Equine Rhinitis B Virus

Overview and Taxonomy of Equine Rhinitis B Virus

Equine rhinitis B virus (ERBV) represents a significant, yet historically underappreciated, viral pathogen within the Picornaviridae family, specifically assigned to the genus Erbovirus. This virus is a primary etiological agent of acute upper respiratory tract disease in equids worldwide, contributing to morbidity, disruption of training and competition schedules, and substantial economic losses within the equine industry. Unlike its more extensively studied counterpart, equine rhinitis A virus (ERAV), which belongs to the genus Aphthovirus and is closely related to foot-and-mouth disease virus (FMDV), ERBV occupies a distinct phylogenetic niche. The taxonomic classification of ERBV has evolved significantly with the advent of advanced molecular diagnostics and metagenomic sequencing, revealing a genetic diversity far greater than initially appreciated. This section provides a comprehensive overview of the virus, its taxonomic position within the picornavirus family, its genomic architecture, serotypic and genotypic diversity, and the clinical and epidemiological context that defines its importance as a pathogen of horses.

Taxonomic Classification and Phylogenetic Position

The family Picornaviridae is a large and diverse group of small, non-enveloped, positive-sense single-stranded RNA viruses that infect a wide range of vertebrate hosts, including humans and numerous animal species. Within this family, ERBV is the sole recognized member of the genus Erbovirus [1, 5]. This genus was established to accommodate viruses that are genetically and antigenically distinct from other picornaviruses, particularly the aphthoviruses (which include ERAV and FMDV) and the enteroviruses. The taxonomic separation of ERBV from ERAV is critical, as these two viruses, despite sharing a common name and causing similar clinical syndromes, are fundamentally different at the molecular and structural levels. ERAV is an acid-labile aphthovirus that dissociates into pentameric subunits at low pH, a mechanism linked to its cell entry and genome release [18]. In contrast, ERBV exhibits variable acid stability; for instance, ERBV3 has been demonstrated to be acid-stable, while ERBV2 is acid-labile, a property that may influence tissue tropism and transmission routes [4].

Phylogenetic analyses based on complete polyprotein sequences and specific structural protein genes, such as VP1, consistently place ERBV in a distinct clade, separate from other picornavirus genera. The virus is most closely related to bovine rhinitis B virus (BRBV), another member of the Erbovirus genus, which is associated with bovine respiratory disease complex (BRDC) [13-15]. This relationship is not merely taxonomic; it suggests a shared evolutionary history and potentially analogous pathogenic mechanisms in their respective hosts. The RNA-dependent RNA polymerase (RdRp) of ERBV, encoded by the 3D gene, shows significant homology to that of BRBV and, interestingly, to FMDV, with up to 67% sequence identity at the amino acid level for the polymerase [16]. However, despite this homology, the overall genomic organization and the structural proteins of ERBV are sufficiently divergent to warrant its classification as a separate genus. The World Organisation for Animal Health (WOAH) recognizes ERBV as a significant pathogen of equids, and its detection is increasingly incorporated into routine diagnostic panels for equine respiratory disease, reflecting its growing recognition within the veterinary community.

Genomic Architecture and Molecular Features

The ERBV genome is a single-stranded, positive-sense RNA molecule of approximately 8,500 to 9,500 nucleotides in length, depending on the strain and serotype. The genome is organized as a single open reading frame (ORF) flanked by a 5' untranslated region (UTR) and a 3' UTR, followed by a poly(A) tail. The 5' UTR is highly structured and contains a type II internal ribosome entry site (IRES), a hallmark of many picornaviruses, which facilitates cap-independent translation of the viral polyprotein [2]. The polyprotein, typically around 2,700 amino acids in length, is co- and post-translationally cleaved by viral proteases into mature structural and non-structural proteins. The canonical picornavirus polyprotein cleavage pattern is L-VP4-VP2-VP3-VP1-2A-2B-2C-3A-3B-3C-3D, where L is the leader protein, VP1-4 are the capsid proteins, and 2A-3D are the non-structural proteins involved in replication and proteolysis.

A defining feature of the ERBV genome is the presence of a leader proteinase (Lpro) at the N-terminus of the polyprotein. This protein is a papain-like cysteine protease that plays a crucial role in shutting off host cell cap-dependent translation by cleaving the eukaryotic translation initiation factor 4G (eIF4G), a strategy common to aphthoviruses and erboviruses. However, recent metagenomic discoveries have revealed significant variability in this region. A highly divergent ERBV strain identified in a diarrheic foal in Japan was found to possess an 87-amino-acid insertion in the Lpro region, a feature not previously described in any known ERBV serotype [2]. This insertion likely alters the structure and potentially the substrate specificity of the protease, suggesting that ERBV may employ a broader range of strategies to subvert host cellular machinery than previously understood. Furthermore, this same strain exhibited atypical cleavage motifs within the polyprotein, indicating that the canonical processing pathways may not be universal across all ERBV genotypes [2].

The 2A protein of ERBV is of particular interest, not only for its role in viral polyprotein processing but also for its application in biotechnology. The 2A peptide from ERBV-1 is a well-characterized "self-cleaving" peptide that mediates a ribosome-skipping event, leading to the efficient separation of proteins expressed from a single open reading frame. This property has been exploited extensively in synthetic biology for polycistronic gene expression in various organisms, including yeast (Schizosaccharomyces pombe and Saccharomyces cerevisiae) and mammalian cells [10, 11]. The high cleavage efficiency (ranging from ~70% to >99%) of the ERBV-1 2A peptide makes it a powerful tool for constructing multi-gene expression vectors, gene digital circuits, and for tagging endogenous proteins [10, 11]. This biotechnological utility underscores the broader impact of fundamental virological research.

Serotypic and Genotypic Diversity

Historically, ERBV was classified into two serotypes, ERBV1 and ERBV2, based on virus neutralization assays using monospecific rabbit antisera [5]. The prototype strains are ERBV1.1436/71 and ERBV2.313/75. For many years, these two serotypes were considered the only antigenic variants of the virus. However, the application of molecular techniques, particularly reverse transcription-PCR (RT-PCR) and next-generation sequencing, has dramatically expanded our understanding of ERBV diversity. The first complete genome sequence of an ERBV3 strain was reported from fecal samples of horses in Dubai, establishing a third distinct genotype [4]. This discovery was pivotal, as it demonstrated that ERBV could be shed in feces, a finding with significant implications for transmission routes and environmental persistence.

The genetic basis for serotypic differentiation lies primarily in the capsid protein VP1, which is the major immunodominant protein and the primary target of neutralizing antibodies. Sequence analysis of the VP1 gene reveals that ERBV1, ERBV2, and ERBV3 share only 47.1–49.8% nucleotide identity at the amino acid level, a degree of divergence that is consistent with distinct serotypes [2]. The novel ERBV strain from Japan, provisionally designated as a potential fourth genotype, showed only 62.5–63.1% identity in the polyprotein and 47.1–49.8% in the VP1 region compared to known ERBV serotypes, strongly suggesting it represents a novel genotype [2]. This finding highlights a critical gap in current surveillance: broad-range molecular assays designed to detect ERBV1-3 may fail to identify highly divergent strains, leading to an underestimation of true prevalence and diversity [2]. The Ka/Ks ratios (ratio of non-synonymous to synonymous substitutions) for all coding regions in ERBV3 genomes are consistently <0.1, indicating that these viruses are under strong purifying selection and are stably evolving within the horse population [4]. Molecular clock analyses of the VP1 gene estimate the most recent common ancestor (MRCA) of ERBV3 to be around 1785, with the MRCAs of ERBV1 and ERBV2 estimated at 1848 [4]. These dates suggest that the diversification of ERBV serotypes has occurred over centuries, long before the advent of modern equine husbandry.

Clinical and Epidemiological Context

ERBV is a primary cause of acute respiratory disease in horses, particularly in young animals. The virus is detected with a notable frequency in horses presenting with acute onset of fever, nasal discharge, ocular discharge, and cough [1, 3]. Large-scale biosurveillance programs in the United States, involving over 10,000 equids, have consistently reported an overall ERBV qPCR-positivity rate of approximately 5.1% in horses with respiratory signs [1, 3, 9]. This rate is comparable to that of other established respiratory pathogens such as equine herpesvirus-4 (EHV-4) and equine influenza virus (EIV) in certain study populations [9]. Importantly, ERBV is detected as a single pathogen in the majority of positive cases (approximately 66%), confirming its role as a primary etiological agent rather than a mere bystander [1]. The virus is most prevalent in horses under one year of age, with the median age of positive horses being three years [1, 3]. This age distribution is consistent with the waning of maternal antibodies and the increased exposure to novel pathogens in training and competition environments.

Coinfection is a hallmark of ERBV infection, with approximately 22-34% of ERBV-positive horses also testing positive for at least one other respiratory pathogen [1, 3]. The most common coinfecting agents are Streptococcus equi subspecies equi (the causative agent of strangles), equine herpesvirus-4 (EHV-4), and equine influenza virus (EIV) [1]. The clinical significance of these coinfections is an area of active investigation. It is hypothesized that ERBV, by damaging the respiratory epithelium and inducing immunosuppression, may predispose horses to secondary bacterial infections, thereby exacerbating clinical disease and prolonging recovery. The detection of ERBV in tracheal washes, but not in nasopharyngeal swabs, has been significantly associated with coughing (odds ratio [OR] 5.3; 95% CI 2.1-14.0; P<0.001), suggesting that lower airway involvement is a key driver of clinical signs [8, 12]. This finding underscores the importance of sampling site for accurate diagnosis and highlights the potential role of ERBV in inflammatory airway disease (IAD).

The virus is not only detected in clinically ill horses but also circulates silently in apparently healthy populations. Studies have shown that a small but consistent percentage of clinically healthy horses (0.8-1.78%) can test qPCR-positive for ERBV, indicating that subclinical shedding occurs and may contribute to the maintenance and spread of the virus within a herd [3, 7, 17]. This silent circulation is particularly relevant in the context of multi-day equestrian events, where the commingling of horses from diverse geographic origins creates a high-risk environment for pathogen transmission. Environmental surveillance at such events has detected ERBV in stall samples, confirming that the virus can be shed into the environment and potentially serve as a source of fomite transmission [6]. The detection of ERBV in fecal samples from both diarrheic and healthy horses further expands the potential transmission routes, suggesting that fecal-oral spread may be more important than previously recognized [2, 4]. This finding has direct implications for biosecurity protocols, emphasizing the need for rigorous hygiene practices beyond just respiratory isolation.

Molecular Pathogenesis and Genomic Organization

Genomic Architecture and Phylogenetic Classification

Equine rhinitis B virus (ERBV) is a positive-sense, single-stranded RNA virus belonging to the family Picornaviridae, genus Erbovirus. The ERBV genome is approximately 8.4–9.4 kilobases in length, featuring a single open reading frame (ORF) that encodes a polyprotein of roughly 2,721 amino acids, which is subsequently processed into structural and nonstructural proteins [2, 4]. The genomic organization of ERBV follows the canonical picornavirus layout: 5′ untranslated region (UTR)–VP4–VP2–VP3–VP1–2A–2B–2C–3A–3B–3C–3D–3′ UTR–poly(A) tail. However, ERBV exhibits distinctive features that set it apart from other picornaviruses, particularly within the leader proteinase (Lpro) region and the internal ribosome entry site (IRES). The 5′ UTR of ERBV contains a type II IRES, a structural element that facilitates cap-independent translation initiation, a hallmark of picornaviruses [2]. This IRES is critical for the virus’s ability to hijack the host translational machinery, ensuring efficient viral protein synthesis even under conditions of host cell shutoff.

Phylogenetically, ERBV is classified into three recognized serotypes, ERBV1, ERBV2, and ERBV3, based on antigenic and genetic divergence, particularly within the VP1 capsid protein [4, 5]. The VP1 region is the most variable among ERBV serotypes, with amino acid identities as low as 47.1–49.8% between novel genotypes and established serotypes [2]. This hypervariability in VP1 is a key driver of antigenic diversity and immune evasion, as VP1 contains major neutralizing epitopes. The most recent common ancestor (MRCA) of ERBV3 has been estimated to date back to approximately 1785 (highest posterior density intervals: 1176–1937), while ERBV1 and ERBV2 MRCAs were estimated at 1848 (1466–1949), indicating a long evolutionary history in equine populations [4]. Notably, the Ka/Ks ratios across all coding regions of ERBV3 genomes are consistently below 0.1, suggesting that these viruses are under strong purifying (negative) selection and are stably evolving in horses [4]. This evolutionary stability contrasts with the rapid antigenic drift observed in other RNA viruses, such as equine influenza virus, and may reflect a well-adapted host-pathogen relationship.

Unique Genomic Features and Proteolytic Processing

A defining characteristic of ERBV is the presence of a 2A peptide sequence that mediates a “cleavage” event via a ribosome-skipping mechanism rather than conventional proteolysis. The ERBV-1 2A peptide is a short, approximately 20-amino-acid sequence that induces the ribosome to skip the formation of a peptide bond between the 2A glycine and the 2B proline, resulting in the separation of the upstream and downstream proteins [10, 11]. This mechanism has been harnessed extensively in synthetic biology for polycistronic gene expression in yeast and mammalian systems, where the ERBV-1 2A peptide demonstrates “cleaving” efficiencies ranging from approximately 70% to 99% depending on the insertion site and cellular context [10]. The 2A peptide from ERBV-1, along with those from other picornaviruses such as Thosea asigna virus (T2A) and Ljungan virus (LV2A), has been successfully employed to construct Boolean logic gates and multi-cistronic vectors in Saccharomyces cerevisiae, underscoring its utility beyond virology [11]. In the context of natural infection, this 2A-mediated separation is essential for the proper stoichiometric release of the nonstructural proteins 2B and 2C, which are involved in membrane rearrangement and RNA replication.

The leader proteinase (Lpro) of ERBV is another distinctive element. Recent metagenomic analysis of a highly divergent ERBV strain from Japan revealed an 87-amino-acid insertion in the Lpro region, a feature not previously described in erboviruses [2]. This insertion may alter substrate specificity or enhance the ability of Lpro to cleave host factors involved in innate immunity, such as translation initiation factors or components of the interferon pathway. Atypical cleavage motifs were also identified in this novel strain, suggesting that the polyprotein processing cascade may be more flexible than previously appreciated [2]. The Lpro of ERBV is functionally analogous to the leader protease of foot-and-mouth disease virus (FMDV), which cleaves eukaryotic initiation factor 4G (eIF4G) to shut off host cap-dependent translation. However, the precise host targets of ERBV Lpro remain to be fully elucidated, representing a critical gap in our understanding of ERBV pathogenesis.

Capsid Structure and Receptor Interactions

The ERBV capsid is composed of 60 copies each of four structural proteins: VP1, VP2, VP3, and VP4, arranged in an icosahedral symmetry typical of picornaviruses. While no high-resolution cryo-electron microscopy (cryo-EM) structure of ERBV has been published to date, insights can be drawn from the closely related equine rhinitis A virus (ERAV), an aphthovirus. The ERAV capsid exhibits a pitted surface topography, with VP1 diverging significantly from FMDV VP1, resulting in a canyon-like structure that may serve as a receptor-binding site [18]. Given the phylogenetic relatedness, ERBV likely shares a similar capsid architecture, although the specific cellular receptor(s) for ERBV remain unidentified. In contrast, the receptor for Eastern equine encephalitis virus (EEEV), an alphavirus, has been identified as the very low-density lipoprotein receptor (VLDLR), which binds to the E1/E2 glycoprotein complex through multiple LA domain interactions [25, 26]. No such receptor has been characterized for ERBV, and the virus’s tropism for respiratory epithelium suggests the involvement of a yet-unknown surface molecule expressed on ciliated epithelial cells and goblet cells of the upper respiratory tract.

The acid stability of ERBV particles varies by serotype, with ERBV3 being acid-stable and ERBV2 being acid-labile [4]. This property has implications for transmission and pathogenesis: acid-stable ERBV3 can survive passage through the gastrointestinal tract and has been detected in fecal samples from horses in the Middle East, with viral loads ranging from 8.28 × 10³ to 5.83 × 10⁴ copies per mL [4]. Fecal shedding of ERBV suggests a potential fecal-oral route of transmission, in addition to the established respiratory route. The detection of ERBV in fecal samples from clinically normal horses also raises the possibility of subclinical gastrointestinal infection or prolonged shedding, which could contribute to environmental contamination and silent spread within equine populations [4, 21].

Molecular Pathogenesis and Host-Virus Interactions

ERBV primarily targets the upper respiratory tract, where it infects ciliated epithelial cells and goblet cells, leading to ciliary dysfunction, mucus hypersecretion, and inflammation. This pattern is consistent with the pathogenesis of bovine rhinitis B virus (BRBV), a closely related aphthovirus, which has been localized to the superficial cilia lining of the upper respiratory tract using in situ hybridization [13]. In BRBV-infected calves, viral RNA was detected in nasal turbinates, trachea, and, sporadically, in the brain, suggesting the potential for neuroinvasion, albeit rarely [13]. By analogy, ERBV may occasionally breach the respiratory epithelium and gain access to the central nervous system, although clinical neurological signs have not been reported in ERBV-infected horses.

The host response to ERBV infection involves a complex interplay of innate and adaptive immunity. Infection of RK-13 cells with ERBV induces distinct changes in the volatile organic compound (VOC) profile, as detected by headspace solid-phase microextraction coupled with gas chromatography–mass spectrometry [20]. These VOCs, including 2-ethyl-1-hexanol, acetophenone, and benzaldehyde, are byproducts of host cellular metabolism, oxidative stress, and apoptosis. Principal component analysis of VOC profiles clearly separated ERBV-infected cells from those infected with equine arteritis virus or equine herpesvirus 1, reflecting virus-specific metabolic reprogramming [20]. This metabolic signature may have diagnostic utility, as VOC profiling could serve as a non-invasive, rapid screening tool for ERBV infection in field settings.

Neutralizing antibodies against ERBV develop within 7–14 days post-infection and can persist for at least one year, as demonstrated by serological surveys showing that few horses two years or older are seronegative [13, 22]. However, the presence of high antibody titers does not necessarily correlate with protection from reinfection, and subclinical shedding can occur in seropositive horses [7, 22]. This phenomenon is reminiscent of the immune evasion strategies employed by other picornaviruses, such as FMDV, where antigenic variation and non-neutralizing antibody responses allow for persistent infection. The cellular immune response to ERBV is less well characterized, but it is likely that CD8+ T cells play a role in clearing infected cells, as observed in other respiratory viral infections.

Coinfection and Synergistic Pathogenesis

A hallmark of ERBV infection is its frequent occurrence as part of polymicrobial respiratory disease. In a large-scale biosurveillance program spanning 2012–2023 in the United States, ERBV was detected as a single pathogen in 65.99% of positive cases and as a coinfection with at least one other respiratory pathogen in 34.01% of cases [1]. The most common coinfecting pathogens were Streptococcus equi subspecies equi (the causative agent of strangles), equine herpesvirus type 4 (EHV-4), and equine influenza virus (EIV) [1, 27]. Coinfection with ERBV and S. equi subsp. equi was the most frequently observed combination in equine respiratory PCR panels [27]. This synergistic relationship may be explained by ERBV-induced damage to the respiratory epithelium, which compromises mucociliary clearance and facilitates bacterial adherence and invasion. Similarly, coinfection with EHV-4, a gammaherpesvirus that establishes latency, may lead to reactivation and enhanced shedding of both viruses, exacerbating clinical signs.

The clinical impact of coinfection is significant. Horses with ERBV coinfection tend to present with more severe clinical signs, including fever, mucopurulent nasal discharge, ocular discharge, and cough, compared to those with single infections [1, 3]. In a prospective longitudinal study of Standardbred racehorses, detection of ERBV RNA in tracheal washes was significantly associated with coughing (odds ratio 5.3; 95% CI 2.1–14.0; P < 0.001), and when only quantifiable viral loads were considered, the odds ratio increased to 15.0 (95% CI 3.7–60.0; P < 0.001) [8, 12]. This strong association underscores the role of ERBV as a primary pathogen in inflammatory airway disease (IAD) and suggests that high viral loads are directly correlated with clinical severity. Notably, detection of ERBV in nasopharyngeal swabs was not associated with clinical signs, indicating that tracheal wash sampling is superior for assessing active lower airway infection [8, 12].

Genomic Diversity and Implications for Diagnostics

The genomic diversity of ERBV is greater than previously recognized, with novel genotypes being discovered through metagenomic sequencing. A highly divergent ERBV strain identified in a diarrheic foal in Japan exhibited only 62.5–63.1% polyprotein identity and 47.1–49.8% VP1 identity to known serotypes, warranting classification as a novel genotype [2]. This strain also featured an 87-amino-acid insertion in Lpro and atypical cleavage motifs, suggesting that the genetic plasticity of ERBV is substantial. Importantly, a broadly reactive RT-qPCR assay targeting ERBV1–3 failed to detect this novel strain, while a strain-specific assay revealed a 10.8% positivity rate in rectal swabs from the same farm [2]. This finding highlights a critical gap in current surveillance: standard diagnostic assays may underestimate the true prevalence of ERBV if they are designed based on conserved regions that are not universally present across all genotypes. The 5′ UTR and 3D polymerase (RNA-dependent RNA polymerase) regions are commonly targeted for molecular detection, but even these regions can exhibit sufficient variability to cause primer-template mismatches [17, 19]. The development of next-generation sequencing-based metagenomic approaches is essential for capturing the full extent of ERBV diversity and for updating diagnostic panels accordingly.

The 3D polymerase of ERBV is a key target for antiviral drug development, given its essential role in viral RNA replication. Homology modeling of the closely related BRBV 3D polymerase has revealed a structure highly similar to that of FMDV 3D polymerase, with approximately 67% amino acid identity [16]. The active site, nucleic acid binding channel, and template/primer binding sites are conserved, but notable differences exist in the C-terminal region, where BRBV lacks a positively charged α-helix present in FMDV [16]. This structural divergence may influence the mechanism of substrate handling and could be exploited for the design of selective inhibitors against erbovirus polymerases. Given the economic importance of equine respiratory disease and the lack of specific antiviral therapies for ERBV, the 3D polymerase represents a promising target for structure-based drug design.

Epidemiological and Pathogenetic Context

ERBV is detected in approximately 5% of nasal swabs from horses with acute onset of fever and respiratory signs, with a similar positivity rate (5.08%) observed in the large US biosurveillance program [1, 3]. Young horses, particularly those less than one year of age, are at highest risk, and the median age of ERBV-positive horses is three years [1, 3]. Horses used for competition are also more likely to test positive, likely due to increased stress, commingling, and transport, which facilitate viral transmission [1]. The virus circulates year-round but shows increased detection during winter months, when horses are more likely to be housed indoors in close confinement [6]. Environmental sampling of stall surfaces has detected ERBV RNA, albeit at low frequencies, indicating that fomite transmission may contribute to spread [6]. Subclinical shedding is well documented, with 0.8% of healthy horses testing positive in one study, and these silent shedders can serve as sources of infection for naive cohorts [3, 7].

The pathogenesis of ERBV is also influenced by host factors such as age, immune status, and concurrent infections. Foals and yearlings are particularly susceptible due to waning maternal antibodies and an immature adaptive immune system. In a study of Standardbred yearlings, the attack rate for respiratory disease associated with ERAV (a related aphthovirus) was 87.9%, and late-born foals took longer to recover [23, 24]. By extension, ERBV likely follows a similar age-dependent pattern, with younger horses experiencing more severe and prolonged disease. The role of stress-induced immunosuppression, such as that caused by transport, training, or competition, cannot be overstated, as it may trigger reactivation of latent coinfections (e.g., EHV-4) and exacerbate ERBV pathogenesis.

In summary, the molecular pathogenesis of ERBV is driven by its unique genomic organization, including a type II IRES, a 2A peptide with ribosome-skipping activity, and a variable Lpro region that may modulate host immune responses. The virus’s ability to coinfect with bacterial and viral pathogens, its acid stability (in the case of ERBV3), and its capacity for subclinical shedding contribute to its widespread circulation and clinical impact. The ongoing discovery of highly divergent genotypes underscores the need for continuous genomic surveillance and the development of broadly reactive molecular diagnostics. Understanding the molecular underpinnings of ERBV infection is essential for designing effective control strategies, including vaccines and antiviral therapeutics, to mitigate the economic and welfare burden of equine respiratory disease.

Genetic Diversity and Emerging Genotypes

The genus Erbovirus, within the family Picornaviridae, has historically been defined by a relatively narrow genetic scope, comprising two principal serotypes, Equine rhinitis B virus 1 (ERBV1) and ERBV2, based upon antigenic characterization and nucleotide sequence analysis of the structural protein (P1) region [5, 19]. However, the application of metagenomic sequencing, expanded surveillance programs, and phylogenetic interrogation of global isolates has profoundly reshaped our understanding of ERBV’s genetic architecture. It is now evident that the virus exists as a far more heterogeneous population than previously appreciated, encompassing at least three distinct genotypes, with strong evidence pointing toward the existence of additional, highly divergent lineages that challenge the limits of current diagnostic frameworks. The genetic diversity of ERBV is not a static catalog of sequence variation; it is a dynamic landscape shaped by evolutionary pressures, geographical segregation, host adaptation, and the emergence of novel strains with unique genomic features that may alter pathogenic potential and transmission dynamics.

Established Genotypes and the Recognition of ERBV3

For decades, classification rested on the prototype strains ERBV1.1436/71 and ERBV2.313/75, which were discriminated by virus neutralization tests and limited partial sequencing of the VP1 gene [5]. These two serotypes exhibited distinct geographical distributions and pathogenic profiles, though both were consistently associated with acute febrile respiratory disease in horses. The paradigm shifted dramatically with the application of next-generation sequencing to fecal samples collected from horses in the Middle East. Woo and colleagues (2016) performed a seminal molecular epidemiological study in Dubai, identifying ERBV RNA in 13.8% of fecal samples and, through complete genome sequencing, characterized three strains that could not be classified as either ERBV1 or ERBV2 [4]. These strains, designated ERBV3, formed a monophyletic clade with robust bootstrap support. The major genetic distinction resided in the VP1 capsid protein, where amino acid sequence identities between ERBV3 and the established serotypes were markedly lower than the intra-serotype variability observed within ERBV1 or ERBV2. This discovery was not merely taxonomic. Evolutionary analyses using a relaxed molecular clock on the VP1 gene estimated the time of the most recent common ancestor (MRCA) for ERBV3 to be approximately 1785 (95% HPD: 1176–1937), suggesting that this lineage has been circulating independently and largely undetected in equine populations for centuries [4]. In contrast, the MRCA dates for ERBV1 and ERBV2 were estimated to be 1848 and 1848, respectively, indicating that the diversification of these lineages is a relatively recent phenomenon on an evolutionary timescale. Critically, the study also demonstrated that the ratio of nonsynonymous to synonymous substitutions (Ka/Ks) for all coding regions in ERBV3 genomes was uniformly below 0.1, a hallmark of strong purifying selection [4]. This indicates that ERBV3 is not a rapidly evolving, error-prone entity in a state of adaptive flux; rather, it is a stably evolving virus, well-adapted to its equine host, and has likely been maintained in a largely cryptic transmission cycle.

The Emergence of Highly Divergent Novel Genotypes: The Japanese Strain

The most compelling evidence for a previously unrecognized echelon of ERBV genetic diversity comes from a 2025 study by Ketphan and colleagues, which identified a highly divergent ERBV strain from a diarrheic foal in Japan through metagenomic analysis of a rectal swab [2]. This strain, which we may refer to provisionally as ERBV-JPN/2022, possesses a 9,448-nucleotide genome encoding a 2,721-amino-acid polyprotein, yet it shares only 62.5–63.1% nucleotide identity in the polyprotein and a striking 47.1–49.8% identity in the VP1 region compared to known ERBV serotypes [2]. By traditional picornavirus taxonomy thresholds, these values place this strain well outside the boundary of established genotypic groups and strongly suggest it represents a novel genotype, potentially a fourth serotype. The genomic divergence is not uniformly distributed. The virus exhibits an 87-amino-acid insertion in the leader proteinase (Lpro) region, a domain critical for antagonizing host innate immune responses and processing the viral polyprotein [2]. This insertion is unprecedented among known erboviruses and may confer unique functional properties, such as altered substrate specificity, enhanced cleavage efficiency, or novel interactions with host cellular machinery. Furthermore, the strain displays atypical cleavage motifs at several polyprotein junction sites, deviating from the canonical patterns observed in ERBV1, ERBV2, and ERBV3 [2]. These molecular anomalies suggest a fundamental divergence in the viral life cycle and potentially a distinct pathogenesis profile.

The implications for diagnostic surveillance are profound. The study developed a strain-specific RT-qPCR assay targeting this novel genotype and screened 37 rectal swabs from Japanese horses, yielding a positivity rate of 10.8% [2]. Crucially, none of these samples tested positive using a broadly reactive “pan-ERBV1-3” RT-qPCR assay designed to detect all three established serotypes [2]. This finding underscores a critical failure point in current surveillance infrastructure. The primers and probes used in virtually all contemporary diagnostic panels, including those deployed in the large-scale US biosurveillance programs [1, 3, 9], are predicated on conserved sequences within the 5’ untranslated region (UTR) or the 3Dpol gene of ERBV1, ERBV2, and ERBV3. These assays possess an inherent genetic blind spot for highly divergent lineages. The Japanese strain is not an isolated curiosity; it represents a sentinel event, signaling the existence of a shadow population of ERBV genotypes that are circulating beneath the detection threshold of standard molecular tools. It is highly plausible that similar divergent strains exist in other global regions but remain undiscovered due to systematic detection bias.

Molecular Epidemiology and Phylogeographic Structure

The genetic diversity of ERBV exhibits a pronounced phylogeographic structure, with distinct clades showing differential distribution across continents and even within nations. Analysis of partial 3Dpol sequences from Polish isolates revealed that while Polish sequences clustered broadly with international ERBV strains, a finer-scale network analysis identified significant segregation of sequences based on the specific stud farm and individual horse [17]. This micro-epidemiological pattern suggests that ERBV transmission dynamics are intensely local, with distinct viral lineages becoming entrenched within specific facilities or management groups, potentially evolving independently for extended periods before spillover occurs. This is consistent with the observation that the detection frequency of ERBV in healthy horses is extremely low (0.8%) compared to clinically ill horses (5.1%), implying that the virus is not maintained in a large, uniformly distributed reservoir but rather circulates in discrete, transient pockets [3].

Contrasting patterns emerge in the United States, where large-scale surveillance of over 10,000 equids has shown ERBV (combined with ERAV) as a single pathogen in 2.3% of acute respiratory cases, with a distinct age predilection for horses under one year of age and those used for competition [1, 9]. The coinfection rate with other respiratory pathogens, particularly Streptococcus equi subsp. equi, EHV-4, and equine influenza virus, is remarkably high, ranging from 21.9% to 34.01% [1, 3]. This frequent co-detection raises a critical question: does ERBV genetic diversity influence the likelihood or severity of coinfection? It is biologically plausible that distinct ERBV genotypes may differentially modulate the host’s mucosal immune response, creating a permissive environment for secondary bacterial or viral invasion. Strains with variant leader proteinase sequences, such as the Japanese isolate with its 87-amino-acid insertion, might exhibit enhanced or altered antagonism of the interferon response, leading to greater susceptibility to superinfection [2].

Implications for Genomic Surveillance and Pathogenesis

The emergence of novel genotypes has direct implications for our understanding of ERBV pathogenesis. Historically, ERBV was considered a pathogen of the upper respiratory tract, with primary isolation from nasopharyngeal swabs and tracheal washes [5, 8, 12]. Detection of ERBV3 and the novel Japanese genotype in fecal samples [2, 4] indicates a broader tissue tropism than previously recognized, potentially involving the gastrointestinal tract. The detection of ERBV in 13.8% of fecal samples from clinically affected horses in Dubai, and in 10.8% of rectal swabs in Japan, challenges the dogma that ERBV is exclusively a respiratory pathogen [2, 4]. Whether this fecal shedding represents true enteric infection, or merely reflects ingestion of respiratory secretions, remains to be determined. However, the presence of both acid-stable (ERBV3) and acid-labile (ERBV2) genotypes in fecal material suggests that at least some ERBV strains possess the structural resilience to survive the harsh gastrointestinal environment, opening the possibility of an alternative fecal-oral transmission route [4].

From a structural and functional perspective, the VP1 protein, the primary determinant of antigenic diversity and a critical target for neutralizing antibodies, exhibits the highest degree of inter-genotype variability. Pairwise comparisons show that VP1 amino acid identity between ERBV3 and other genotypes is below 50% [2, 4]. This degree of divergence is comparable to that seen between different serotypes of foot-and-mouth disease virus (FMDV), a related aphthovirus [16]. Such profound antigenic variation has significant implications for vaccine development. While no licensed ERBV vaccine currently exists, any future immunization strategy must account for this serotypic diversity to ensure broad cross-protection. The antigenic distance between the Japanese novel genotype and established serotypes is so vast that it is highly improbable that natural infection or vaccination with one genotype would confer sterilizing immunity against the other.

Finally, the discovery of novel genotypes mandates a re-evaluation of the evolutionary dynamics of erboviruses. The low Ka/Ks ratios observed for ERBV3 suggest that these lineages are not engaged in rapid antigenic drift under positive selection, but rather are evolving in a stable, host-adapted manner [4]. However, the presence of unique genetic elements, such as the leader proteinase insertion in the Japanese strain, implies that the ERBV genome is capable of accommodating significant structural innovations without compromising viability. This plasticity may allow the virus to adapt to new host environments or evade host immune pressure through mechanisms distinct from simple point mutation. The field must now move beyond a serotype-based classification system and embrace a phylogenomic framework that recognizes ERBV as a complex of multiple, genetically distinct lineages. Comprehensive genomic surveillance, incorporating metagenomic tools that do not rely on targeted PCR, is essential to map the true extent of ERBV diversity and to preempt the emergence of strains with altered pathogenesis or host range.

Epidemiological Patterns and Risk Factors

Equine rhinitis B virus (ERBV) is now recognized as a globally distributed respiratory pathogen of equids, yet its epidemiological profile has remained comparatively neglected relative to other equine respiratory viruses such as equine influenza virus (EIV) or equine herpesvirus type 1 (EHV-1). The cumulative evidence from the past two decades, however, has begun to delineate a distinct epidemiological signature characterized by high prevalence in young, athletic horses, pronounced seasonality in temperate climates, frequent coinfection with other respiratory pathogens, and the existence of both subclinical shedding and clinically apparent disease. A comprehensive understanding of these patterns is essential for informing surveillance strategies, biosecurity protocols, and ultimately, disease control measures.

Global Prevalence and Temporal Trends

The most robust estimates of ERBV prevalence in clinically affected horses derive from large-scale qPCR-based surveillance programs, predominantly conducted in the United States. A landmark analysis of 8,684 nasal swab submissions collected over an 11-year period (2012–2023) through a voluntary upper respiratory biosurveillance program revealed an overall ERBV qPCR-positivity rate of 5.08% (441/8,684) [1]. This figure is remarkably consistent with an earlier 7-year retrospective study (2013–2019) that reported a 5.1% positivity rate (333/6,568) among horses presenting with acute respiratory signs [3]. Collectively, these data indicate that approximately 1 in 20 equids presenting with acute febrile respiratory disease will test positive for ERBV when molecular diagnostics are applied. Critically, the frequency of detection has shown an increasing yearly trend since the introduction of the qPCR assay in 2013, a phenomenon that likely reflects enhanced clinician awareness, improved diagnostic modalities, and possibly true viral emergence or circulation dynamics [3].

The prevalence of ERBV in clinically healthy horses is substantially lower. In the aforementioned 7-year study, only 3 of 356 (0.8%) healthy horses tested qPCR-positive for ERBV, a stark contrast to the 5.1% positivity in clinically ill cohorts [3]. This differential underscores the pathogenic potential of ERBV and indicates that subclinical shedding, while documented, is far less common than in horses with evident respiratory disease. However, the detection of ERBV in 10.8% of fecal samples from a single farm in Japan, as identified through metagenomic analysis employing a novel strain-specific RT-qPCR assay, suggests that subclinical or enteric-associated infection may be underappreciated in certain contexts [2]. Furthermore, the finding that 4 of 29 (13.8%) fecal samples from horses in Dubai tested positive for ERBV, with viral loads ranging from 8.28 × 10³ to 5.83 × 10⁴ copies per mL, introduces the possibility of fecal-oral transmission and raises questions about the role of ERBV in equine gastrointestinal disease [4].

Globally, ERBV has been detected on multiple continents, including North America [1, 3, 9], Europe [8, 12, 17], Asia [2, 4], and Australia [5]. The overall seroprevalence, as measured by complement fixation or neutralizing antibody assays, is exceptionally high. In a longitudinal study of elite Standardbred trotters in Sweden, nearly all horses two years of age or older were seropositive to ERBV, and antibody titers remained elevated for at least one year following exposure [22]. Similar findings have been reported in Poland, where ERBV RNA was detected in 1.78% (11/621) of nasal swabs from horses at national studs, with the majority of positive samples (10/11) originating from healthy horses, indicating that endemic circulation is the norm rather than the exception [17]. A serological survey of 300 horses across multiple regions of Mongolia in 2007 provided additional evidence of widespread exposure, with ERBV antibodies detected in a high proportion of sampled animals [28]. These seroprevalence data collectively suggest that ERBV infection is nearly ubiquitous in equine populations worldwide, yet the detection of active viral shedding in only a minority of horses implies that infection is followed by a period of immune protection, during which reinfection is unlikely to result in detectable virus excretion.

Demographic and Host-Level Risk Factors

Age is the most consistently identified risk factor for ERBV detection. Young horses, particularly those less than one year of age, are significantly more likely to test qPCR-positive for ERBV compared to older cohorts [1]. In the large US biosurveillance cohort, the median age of ERBV-positive horses was 3 years, and the highest detection frequency occurred in foals and weanlings [3]. This age distribution is biologically plausible: young horses lack prior exposure and thus possess waning maternal antibodies coupled with naive adaptive immune systems, rendering them susceptible to primary infection. The epidemiological pattern mirrors that observed for other equine picornaviruses, notably equine rhinitis A virus (ERAV), for which yearling Standardbred racehorses exhibited an attack rate of 87.9% during a respiratory disease outbreak at a training facility in Ontario, Canada [23, 24]. For ERAV, increasing age was associated with a significantly reduced risk of clinical disease (OR = 0.011, p < 0.001) [23], and it is probable that a similar age-related protective effect operates for ERBV, given the high seroprevalence in adult horses.

The influence of sex on ERBV detection appears minimal, with no consistent differences reported between males and females across multiple studies [1, 3]. Breed associations have been identified, with certain breeds showing higher odds of ERBV positivity, although confounding by geographic region and intended use complicates interpretation [9]. More informative is the association between ERBV detection and the horse’s primary use. Horses employed in competitive disciplines, including racing, show jumping, and dressage, are at elevated risk for qPCR-positivity compared to pleasure or breeding horses [1]. This likely reflects several interconnected factors: the commingling of large numbers of horses from diverse geographic origins at competitions, the physiological stress imposed by training and transport, and the increased likelihood of clinical sampling in high-value athletic animals. The concentration of young, immunologically naive horses at training facilities, where they are exposed to older, potentially shedding animals, creates a perfect epidemiological storm for ERBV transmission [23, 24].

Seasonal and Environmental Patterns

Seasonality is a notable feature of ERBV epidemiology, particularly in temperate regions. Environmental surveillance conducted at a multi-week equestrian show during winter months in the United States revealed the detection frequency of true respiratory pathogens, including ERBV, was higher compared to similar studies performed during spring and summer [6]. This winter peak is characteristic of many respiratory viruses in both human and veterinary medicine, likely driven by increased indoor crowding, reduced ventilation in stables, and environmental conditions that favor viral stability. The virus itself is an erbovirus, a genus within the Picornaviridae, and is known to be relatively resistant to environmental degradation, particularly at low temperatures and in the presence of organic material, though systematic studies of ERBV environmental persistence are lacking.

The role of environmental contamination in ERBV transmission has been investigated through stall sponge sampling. In pooled samples from 53 barns at a single equestrian event, ERBV was detected at a low but quantifiable frequency (0.2–1.4% of samples), alongside more prevalent pathogens such as Streptococcus zooepidemicus (28.69%) and EHV-2 (14.45%) [6]. Although the detection of ERBV in environmental samples was infrequent, the fact that it could be recovered from stalls suggests that contaminated fomites, including feed buckets, waterers, and stall walls, could serve as a source of indirect transmission, particularly in facilities with high horse turnover. The identification of ERBV in 10.8% of rectal swabs from a single farm in Japan over a two-year period (2022–2024) further implicates environmental shedding as a potential route of exposure [2].

Coinfection Patterns

A defining feature of ERBV infection is the high frequency of co-detection with other respiratory pathogens. In the US biosurveillance program, 34.01% (150/441) of ERBV-positive samples contained at least one additional respiratory pathogen, with Streptococcus equi subspecies equi (the causative agent of strangles), EHV-4, and EIV being the most commonly identified coinfecting agents [1]. A similar proportion was reported in the 7-year retrospective study, where coinfections were identified in 21.9% (73/333) of ERBV qPCR-positive samples [3]. In a separate analysis of 1,008 equine respiratory PCR panels performed at the New York State Animal Health Diagnostic Center during 2023, the most common coinfection detected was S. equi subspecies equi with equine rhinitis B virus [27]. This striking association between ERBV and a bacterial pathogen of major clinical and economic importance warrants particular attention, as it raises the possibility that ERBV infection may predispose the equine respiratory tract to secondary bacterial invasion, analogous to the well-established synergism between influenza virus and Streptococcus pneumoniae in humans.

The clinical implications of coinfection are significant. Horses with ERBV coinfection presented with a more severe clinical picture, often including fever, nasal discharge, ocular discharge, and cough [1]. In contrast, ERBV as a single pathogen was associated with a milder, self-limiting disease course. Multivariate analysis of clinical data from the larger 2012–2023 cohort revealed that coinfection with S. equi and EHV-4 independently increased the odds of severe clinical disease compared to ERBV mono-infection [1]. This finding has practical diagnostic implications: the detection of ERBV in a respiratory panel should prompt careful evaluation for concurrent pathogens, particularly S. equi, which requires specific therapeutic and biosecurity interventions.

Transmission Dynamics and Subclinical Shedding

The detection of ERBV in clinically healthy horses, albeit at low frequency, is a critical epidemiological consideration. In a study of clinically healthy sport horses undergoing quarantine following a multi-county outbreak of EHV-1 myeloencephalopathy in California, ERBV was identified at low but stable frequencies within previously reported ranges, alongside EIV and EHV-4 [7]. Similarly, in 97 apparently healthy horses sampled at a multi-day show in the United States, ERBV was detected in fecal samples via nanoscale real-time PCR [21]. These findings indicate that subclinically infected horses can serve as silent shedders, perpetuating viral circulation within populations and potentially initiating outbreaks when naive animals are introduced. The phenomenon is particularly relevant at equestrian events, where horses from disparate geographic origins are transiently housed in close proximity.

The duration of viral shedding in naturally infected horses has not been rigorously defined, but limited data from longitudinal studies suggest that ERBV RNA can be detected in nasal secretions for up to 21 days following experimental infection. In the field, the detection of ERBV in serial tracheal wash samples from racehorses over consecutive months implies that either prolonged shedding or reinfection is common in high-turnover training populations [8, 12]. The virus’s ability to establish persistent infection, if it indeed does so, remains unknown, although the high seroprevalence in adult horses suggests that most infections are acute and self-limiting, followed by durable immunity.

Molecular Epidemiology and Emerging Genotypes

The epidemiological landscape of ERBV is complicated by the existence of at least three established serotypes (ERBV1, ERBV2, and ERBV3) and the recent discovery of highly divergent strains that challenge detection by conventional PCR assays. In a metagenomic analysis of a diarrheic foal in Japan, a novel ERBV strain was identified that shared only 62.5–63.1% polyprotein amino acid identity and 47.1–49.8% VP1 identity with known serotypes [2]. This strain, proposed as a novel genotype, was detected in 4 of 37 (10.8%) rectal swab samples from horses on a single farm, yet none of these samples were positive using broad-range primers designed to detect ERBV1–3, indicating that current diagnostic assays may systematically underestimate the prevalence of genetically diverse ERBV strains [2]. The phylogenetic analysis of ERBV3 strains from Dubai revealed a most recent common ancestor (MRCA) estimated to the year 1785 (95% HPD: 1176–1937), with an estimated evolutionary rate that suggests the virus has been stably circulating in equine populations for centuries [4]. The Ka/Ks ratios for all coding regions of ERBV genomes are consistently less than 0.1, indicating strong purifying selection and a stably evolving virus that is well-adapted to its host [4].

The clinical significance of these divergent genotypes remains uncertain. The novel Japanese strain was originally identified in a foal with diarrhea, not respiratory disease, and the four additional positive samples were from horses on the same farm, suggesting a localized outbreak possibly associated with enteric disease [2]. Whether this genotype is capable of causing respiratory disease or is primarily enterotropic is unknown, but the finding underscores the need for expanded surveillance using metagenomic or broadly reactive molecular tools. The World Organisation for Animal Health (WOAH) recognizes ERBV as a pathogen of equids, though it is not currently listed as a notifiable disease; the emergence of antigenically divergent strains may prompt a reassessment of this status.

Risk Factors for Clinical Disease

While many horses are exposed to ERBV and seroconvert without overt signs of illness, a subset develops clinically apparent respiratory disease. Factors that distinguish clinical from subclinical infection are incompletely understood, but evidence points to the interplay of host age, immune status, viral dose, and environmental stressors. Horses with acute onset of fever and respiratory signs, particularly those ≤1 year of age, are significantly more likely to test qPCR-positive for ERBV [1]. In tracheal wash samples from Standardbred racehorses in training, the detection of ERBV RNA was significantly associated with coughing (OR 5.3; 95% CI 2

Clinical Manifestations and Coinfections

Clinical Manifestations of Primary ERBV Infection

Equine rhinitis B virus (ERBV) infection presents a spectrum of clinical manifestations ranging from subclinical carriage to acute febrile respiratory disease, with disease expression heavily influenced by host age, immune status, and the presence of concurrent pathogens. The most comprehensive longitudinal surveillance data from the United States, spanning over a decade (2012–2023), reveals that ERBV is detected via qPCR in approximately 5.08% of nasal swabs submitted from clinically ill horses, with young horses less than one year of age significantly overrepresented among positive cases [1]. This age predilection is corroborated by a seven-year retrospective analysis of 6,568 horses presenting with acute respiratory signs, which identified a median age of three years among ERBV-positive animals and documented that only 0.8% of healthy horses harbored the virus [3]. The marked disparity in detection rates between clinically ill (5.1%) and healthy (0.8%) populations underscores the pathogenic potential of ERBV, though the existence of subclinical shedding complicates the clinical interpretation of a positive qPCR result in individual cases [3].

The cardinal clinical signs associated with acute ERBV infection include fever, serous to mucopurulent nasal discharge, and coughing, which together constitute the classic triad of equine viral respiratory disease [1, 3, 29]. The febrile response is typically acute in onset, often exceeding 38.5°C, and may be accompanied by lethargy, anorexia, and depression [29]. Ocular discharge, ranging from serous to mucoid, has been reported in a substantial proportion of cases, and some affected horses develop peripheral lymphadenopathy and edema of the distal limbs [1, 29]. The cough associated with ERBV infection is noteworthy for its clinical significance: in a prospective longitudinal study of 52 Standardbred racehorses followed over 27 consecutive months, detection of ERBV RNA in tracheal wash (TW) samples was strongly and independently associated with coughing, yielding an odds ratio (OR) of 5.3 (95% CI 2.1–14.0; P < 0.001) compared to horses without ERBV detection [8, 12]. When only quantifiable viral loads were considered, the association became even more pronounced (OR 15.0; 95% CI 3.7–60.0; P < 0.001), suggesting a dose-dependent relationship between viral replication in the lower airways and the induction of cough reflex [8]. Importantly, this association was specific to lower airway sampling: detection of ERBV in nasopharyngeal swabs (NS) showed no significant correlation with any clinical sign, highlighting the critical importance of sampling site in assessing the role of ERBV in respiratory disease [12].

The clinical course of uncomplicated ERBV infection is generally regarded as mild to moderate, with spontaneous resolution of fever within 2–4 days and gradual improvement of respiratory signs over 7–14 days. However, the duration of clinical disease can be highly variable, a phenomenon that may reflect differences in host immune competence, viral strain virulence, or the presence of undetected co-pathogens. The pathophysiological basis for the cough-predominant phenotype may relate to ERBV’s tropism for ciliated epithelial cells in the upper and lower respiratory tract, as demonstrated in the closely related bovine rhinitis B virus (BRBV), which infects ciliated epithelial and goblet cells, leading to focal defects in the superficial cilia lining [13]. This cytopathic effect likely impairs mucociliary clearance, predisposing to persistent cough and secondary bacterial colonization.

Subclinical Infection and Silent Shedding

A critical aspect of ERBV epidemiology is the occurrence of subclinical infection, which has profound implications for disease transmission and biosecurity. Detection of ERBV in clinically healthy horses has been documented in multiple studies, including a surveillance program of sport horses returning to competition following an EHV-1 myeloencephalopathy outbreak, where ERBV was found at low but stable frequencies consistent with background circulation [7]. Similarly, environmental sampling at a multi-week equestrian show detected ERBV in pooled stall sponges, confirming that apparently healthy horses can shed the virus and contaminate their surroundings [6]. The phenomenon of subclinical shedding is further supported by a Polish study in which 10 of 11 ERBV-positive horses were clinically healthy [17], and by a fecal survey from the Middle East that identified ERBV RNA in 13.8% of fecal samples from Dubai horses [4]. These findings indicate that ERBV circulates within equine populations through a reservoir of asymptomatic carriers, a pattern reminiscent of other picornaviruses such as foot-and-mouth disease virus (FMDV). The World Organisation for Animal Health (WOAH) classifies respiratory pathogens with subclinical shedding potential as particularly challenging for disease control, given the difficulty of identifying and isolating infectious animals. The demonstration that ERBV can be shed in feces raises the possibility of fecal-oral transmission, a route not traditionally emphasized for equine respiratory viruses but one that could facilitate contamination of shared water sources, feed, and bedding [4]. The detection of a highly divergent ERBV strain in a diarrheic foal from Japan further suggests that enteric involvement, though likely underappreciated, may be part of the clinical spectrum in some cases [2].

Coinfections: Prevalence, Patterns, and Clinical Implications

Coinfection is a defining feature of ERBV infection, with profound implications for disease severity, diagnostic interpretation, and therapeutic management. The largest biosurveillance study to date, encompassing 8,684 nasal swab submissions from the United States, reported that ERBV was detected as a single pathogen in 65.99% of positive cases, while 34.01% of ERBV-positive horses harbored at least one additional respiratory pathogen [1]. Among coinfected horses, the most frequently copathogens were Streptococcus equi subspecies equi (S. equi), equine herpesvirus type 4 (EHV-4), and equine influenza virus (EIV) [1]. This finding is strikingly consistent with a seven-year retrospective analysis that identified a 21.9% coinfection rate, with S. equi and EHV-4 again predominating [3]. The clinical significance of these co-detections cannot be overstated: bacterial superinfection, particularly with S. equi, can transform a self-limiting viral illness into a prolonged, suppurative condition requiring antibiotic therapy and carrying a risk of guttural pouch empyema or metastatic abscessation (strangles). Similarly, dual viral infection with EIV or EHV-4 may synergistically amplify airway inflammation through overlapping mechanisms of epithelial damage and immune dysregulation.

A more recent retrospective analysis of 1,008 equine respiratory PCR panels performed at the New York State Animal Health Diagnostic Center in 2023 confirmed that the most common coinfection identified was S. equi with equine rhinitis B virus, underscoring the reproducibility of this association across different geographic regions and time periods [27]. The biological plausibility of ERBV–S. equi synergy is supported by the known ability of respiratory viruses to disrupt epithelial barriers, expose basement membrane components that facilitate bacterial adherence, and impair neutrophil and macrophage function. The bovine literature on rhinitis B virus (BRBV) provides a useful comparative framework: BRBV is the most prevalent virus detected in nasal swabs from cattle with acute bovine respiratory disease (BRD), with detection rates reaching 40.8% in affected calves, and is increasingly recognized as an initiator of the viral-bacterial cascade that culminates in bronchopneumonia [13, 14]. Although analogous challenge studies have not been performed in horses, the virological and clinical parallels suggest that ERBV may play a comparable priming role in equine respiratory disease complex.

The clinical impact of coinfection is further illuminated by a 13-year prospective surveillance program (2008–2021) that evaluated 10,296 equids with acute fever or respiratory signs. This study reported that multiple pathogens were detected in 274 horses (2.7%), with ERBV being one of several viruses identified in mixed infections [9]. Importantly, the clinical signs associated with single versus multiple pathogen detection were largely overlapping, making it impossible to distinguish coinfected horses based on physical examination alone [9]. This diagnostic ambiguity highlights the indispensability of comprehensive molecular panels for accurate etiological diagnosis and appropriate antimicrobial stewardship. While specific prevalence factors such as age, breed, and use differed among pathogens, the clinical presentation remained fairly homogeneous, reinforcing the concept that acute febrile respiratory disease in horses is a syndromic entity with multiple potential etiologies [9].

Cough, Tracheal Mucus, and Inflammatory Airway Disease

Perhaps the most clinically actionable finding from the literature is the robust association between ERBV detection in the lower airways and inflammatory airway disease (IAD), a condition of immense importance to equine athletes. In a rigorously designed longitudinal study of Standardbred racehorses, detection of ERBV RNA in tracheal wash samples was significantly associated not only with coughing but also with excess tracheal mucus accumulation when present as a coinfection with EHV-2 [8, 12]. The odds of coughing were 5.3 times higher in horses with ERBV-positive TW compared to negative horses, and when ERBV was quantifiable, the odds increased 15-fold [8]. This dose-response relationship strongly supports a causal role for ERBV in the pathogenesis of IAD, rather than mere bystander detection. Conversely, detection of the same virus in nasopharyngeal swabs showed no association with any clinical parameter, indicating that ERBV must reach the lower airways to exert its pathogenic effects [12]. This finding has direct clinical relevance: a negative nasopharyngeal swab does not rule out ERBV involvement in IAD, and tracheal wash sampling should be considered when evaluating horses with chronic cough, poor performance, or abnormal tracheal mucus scores.

Fecal Shedding and the Potential for Enteric Manifestations

The discovery of ERBV in fecal samples from apparently healthy horses in the Middle East, with viral loads ranging from 8.28 × 10³ to 5.83 × 10⁴ copies/mL, raises intriguing questions about the virus’s tropism and transmission dynamics [4]. Both acid-stable (ERBV3) and acid-labile (ERBV2) serotypes were identified in feces, indicating that the virus can survive gastrointestinal transit regardless of serotype-specific acid resistance [4]. The phylogenetic analysis of these fecal strains revealed that ERBV3 has been stably evolving in horses, with the most recent common ancestor estimated to 1785, suggesting a long history of host adaptation [4]. The detection of a highly divergent ERBV strain in a diarrheic foal that was also rotavirus A-positive further complicates the picture: whether ERBV contributed to the enteric signs or was merely an incidental finding remains unknown, but the case highlights the need for prospective studies to evaluate the role of ERBV in equine gastrointestinal disease [2]. Fecal sampling may represent an underutilized, non-invasive diagnostic tool for surveillance, particularly in young horses where nasopharyngeal collection is challenging. However, the clinical significance of fecal shedding in the absence of respiratory signs warrants further investigation.

Diagnostic Approaches and Detection Methods

The accurate and timely diagnosis of equine rhinitis B virus (ERBV) infection is paramount for understanding its epidemiology, implementing effective biosecurity measures, and differentiating it from other respiratory pathogens with overlapping clinical presentations. The diagnostic landscape for ERBV has evolved considerably from early reliance on virus isolation (VI) and serology to the current dominance of highly sensitive and specific nucleic acid amplification techniques. However, challenges remain, particularly concerning viral genetic diversity, the distinction between active infection and subclinical shedding, and the interpretation of molecular results in the context of coinfections.

Molecular Detection: The Cornerstone of Contemporary Diagnosis

The advent of molecular diagnostics has revolutionized the detection of ERBV, addressing the inherent limitations of traditional methods. The virus is frequently fastidious in cell culture, and its growth may be inconsistent or fail to produce a discernible cytopathic effect (CPE), leading to false-negative results [5, 19]. Consequently, reverse transcription-polymerase chain reaction (RT-PCR) and its real-time quantitative variant (RT-qPCR) have become the gold standard for diagnosing acute infections.

Development and Target Regions of PCR Assays

Several robust assays have been developed, primarily targeting highly conserved genomic regions to ensure broad reactivity across ERBV serotypes. The 5′ untranslated region (5′-UTR) is a favored target due to its critical role in viral translation and its high degree of sequence conservation within the Picornaviridae family. Lu et al. (2012) developed a suite of assays, including a one-step TaqMan® real-time RT-PCR and conventional RT-PCR assays specifically targeting the 5′-UTR of ERBV, demonstrating high sensitivity (detection limit of 1 plaque-forming unit per mL) and specificity, with no cross-reactivity against other common equine respiratory viruses such as equine influenza virus (EIV) or equine herpesvirus-1 (EHV-1) [19]. Similarly, the 3D polymerase (3Dpol) region, encoding the RNA-dependent RNA polymerase, has been successfully used for both detection and phylogenetic characterization. Black et al. (2006) employed a nested RT-PCR amplifying a product within the 3Dpol and 3′ non-translated region, successfully detecting all tested ERBV1 and ERBV2 isolates with a limit of detection of 0.1 median tissue culture infectious dose (TCID₅₀) [5].

The diagnostic power of these assays is underscored by large-scale surveillance programs. A voluntary upper respiratory biosurveillance program in the United States, analyzing over 8,600 nasal swabs from 2012–2023, utilized qPCR to identify an overall ERBV-positivity rate of 5.08% [1]. This rate was corroborated by a study of 6,568 horses from 2013–2019, which found a 5.1% ERBV qPCR-positivity rate, with a notable increase in detection frequency over the study period following the introduction of the assay [3]. These data not only confirm the utility of qPCR but also highlight the significant burden of ERBV in populations with acute respiratory signs.

Challenges of Viral Genetic Diversity and Assay Coverage

A critical caveat to molecular diagnostics is the potential for assays to fail in detecting divergent strains. The inherent genetic plasticity of RNA viruses means that primer or probe mismatches can lead to false-negative results, particularly for novel or uncharacterized genotypes. Ketphan et al. (2025) demonstrated this problem when performing metagenomic analysis on a fecal sample from a diarrheic foal in Japan [2]. They identified a highly divergent ERBV strain with only 62.5–63.1% polyprotein identity and 47.1–49.8% VP1 identity to known serotypes. Crucially, while they developed a strain-specific RT-qPCR that detected a 10.8% positivity rate in subsequent fecal samples, a simultaneously used "broadly reactive" assay targeting ERBV1-3 failed to detect any of these cases [2]. This finding underscores a significant surveillance gap and suggests that many currently used diagnostic assays may systematically underestimate the true prevalence of ERBV by missing highly divergent, potentially novel genotypes. The detection of ERBV in fecal samples from the Middle East via RT-PCR further illustrates the need for assays that can handle varied sample matrices and potentially different viral quasispecies [4]. Future assay design must incorporate primers from the most highly conserved genomic regions, such as specific motifs within the 3Dpol or internal ribosome entry site (IRES), and be periodically re-evaluated against a growing database of global ERBV sequences.

Sample Selection and Clinical Correlates

The diagnostic sensitivity and clinical interpretation of ERBV detection are profoundly influenced by the type of sample collected. While nasal or nasopharyngeal swabs are the most common and practical specimens for field collection, their utility in establishing a causal link between the virus and clinical disease is limited.

The Superiority of Lower Respiratory Tract Sampling

A landmark longitudinal study by Doubli-Bounoua et al. (2016) on 52 Standardbred racehorses provided critical insights into this issue. The study directly compared the detection of ERBV RNA in nasal swabs (NS) versus tracheal washes (TW) collected monthly over two years [8, 12]. The monthly incidence of ERBV in TW was 7.1%, whereas detection in NS was not significantly associated with any clinical sign. In stark contrast, the detection of ERBV in TW was strongly and significantly associated with coughing (Odds Ratio [OR] = 5.3; 95% CI 2.1–14.0; P < 0.001). When quantitative analysis was applied, the association was even stronger: quantifiable ERBV detection in TW had an OR of 15.0 for coughing [8]. This suggests that virus present in the lower airways is a major risk factor for clinical inflammation and disease, whereas nasal shedding may reflect transient mucosal carriage, early-stage infection, or even passive contamination. For evaluating horses with suspected infectious inflammatory airway disease (IAD), lower respiratory tract sampling via tracheal wash or bronchoalveolar lavage is therefore diagnostically superior.

Alternative and Surveillance Sampling Strategies

Beyond direct sampling of the respiratory tract, ERBV RNA has been detected in other biological and environmental matrices. Molecular epidemiology studies have confirmed the presence of ERBV in fecal samples, with detection rates of 13.8% in horses from Dubai, including both acid-stable (ERBV3) and acid-labile (ERBV2) serotypes [4]. This raises important questions regarding alternative transmission routes (fecal-oral) and the potential role of ERBV in gastrointestinal disease. Environmental surveillance has also emerged as a non-invasive tool. Lawton et al. (2023) detected ERBV in a very small percentage of pooled environmental stall sponge samples collected at a multi-week equestrian show, demonstrating that the virus can be present in the horse’s immediate environment [6]. While of low diagnostic sensitivity for individual animals, this approach can be valuable for monitoring pathogen circulation and biosecurity efficacy within a facility. qPCR panels applied to bronchoalveolar lavage fluid (BALF) from healthy horses have further confirmed the presence of ERBV in subclinical carriers, highlighting the complexity of interpreting a positive result without clinical context [30].

Serological Approaches and Their Limitations

Serological testing, including virus neutralization tests (VNT) and enzyme-linked immunosorbent assays (ELISA), has been instrumental in establishing the global seroprevalence of ERBV. However, its role in diagnosing acute clinical disease is limited.

High Seroprevalence and the Challenge of Acute Diagnosis

Extensive serosurveillance consistently demonstrates that ERBV infection is ubiquitous. For instance, serological surveys in Mongolia and other regions show that a high proportion of adult horses have antibodies against ERBV [28]. While a single positive serological result confirms past exposure, it is of little diagnostic value in an acutely ill horse due to the high background seropositivity in the population. A four-fold rise in antibody titer between acute and convalescent sera is more informative but is retrospective and impractical for clinical decision-making. Back et al. (2015) explicitly stated that "in absence of clinical signs, serology to common respiratory viruses appears to have little diagnostic benefit in evaluation of poor performance in young athletic horses" [22]. Furthermore, while seroconversion to ERAV has been strongly associated with clinical respiratory disease and subsequent failure to race, a similar robust association with ERBV has been more difficult to establish using serology alone [31].

Cross-Reactivity and Serotype Specificity

The presence of multiple ERBV serotypes (ERBV1, 2, and 3) complicates serological interpretation. While complement fixation and VNT can be serotype-specific, they require a panel of antigens and can be labor-intensive. The recent identification of a highly divergent ERBV genotype in Japan [2] raises the possibility that current serological assays, based on reference strains, may underestimate the breadth of infection. The interpretation of serological data must be cautious, as it often reflects cumulative lifetime exposure rather than a current infection event.

Emerging and Ancillary Diagnostic Technologies

The diagnostic armamentarium for ERBV is expanding with innovative approaches that move beyond simple pathogen detection.

Volatile Organic Compound (VOC) Profiling

A novel, non-invasive diagnostic approach based on host metabolomics is under investigation. Matczuk et al. (2025) explored the production of volatile organic compounds (VOCs) in RK-13 cell cultures infected with ERBV, among other equine viruses [20]. Using headspace solid-phase microextraction (HS-SPME) coupled with gas chromatography-mass spectrometry (GC-MS), they demonstrated that ERBV infection induces distinct VOC signatures, likely reflecting virus-specific metabolic reprogramming of the host cell. Principal component analysis (PCA) of the VOC profiles allowed clear separation of ERBV-infected cultures from those infected with other virus families [20]. While still at an experimental in vitro stage, this technology holds promise for developing rapid, point-of-care breath tests that could screen for respiratory viral infections in horses, providing a result in minutes rather than hours.

Metagenomic Next-Generation Sequencing (mNGS)

As highlighted by the discovery of a novel ERBV genotype in Japan, metagenomic analysis is a powerful, albeit specialized, tool for virus discovery and surveillance [2]. mNGS is an unbiased approach that can detect any nucleic acid in a sample, including unexpected or highly divergent pathogens without the need for specific primers. This technique is invaluable for identifying novel strains that escape routine PCR panels and for providing comprehensive genomic data that inform phylogenetic, evolutionary, and structural studies [2, 15]. Its current limitations include high cost, complex bioinformatics requirements, and lower sensitivity for detecting low-abundance targets compared to targeted qPCR.

Integration into Multiplex Panels

Given the high rate of coinfection (34.01% in one large US study) [1], the diagnosis of ERBV is most clinically meaningful when performed as part of a comprehensive respiratory panel. Multiplex qPCR assays capable of simultaneously detecting ERBV alongside EIV, EHV-1, EHV-4, equine rhinitis A virus (ERAV), and Streptococcus equi subsp. equi are now standard in many diagnostic laboratories [1, 7, 9, 27]. This approach provides a holistic view of the respiratory pathogen complex, enabling clinicians to identify not only the primary agent but also contributing copathogens, which may influence prognosis and treatment. The data from such panels have been crucial in defining the epidemiological landscape of ERBV, identifying young horses and those used for competition as being at higher risk, and demonstrating that S. equi, EHV-4, and EIV are the most common coinfecting pathogens [1, 3, 27].

Surveillance and Control Strategies

The effective management of Equine Rhinitis B Virus (ERBV) within equine populations necessitates a multifaceted approach that integrates robust surveillance infrastructure, advanced molecular diagnostics, and evidence-based biosecurity protocols. Unlike WOAH-listed pathogens such as equine influenza virus or African horse sickness, ERBV currently lacks internationally mandated reporting frameworks or standardized control programs. However, the cumulative evidence from the last decade has illuminated both the prevalence and clinical significance of this pathogen, compelling a re-evaluation of surveillance paradigms and the development of targeted control strategies. The following analysis delineates the current state of ERBV surveillance, the diagnostic tools available, and the strategic imperatives for controlling its spread.

Epidemiological Surveillance Infrastructure and Data Synthesis

The cornerstone of contemporary ERBV surveillance has been the establishment of voluntary, laboratory-based biosurveillance programs. The most extensive dataset originates from a United States-based program initiated in 2008, which has analyzed over 10,000 nasal swab submissions from equids presenting with acute fever or respiratory signs [9]. A subsequent analysis of this program between 2012 and 2023, encompassing 8,684 submissions, revealed a consistent ERBV qPCR-positivity rate of 5.08%, with detections occurring as a single pathogen in 65.99% of cases and as a coinfection in 34.01% [1]. This longitudinal dataset is invaluable, providing not only a baseline prevalence but also critical demographic risk factors. Notably, young horses under one year of age and those used for competition are significantly more likely to test positive [1, 3]. The finding that competition horses are at elevated risk underscores the role of high-density housing, frequent transport, and mixing of animals from diverse geographical origins as potent amplifiers of transmission.

The temporal dynamics of ERBV detection are also revealing. A seven-year review (2013–2019) of the same program documented a clear increasing yearly frequency of ERBV detection since the assay's introduction in 2013 [3]. This trend may reflect genuine epidemiological expansion, increased clinician awareness and testing, or a combination of both. However, the detection rate among clinically healthy horses remains low (0.8%), which is a critical benchmark for interpreting positive results in sick animals [3]. This stark difference emphasizes that ERBV is not a commensal organism; its detection in a symptomatic horse is highly suggestive of a causal or contributory role in disease. The high rate of coinfection, most frequently with Streptococcus equi subsp. equi, equine herpesvirus-4 (EHV-4), and equine influenza virus (EIV), complicates clinical attribution but is a consistent feature of ERBV epidemiology [1, 27]. From a control perspective, this implies that interventions targeting ERBV must be integrated into broader respiratory disease management protocols.

Molecular Diagnostic Arsenal and Genomic Surveillance

The sensitivity and specificity of ERBV detection have been revolutionized by the development of nucleic acid amplification techniques. Traditional virus isolation, hampered by fastidious growth requirements and inconsistent cytopathic effect, has been largely superseded by real-time quantitative reverse transcription PCR (RT-qPCR) assays. A landmark study by Lu et al. (2012) developed and validated one-step TaqMan® RT-qPCR and conventional RT-PCR assays targeting the 5’ untranslated region (UTR) of ERAV and ERBV [19]. These assays demonstrated high specificity, detecting only the target virus without cross-reactivity to other common equine respiratory pathogens, and achieved a sensitivity of 1 plaque-forming unit per ml [19]. This level of analytical sensitivity is crucial for detecting low-level shedding, particularly in subclinical or early-stage infections.

However, the recent discovery of a highly divergent novel ERBV genotype in Japan, exhibiting only 62.5–63.1% polyprotein identity and 47.1–49.8% VP1 identity to known serotypes, has exposed a critical vulnerability in current surveillance strategies [2]. The novel strain was detected using metagenomic sequencing and a strain-specific RT-qPCR, yet it was entirely missed by a broadly reactive assay designed to detect ERBV1–3. Importantly, screening of 37 rectal swab samples with this broadly reactive assay yielded zero positives, while the new specific assay found a 10.8% positivity rate [2]. This finding provides direct evidence of significant surveillance gaps. It strongly suggests that current diagnostic panels, which are often designed based on historically circulating strains, may systematically fail to detect a reservoir of genetically distinct ERBVs. Consequently, a control strategy predicated solely on existing PCR panels will be incomplete. There is an urgent need for ongoing metagenomic surveillance and periodic re-validation of diagnostic primers and probes against a continually expanding genomic database. The integration of pan-picornavirus or targeted next-generation sequencing approaches into routine diagnostic workflows would enhance the capacity to detect these cryptic strains.

Furthermore, the tissue tropism of ERBV may be broader than previously appreciated, complicating standard surveillance sampling. ERBV has been detected in fecal samples from diarrheic foals and apparently healthy horses, with viral loads ranging from 8.28 × 10³ to 5.83 × 10⁴ copies per ml in one Middle Eastern study [4]. The detection of both acid-stable (ERBV3) and acid-labile (ERBV2) serotypes in feces suggests that oro-fecal transmission may be a viable, albeit likely secondary, route of spread. This has profound implications for surveillance: reliance solely on nasal swabs could underestimate the true prevalence. Moreover, the detection of ERBV in fecal samples introduces the possibility of using non-invasive fecal sampling as an alternative surveillance tool, particularly in foals or in situations where nasal swabbing is logistically challenging.

Environmental and Subclinical Surveillance

Beyond individual animal testing, environmental surveillance has emerged as a powerful tool for assessing pathogen circulation within a group without the need for handling individual horses. A study conducted at a multi-week equestrian show during winter months employed environmental sponge sampling from stall surfaces, detecting ERBV RNA in a small but notable percentage of pooled samples [6]. While ERBV was not the most prevalent pathogen detected (that distinction belonged to S. zooepidemicus and EHV-2), its presence in the environment confirms that contaminated fomites (stall walls, feed buckets, waterers) can serve as a reservoir for transmission. The study also reported a higher detection frequency of respiratory pathogens during winter months, correlating with increased time spent in closed, poorly ventilated barns [6]. From a control perspective, this underscores the importance of seasonal biosecurity intensification, including enhanced disinfection protocols during the cold season.

The phenomenon of subclinical shedding in apparently healthy horses is a major challenge for control. Following a multi-county outbreak of EHV-1 myeloencephalopathy in California, mandatory qPCR testing of clinically healthy sport horses during quarantine revealed low but stable frequencies of ERBV shedding [7]. This silent circulation means that horses can introduce ERBV into a naïve population without displaying overt clinical signs, acting as "Trojan horses" that undermine quarantine effectiveness. The study also demonstrated that a strategy of quarantine coupled with qPCR testing for EHV-1 was successful in preventing further EHM outbreaks [7], but ERBV was not the target of this intervention. This highlights a critical operational gap: current quarantine protocols rarely include testing for ERBV. Given its prevalence and potential to cause coughing and reduced performance, the cost-benefit of including ERBV in pre- or post-movement testing panels should be evaluated, especially for high-value equine athletes.

Biosecurity and Control Strategies: Vaccination, Isolation, and Management

Currently, there is no licensed commercial vaccine for ERBV. This absence represents the most significant gap in the control armamentarium. The development of an effective vaccine is hampered by the antigenic diversity of the virus, as evidenced by the existence of multiple serotypes (ERBV1, ERBV2, ERBV3) and the newly discovered divergent genotype [2, 4]. Any successful vaccine would likely need to be multivalent or based on conserved, immunogenic epitopes, such as those within the more conserved 3D polymerase region or structural proteins that elicit cross-neutralizing antibodies. The ERBV 2A peptide, which mediates ribosomal skipping during polyprotein translation, has been extensively studied for biotechnological applications [10, 11], but its immunogenicity as a vaccine antigen remains uncharacterized.

In the absence of vaccination, control strategies must rely on rigorous biosecurity, early detection, and isolation. The core principles are analogous to those for other respiratory pathogens:

1. Isolation of New Arrivals: New horses entering a facility should be quarantined for a minimum of 14–21 days. Given that subclinical shedding is documented [7] and that young horses (<1 year) are high-risk shedders [1], isolation protocols should be strictly enforced for all incoming animals, particularly yearlings and two-year-olds entering training yards.
2. Cohorting by Age: Consistent epidemiological data demonstrate that young horses are more susceptible to ERBV infection and are more likely to test positive [1, 9, 23]. Training and housing facilities should stratify animals by age to reduce pathogen transmission from younger, more infectious cohorts to older, potentially immunologically naïve or recovered horses.
3. Environmental Decontamination: ERBV, as a non-enveloped picornavirus, is relatively resistant to inactivation by many common disinfectants. The virus requires virucidal agents effective against non-enveloped viruses, such as accelerated hydrogen peroxide, sodium hypochlorite (bleach), or potentiated peracetic acid. Given the detection of ERBV RNA on environmental surfaces [6], routine disinfection of stalls, shared equipment (bits, tack, grooming tools), and transport vehicles is essential.
4. Ventilation and Air Quality: ERBV is transmitted primarily via the respiratory route. Poorly ventilated barns, especially during winter, facilitate aerosol and droplet transmission. Improving air exchange rates and reducing stocking density can significantly lower the infectious dose to which susceptible animals are exposed.

The role of serology in control strategies remains limited. Serological surveys have confirmed that ERBV infection is ubiquitous, with a high proportion of adult horses being seropositive [22]. Seroconversion is typically slow, and antibody titers can persist for at least a year, making it difficult to distinguish between recent and historical infection using a single sample [22]. In the context of poor performance investigations, serology has shown little diagnostic benefit in the absence of clinical signs [22]. Therefore, acute and convalescent serology is rarely practical for outbreak management. The diagnostic focus must remain on RT-qPCR detection of viral RNA in nasal secretions or tracheal washes.

Finally, a strategic imperative for the future is the integration of ERBV surveillance into a broader, syndromic One Health framework. While ERBV is not zoonotic, its behavior, including its potential for fecal shedding and its role as a co-pathogen in exacerbating bacterial infections, mirrors patterns seen in other picornaviruses of veterinary and public health concern. The development of a standardized, internationally recognized case definition for ERBV-associated respiratory disease, coupled with the establishment of a shared genomic database, would be of immense value. Such a database would facilitate real-time monitoring of strain emergence, enable rapid updates to diagnostic assays, and inform the rational design of future vaccines. Without this global coordination and investment in molecular surveillance infrastructure, the true impact and evolution of ERBV will remain incompletely understood, and control efforts will remain reactive rather than proactive.

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