentally determined to be as low as 13 RNA copies, with lower doses paradoxically associated with prolonged viremia and delayed seroconversion, suggesting that low-dose exposure may facilitate viral persistence [18].

Phylogenetic Diversity and Subtype Distribution

EqHV exhibits a relatively conserved genome compared to HCV, with all global isolates classified into a single genotype comprising three distinct subtypes (subtypes 1–3) [13, 29]. Phylogenetic analyses based on partial NS5B, NS3, and 5'UTR sequences have consistently demonstrated this tripartite structure, with subtypes circulating across multiple continents [10, 13, 42]. A comprehensive phylogenetic study of Chinese EqHV strains provided strong evidence for natural recombination events within the NS5A and NS5B genes of subtype 1 strains, a phenomenon previously unrecognized in EqHV but well-documented in HCV [13]. This recombination event was identified in American strains but not in Chinese or other global isolates, suggesting geographic restriction of recombinant variants [13].

Recent evidence from Italy has suggested the possible existence of a fourth EqHV subtype, identified through phylogenetic analysis of the NS3 gene in a national survey of 1,801 horses [3]. This putative fourth subtype requires further confirmation through full-genome sequencing and expanded geographic sampling. The NS3 protein sequences of Italian isolates revealed mutational profiles that could potentially affect binding affinity to viral RNA, though the functional consequences remain to be determined [3].

Geographic clustering of EqHV strains is evident in some regions but not others. Australian strains form a tight genomic cluster, suggesting limited viral diversity and potentially a single introduction event [36, 39]. In contrast, French strains are more broadly distributed across the phylogenetic tree, indicating multiple introductions or longer-standing viral circulation [10, 36, 39]. Mongolian strains form two distinct clusters based on geographic origin, with unique mutations in the 5'UTR that may represent adaptive changes to local horse populations [9]. South African strains do not exhibit in-depth clustering with isolates from particular continents, suggesting a complex evolutionary history and potential long-standing circulation in African equids [11].

Implications for Equine Health and Biological Product Safety

The global distribution of EqHV has significant implications for equine health management and the safety of equine-derived biological products. The virus has been detected in commercial horse serum batches from New Zealand, Brazil, and the United States, as well as in pregnant mare serum gonadotropin (PMSG) products [16, 28]. The presence of EqHV in biological products poses a risk of iatrogenic transmission to recipient horses and potentially contaminates cell culture systems used for vaccine production and research [16, 28]. The Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) have not issued specific guidelines for EqHV screening in equine biologicals, though the virus's close relationship to HCV and its demonstrated pathogenicity in some horses warrant consideration of routine testing protocols.

The economic impact of EqHV on the global equine industry remains poorly quantified but is potentially substantial. Chronic EqHV infection has been associated with subclinical hepatitis, elevated liver enzymes, and, in a subset of cases, progressive liver fibrosis and failure [1, 7, 14]. The median duration of documented hepatitis in persistently infected horses has been reported as 18.4 months, with some animals maintaining viremia for over 10 years [1]. Histopathological findings in chronic EqHV infection mirror those seen in human HCV, including portal lymphocytic infiltration, lymphoid aggregates, bridging fibrosis, and individual hepatocyte necrosis [1]. These pathological changes may contribute to reduced performance in athletic horses, though direct evidence linking EqHV infection to poor performance remains limited [34, 46].

The global distribution of EqHV, with prevalence rates ranging from less than 2% in some European populations to over 40% in Mongolian horses, underscores the need for continued surveillance and research. The virus's capacity for persistent infection, its association with chronic hepatitis, and its potential for iatrogenic transmission through biological products position EqHV as a pathogen of emerging importance in equine medicine. Future epidemiological studies should prioritize standardized diagnostic approaches, expanded geographic sampling, and longitudinal cohort designs to elucidate the natural history of infection and identify modifiable risk factors for disease progression.

Transmission Routes and Vector Involvement

The elucidation of transmission pathways for Equine Hepacivirus (EqHV) remains one of the most critical and complex challenges in equine virology. As a member of the Flaviviridae family, a taxon renowned for employing diverse and often arthropod-mediated transmission strategies, the natural history of EqHV presents a paradox. While the virus is globally distributed with a combined RNA prevalence estimated at 7.88% (95% CI: 5.23–11.69%) [17], and seroprevalence rates reaching as high as 83.7% in some populations [11], the mechanisms sustaining this high level of transmission in the absence of a clearly defined vector have been a subject of intense investigation. Current evidence points toward a multifactorial transmission model involving iatrogenic, vertical, and likely horizontal routes, with the role of arthropod vectors remaining controversial and context-dependent.

Iatrogenic and Parenteral Transmission: The Role of Biological Products

The most definitively characterized route of EqHV transmission is iatrogenic, occurring through the administration of contaminated equine-origin biological products. This pathway is inextricably linked to the history of Theiler’s disease (serum hepatitis), a fulminant and often fatal hepatic necrosis. Early epidemiological investigations into serum hepatitis outbreaks consistently identified a temporal association with the administration of equine-derived antisera, such as tetanus antitoxin (TAT) and botulism antitoxin. Prospective studies have since confirmed that EqHV, alongside equine parvovirus-hepatitis (EqPV-H), is a key contaminant of these products. In a landmark prospective study of 18 consecutive serum hepatitis cases, EqHV RNA was detected in the serum of 2 of 14 cases, while EqPV-H was present in all 18 [23]. Critically, the TAT of the same lot number administered to 10 of the 12 TAT-associated cases was uniformly positive for EqPV-H, underscoring the direct link between contaminated biologics and disease [23].

The contamination of commercial equine serum products is not a rare event. High-throughput sequencing of commercial horse serum batches from New Zealand, Brazil, and the United States has consistently identified EqHV, equine pegivirus (EPgV), and Theiler’s disease-associated virus (TDAV) [28]. Similarly, a study in China confirmed the presence of EqHV, EPgV, and TDAV in commercially available equine sera used for cell culture propagation, highlighting a global biosecurity concern for both veterinary medicine and biological manufacturing [16]. The risk extends beyond traditional antisera to include modern regenerative therapies. Allogenic stem cell products, which are increasingly used in equine practice, have also been implicated as a source of EqHV transmission, with cases of serum hepatitis reported following their administration [23]. This iatrogenic route is particularly insidious because infected donor horses may be clinically asymptomatic, exhibiting no biochemical evidence of liver disease despite harboring high viral loads [10, 44]. The World Organisation for Animal Health (WOAH) and national veterinary authorities have thus emphasized the critical need for screening donor animals and biological products for EqHV and other hepatotropic viruses to mitigate this preventable cause of disease.

Vertical Transmission: In Utero and Perinatal Pathways

Evidence for vertical transmission of EqHV has accumulated, suggesting that the virus can be passed from mare to foal both in utero and potentially during the perinatal period. The most compelling evidence for in utero transmission comes from a retrospective study of 394 dead foals or fetuses in France. EqHV RNA was detected in three foals, and for the first time, the viral genome was identified in two corresponding allantochorion samples [12]. The detection of viral RNA in the placenta provides a direct anatomical conduit for maternal-fetal transmission, mirroring the known vertical transmission of HCV in humans, albeit at a lower frequency. Further support for this route is provided by the detection of EqHV entry receptors, including CD-81, occludin (OCLN), and claudin-1 (CLDN-1), on the allantochorionic region of the Thoroughbred placenta, demonstrating the molecular feasibility of transplacental infection [31].

The clinical significance of vertical transmission appears to be variable. While some foals may be born viremic and clear the infection, others may develop persistent infection. A study in Mongolia, where EqHV infection is hyperendemic, found that 17 of 19 horses retested after a 7-month interval remained positive, suggesting a high rate of persistent infection that could be established early in life [9]. However, the overall contribution of vertical transmission to population-level prevalence is likely modest compared to horizontal routes. The relatively low RNA prevalence in foals (e.g., 7.93% in South African Thoroughbred foals aged 58–183 days) compared to the high seroprevalence (83.70%) in the same population suggests that horizontal exposure after birth is the dominant driver of infection [11]. The presence of maternal antibodies may provide partial protection in very young foals, but waning passive immunity coincides with increasing RNA detection as foals age [11].

Horizontal Transmission: Evidence for Direct and Indirect Contact

The high global seroprevalence of EqHV, often exceeding 40–60% in adult horse populations [9, 11], cannot be adequately explained by iatrogenic and vertical routes alone. This has driven the search for horizontal transmission mechanisms, including direct contact, fomites, and environmental contamination. Epidemiological evidence for horizontal spread is compelling. A detailed investigation of an EqHV cluster in a small stable in southern Italy documented the sequential infection of 6 out of 13 horses (46.2%) over a one-year period. Phylogenetic analysis of the NS5B, 5’UTR, and NS3 genes revealed that all viruses were genetically highly related (100% nucleotide identity), strongly supporting a point-source introduction followed by horizontal transmission within the herd [33]. The authors concluded that direct horse-to-horse contact, likely through sharing of feed or water buckets or through nasal secretions, was the most plausible mechanism.

The potential for fecal-oral transmission has also been raised. A study in Korea detected EqHV RNA in 8.1% of serum samples from 160 horses, but notably, the virus was not detected in 114 corresponding fecal samples [37]. While this negative finding for EqHV in feces contrasts with the detection of EqPV-H in 5.3% of the same fecal samples, it does not definitively rule out the fecal-oral route for EqHV, as viral shedding may be intermittent or below the limit of detection. The closely related HCV is not typically transmitted via the fecal-oral route, but other flaviviruses can be, and the high stability of enveloped RNA viruses in the environment warrants further investigation. Management factors are also strongly associated with infection risk. A study in Brazil found that the chance of EqHV infection was 4.9 times higher in horses raised under intensive production systems compared to extensive management [40]. This likely reflects increased stocking density, shared feeding and watering equipment, and greater opportunities for contact transmission. Similarly, a meta-analysis of global prevalence data identified management-related variables, such as transport, reproductive practices, and communal housing, as significant risk factors for EqHV infection [17].

Vector Involvement: The Enigma of Arthropod Transmission

Given that EqHV is a member of the Flaviviridae, a family that includes archetypal arthropod-borne viruses (arboviruses) such as West Nile virus (WNV), dengue virus, and yellow fever virus, the hypothesis that EqHV is also vector-borne has been a central focus of research. However, the evidence to date is contradictory and suggests that the role of vectors is either absent, highly inefficient, or restricted to specific ecological niches.

The Case Against Mosquito Vectors: The most comprehensive investigation of mosquito involvement was conducted in Austria, where over 5,000 mosquitoes were collected from horse stables and analyzed for EqHV RNA by RT-qPCR. Despite a concurrent EqHV antibody prevalence of 45.9% and RNA prevalence of 4.15% in the local horse population, no EqHV RNA was detected in any of the mosquito pools [8]. This study included multiple mosquito genera known to be competent vectors for other flaviviruses, providing strong evidence that mosquitoes are unlikely to play a significant role in EqHV transmission in temperate Europe. The authors concluded that the absence of viral RNA in mosquitoes, despite high equine infection pressure, argues against a mosquito-borne cycle for EqHV.

The Case for Stomoxys calcitrans (Stable Flies): In a striking contrast, a subsequent study from the same geographic region (eastern Austria) provided the first evidence of EqHV RNA in a hematophagous dipteran. From 2021 to 2022, 783 Stomoxys calcitrans (stable flies) were collected from three horse barns. EqHV RNA was detected in 14 of 189 pools of heads and thoraxes (including wings and legs), with a minimum infection rate (MIR) of 1.2% in 2021 and 3.9% in 2022 [35]. Critically, the corresponding abdomens from these positive pools were individually analyzed, and EqHV RNA was confirmed in 34 of 40 abdomens from 2021 and 20 of 40 abdomens from 2022 [35]. The detection of viral RNA in the head and thorax region is particularly significant, as it suggests the virus may have migrated beyond the gut, a prerequisite for biological transmission. However, the Ct values were close to the presumed limit of detection, indicating very low viral RNA copy numbers [35]. This could represent residual infectious blood in the fly’s mouthparts (mechanical transmission) or a true, albeit low-level, biological infection. The study’s findings are biologically plausible: S. calcitrans is a persistent and painful biter of horses, is abundant in stables, and is a known mechanical vector for other equine pathogens, including equine infectious anemia virus (EIAV). The seasonal peak of EqHV detection in flies during autumn also aligns with peak stable fly populations.

Synthesis and Unresolved Questions: The divergent results between the mosquito and stable fly studies highlight the complexity of EqHV transmission. The negative mosquito data [8] are robust and suggest that culicid mosquitoes are not competent vectors. The positive stable fly data [35] are intriguing but require cautious interpretation. The low viral loads and the fact that the study only detected RNA, not infectious virus, mean that the vector competence of S. calcitrans remains unproven. It is possible that stable flies act as mechanical vectors, transferring virus on their mouthparts from a viremic horse to a naïve horse during interrupted feeding, a mechanism well-documented for EIAV. Alternatively, the detection of RNA in the head/thorax could be an artifact of contamination during the dissection process. Further research is urgently needed to: (1) attempt to isolate infectious EqHV from field-caught S. calcitrans; (2) perform controlled laboratory feeding experiments to assess vector competence; and (3) investigate other potential arthropod vectors, such as ticks or other biting flies (e.g., Culicoides spp.), which have not been systematically evaluated. The European College of Equine Internal Medicine consensus statement on equine flaviviridae infections currently states that the natural route of transmission for EqHV remains speculative [41], a position that remains accurate despite the recent stable fly data.

Infectious Dose and Host Factors Influencing Transmission

The probability of successful transmission is not solely a function of the route but is also critically dependent on the infectious dose and the host’s immune status. Experimental inoculation studies have elegantly demonstrated that the infectious dose of EqHV is remarkably low. In a dose-titration experiment, the minimum infectious dose was estimated to be approximately 13 RNA copies, whereas 6–7 copies were insufficient to establish infection [18]. This low infectious dose has profound implications for transmission dynamics: it means that even minute quantities of contaminated blood, saliva, or other bodily fluids could initiate a new infection. This finding helps explain the efficiency of iatrogenic transmission via contaminated needles or biological products, where even trace amounts of residual blood can harbor a sufficient viral load.

The dose of infection also dictates the subsequent immune response and infection outcome. High-dose inoculations (e.g., 1.3 × 10⁶ copies) were associated with robust innate and adaptive immune activation, earlier seroconversion, and often rapid viral clearance. In contrast, low-dose inoculations (13–130 copies) led to prolonged viremia and delayed or undetectable seroconversion, potentially facilitating the establishment of persistent infection [18]. This dose-dependent effect suggests that natural transmission events, which likely involve low-dose exposures through contact or fomites, may be more likely to result in chronic infection than high-dose iatrogenic exposures. Furthermore, the host’s prior immune status is a major determinant. Experimental challenge studies have demonstrated that horses which have cleared a primary EqHV infection are protected against rechallenge with both homologous and heterologous isolates, with only minute amounts of circulating virus detectable [19]. This natural immunity likely contributes to the age-related prevalence patterns observed in the field, where older horses are more likely to be seropositive and less likely to be RNA-positive [11, 38].

In conclusion, the transmission of EqHV is a multifaceted process. Iatrogenic transmission via contaminated biological products is the most clearly defined and preventable route. Vertical transmission occurs but is likely a minor contributor to population-level prevalence. Horizontal transmission, likely through direct contact and possibly fomites, is the dominant mechanism sustaining the virus in equine populations. The role of arthropod vectors remains an open and critical question; while mosquitoes appear to be irrelevant, the recent detection of EqHV RNA in Stomoxys calcitrans opens a new avenue of investigation, though definitive proof of vector competence is lacking. The remarkably low infectious dose of EqHV facilitates all of these routes, and the dose-dependent nature of the host immune response may explain the high prevalence of persistent infection observed globally. Future research must focus on isolating infectious virus from potential vectors, characterizing the stability of EqHV in the environment, and identifying the specific bodily fluids (e.g., saliva, urine, nasal secretions) involved in horizontal transmission to inform evidence-based biosecurity protocols.

Clinical Manifestations and Chronic Hepatitis

The clinical consequences of equine hepacivirus (EqHV) infection span a remarkably broad spectrum, ranging from entirely subclinical viremia to a progressive, debilitating chronic hepatitis that can culminate in hepatic failure and death. Understanding this continuum is essential for equine clinicians, as the distinction between transient, self-limiting infection and persistent, pathogenic viral carriage carries profound prognostic and therapeutic implications. The constellation of clinical and pathological findings associated with EqHV-induced chronic hepatitis, as delineated in recent prospective and retrospective investigations, bears striking parallels to the natural history of hepatitis C virus (HCV) infection in humans, reinforcing the unique position of EqHV as both a naturally occurring pathogen of horses and a comparative model for human hepatology [1, 4, 5, 24].

The Spectrum of Clinical Disease: From Subclinical to Hepatic Decompensation

A critical observation emerging from the literature is that the majority of EqHV-infected horses do not exhibit overt clinical signs of liver disease. Numerous cross-sectional and longitudinal prevalence studies have documented active EqHV infection, defined by the presence of viral RNA in serum, in horses that appear clinically healthy and demonstrate no biochemical evidence of hepatopathy [3, 8, 10, 11]. For instance, a large-scale French study of 1,033 horses found that the presence of circulating EqHV was not significantly associated with elevations in standard liver-specific biomarkers, including glutamate dehydrogenase (GLDH), gamma-glutamyl transferase (γ-GT), or bile acids, in the sampled population [10]. Similarly, investigations of racing Thoroughbreds, a population with a known high prevalence of EqHV exposure, have failed to establish a consistent link between EqHV RNA positivity and clinically significant elevations in liver enzyme activity, such as γ-GT and sorbitol dehydrogenase (SDH) [34, 46]. This silent carriage phase is reminiscent of the early, often asymptomatic decades of chronic HCV infection in humans, during which ongoing low-grade hepatocellular injury and fibrosis may proceed undetected without routine biochemical screening [1, 24].

However, this benign picture is not universal. A subset of persistently infected horses develops a clinically significant chronic hepatitis characterized by persistent or intermittent elevations in serum liver biomarkers over extended periods. The most comprehensive characterization of this phenomenon comes from a mixed retrospective and prospective case series by Jager et al. (2025), which meticulously documented 19 horses with chronic hepatitis and persistent EqHV infection [1]. In this cohort, the median duration of documented hepatitis was 18.4 months (range: 5.2–120 months), and the median period of documented viremia was 14.8 months (range: 6.9–55.6 months), definitively establishing that EqHV can establish a long-term, productive infection in the equine liver [1]. Clinically, affected horses may present with non-specific signs of chronic illness, including progressive weight loss, lethargy, inappetence, and poor body condition, as described in detailed case reports of EqHV-infected horses with chronic wasting and severe hepatopathy [7, 14]. In more advanced stages, icterus, hepatic encephalopathy (manifesting as neurologic signs such as depression, ataxia, or head pressing), and photosensitization can develop, reflecting significant synthetic and metabolic failure of the liver [1, 7, 41]. The terminal event in severe cases is acute-on-chronic liver failure, which can be fatal [1].

Histopathological Hallmarks of EqHV-Associated Chronic Hepatitis

The histopathological changes observed in the livers of horses with persistent EqHV infection are remarkably congruent with those seen in human chronic hepatitis C, providing compelling evidence for a shared pathogenic mechanism [1, 5]. In the landmark case series by Jager et al. (2025), liver biopsies from persistently infected horses were independently reviewed and revealed a consistent pattern of chronic inflammatory liver disease [1]. Key findings include:

• Portal and Lobular Inflammation: A predominantly lymphocytic infiltrate is a near-universal feature, affecting both the portal tracts and the hepatic lobules [1]. This cellular composition, dominated by T lymphocytes, is a hallmark of HCV-induced hepatitis and suggests an ongoing host adaptive immune response directed against viral antigens within the liver. Immunohistochemical studies in experimentally infected horses have confirmed that this influx is mediated by CD3+ T cells, with significant increases in both CD4 and CD8 mRNA levels observed in liver tissue at the peak of hepatic injury [22].

• Lymphoid Aggregates and Follicles: The formation of lymphoid aggregates or even well-organized lymphoid follicles within the portal tracts is a distinctive feature shared between EqHV and HCV infections [1]. These structures represent foci of B- and T-cell activation and proliferation, indicative of a chronic, antigen-driven immune response that fails to eradicate the virus.

• Hepatocyte Injury and Necrosis: Individual hepatocyte necrosis, often with associated apoptotic bodies, is a consistent finding, reflecting the direct and immune-mediated cytopathic effects of the virus [1, 49]. This injury is generally mild and piecemeal, rather than the massive, confluent necrosis that characterizes acute hepatotoxins or severe viral hepatitis like Theiler's disease (often associated with equine parvovirus-hepatitis).

• Fibrosis: A critical sequela of chronic inflammation is hepatic fibrosis. In the EqHV-positive cohort, bridging fibrosis, the formation of fibrous septa linking portal tracts and central veins, was documented [1]. This architectural distortion is the precursor to cirrhosis and represents the most significant long-term consequence of chronic hepatitis. The progression of fibrosis in EqHV infection mirrors the slow, insidious fibrotic progression seen in a substantial proportion of untreated chronic HCV patients [1, 24].

Importantly, Jager et al. (2025) specifically excluded horses that died or were euthanized within six months of EqHV detection, as well as those that cleared the virus, to focus on the chronic disease phenotype [1]. This study design underscores that while many horses may clear the infection, those that do not are at risk for a progressive, potentially life-threatening liver disease.

Clinical Biochemistry, Viral Dynamics, and Immunological Correlates

The biochemical profile of EqHV-associated chronic hepatitis is characterized by variable and often intermittent elevations in hepatocellular injury and cholestatic markers. Glutamate dehydrogenase (GLDH) is considered a highly sensitive marker of acute hepatocellular damage in horses, and several studies have demonstrated significantly higher serum GLDH levels in EqHV RNA-positive horses compared to negative controls [8, 18]. Gamma-glutamyl transferase (GGT), an indicator of cholestasis or biliary injury, can also be elevated, although this is less consistently associated with EqHV infection [34, 46]. In a large study of Austrian horses, EqHV RNA-positive animals had significantly higher GLDH levels, despite lacking other clinical signs of hepatitis, suggesting that even subclinical infections impose a measurable burden of hepatocyte turnover [8].

The dynamics of viremia are highly variable and influenced by host immune status and inoculum size. Experimentally infected horses typically develop detectable viremia within one to two weeks post-inoculation [5, 18, 19]. Peak viral loads can reach high titers (up to 10⁸ copies/mL), but the duration of viremia is a key determinant of clinical outcome. Horses that mount a robust and effective immune response clear the virus within several weeks to months, often with no or only transient, mild hepatitis [5, 19]. In contrast, horses that develop persistent infection maintain detectable viral RNA in serum for months to years, and it is within this cohort that chronic hepatitis and fibrosis are observed [1]. Interestingly, the infectious dose of EqHV appears to influence this trajectory: high-dose inoculations have been associated with stronger innate and adaptive immune responses and more rapid clearance, while low-dose exposure may favor the establishment of persistent, low-level viremia that is poorly detected by standard serological assays and evades immune clearance [18].

A notable immunological feature of resolving EqHV infection is the development of protective immunity. Studies have demonstrated that horses that have cleared a primary infection are protected against reinfection with both homologous and distinct viral isolates, although sterilizing immunity is not always achieved, and minute amounts of virus may be transiently detectable after challenge [19]. This finding suggests that a prophylactic vaccine against EqHV (and by extension, HCV) is a biologically plausible goal [19, 30]. The detection of EqHV-specific T-cell responses, particularly within the liver rather than the periphery, further highlights the central role of cellular immunity in controlling hepaciviral infection [22].

Chronic Hepatitis in the Context of Co-Infections and Other Hepatopathies

The clinical interpretation of EqHV infection in a given horse is frequently complicated by the presence of other hepatotropic agents. Co-infections with equine parvovirus-hepatitis (EqPV-H), the primary cause of Theiler's disease, are common and have been reported in horses with chronic hepatitis [1, 37, 44, 48]. In the Jager et al. (2025) cohort, EqPV-H co-infection was identified as a frequent comorbidity, alongside bacterial cholangiohepatitis and equine multinodular pulmonary fibrosis [1]. Distinguishing the relative contribution of each agent to the clinical and histopathological picture can be challenging, as EqPV-H itself is associated with a wide spectrum of hepatic lesions, including centrilobular necrosis, individual hepatocyte death, and lobular and portal infiltrates [49]. It is plausible that synergistic interactions between EqHV and EqPV-H, or other stressors, accelerate the progression of liver disease. For instance, horses co-infected with EqPV-H may have significantly higher viral loads of one or both agents, potentially overwhelming host immune defenses and leading to more severe pathology [1].

Furthermore, EqHV must be differentiated from other causes of hepatitis and elevated liver enzymes in horses. The "high GGT syndrome" observed in racing Thoroughbreds, characterized by marked and unexplained elevations in GGT, has been specifically investigated for a viral etiology. Large case-control studies have found no significant association between EqHV (or EqPV-H) RNA positivity and high GGT activity, indicating that this syndrome is likely a metabolic or exercise-induced phenomenon rather than a sequelae of viral hepatitis [34, 46]. The European College of Equine Internal Medicine consensus statement on equine flaviviridae emphasizes that while EqHV is associated with mild acute and chronic hepatitis, a definitive causal link between viremia and the full spectrum of clinical liver disease in individual cases requires the exclusion of other etiologies, including toxins, bacterial infections, and other hepatotropic viruses [41].

In conclusion, the clinical manifestations of EqHV infection are highly heterogeneous, ranging from asymptomatic viremia to a slowly progressive chronic hepatitis that can culminate in liver failure. The histopathological similarities to human HCV, including lymphocytic infiltration, lymphoid aggregates, and fibrosis, are striking and underscore the potential utility of the equine model for understanding hepaciviral pathogenesis. The emergence of detailed case series documenting chronic disease [1] has shifted the paradigm from viewing EqHV as an innocuous, subclinical infection to recognizing it as a significant, albeit underdiagnosed, cause of progressive liver disease in horses. Its role within the complex landscape of equine hepatopathies, particularly in the context of co-infections with EqPV-H, demands a high index of clinical suspicion and the routine integration of molecular diagnostics into the workup of chronic hepatitis.

Diagnostic Approaches for EqHV Detection

The accurate and timely diagnosis of equine hepacivirus (EqHV) infection is a multifaceted challenge that requires a nuanced understanding of viral kinetics, host immune responses, and the limitations of current methodologies. As EqHV is recognized as the closest genetic relative of hepatitis C virus (HCV) and is increasingly implicated in both subclinical and chronic hepatitis in horses [1, 4, 5], diagnostic approaches must be robust enough to detect active viremia, differentiate between acute and persistent infection, and rule out other hepatotropic pathogens such as equine parvovirus-hepatitis (EqPV-H) and various pegiviruses [23, 45]. The diagnostic armamentarium currently includes molecular detection of viral RNA, serological profiling of host antibodies, and emerging techniques for direct visualization of viral nucleic acids in tissue. Each approach carries distinct biological and clinical implications that demand careful interpretation within the epidemiological context of the individual horse and the population at large.

Molecular Detection of Viral RNA: The Gold Standard

The cornerstone of diagnosing active EqHV infection is the detection of viral RNA in serum, plasma, or liver tissue using reverse transcription quantitative polymerase chain reaction (RT-qPCR). This technique targets highly conserved regions of the viral genome, most commonly the 5′ untranslated region (5′UTR), the non-structural protein 3 (NS3) gene, or the NS5B gene [2, 10, 42]. The choice of target region is critical; the 5′UTR is exceptionally conserved across EqHV subtypes due to its essential role in cap-independent translation mediated by an internal ribosome entry site (IRES) structure, making it an ideal target for broad-spectrum detection [15, 20]. In contrast, sequencing of the NS5B and NS3 regions provides the phylogenetic resolution necessary for subtyping and molecular epidemiological investigations [3, 10, 13].

Quantitative RT-qPCR offers several advantages beyond mere presence or absence of the virus. Viral load quantification, expressed as RNA copies per milliliter of serum, has been correlated with disease severity and persistence. In a large prospective case series of horses with chronic hepatitis and persistent EqHV infection, median viremia persisted for 14.8 months (range 6.9–55.6 months), and viral loads were sufficiently high to be reliably detected by standard RT-qPCR protocols [1]. However, the dynamics of viremia are not static; experimental inoculation studies have demonstrated that infectious dose profoundly influences the kinetics of detectable RNA. Gömer et al. (2022) established that the minimal infectious dose is approximately 13 RNA copies, with doses as low as 6–7 copies failing to establish productive infection but still capable of eliciting a strong host immune response detectable at the transcriptomic level [18]. This finding has profound implications for diagnostic sensitivity: horses exposed to very low viral inocula may harbor transient, low-level viremia that falls below the limit of detection of conventional assays, potentially leading to false-negative results in early or abortive infections.

The diagnostic performance of RT-qPCR is also influenced by sample type and timing. Serum is the most commonly used specimen due to its ease of collection and high viral titers during the acute and chronic phases of infection [8, 41]. However, liver tissue may be required in cases where serum viral loads have declined to undetectable levels, particularly in chronic infections where viral replication may be compartmentalized within the hepatic parenchyma. Tegtmeyer et al. (2019) demonstrated this phenomenon in a horse with severe chronic hepatopathy where EqHV RNA was readily detectable in liver tissue via fluorescent in situ hybridization (FISH) even as serum PCR results became negative over time [7]. This suggests that reliance solely on serum RT-qPCR may underestimate the true prevalence of persistent infection, especially in horses with long-standing disease.

Genotyping and Sequencing for Molecular Epidemiology

Beyond simple detection, sequencing of EqHV genomes has become an indispensable tool for understanding transmission dynamics, viral evolution, and the emergence of novel subtypes. Phylogenetic analysis of partial or complete genomic sequences has revealed that EqHV exists as a single genotype with at least three, and possibly four, distinct subtypes [3, 13, 27]. The NS5B gene, due to its relatively conserved nature yet sufficient variability for phylogenetic discrimination, has been the workhorse of subtyping efforts. Nardini et al. (2024) sequenced the NS3 fragment of Italian EqHV strains and identified a potential fourth subtype candidate, underscoring the ongoing diversification of this virus [3]. These findings are not merely taxonomic; subtype-specific differences in viral load have been observed. Pronost et al. (2017) reported that horses infected with the predominant EqHV subtype in France exhibited viral loads approximately fourfold higher than those infected with the second subtype, a difference that could be linked to amino acid substitutions in the palm domain of NS5B affecting polymerase processivity [10].

Next-generation sequencing (NGS) and metagenomic approaches have further revolutionized our ability to characterize EqHV in complex biological samples. In a horse with chronic wasting and high-titer viremia, NGS of serum revealed an overwhelming abundance of EqHV reads, effectively ruling out other infectious agents and confirming EqHV as the predominant pathogen [7]. NGS has also been instrumental in detecting recombination events, which are relatively rare among hepaciviruses but have been documented in EqHV subtype 1 within the NS5A and NS5B genes [13]. These recombination events can complicate phylogenetic interpretation and may have implications for diagnostic assay design if they occur within primer binding sites. Additionally, deep sequencing of the E1 and E2 glycoprotein-encoding regions has revealed striking differences in intra-host viral diversity between acutely and chronically infected horses, mirroring patterns seen in HCV infection [25]. Such analyses require specialized bioinformatic pipelines but offer unparalleled insight into the evolutionary pressures driving persistence versus clearance.

Isothermal Amplification and Point-of-Care Testing

While RT-qPCR remains the reference standard, its requirement for sophisticated laboratory infrastructure, cold chain management of reagents, and skilled personnel limits its utility in field settings, particularly in low-resource regions where EqHV prevalence may be highest. Isothermal amplification techniques, such as reverse transcription loop-mediated isothermal amplification (RT-LAMP), offer a promising alternative. These methods operate at a constant temperature, eliminating the need for thermal cyclers, and can provide results in under an hour with minimal sample preparation. Although no commercial RT-LAMP assay for EqHV is currently available, the global prevalence of EqHV in Asia has been estimated at 16.13% (95% CI 7.79–30.43%), with substantial pockets of infection in Mongolia, China, and Korea [9, 17, 37, 42]. In Mongolia alone, seroprevalence approaches 40%, and persistent infection was documented in 17 of 19 horses retested after seven months [9]. These epidemiological data highlight the urgent need for field-deployable diagnostic tools to support surveillance efforts and inform herd management decisions.

Serological Approaches: Detecting Past and Present Exposure

Serological detection of antibodies against EqHV provides complementary information to molecular testing, distinguishing between active infection (RNA-positive) and past exposure (seropositive, RNA-negative). The luciferase immunoprecipitation system (LIPS) targeting the non-structural protein 3 (NS3) has emerged as a sensitive and specific platform for anti-EqHV antibody detection [8, 11]. This assay exploits the high immunogenicity of NS3, which elicits robust humoral responses during natural infection. In a comprehensive study of Austrian horses, the LIPS assay yielded an EqHV antibody prevalence of 45.9% (177/386), far exceeding the RNA prevalence of 4.15% (16/386), indicating that the majority of infections are cleared by the host immune system [8]. Similarly, a South African study of Thoroughbred foals reported an astonishing seroprevalence of 83.70% in animals aged 58–183 days, with RNA detected in only 7.93% [11]. The high seroprevalence in very young foals raises questions about the timing of exposure and the potential role of maternal antibodies or in utero transmission [12].

The kinetics of seroconversion are dose-dependent and may lag behind viremia by several weeks. In experimental infections, horses receiving high doses of EqHV (≥1.3 × 10⁶ RNA copies) seroconverted rapidly and cleared the virus, whereas those receiving low doses (13–1,300 copies) exhibited prolonged viremia without detectable seroconversion within the surveillance period of 40–50 days [18]. This finding has critical diagnostic implications: a negative serology result in a horse with suspected acute EqHV infection does not rule out active viremia, particularly if the infectious dose was low or if sampling occurred early in the course of infection. Conversely, a seropositive result in the absence of detectable RNA is consistent with resolved infection and likely confers at least partial protection against reinfection, as demonstrated in challenge experiments where previously infected horses were protected against homologous and heterologous EqHV isolates [19].

Enzyme-linked immunosorbent assays (ELISAs) using recombinant E2 glycoprotein have also been developed but are less widely adopted than LIPS-based assays. The E2 protein is the major envelope glycoprotein and a primary target of neutralizing antibodies, making it an attractive antigen for serodiagnosis. Badenhorst et al. (2022) used an E2-specific IgG ELISA to monitor humoral responses in a vaccination study and demonstrated that vaccinated ponies produced isotype-switched IgG responses [30]. However, the correlation between E2-specific antibodies and protective immunity remains incompletely defined. It is important to note that no standardized, commercially available serological test for EqHV has received regulatory approval from organizations such as the World Organisation for Animal Health (WOAH) or the European Medicines Agency. This lack of standardization complicates inter-study comparisons and underscores the need for harmonized diagnostic guidelines.

In Situ Hybridization and Immunohistochemistry: Localizing Viral RNA and Antigen in Tissue

For horses with histopathological evidence of hepatitis in whom serum testing is inconclusive, direct visualization of EqHV RNA or antigen in liver biopsy specimens can provide definitive evidence of hepatic infection. Fluorescent in situ hybridization (FISH) using probes complementary to the viral genome has been successfully applied to formalin-fixed, paraffin-embedded (FFPE) liver tissues [7, 50]. Pfankuche et al. (2018) compared multiple in situ hybridization techniques and found that commercially available FISH-RNA probe mixes consistently detected EqHV in equine liver sections, whereas self-designed digoxigenin-labeled RNA probes failed to yield a signal [50]. The superior sensitivity of the FISH approach likely reflects the use of multiple overlapping probes that amplify the signal and tolerate some degree of RNA degradation inherent in archived tissues.

In the chronic hepatitis case series by Jager et al. (2025), liver biopsies from horses with persistent EqHV infection revealed histopathological changes remarkably similar to those seen in human HCV infection, including portal lymphocytic infiltrates, lymphoid aggregates, piecemeal necrosis, and variable degrees of fibrosis [1]. Importantly, EqPV-H was also detected by in situ hybridization in a separate study of 98 archived FFPE liver samples, and 48% of these samples were positive for EqPV-H, with a strong association between high viral load and histologic lesions such as centrilobular necrosis and portal infiltrates [49]. These findings highlight the critical need for multiplexed diagnostic approaches that can distinguish between EqHV and EqPV-H, particularly in co-infected horses. Immunohistochemistry targeting viral proteins such as NS3 or E2 could theoretically provide similar localization, but the availability of validated antibodies for EqHV remains extremely limited, and cross-reactivity with host proteins or other flaviviruses is a persistent concern.

Viral Metagenomics and Pan-Viral Detection

In cases of acute hepatic necrosis or Theiler's disease, where EqPV-H is the most commonly identified etiologic agent, the role of EqHV may be obscured by co-infections. Tomlinson et al. (2018) prospectively tested 18 consecutive cases of equine serum hepatitis and found that EqPV-H was present in all 18 cases, whereas EqHV was detected in only 2 of 14 serum samples and no liver samples [23]. This apparent dominance of EqPV-H in fulminant hepatitis does not diminish the importance of EqHV testing; rather, it emphasizes that a comprehensive virologic workup should include assays for both viruses, as well as equine pegivirus (EPgV) and Theiler's disease-associated virus (TDAV), to fully characterize the infectious landscape [40, 43, 48]. Viral metagenomics using NGS offers a hypothesis-free approach to pathogen discovery and can detect unexpected or novel viruses, such as the recently identified equine circovirus 1 in a febrile mare with hepatitis [51]. While NGS is not yet practical for routine diagnostic use due to cost and complexity, it serves as a powerful tool for outbreak investigations and for validating targeted molecular assays.

Clinical Integration and Interpretation

The diagnostic approach to EqHV must be integrated with clinical and biochemical assessments. Horses with chronic EqHV infection often exhibit persistently elevated serum liver biomarkers, particularly glutamate dehydrogenase (GLDH) and gamma-glutamyl transferase (GGT), although these elevations may be mild and intermittent [1, 8, 34]. In a cohort of Thoroughbred racehorses with high GGT syndrome, no significant association was found between EqHV infection and elevated liver enzymes, indicating that viral infection alone does not explain the high prevalence of biochemical abnormalities in athletic horses [34, 46]. This dissociation between viremia and liver injury underscores the importance of liver biopsy for definitive diagnosis of EqHV-associated hepatitis. The World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) have long emphasized the gold standard role of histopathology in diagnosing HCV-related liver disease in humans, and the same principle applies to EqHV in horses. Histopathological features of chronic EqHV infection, including portal fibrosis, lymphoid aggregates, and individual hepatocyte necrosis, are not pathognomonic but are highly suggestive in the context of persistent viremia [1, 7].

Finally, the detection of EqHV RNA in commercial equine serum products and biologicals has raised significant biosafety concerns. Several studies have identified EqHV, along with EPgV and TDAV, in batches of commercial horse serum used for cell culture and vaccine production [16, 28]. This contamination poses a risk of iatrogenic transmission to recipient horses and potentially confounds research results. The WOAH recommends rigorous screening of all equine-derived biological products for known hepatotropic viruses, and the adoption of EqHV RT-qPCR as a routine quality control measure is strongly advised [41, 47]. For clinicians, a positive EqHV RT-qPCR result in a horse with unexplained hepatitis should prompt an investigation into recent exposure to blood products, antitoxins, or stem cell therapies, as these represent well-documented routes of iatrogenic transmission [23, 47]. The integration of molecular, serological, and histopathological data within a framework of rigorous biosecurity will continue to be the foundation of effective EqHV diagnosis and control.

Therapeutic Strategies and Prognosis

The clinical management of equine hepacivirus (EqHV) infection remains a formidable challenge, shaped fundamentally by the absence of licensed direct-acting antiviral (DAA) agents and the nuanced, often subclinical, trajectory of the disease. As of the current consensus, no specific antiviral therapy is approved for EqHV in horses, and the cornerstone of intervention is supportive medical management aimed at mitigating hepatic injury and maintaining metabolic homeostasis [41]. This therapeutic vacuum stands in stark contrast to the robust armamentarium available for hepatitis C virus (HCV) in human medicine, yet the close phylogenetic relationship between EqHV and HCV offers a tantalizing, albeit still largely theoretical, roadmap for future equine-specific drug development.

Current Antiviral Therapeutic Landscape and the Case for Direct-Acting Antivirals

The European College of Equine Internal Medicine (ECEIM) consensus statement unequivocally states that no direct antiviral treatment against hepacivirus infections in horses is currently available, relegating therapy to supportive care [41]. This supportive framework typically includes judicious fluid therapy to correct dehydration and electrolyte imbalances, administration of hepatoprotective agents such as S-adenosylmethionine (SAMe) or vitamin E, dietary modifications to reduce protein and ammonia loads in cases of hepatic encephalopathy, and the管理和 of concurrent infections or complications. The rationale for this conservative approach is rooted in the observation that the majority of EqHV infections are self-limiting and subclinical, with many horses clearing the virus spontaneously without intervention [4, 45].

However, the paradigm of therapeutic nihilism is being challenged by emerging in silico evidence. Structural modeling of the EqHV NS5B RNA-dependent RNA polymerase (RdRp), the enzymatic engine of viral replication, has revealed striking three-dimensional conservation with its HCV counterpart. Molecular docking simulations demonstrate that sofosbuvir, a nucleoside analogue inhibitor of HCV NS5B, exhibits a molecular interaction pattern with EqHV NS5B that is highly similar to its binding mode in HCV [32]. This finding is mechanistically significant because sofosbuvir targets the highly conserved active site of the polymerase, a region where resistance mutations are less likely to emerge. In contrast, dasabuvir, a non-nucleoside inhibitor that binds to a less conserved allosteric site, showed less favorable interaction profiles with EqHV NS5B, suggesting that not all DAAs will be cross-reactive [32]. These computational predictions provide a strong biological rationale for future in vitro and in vivo efficacy studies of sofosbuvir or its derivatives in EqHV-infected horses. The potential repurposing of sofosbuvir represents a high-value translational opportunity, given its well-characterized safety profile in humans and its mechanism of action against a structurally homologous viral target. The path forward requires rigorous pharmacokinetic studies in horses to establish appropriate dosing regimens and evaluate hepatic drug penetration, followed by controlled challenge studies to assess virological and clinical endpoints.

Immunomodulatory Strategies and Vaccine Development

Given the lack of DAAs, harnessing the host immune response represents the most promising avenue for therapeutic intervention and prophylaxis. The equine immune response to EqHV is characterized by a complex interplay between innate and adaptive immunity, with the outcome of infection, clearance versus persistence, hinging on the quality, magnitude, and kinetics of these responses. Experimental inoculation studies have demonstrated that the infectious dose profoundly shapes the immune trajectory. High-dose inocula (≥1.3 × 10⁶ RNA copies) elicit robust upregulation of innate and adaptive immune pathways, including inflammatory responses, and are associated with early viral clearance. Conversely, low-dose inocula (as few as 13 RNA copies, the minimal infectious dose) result in prolonged viremia and delayed or undetectable seroconversion, suggesting that a low initial viral burden may allow EqHV to evade early immune detection and establish persistence [18]. This dose-dependent immunopathogenesis has direct therapeutic implications: if natural or iatrogenic exposures are typically low-dose, interventions that amplify early innate immune sensing, such as toll-like receptor agonists or interferons, might tip the balance toward clearance.

The most advanced immunomodulatory strategy explored to date is vaccination. A landmark study using recombinant EqHV E2 envelope glycoprotein formulated with adjuvant in ponies provided critical proof-of-concept data [30]. While vaccination did not confer sterilizing immunity, all vaccinated ponies became viremic upon experimental challenge, it significantly accelerated viral clearance compared to unvaccinated controls. Vaccinated ponies cleared serum EqHV RNA earlier and exhibited faster recovery from EqHV-associated liver insult, as assessed by histopathology and serum biochemistry [30]. Mechanistically, the vaccine induced E2-specific immunoglobulin G (IgG) responses, and while these antibodies were not sufficient to block infection entirely, they likely contributed to reducing the duration and severity of hepatitis. This is consistent with the hypothesis that neutralizing antibodies, even if not sterilizing, can limit viral dissemination and facilitate T-cell-mediated clearance. Furthermore, studies of natural immunity have shown that horses that have cleared a primary EqHV infection are protected against rechallenge with both homologous and heterologous isolates, with only trace amounts of circulating virus detectable upon secondary challenge [19]. This robust natural protection suggests that a multi-epitope vaccine capable of inducing both humoral and cellular immunity, including CD4+ and CD8+ T-cell responses, could achieve sterilizing immunity. Intriguingly, transcriptomic and flow cytometric analyses of acute EqHV infection have revealed that virus-specific T-cell responses are largely sequestered within the liver, with a mean 15-fold increase in CD3+ T-cell infiltration observed in liver biopsies at the peak of hepatic enzyme elevation, even when peripheral T-cell responses are undetectable by interferon-gamma ELISpot [22]. This compartmentalization of the immune response underscores the need for vaccine strategies that effectively prime liver-homing T cells.

Management of Chronic Infection and Hepatic Complications

A subset of EqHV-infected horses, estimated to be a minority but clinically significant, develop persistent viremia and chronic hepatitis, mirroring the natural history of HCV in humans. In a detailed case series of 19 horses with documented persistent EqHV infection and chronic hepatitis, the median duration of viremia was 14.8 months (range 6.9–55.6 months), and the median duration of biochemical hepatitis was 18.4 months (range 5.2–120 months) [1]. Liver histopathology in these chronic cases revealed a striking resemblance to HCV-induced liver disease, including portal and lobular lymphocytic infiltration, lymphoid aggregate formation, individual hepatocyte necrosis, and progressive fibrosis [1]. These findings establish that EqHV is not merely a transient, benign infection but can drive a progressive, fibrotic liver disease with the potential for cirrhosis and liver failure. The management of chronic EqHV infection, therefore, requires a shift from acute supportive care to longitudinal monitoring and complication prevention.

Serial assessment of serum liver biomarkers, particularly gamma-glutamyl transferase (GGT), glutamate dehydrogenase (GLDH), and bile acids, is essential for tracking disease progression and guiding therapeutic decisions [8, 10, 46]. Notably, GLDH appears to be a sensitive marker of hepatocellular injury in EqHV infection, with EqHV RNA-positive horses showing significantly higher GLDH levels compared to uninfected controls [8]. However, the interpretation of these biomarkers must be contextualized within the broader epidemiological landscape. Large-scale cross-sectional studies have found that viral infection (EqHV or EqPV-H) does not fully explain the high prevalence of elevated liver enzyme activity observed in racing Thoroughbreds; indeed, pegivirus E (PgV E) infection was associated with a reduced risk of having concurrently increased liver enzyme activity, underscoring the multifactorial nature of equine hepatopathy [34, 46]. In horses with documented chronic EqHV infection and evidence of fibrotic progression, empirical use of anti-fibrotic agents such as colchicine or angiotensin receptor blockers, though extrapolated from human hepatology, has not been systematically evaluated in equine patients and cannot be recommended outside of controlled clinical trials.

The management of comorbidities is particularly critical in chronic EqHV cases. In the aforementioned case series, concurrent infections were frequent, including bacterial cholangiohepatitis, equine parvovirus-hepatitis (EqPV-H), and equine multinodular pulmonary fibrosis [1]. The presence of co-infections likely exacerbates hepatic inflammation and accelerates fibrosis progression, necessitating a comprehensive diagnostic workup, including PCR panels for EqPV-H, equine pegivirus, and Theiler's disease-associated virus, in any horse with chronic hepatitis [23, 36, 37, 43, 48]. From a prognostic standpoint, the development of overt liver failure, characterized by icterus, hepatic encephalopathy, and coagulopathy, carries a grave prognosis. In the retrospective cohort, 8 of 29 initially enrolled horses died or were euthanized within 6 months of EqHV detection, attesting to the potential lethality of this infection [1].

Prognostic Indicators and Disease Outcomes

Prognostication in EqHV infection is nuanced and must account for viral, host, and environmental factors. As a general principle, the prognosis for acute, subclinical EqHV infection in immunocompetent adult horses is excellent, with spontaneous clearance occurring within weeks to months in the majority of cases. Experimental infections in horses and donkeys have demonstrated that viremia typically resolves by 12 weeks post-inoculation, coincident with seroconversion and a mild, self-limited elevation in liver enzymes [6, 19]. However, certain factors portend a less favorable outcome. Persistent viremia beyond 6 months, particularly when accompanied by sustained elevations in GLDH, GGT, or bile acids, signals the establishment of chronic infection and warrants long-term monitoring for fibrotic progression [1, 7].

Virological factors also influence prognosis. Deep-sequencing analyses of intra-host viral populations have revealed that chronic EqHV infection is associated with higher genetic diversity within the E1 and E2 glycoprotein-encoding regions compared to acute, resolving infections, mirroring the evolutionary dynamics observed in HCV [25]. This diversification likely reflects ongoing immune selection pressure and may serve as a molecular marker of immune escape and viral persistence. Additionally, the degree of miR-122 dependence, a liver-specific microRNA that EqHV, like HCV, requires for efficient replication, may modulate pathogenesis. Mutagenesis studies have shown that EqHV/HCV chimeric viruses can evolve to replicate independently of miR-122, but such variants are not dominant in natural populations, suggesting that miR-122 dependence confers a selective advantage, likely by reinforcing hepatotropism and exploiting the tolerogenic liver microenvironment to promote chronicity [15, 20]. The clinical significance of this molecular dependency is that therapeutic strategies aimed at sequestering miR-122 (e.g., antisense oligonucleotides) could theoretically impair EqHV replication, though this approach remains entirely preclinical.

Host demographic factors are also emerging as important prognostic modifiers. Breed-specific susceptibilities have been documented, with Thoroughbreds having 9.64 times higher odds of testing positive for EqHV compared to other breeds, and Quarter Horses also showing increased risk (odds ratio 4.16) [38]. While the mechanisms underlying these breed predilections are unknown, they may reflect genetic differences in immune receptor expression, management practices, or exposure intensity, they suggest that certain subsets of the equine population may be at elevated risk for infection and its complications. Age also plays a role; seroprevalence tends to increase with age, reflecting cumulative exposure, while RNA prevalence peaks in younger animals, indicating that primary infection typically occurs early in life [11, 40]. In donkeys, a cross-species experimental model demonstrated that EqHV infection kinetics are nearly identical to those in horses, but the transcriptomic response, including distinct patterns of viral host factor and immune gene expression, suggests species-specific mechanisms of immune control that may affect clearance rates [6]. This is particularly relevant given that donkeys represent a large global equid population with potential for viral maintenance and transmission.

Perhaps the most critical prognostic determinant is the distinction between EqHV and EqPV-H as etiological agents of hepatitis. While EqHV is associated with mild-to-moderate, often subclinical hepatitis, EqPV-H is the primary cause of Theiler's disease, a fulminant, often fatal hepatic necrosis with mortality rates exceeding 50% in clinical cases [47, 49]. Crucially, co-infection with EqHV and EqPV-H is not uncommon; in Korean and Chinese horse populations, co-infection rates among EqPV-H-positive horses have been reported at 35.3% and 58.8%, respectively [37, 44]. The presence of EqPV-H co-infection in a horse with EqHV viremia dramatically worsens the prognosis, as it introduces the risk of acute, severe liver failure superimposed on chronic hepatitis. Therefore, any horse with EqHV infection presenting with markedly elevated liver enzymes, icterus, or neurological signs should be urgently tested for EqPV-H, and the prognosis should be guarded pending the results.

Horizontal Transmission and Biosecurity Implications for Prognosis

The prognosis for individual horses must also be considered within the broader context of herd health and transmission dynamics. EqHV is known to be transmitted vertically in utero, iatrogenically via contaminated blood products, and horizontally through mechanisms that are not fully elucidated [12, 16, 28, 33]. The recent detection of EqHV RNA in the head and thorax of Stomoxys calcitrans (stable flies) raises the possibility of mechanical vector-borne transmission, which would have profound implications for biosecurity in stable environments [35]. Minimum infection rates in stable flies ranged from 1.2% to 3.9%, and viral RNA was detected in both the abdomen and the head/thorax compartments, indicating that flies can harbor the virus in a location compatible with subsequent transmission via biting [35]. In contrast, extensive mosquito surveillance in Austria failed to detect EqHV RNA in over 5,000 mosquitoes, suggesting that Culicidae are not significant vectors [8]. Until the relative contribution of vector-borne, iatrogenic, and contact transmission is clarified, sound biosecurity practices, including sterile needle use, screening of blood donors, and isolation of viremic horses, remain the most effective tools for preventing new infections and improving population-level prognosis.

Emerging Therapeutic Targets and the Horse as a Surrogate Model

The therapeutic strategies for EqHV are inextricably linked to its role as a surrogate model for HCV. The equine host provides an outbred, immunocompetent, and naturally permissive system for studying hepaciviral pathogenesis, immune evasion, and vaccine efficacy [19, 22, 30]. The development of prophylactic and therapeutic vaccines in this model has direct relevance to both equine and human medicine. The observation that natural immunity protects against rechallenge [19], coupled with the proof-of-concept that E2 subunit vaccination accelerates clearance [30], provides a strong foundation for next-generation vaccine designs incorporating multiple viral antigens (e.g., E1, E2, NS3) and potent adjuvants capable of eliciting liver-resident memory T cells.

From a pharmacological perspective, the conserved structure of the NS5B polymerase and its interaction with sofosbuvir [32] positions this drug as a prime candidate for repurposing trials in horses. Such trials would not only benefit equine patients but also provide critical safety and efficacy data that could inform human HCV eradication efforts in settings where DAAs are less accessible. The translational pipeline from in silico modeling to in vivo equine trials to human application represents a unique bidirectional opportunity. Furthermore, the identification of glycosylation patterns, entry receptor usage (CD81, occludin, claudin-1, SCAR-B1), and the role of miR-122 in EqHV replication [20, 31] opens additional avenues for targeted intervention. Small molecule inhibitors of viral entry, for instance, could be evaluated in equine hepatocyte culture systems before advancing to in vivo challenge studies. The strategic integration of equine clinical care with comparative hepatology research will ultimately accelerate the development of effective countermeasures against EqHV and its human counterpart.

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