Equine Coronavirus

Overview and Taxonomy of Equine Coronavirus

Equine coronavirus (ECoV) represents a significant, re-emerging enteric pathogen of adult horses and foals, belonging to a lineage of viruses with notable zoonotic potential and complex evolutionary histories. As a member of the Coronaviridae family, ECoV is classified within the genus Betacoronavirus, subgenus Embecovirus [3, 11, 13, 18]. This subgenus is of particular public health and veterinary importance, as it includes not only ECoV but also bovine coronavirus (BCoV), porcine hemagglutinating encephalomyelitis virus (PHEV), and the human coronaviruses HCoV-OC43 and HCoV-HKU1, both of which are known to have originated from animal reservoirs [3, 12, 18]. The phylogenetic placement of ECoV within the Embecovirus subgenus is critical for understanding its biology, pathogenesis, and potential for cross-species transmission. The World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) have closely monitored embecoviruses due to their history of host-switching events, underscoring the relevance of studying ECoV within a One Health framework [3, 15].

The taxonomic classification of ECoV is rooted in its genomic architecture and antigenic relationships. The virus possesses a large, single-stranded, positive-sense RNA genome, approximately 30.9 to 31.0 kilobases in length, excluding the poly(A) tail [8, 11]. The complete genome sequence of the prototype strain, ECoV-NC99, first isolated from a diarrheic foal in the United States in 1999, established the foundational genomic map for the species [11]. This genome encodes the canonical coronavirus replicase polyproteins (pp1a and pp1ab), which are proteolytically processed into 16 non-structural proteins (nsp1–16) that form the viral replication-transcription complex [11]. The structural protein repertoire of ECoV is characteristic of embecoviruses and includes the spike (S) glycoprotein, hemagglutinin-esterase (HE), envelope (E), membrane (M), and nucleocapsid (N) proteins [11]. Additionally, the ECoV genome encodes several accessory proteins, NS2, p4.7, p12.7, and I, whose functions are not fully elucidated but are believed to modulate host immune responses and viral pathogenesis [8, 11]. The organization of these open reading frames (ORFs) and the presence of the HE gene, which is a hallmark of embecoviruses, firmly anchor ECoV within this subgenus [11, 13].

Genetic and antigenic analyses have revealed that ECoV is most closely related to BCoV and HCoV-OC43, with nucleotide sequence identities in the range of 98–99% for certain structural genes [10, 11]. However, distinct genomic features differentiate ECoV from its closest relatives. Notably, the nsp3 protein of ECoV contains considerable amino acid deletions and insertions compared to BCoV, HCoV-OC43, and PHEV, suggesting unique evolutionary pressures and potential functional adaptations [11]. Furthermore, the accessory protein p4.7 exhibits remarkable genetic plasticity, with deletions, insertions, and sequence variations observed among ECoV strains from different geographic regions and outbreaks [6, 8-10]. For instance, the Japanese isolate Tokachi09 was found to have a 185-nucleotide deletion in the region encoding p4.7, resulting in the absence of this protein, yet the virus remained replication-competent in cell culture [10]. This variability in the p4.7 region has been proposed as a useful molecular marker for epidemiological tracking of ECoV strains [9]. The spike and nucleocapsid genes, in contrast, are highly conserved across ECoV isolates, with amino acid identities often exceeding 97% between strains from the United States, Japan, and Europe [1, 6, 10]. This conservation suggests strong functional constraints on these structural proteins, which are critical for receptor binding, membrane fusion, and viral assembly.

The evolutionary dynamics of ECoV are characterized by both genetic drift and recombination, mechanisms that are well-documented within the Embecovirus subgenus [3]. A comprehensive molecular evolutionary analysis of ECoV genomes identified 12 putative recombination events, the majority (11 of 12) occurring within the large ORF1ab replicase gene [3]. This finding indicates that recombination is a significant driver of genetic diversity in ECoV, potentially facilitating the emergence of novel strains with altered virulence or host tropism. Evidence of intra-host evolution, suggestive of quasispecies development, has also been detected within the nucleocapsid (N) gene of ECoV from infected horses [3]. The presence of quasispecies is a hallmark of RNA virus evolution and can contribute to rapid adaptation under selective pressures, such as immune responses or antiviral interventions. Furthermore, positive selection analysis has identified specific sites under adaptive evolution on the ancestral branches of major embecovirus lineages, including those that led to the zoonotic HCoV-OC43 and HCoV-HKU1 [3]. The fact that HCoV-OC43 has 42 sites under positive selection, compared to only 2 for HCoV-HKU1, may reflect the more complex ancestral evolutionary history of HCoV-OC43, which involved multiple animal hosts [3]. These insights underscore the importance of studying ECoV evolution to better understand the mechanisms that govern coronavirus host-switching and emergence.

The host range of ECoV is primarily restricted to equids, including horses, donkeys, and possibly other members of the family Equidae [4, 6, 16]. However, the close antigenic and genetic relationship between ECoV and BCoV raises questions about potential cross-species transmission. Experimental inoculation of horses with a modified-live BCoV vaccine resulted in the induction of cross-reactive neutralizing antibodies against ECoV, albeit at lower titers than against BCoV [7, 17]. This antigenic cross-reactivity has practical implications for serological diagnosis and vaccine development. Conversely, BCoV-like sequences have been detected in fecal samples from horses with colic, suggesting that spillover infections from cattle may occur, although the pathogenic significance of such events remains unclear [14]. The World Organisation for Animal Health (WOAH) recognizes the importance of monitoring such interspecies transmission events, as they can have implications for both animal health and food security. The detection of ECoV in a donkey foal with diarrhea in Ireland [6] and the identification of a recombinant ECoV strain in a donkey in China [4] further expand the known host range and highlight the potential for viral adaptation to new equid species. The Chinese donkey strain, which shared 100% genome identity between two samples, was identified as a novel genetic variant with evidence of recombination near the NS2 gene, suggesting that recombination events may facilitate host adaptation [4].

The tissue tropism of ECoV is predominantly enteric, with the virus primarily infecting the epithelial cells lining the intestinal tract. Experimental infection studies using in situ hybridization and quantitative RT-PCR have demonstrated that ECoV RNA is present throughout the entire intestinal tract, from the duodenum to the colon, with infected cells predominantly located on the surface epithelium [5]. This broad distribution of viral RNA along the gastrointestinal tract is consistent with the clinical presentation of enteric disease, including diarrhea and colitis. Interestingly, while ECoV RNA has been detected in nasal swabs and occasionally in lung tissue from infected horses, in situ hybridization studies have failed to identify infected cells in the respiratory tract, suggesting that the virus does not productively replicate in the lungs [5]. The detection of ECoV RNA in the lung of one viremic horse was attributed to the presence of viral RNA in the blood within pulmonary capillaries rather than true infection of respiratory epithelial cells [5]. This finding supports the conclusion that ECoV is primarily an enteric pathogen with minimal respiratory involvement, a distinction that is important for diagnostic sampling and biosecurity protocols. The development of equine intestinal enteroids (EIEs) as an in vitro model has confirmed that ECoV can infect and replicate in intestinal epithelial cells derived from the duodenum, jejunum, and ileum, providing a valuable tool for studying host-pathogen interactions at the cellular level [2].

In summary, ECoV is a genetically and antigenically distinct member of the Betacoronavirus genus, subgenus Embecovirus, with a genome organization and evolutionary history that place it in close relation to BCoV and HCoV-OC43. Its genetic diversity is shaped by recombination, positive selection, and intra-host evolution, particularly within the ORF1ab and accessory protein genes. The virus exhibits a primary tropism for the equine intestinal epithelium, with minimal evidence of respiratory tract infection. Understanding the taxonomy and evolutionary biology of ECoV is essential for interpreting epidemiological patterns, developing diagnostic tools, and assessing the risk of cross-species transmission to other animals or humans.

Molecular Pathogenesis of Equine Coronavirus

Equine coronavirus (ECoV), a member of the Betacoronavirus genus within the subgenus Embecovirus, represents a sophisticated pathogen whose molecular machinery orchestrates a complex interplay between viral replication, host cell manipulation, and immune evasion. Understanding the molecular pathogenesis of ECoV requires a deep dissection of its genomic architecture, entry mechanisms, replication strategies, and the downstream consequences of infection at the cellular and tissue level. This section provides an exhaustive analysis of these molecular events, drawing upon the most current research to illuminate how ECoV establishes infection, causes disease, and interacts with the equine host.

Virion Architecture and Genomic Organization

The ECoV virion is an enveloped, pleomorphic particle characterized by the hallmark crown-like surface projections composed of the spike (S) glycoprotein, which is critical for host cell attachment and entry [11, 12]. The complete genome of the prototype NC99 strain, first isolated in the United States, is a single-stranded, positive-sense RNA molecule of 30,992 nucleotides, excluding the poly(A) tail [11]. This genome is organized into a canonical coronavirus architecture: a large 5′ replicase gene (ORF1ab) encoding polyproteins pp1a and pp1ab, which are proteolytically processed into 16 non-structural proteins (nsps), followed by the structural protein genes and a suite of accessory proteins [11]. The structural proteins include the hemagglutinin esterase (HE), spike (S), envelope (E), membrane (M), and nucleocapsid (N) proteins, while the accessory proteins, NS2, p4.7, p12.7, and the I protein, are variably present among different ECoV isolates [11, 22]. This genomic plasticity, particularly within the accessory protein region, is a hallmark of ECoV evolution and has profound implications for its virulence and host adaptation.

Comparative genomic analyses of ECoV strains from Japan, the United States, and China have revealed that while the S and N genes are highly conserved among strains, substantial variations exist in ORF1a, particularly within nsp3, and in the accessory protein region encompassing NS2 and p4.7 [8, 10]. The Japanese isolate Tokachi09, for instance, possesses a 185-nucleotide deletion in the p4.7 region, effectively eliminating this open reading frame, yet the virus remains fully replication-competent in cell culture [10]. This observation suggests that p4.7 is not essential for basic viral replication but may play a role in modulating host interactions or pathogenesis in vivo. Furthermore, recent genomic surveillance of the 2025 outbreak at Obihiro Racecourse in Japan identified a novel ECoV strain phylogenetically distinct from those responsible for previous outbreaks at the same facility, providing strong evidence that multiple, genetically diverse ECoV lineages can circulate and cause disease in equine populations [1].

Receptor Binding and Cellular Entry

The molecular pathogenesis of ECoV begins with the interaction between the S1 subunit of the spike protein and its cognate cellular receptor. As a member of the embecoviruses, ECoV employs a dual-receptor binding strategy involving both sialic acids and a protein receptor, likely the equine homolog of the angiotensin-converting enzyme 2 (ACE2) or, alternatively, a yet-unidentified equine-specific receptor. The hemagglutinin esterase (HE) protein, a second surface glycoprotein, possesses sialate-O-acetylesterase activity that facilitates reversible binding to O-acetylated sialic acids, promoting viral attachment to the mucus-rich intestinal epithelium and potentially aiding in cell-to-cell spread [11]. The spike protein itself contains a receptor-binding domain (RBD) within the S1 subunit that mediates high-affinity binding to the host receptor, triggering a conformational change that allows the S2 subunit to mediate membrane fusion [12].

Cryo-electron microscopy (cryo-EM) studies of the stabilized ECoV spike protein have provided the first atomic-resolution insights into its prefusion conformation [12]. These structural analyses revealed that stabilization of the prefusion state is achieved by arresting the release of the fusion peptide and maintaining the S1B receptor-binding domain in the ‘down’ state through complex polar interactions between neighboring S1B domains and bound free fatty acids at the interprotomer interface [12]. This structural mastery is critical for vaccine antigen design and for understanding the conformational dynamics that govern receptor engagement. The importance of the spike protein in host range and tissue tropism cannot be overstated; subtle changes in the RBD can dramatically alter receptor affinity and species specificity, a lesson underscored by the zoonotic potential of embecoviruses such as HCoV-OC43 and HCoV-HKU1 [13, 24].

Viral Replication in the Intestinal Epithelium

Once membrane fusion is accomplished, the viral genome is released into the cytoplasm, where it serves as a template for translation of the replicase polyproteins and the assembly of the replication-transcription complex (RTC). The nsps produced from ORF1ab, including the RNA-dependent RNA polymerase (RdRp, nsp12), the helicase (nsp13), and various RNA-processing enzymes, orchestrate genome replication and subgenomic mRNA synthesis [11, 27]. Studies have shown that zinc ions (Zn²⁺) can potently inhibit the RdRp activity of coronaviruses, including SARS-CoV, by blocking elongation and reducing template binding [27]. While this has not been explicitly demonstrated for ECoV, the close phylogenetic relationship and conserved nsp12 structure suggest a similar susceptibility, raising the possibility that zinc ionophores could be explored as antiviral agents in equine medicine.

ECoV exhibits a marked tropism for the intestinal epithelium. Experimental infection studies in horses have demonstrated that ECoV RNA is broadly distributed throughout the intestinal tract, from the duodenum to the colon, with infected cells primarily localized to the surface epithelium of the villi [5]. This pattern of infection is consistent with the clinical presentation of enteritis and diarrhea. In situ hybridization (ISH) studies confirmed the presence of viral RNA in enterocytes, but notably, ECoV was not detected in the respiratory tract tissues (trachea or lung) of experimentally infected horses, despite occasional detection of viral RNA in nasal swabs [5]. This finding strongly supports the conclusion that ECoV is primarily an enteric pathogen, with the lung tissue being negative for viral infection even when viremia is present [5]. The development of equine intestinal enteroids (EIEs) from the duodenum, jejunum, and ileum has provided a powerful ex vivo model to study ECoV infection at the cellular level. These three-dimensional cultures recapitulate the diversity of intestinal epithelial cell types and support productive ECoV infection and replication, as demonstrated by quantitative RT-PCR, virus titration, and electron microscopy [2]. This model is invaluable for dissecting the molecular interactions between ECoV and its primary target cells, including the role of specific host factors in viral entry, replication, and innate immune evasion.

Cytopathology and Host Cell Dysregulation

Infection of epithelial cells with ECoV leads to a cascade of cellular dysfunctions that culminate in cell death and disruption of the intestinal barrier. In vitro studies in MDBK cells have established that ECoV induces apoptosis via a caspase-dependent pathway, with both the death receptor-mediated (extrinsic) and mitochondrial (intrinsic) pathways being activated, as evidenced by increased caspase-8, caspase-9, and caspase-3/7 activities [23]. Apoptotic cell death is a double-edged sword: it can limit viral spread by eliminating infected cells prematurely, but it also contributes to tissue damage and the loss of intestinal epithelial integrity. In the context of ECoV infection, apoptosis likely underlies the histopathological lesions of acute necrotizing enteritis and colitis observed in severe cases [19, 23]. The resulting breach in the gastrointestinal barrier allows for the translocation of luminal contents, including bacteria and endotoxins, into the systemic circulation, precipitating the life-threatening complications of endotoxemia, septicemia, and hyperammonemia-associated encephalopathy [20, 21, 26].

The disruption of the intestinal barrier also has profound metabolic consequences. Hyperammonemia, a key factor in the encephalopathic signs observed in some ECoV cases, arises from the absorption of ammonia produced by urease-positive bacteria in the gut, combined with impaired hepatic clearance due to portosystemic shunting or hepatocellular dysfunction secondary to endotoxemia [21, 26]. Fecal viral load has been identified as a significant prognostic indicator, with non-surviving horses exhibiting significantly higher genome equivalents per gram of feces compared to survivors [21]. This finding suggests that the magnitude of viral replication directly correlates with disease severity and the likelihood of developing fatal complications.

Host-Pathogen Interactions: Immune Evasion and Quasispecies Dynamics

ECoV has evolved sophisticated strategies to evade the host immune response. The non-structural proteins, particularly nsp1, nsp3, and nsp16, are known in other coronaviruses to antagonize interferon signaling and block the innate antiviral response, although these specific mechanisms have yet to be fully characterized in ECoV. The nucleocapsid (N) protein is a multifunctional protein that not only packages the viral genome but also modulates host cell processes. A critical post-translational modification of the N protein, ADP-ribosylation, has been identified in multiple coronaviruses, including mouse hepatitis virus, SARS-CoV, and MERS-CoV, and is also highly likely to occur in ECoV [25]. This modification, which is dependent on active viral infection and not present in plasmid-derived N protein, may play a regulatory role in viral RNA synthesis or in counteracting host ADP-ribosyltransferases that are part of the antiviral response [25].

The quasispecies nature of RNA viruses, driven by the error-prone RdRp, is a key feature of ECoV evolution and pathogenesis. Intra-host evolution of the N gene has been documented in naturally infected horses, with evidence of quasispecies development suggesting active viral adaptation within a single host [3]. This genetic diversity provides the raw material for natural selection, allowing the virus to escape immune pressure and adapt to new hosts. Furthermore, genetic recombination is a major driver of ECoV diversity. At least 12 putative recombination events have been identified across the ECoV genome, with 11 of these falling within ORF1ab [3]. These recombination events, likely occurring during co-infection of a single cell with genetically distinct ECoV strains, can generate novel viral variants with altered pathogenic potential, as demonstrated by the detection of a recombinant ECoV in donkey foals in China, where the recombination breakpoint localized to the NS2 gene region [4]. This capacity for genetic exchange underscores the importance of continued genomic surveillance to monitor the emergence of new strains that could pose increased risk to equine health.

Comparative and Zoonotic Considerations

The molecular pathogenesis of ECoV is best understood within the broader context of the Embecovirus subgenus, which includes bovine coronavirus (BCoV), porcine hemagglutinating encephalomyelitis virus (PHEV), and the human coronaviruses HCoV-OC43 and HCoV-HKU1. The high degree of sequence and antigenic relatedness between ECoV and BCoV is particularly noteworthy. Cross-neutralization studies have demonstrated that antibodies generated against BCoV can recognize ECoV, albeit with lower titers, and modified-live BCoV vaccines can induce a measurable antibody response in horses [7, 17]. This antigenic similarity also complicates serological diagnosis, as horses may have antibodies that react with both viruses [14]. Indeed, the detection of BCoV-like sequences in the feces of horses with colic raises the question of whether BCoV can cross the species barrier and cause disease in equids, or whether these represent yet-unrecognized ECoV variants [14].

The zoonotic potential of ECoV remains a topic of active investigation. While ECoV is not known to cause disease in humans, its close relationship to HCoV-OC43, a common cause of the common cold that originated from a bovine-to-human spillover event, highlights the theoretical possibility of future zoonotic emergence [3, 13]. Molecular evolutionary analyses have identified 42 sites subject to positive selection on the ancestral branch of the HCoV-OC43 clade, likely reflecting the complex series of host jumps it underwent before becoming established in humans, compared to only two such sites for HCoV-HKU1 [3]. Understanding the selective pressures that shape ECoV evolution, particularly in response to changes in host ecology and immunity, is critical for assessing the risk of cross-species transmission and for informing public health surveillance efforts. From a biosecurity perspective, the World Organisation for Animal Health (WOAH) recognizes the importance of monitoring emerging coronaviruses in animal populations, and ECoV serves as a prime example of a pathogen whose molecular evolution must be tracked to safeguard both animal and potential human health.

Epidemiology and Transmission Dynamics of Equine Coronavirus

Equine coronavirus (ECoV), a member of the Betacoronavirus genus within the subgenus Embecovirus, has emerged as a globally significant enteric pathogen of adult horses, with its epidemiological footprint expanding markedly since the initial description of the prototype strain NC99 in the United States in 1999 [11, 20]. The epidemiology of ECoV is characterized by a complex interplay of host factors, management practices, viral genetic diversity, and environmental conditions that collectively govern its transmission dynamics. Understanding these patterns is critical for implementing effective biosecurity measures, particularly given the virus’s capacity to cause outbreaks with variable morbidity and, in some instances, substantial case fatality rates [21, 44].

Global Distribution and Seroprevalence Patterns

ECoV has been documented across multiple continents, including North America, Europe, Asia, the Middle East, and Australia, indicating a widespread, if not ubiquitous, distribution in equine populations [1, 4, 28, 32, 35, 43, 45]. Seroprevalence studies provide the most comprehensive view of population-level exposure, revealing striking geographic and demographic variability. In Japan, seropositivity rates among riding horses at four facilities ranged from 79.2% to 94.6%, suggesting intense, widespread circulation within these closed populations [31]. Similarly, a longitudinal study of Thoroughbred yearlings in Japan found that 44.1% were already seropositive upon entering training farms in August, with infection rates spiking to 60.9% between August and December [33]. Historical serosurveys from the Tokachi area of Hokkaido, Japan, demonstrate that ECoV has been enzootic in that region since at least 1989, with annual seroprevalence rates consistently between 84.4% and 100% [41]. This long-standing endemicity underscores the virus’s ability to persist within populations, likely maintained through a combination of subclinical shedding and waning maternal immunity in young stock.

In contrast, seroprevalence in other regions appears substantially lower. A large-scale US study of 5,247 healthy adult horses from 18 states reported an overall seroprevalence of only 9.6%, with significant associations with the Mid-West region, draft breeds, and ranch/farm or breeding use [39]. A study of horses recently imported from Europe to the United States found a mere 2.3% seropositivity for ECoV [30], while a survey of healthy horses in Israel detected antibodies in 12.3% of animals, with seropositive horses found on 58.6% of farms [35]. In the Netherlands, a seroprevalence of 25.9% was observed in young horses, rising to 82.8% in adults, and notably, 62% of Icelandic horses, a population with limited importation, were seropositive, suggesting either historical exposure or cross-reactivity with related coronaviruses [36]. These disparities likely reflect differences in population density, management intensity, testing methodologies (virus neutralization versus ELISA), and the true force of infection in various ecological niches. The World Organisation for Animal Health (WOAH) recognizes ECoV as an emerging disease of equids, and the variable seroprevalence highlights the need for standardized surveillance protocols to accurately map its global burden.

Risk Factors for Infection and Clinical Disease

Epidemiological investigations have identified several robust risk factors for ECoV infection, many of which are intrinsically linked to intensive management practices. A retrospective cohort study of an outbreak on a large farm in North Carolina provided compelling quantitative evidence: horses that were primarily stalled had an odds ratio of 167.1 for testing PCR-positive compared to those on pasture [29]. Housing next to a positive horse (OR 7.5), being in active work (OR 26.9), and being fed rationed hay versus ad libitum (OR 1,558) were also significant predictors [29]. The extraordinarily high odds ratio for rationed hay likely reflects increased fomite transmission via shared feed tubs or hay nets, as well as the stress of competition for limited resources. The same study identified feeding alfalfa hay as a risk factor, potentially due to its high protein content altering the intestinal microenvironment or simply serving as a proxy for specific management systems [29].

Breed and use also modulate risk. Draft horses have been consistently overrepresented in outbreak reports, both in Japan and the United States [1, 9, 39, 40]. A study of 277 horses with acute fever found that draft horses, pleasure horses, and those on premises with multiple affected animals were significantly more likely to test qPCR-positive for ECoV [40]. The draft horse predilection may relate to breed-specific management (e.g., group housing, shared equipment) or potentially to genetic susceptibility, though the latter remains speculative. Age is another critical factor; while foals can be infected, clinical disease is far more common in adult horses, particularly those aged 2–4 years [10, 33]. In a longitudinal study of Japanese Thoroughbreds, the morbidity rate during estimated exposure periods was 39.2% in yearlings but only 4% in active racehorses, suggesting that age-related immunity or prior exposure protects older animals [33]. However, the same study noted that infection rates in racehorses were significantly higher between November and May (15.5%) than during the summer months (0%), indicating a seasonal component that may be driven by indoor housing and increased viral stability in colder temperatures [33].

Transmission Dynamics: Routes, Shedding, and Environmental Persistence

The primary mode of ECoV transmission is the fecal-oral route, a conclusion strongly supported by experimental infection studies and field observations [20, 38, 42]. Experimentally infected horses begin shedding virus in feces within 1–3 days post-inoculation, with peak shedding occurring 3–4 days after the onset of clinical signs [37, 42]. Viral RNA can be detected in feces for up to 9–11 days, and in some asymptomatic carriers, shedding may persist even longer [26, 37, 42]. The viral load in feces can be substantial; one study found that nonsurviving miniature horses had significantly higher fecal viral loads than survivors, suggesting that shedding intensity correlates with disease severity and may serve as a prognostic indicator [21].

The role of subclinical shedders in transmission dynamics cannot be overstated. In an outbreak at a riding stable in Tokyo, all 41 horses were infected based on seroconversion, yet only 15 (37%) showed clinical signs; the remaining 26 horses (63%) were subclinical but still shed virus [34]. This phenomenon was also observed in a Swiss outbreak, where two ponies tested positive for ECoV without any clinical signs [19]. Subclinical shedders, particularly those with prolonged shedding, act as silent amplifiers of the virus, maintaining transmission chains even when clinical cases are absent. This is especially problematic in training centers, boarding facilities, and racetracks where horses are commingled and frequently moved. The duration of shedding may also be breed-dependent; in the Tokyo outbreak, non-Thoroughbreds shed ECoV for significantly longer periods than Thoroughbreds, potentially contributing to differential transmission risks within mixed-breed populations [34].

While fecal-oral transmission is paramount, evidence for respiratory transmission is emerging. Experimental inoculation of Japanese draft horses detected ECoV RNA in nasal swabs from all three inoculated animals, and viremia was observed in the two that developed clinical signs [42]. A field study of febrile horses found that 4 of 20 fecal-positive horses also tested qPCR-positive for ECoV in nasal secretions [40]. However, experimental infection studies using in situ hybridization have failed to demonstrate ECoV infection of respiratory epithelial cells, suggesting that detection in nasal swabs may reflect contamination from fecal material or viremia rather than true respiratory replication [5]. The same study found that ECoV RNA was present throughout the intestinal tract, with positive cells predominantly on the intestinal surface, and that lung tissue was negative by in situ hybridization even when real-time RT-PCR was positive, likely due to detection of blood-borne virus [5]. Thus, while aerosol or droplet transmission cannot be entirely excluded, the preponderance of evidence supports the fecal-oral route as the primary mechanism, with fomites, contaminated bedding, feed, water, and equipment, playing a major role in indirect transmission [20, 44].

Environmental persistence of ECoV is a critical but understudied aspect of its epidemiology. As an enveloped virus, ECoV is generally susceptible to desiccation and common disinfectants, but it can survive for days to weeks in moist, cool environments such as soiled bedding or fecal material. The virus’s lipid envelope renders it sensitive to heat and ultraviolet light, which may explain the observed seasonality of outbreaks, with most occurring during the winter and early spring months in both Japan and the United States [1, 9, 33, 44]. The 2025 outbreak at Obihiro Racecourse in Japan occurred in January, consistent with this pattern [1]. The role of wildlife or other domestic animals as mechanical vectors is unknown, but the detection of ECoV in donkeys in China and Ireland suggests that the virus can circulate within and potentially between equid species [4, 6]. Furthermore, the detection of bovine coronavirus (BCoV)-like sequences in horses with colic raises the possibility of interspecies transmission, though the clinical significance of such spillover events remains to be determined [14].

Molecular Epidemiology and Evolutionary Dynamics

The molecular epidemiology of ECoV reveals a virus undergoing active evolution, with implications for its transmission potential and antigenic diversity. Complete genome sequencing of Japanese isolates from 2009 and 2012 (Tokachi09, Obihiro12-1, Obihiro12-2) showed high genetic similarity to the US prototype NC99 (98.2–98.7% nucleotide identity), but with notable deletions and insertions in the nsp3 region of ORF1a, the NS2 gene, and the p4.7 accessory gene [8]. The p4.7 region, in particular, is highly variable; Japanese isolates from 2009 had a 185-nucleotide deletion compared to NC99, resulting in the absence of the predicted p4.7 protein, while Irish isolates from 2011 and 2013 showed deletions or insertions in the same region [6, 10]. This genetic plasticity suggests that p4.7 is not essential for viral replication in vitro, but its variability may influence host range or transmission efficiency in vivo [10].

Phylogenetic analyses have demonstrated that ECoV strains do not cluster strictly by geography or time. The 2025 outbreak strain at Obihiro Racecourse, for instance, did not group with the three previous outbreak strains from the same facility (2004, 2009, 2012), indicating that it was introduced from a distinct lineage rather than representing a re-emergence of an endemic strain [1]. Similarly, the first Chinese ECoV isolate from a donkey foal in Shandong Province shared only 97.05% genome-wide identity with NC99 and was identified as a recombinant, with the recombination breakpoint located near the NS2 gene [4]. This finding underscores the capacity of ECoV to undergo recombination, a hallmark of coronavirus evolution that can generate novel variants with altered pathogenic or transmission phenotypes.

Intra-host evolution has also been documented, with quasispecies development observed within the nucleocapsid (N) gene in two of four US horses analyzed [3]. The N gene is a known target of host immune pressure, and the presence of minor variant populations within individual hosts may facilitate immune evasion and contribute to the virus’s ability to persist in populations. Furthermore, a comprehensive analysis of the Embecovirus subgenus identified 12 putative recombination events within the ECoV genome, 11 of which fell within ORF1ab, a region encoding nonstructural proteins critical for replication [3]. This high recombination frequency, combined with positive selection acting on specific codons, suggests that ECoV is continuously adapting, with potential implications for cross-species transmission. The close phylogenetic relationship between ECoV and human coronaviruses OC43 and HKU1, both of which originated from animal reservoirs, highlights the zoonotic potential of embecoviruses and underscores the importance of surveillance in equine populations as a component of One Health pandemic preparedness [3, 13].

Clinical Manifestations and Disease Spectrum

Equine coronavirus (ECoV) infection presents a remarkably heterogeneous clinical picture, ranging from subclinical carriage to fulminant, fatal enterocolitis. The disease spectrum is influenced by a complex interplay of viral strain characteristics, host factors (including age, breed, and immune status), management practices, and the presence of co-infections. A comprehensive understanding of this spectrum is paramount for clinicians, as the initial clinical presentation is often non-specific, and the potential for rapid decompensation in a subset of cases necessitates a high index of suspicion.

The Core Clinical Triad: Fever, Anorexia, and Lethargy

The most consistently reported clinical manifestations across all outbreaks and case series are the triad of pyrexia, anorexia, and lethargy or depression. These signs are often the earliest indicators of infection and may be the only abnormalities observed in a substantial proportion of affected horses. In the seminal outbreak investigations, these signs are nearly ubiquitous. For instance, during the 2025 outbreak among draft horses at Obihiro Racecourse in Japan, anorexia was documented in an extraordinary 98.9% (174/176) of clinically affected horses, while fever was present in 83.5% (147/176) [1]. This pattern is echoed in earlier outbreaks, where fever and anorexia are consistently reported as the most common findings, often preceding the development of enteric signs by 24-48 hours [29, 44, 46]. The fever is typically moderate to high, and the lethargy can be profound, with affected animals showing marked reluctance to move, a dropped head carriage, and diminished interest in their environment [20, 50]. This prodromal phase is critical for clinicians to recognize, as it represents a window for early intervention and biosecurity implementation before the virus has spread widely within a population.

Enteric Manifestations: From Soft Feces to Severe Colitis

While the core triad is consistent, the development and severity of gastrointestinal signs are highly variable. Diarrhea, often considered a hallmark of enteric coronavirus infections, is not a universal finding. In many outbreaks, the prevalence of overt diarrhea is surprisingly low. The 2025 Japanese outbreak reported enteric signs in only 10.2% (18/176) of affected horses [1]. Similarly, a large-scale study of 277 horses with acute fever found that only a minority of ECoV-positive horses presented with diarrhea [40]. This has led to the critical clinical insight that the absence of diarrhea does not rule out ECoV infection.

When enteric signs do occur, they range from mild, soft, unformed manure to profuse, watery diarrhea and acute colitis. In experimental infections, horses often develop only soft feces without progressing to full-blown diarrhea [38, 42]. However, in clinical settings, a subset of horses can develop severe, acute colitis characterized by profuse, watery, and sometimes hemorrhagic diarrhea, which can rapidly lead to dehydration, electrolyte imbalances, and metabolic acidosis [19, 47]. A particularly dangerous presentation is the development of colic signs, which can be severe. In a retrospective study of an outbreak on a large farm, 35.3% (6/17) of clinically affected horses showed signs of colic, and critically, three of these horses had small colon impactions, two of which required surgical intervention [29]. This finding is corroborated by a case series from Australia, where horses presented with mild to severe colic, including one miniature pony that developed severe caecal distension unresponsive to medical therapy, necessitating surgical exploration [28]. These reports underscore that ECoV must be considered a differential diagnosis for acute colic, even in the absence of diarrhea, and that the associated gastrointestinal dysmotility can lead to surgical lesions.

Severe Complications and Fatal Outcomes

Although the majority of ECoV infections are self-limiting and resolve with supportive care within 3-7 days, a small but significant proportion of cases progress to life-threatening complications. The pathophysiological basis for these severe outcomes is the disruption of the gastrointestinal mucosal barrier. This breach allows for the translocation of bacteria and their products, primarily endotoxins (lipopolysaccharides), into the systemic circulation, leading to endotoxemia and potentially septicemia [20, 44, 50].

Endotoxemia and Septic Shock: This is the most common pathway to a fatal outcome. The resulting systemic inflammatory response syndrome (SIRS) can manifest as tachycardia, tachypnea, injected mucous membranes, and profound cardiovascular collapse. In a Swiss outbreak, one horse died due to severe endotoxemia and circulatory shock secondary to severe acute necrotizing enteritis and colitis [19]. Post-mortem findings in horses that died or were euthanized during outbreaks have included septicemia secondary to gastrointestinal translocation [44].

Hyperammonemia and Encephalopathy: A unique and devastating neurological complication of ECoV infection is hyperammonemia-associated encephalopathy. This syndrome is thought to arise from the massive breakdown of blood and protein in the gastrointestinal tract following severe enteritis, leading to an overwhelming production of ammonia that the liver cannot adequately clear. The resulting hyperammonemia can cause cerebral edema and neurological signs, including ataxia, circling, head pressing, blindness, seizures, and coma. This complication has been documented in several reports and carries a grave prognosis. In a particularly severe outbreak among miniature horses, 27% (4/15) of ECoV-positive animals died or were euthanized, and severe hyperammonemia (677 μmol/L, reference range ≤60 μmol/L) was identified in one animal with encephalopathic signs that subsequently died [21]. This highlights that ECoV should be a differential diagnosis for horses presenting with acute neurological signs, especially in the context of a concurrent febrile or enteric illness.

Hematologic and Biochemical Abnormalities: The clinical picture is often supported by characteristic clinicopathologic findings. The most consistent hematologic abnormality is leukopenia, primarily due to lymphopenia and/or neutropenia [20, 44, 46, 48, 49]. This finding is so characteristic that it can be a key diagnostic clue, helping to differentiate ECoV from other causes of fever and colic. Studies have shown that the degree of leukopenia and neutropenia can be more severe in ECoV-infected horses compared to those with other febrile illnesses [46]. In a comparative study, horses with ECoV had significantly decreased neutrophil counts compared to horses with an unknown diagnosis, although they were not statistically different from horses with Salmonella infection, highlighting the clinical overlap between these two important enteric pathogens [48]. Other reported abnormalities include elevated serum amyloid A (SAA), a major acute-phase protein, which correlates with the severity of inflammation [42]. Additionally, clinicopathologic evidence of liver dysfunction, such as elevated liver enzymes, has been observed in some cases [47]. The presence of ventricular tachycardia has also been documented, likely secondary to the systemic inflammatory response and electrolyte disturbances [47].

The Spectrum of Disease: Subclinical Carriers and Breed Susceptibility

A critical aspect of the ECoV disease spectrum is the high prevalence of subclinical infections. In many outbreaks, a substantial proportion of horses that test positive for the virus via fecal PCR or seroconversion show no clinical signs whatsoever. In the Tokyo riding stable outbreak, 63% (26/41) of infected horses were subclinical [34]. These asymptomatic shedders play a pivotal role in the silent dissemination of the virus within a population, making outbreak control challenging. The duration of fecal shedding can be prolonged, lasting up to 9-11 days or more, even in subclinical animals, and this shedding can occur before the onset of clinical signs in those that do become sick [26, 37, 42].

Breed and management factors appear to influence both the risk of infection and the severity of clinical disease. Draft horses have been disproportionately represented in several major outbreaks, particularly in Japan [1, 9, 10]. While this may be partly due to management practices (e.g., intensive housing, group feeding), it may also reflect a genuine breed predisposition. A large-scale seroprevalence study in the USA found that draft horses were significantly more likely to be seropositive than other breeds [39]. Furthermore, the case fatality rate can vary dramatically. While most outbreaks in adult horses of various breeds have a low mortality rate (typically <5%), outbreaks in miniature horses have been associated with a much higher case fatality rate, reaching 27% in one report [21]. This suggests that miniature horses may be particularly susceptible to severe disease, possibly due to genetic factors or a higher susceptibility to hyperammonemia. The risk of severe disease is also amplified by intensive management practices, such as stalled housing, rationed hay feeding, and high levels of commingling, which are risk factors for infection and likely contribute to a higher infectious dose [29].

Co-infections and Differential Diagnoses

The clinical picture of ECoV can be complicated by concurrent infections. A study using an equine fever diagnostic panel found that beta coronavirus was one of the most common pathogens identified, and co-infections with two or three other pathogens (e.g., Anaplasma phagocytophilum) were identified in 13% of positive panels [51]. This highlights the importance of comprehensive diagnostic testing in febrile horses. The clinical and clinicopathologic similarities between ECoV and Salmonella spp. infection are particularly noteworthy. Both can present with fever, colic, diarrhea, and leukopenia, making it impossible to differentiate them on clinical grounds alone [48]. Therefore, the World Organisation for Animal Health (WOAH) and other international health bodies recommend that both ECoV and Salmonella be included in the differential diagnosis for any adult horse presenting with acute fever and enteric signs, and that fecal PCR testing for both pathogens be performed. Other differentials include Potomac Horse Fever (Neorickettsia risticii), clostridial enterocolitis, and non-infectious causes of colitis.

Diagnostics and Laboratory Detection of Equine Coronavirus

The accurate and timely laboratory detection of equine coronavirus (ECoV) is paramount for implementing appropriate biosecurity measures, guiding therapeutic interventions, and elucidating the epidemiological dynamics of this emerging enteric pathogen. The diagnostic landscape for ECoV has evolved substantially since the virus was first identified in 1999, moving from reliance on virus isolation and electron microscopy to a suite of highly sensitive molecular and serological platforms. Given that the clinical presentation of ECoV infection, characterized by fever, anorexia, lethargy, and variable gastrointestinal signs, overlaps considerably with other enteric pathogens such as Salmonella spp., Clostridioides difficile, and Lawsonia intracellularis, laboratory confirmation is not merely adjunctive but essential for definitive diagnosis [44, 46, 48]. Diagnostic approaches can be broadly categorized into direct pathogen detection methods (molecular assays, virus isolation, antigen detection) and indirect methods that assess the host serological response. The choice of assay, sample type, and timing of collection critically influence diagnostic sensitivity and specificity, and understanding these nuances is fundamental for both clinicians and epidemiologists.

Molecular Detection: Real-Time RT-PCR and Conventional PCR

Quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR) targeting conserved regions of the ECoV genome, most commonly the nucleocapsid (N) gene or the spike (S) gene, has emerged as the cornerstone diagnostic modality and is widely regarded as the gold standard for antemortem confirmation of ECoV infection [20, 28, 44, 50]. The analytical sensitivity of qRT-PCR is exceptionally high, capable of detecting as few as 10–100 viral RNA copies per reaction, which permits identification of infected horses even during periods of low viral shedding or in subclinically infected animals. Several validated one-step real-time RT-PCR protocols have been described, and these assays demonstrate robust performance across a range of clinical matrices, including feces, rectal swabs, nasal secretions, and whole blood [5, 40, 54].

Fecal samples represent the primary diagnostic specimen for molecular testing, as ECoV tropism is predominantly enteric. Experimental inoculation studies have demonstrated that viral RNA is shed in feces beginning as early as 1–2 days post-inoculation, with peak shedding occurring approximately 3–4 days after the onset of clinical signs [37, 38, 42]. The duration of fecal shedding typically ranges from 7 to 14 days, although prolonged shedding for up to 21 days has been documented in some asymptomatic carriers [26]. In a landmark outbreak investigation involving 161 horses across four facilities, Pusterla et al. (2012) demonstrated that 86% (38/44) of clinically ill horses tested qRT-PCR-positive for ECoV in feces, while 93% (89/96) of healthy herdmates tested negative, yielding an overall concordance of 91% between clinical status and PCR result [44]. This high level of agreement underscores the diagnostic utility of fecal qRT-PCR in both outbreak and sporadic case settings.

The sampling strategy must account for the intermittent nature of fecal shedding, particularly in early infection or mildly affected animals. Repeat testing of initially negative samples collected 24–48 hours later has been recommended, as a subset of horses may test negative during the prodromal phase and subsequently convert to positive [37]. Importantly, qRT-PCR does not differentiate between viable infectious virus and non-infectious RNA remnants; however, the detection of ECoV RNA in a horse with compatible clinical signs is strongly suggestive of active infection, given the low background prevalence of ECoV in healthy hospitalized populations. Sanz et al. (2019) reported that only 1 of 258 fecal samples (0.4%) from 130 hospitalized horses tested qRT-PCR-positive for ECoV, confirming that incidental detection in asymptomatic horses is rare and that a positive result in a clinically ill horse is highly diagnostically relevant [52].

Nasal secretions have also been explored as an alternative sample type, particularly for febrile horses without overt enteric signs. Pusterla et al. (2019) tested paired fecal and nasal swab samples from 277 horses presenting with acute-onset fever and found that 20 horses (7.2%) tested qRT-PCR-positive for ECoV in feces, but only 4 of these 20 (20%) also tested positive in nasal secretions [40]. This finding indicates that while ECoV RNA can occasionally be detected in the upper respiratory tract, fecal testing is markedly more sensitive and should remain the preferred sampling approach. The detection of ECoV in nasal secretions may reflect environmental contamination, regurgitation, or transient viremic spread rather than true respiratory epithelial infection, a hypothesis supported by experimental studies using in situ hybridization that failed to demonstrate ECoV RNA in lung parenchyma despite concurrent detection in nasal swabs [5].

Whole blood represents a third potential matrix for molecular testing, particularly in the context of viremia. Experimental inoculation of draft horses by Nemoto et al. (2014) demonstrated that viremia, detectable by RT-PCR in whole blood, occurred in symptomatic but not asymptomatic horses, suggesting that blood-based testing may have utility in distinguishing clinically affected from subclinically infected individuals [42]. However, the sensitivity of blood qRT-PCR relative to fecal qRT-PCR is lower, and viremia appears to be transient, typically coinciding with the peak febrile response.

Alternative Nucleic Acid Amplification Technologies

Beyond conventional qRT-PCR, reverse transcription loop-mediated isothermal amplification (RT-LAMP) has been developed as a rapid, field-deployable alternative for ECoV detection. Nemoto et al. (2015) designed an RT-LAMP assay targeting the N gene and demonstrated that it was more sensitive than conventional endpoint RT-PCR while maintaining comparable specificity [53]. The principal advantage of RT-LAMP lies in its isothermal amplification requirement (60–65°C), which eliminates the need for thermal cycling equipment and reduces reaction times to approximately 30–60 minutes. However, the analytical sensitivity of RT-LAMP remains approximately one log lower than that of qRT-PCR, and the assay is more prone to amplicon contamination due to its high amplification efficiency [53]. For resource-limited settings or point-of-care applications where rapid turnaround is critical, RT-LAMP represents a viable screening tool, but qRT-PCR remains the reference standard for confirmatory testing.

Virus Isolation and Genomic Characterization

Virus isolation in cell culture, while not routinely employed for clinical diagnostics, remains an important tool for research, antigenic characterization, and vaccine development. ECoV replicates efficiently in several cell lines, including HRT-18 (human rectal tumor cells), MDBK (Madin-Darby bovine kidney cells), and Vero cells, with cytopathic effect characterized by cell rounding, syncytium formation, and apoptosis [10, 23]. Oue et al. (2011) successfully isolated the Tokachi09 strain from fecal samples of febrile adult horses using HRT-18 cells, and subsequent one-step growth curves demonstrated peak viral titers of approximately 10^6 TCID50/mL at 48–72 hours post-inoculation [10]. The isolation of virus from clinical samples is, however, hampered by the labile nature of coronaviruses, the requirement for fresh or appropriately cryopreserved specimens, and the relatively low viral loads present in some samples.

Genomic characterization of ECoV isolates has provided critical insights into viral evolution, recombination events, and epidemiological linkages. Full-genome sequencing of ECoV strains from Japan, the United States, and Europe has revealed a genome of approximately 30,800–31,000 nucleotides, organized into 11 open reading frames encoding the replicase polyproteins (ORF1ab), structural proteins (HE, S, E, M, N), and accessory proteins (NS2, p4.7, p12.7, I) [8, 11]. Next-generation sequencing (NGS) and Sanger sequencing of amplicons spanning the S, N, and p4.7 genes have been employed to trace outbreak origins and identify genetic markers associated with virulence or host adaptation [1, 4, 6]. Zehr et al. (2024) identified 12 putative recombination events within the ECoV genome, 11 of which localized to ORF1ab, and documented evidence of intra-host evolution (quasispecies development) within the N gene of two infected horses [3]. Such findings underscore the value of genomic surveillance for monitoring the emergence of novel variants that may impact diagnostic assay performance or vaccine efficacy.

Serological Assays: Virus Neutralization and ELISA

Serological testing provides retrospective evidence of ECoV exposure and is indispensable for epidemiological studies, outbreak investigations, and vaccine immunogenicity assessments. The virus neutralization (VN) test, which measures the ability of serum antibodies to inhibit viral replication in cell culture, has historically been the reference serological method. The VN test is highly specific, as it detects only antibodies capable of neutralizing viral infectivity, and is typically performed using a constant virus-varying serum dilution format. A four-fold or greater rise in VN titer between paired acute and convalescent sera, collected 14–21 days apart, is considered confirmatory of recent infection [1, 9, 33, 34]. In the context of the 2025 Obihiro Racecourse outbreak, Fukumoto et al. (2025) demonstrated seroconversion (VN titer ≥1:8) in 25 of 26 tested horses (96.2%), confirming widespread ECoV infection even among horses that were qRT-PCR-negative at the time of sampling [1].

However, the VN test has several limitations that preclude its use as a routine screening tool: it requires cell culture facilities, is labor-intensive, takes 3–5 days to complete, and is subject to inter-laboratory variability. To address these limitations, enzyme-linked immunosorbent assays (ELISAs) based on recombinant ECoV spike protein subunit 1 (S1) have been developed and validated. Zhao et al. (2019) described an indirect S1-based ELISA that demonstrated 98.2% sensitivity and 99.1% specificity relative to the VN test using a panel of 27 paired sera from an ECoV outbreak [36]. The S1 domain was selected as the coating antigen because it contains the receptor-binding domain and elicits a robust, virus-specific antibody response with minimal cross-reactivity to other betacoronaviruses such as bovine coronavirus (BCoV) and human coronavirus OC43. The S1 ELISA has been instrumental in large-scale serosurveys, revealing seroprevalence rates ranging from 2.3% in recently imported European horses to 82.8% in adult Dutch horse populations, depending on geographic region, management practices, and prior outbreak history [30, 36, 39].

Salivary antibody detection represents a novel, non-invasive serological approach that may facilitate population-level surveillance without the need for venipuncture. Bannai et al. (2023) adapted the VN test for use with saliva samples and found that seropositivity rates in saliva (67.6–71.4%) were significantly higher at facilities that had experienced recent ECoV outbreaks compared to those without documented outbreaks (41.7–45.2%) [31]. The detection of neutralizing antibodies in saliva likely reflects transudation of serum IgG into the oral cavity during active or recent infection, and this approach may be particularly useful for screening large numbers of horses in outbreak settings where rapid identification of exposed individuals is critical for biosecurity decision-making.

Hematological and Biochemical Markers as Supportive Diagnostics

While not a replacement for direct pathogen detection, complete blood count (CBC) and serum biochemistry findings can provide strong supportive evidence for ECoV infection and help guide the diagnostic workup. The most consistently reported hematological abnormality is leukopenia, driven by lymphopenia and/or neutropenia, which has been documented in 60–80% of clinically affected horses during the acute phase of illness [44, 46, 48, 49]. Berryhill et al. (2019) compared 33 ECoV-positive horses to horses with similar clinical signs that tested negative for ECoV and found that the ECoV-positive group had significantly lower median neutrophil counts (2.1 × 10^3/μL vs. 4.8 × 10^3/μL; P < 0.001), with neutropenia being the most discriminating hematological feature [46]. Importantly, the degree of leukopenia in ECoV infection is often more profound than that observed in horses with uncomplicated colic or other febrile illnesses, and the combination of fever, anorexia, and neutropenia should prompt inclusion of ECoV in the differential diagnosis.

Serum amyloid A (SAA), an acute-phase protein synthesized by hepatocytes in response to pro-inflammatory cytokines, has been shown to rise markedly in ECoV-infected horses, with concentrations peaking 2–4 days after symptom onset and correlating with the severity of clinical signs [42]. In experimental infection studies, SAA concentrations increased from baseline values of <5 μg/mL to >200 μg/mL in symptomatic horses, while asymptomatic infected horses showed minimal SAA elevation [42]. The measurement of SAA, while not specific for ECoV, can aid in monitoring disease progression and response to therapy.

Hyperammonemia, defined as blood ammonia concentration >60 μmol/L (reference range ≤60 μmol/L), has been identified as a potentially grave prognostic indicator in ECoV infection. Fielding et al. (2014) reported severe hyperammonemia (677 μmol/L) in a miniature horse that died from ECoV-associated encephalopathy, and fecal viral load was significantly higher in non-survivors compared to survivors (P = 0.02) [21]. The pathogenesis of hyperammonemia in ECoV infection is presumed to involve disruption of the intestinal mucosal barrier, leading to absorption of ammonia produced by bacterial urease activity, combined with hepatic dysfunction secondary to endotoxemia and systemic inflammation. Although hyperammonemia is an infrequent complication, its presence in a horse with enteric disease should prompt aggressive monitoring and consideration of ECoV testing.

Differential Diagnosis and Diagnostic Algorithms

The clinical and clinicopathological overlap between ECoV infection and other common enteric pathogens necessitates a structured diagnostic approach. Salmonellosis, in particular, shares many features with ECoV, including acute-onset fever, anorexia, colic, diarrhea, and leukopenia. Manship et al. (2019) conducted a retrospective comparison of 43 horses with ECoV, salmonellosis, or an unknown diagnosis and found no significant differences in clinical presentation, CBC parameters, or serum biochemistry between the ECoV-positive and Salmonella-positive groups [48]. This finding underscores the inadequacy of relying on clinical judgment alone and highlights the imperative for laboratory confirmation.

A comprehensive diagnostic algorithm for horses presenting with acute fever, anorexia, and enteric signs should therefore incorporate the following elements: (1) collection of feces for qRT-PCR for ECoV, bacterial culture for Salmonella spp., and toxin detection for C. difficile; (2) whole blood for CBC, SAA, and qRT-PCR for ECoV (if viremia is suspected); (3) nasal swab for respiratory pathogen panel (equine herpesvirus-1 and -4, equine influenza virus, Streptococcus equi subsp. equi) if respiratory signs are absent, as ECoV is a recognized cause of fever without respiratory involvement [40]; and (4) paired acute and convalescent sera for VN or S1 ELISA to confirm seroconversion when initial qRT-PCR results are negative but clinical suspicion remains high. The inclusion of ECoV in broader multiplex fever panels is increasingly common; Pinn-Woodcock et al. (2025) reported that among 961 equine fever diagnostic panels submitted between 2019 and 2023, betacoronaviruses were the second most frequently detected pathogen, identified in approximately 8–10% of cases [51]. This finding reinforces the importance of routine ECoV testing in febrile horses and supports the integration of ECoV qRT-PCR into standardized diagnostic protocols.

In conclusion, the laboratory detection of ECoV requires a multi-modal approach that leverages the high sensitivity of qRT-PCR for active infection, the specificity of serological assays for retrospective exposure assessment, and the supportive value of hematological and biochemical markers. The selection of appropriate sample types, feces as the primary matrix, supplemented by nasal swabs and whole blood in specific clinical contexts, and the timing of collection relative to disease onset are critical determinants of diagnostic yield. As ECoV continues to be recognized as a significant cause of enteric disease in adult horses worldwide, the refinement and standardization of diagnostic protocols will remain a priority for both clinical practice and epidemiological surveillance.

Treatment, Management, and Prognosis of ECoV Infection

The clinical approach to equine coronavirus (ECoV) infection is fundamentally predicated on the self-limiting nature of the disease in the vast majority of affected horses, combined with an aggressive, anticipatory stance toward the early recognition and management of potentially life-threatening complications. As no specific antiviral therapy is currently approved or commercially available for ECoV, the cornerstone of intervention remains rigorous, multi-modal supportive care tailored to the severity of clinical signs, the presence of complications, and the individual patient’s physiological status. The management framework must also integrate robust infection control protocols to curtail nosocomial and farm-level spread, given the virus’s high infectivity and the existence of subclinically shedding animals [34, 44].

Supportive Care and Medical Management

The primary therapeutic objectives for a horse with confirmed or suspected ECoV infection are the restoration and maintenance of fluid, electrolyte, and acid-base homeostasis; the mitigation of systemic inflammation and endotoxemia; the provision of caloric support; and the vigilant monitoring for and treatment of secondary complications such as hyperammonemic encephalopathy and septic shock.

Fluid Therapy and Electrolyte Balance: Dehydration and electrolyte derangements are common findings, particularly in horses exhibiting diarrhea or reduced water intake due to pyrexia and lethargy [46, 50]. The cornerstone of treatment is the administration of balanced isotonic crystalloid solutions (e.g., lactated Ringer’s solution or Plasma-Lyte A) administered intravenously. The rate and volume of fluid administration must be guided by serial assessments of hydration status (packed cell volume, total solids, skin turgor, mucous membrane moisture), central venous pressure if accessible, and urine output. For horses with profound hypovolemia or evidence of hypovolemic shock, initial rapid fluid resuscitation (e.g., 20-40 mL/kg over 15-30 minutes) may be necessary. Electrolyte monitoring should focus on sodium, potassium, chloride, and calcium, with aggressive replacement of deficits as needed. Hypokalemia, in particular, is a frequent sequela of anorexia and gastrointestinal losses and must be addressed cautiously to avoid cardiac dysrhythmias, especially in horses with concurrent myocardial involvement [47].

Management of Inflammation, Endotoxemia, and Coagulopathy: Disruption of the intestinal mucosal barrier, secondary to viral-induced enteritis, is a critical pathophysiologic event that can lead to the translocation of luminal bacteria and their products (e.g., lipopolysaccharide [LPS]) into the systemic circulation [20, 21, 28, 46]. This endotoxemia is a primary driver of systemic inflammatory response syndrome (SIRS), which can progress to septic shock, disseminated intravascular coagulation (DIC), and multi-organ dysfunction. Consequently, anti-inflammatory therapy is a mainstay of treatment.

• Non-Steroidal Anti-Inflammatory Drugs (NSAIDs): Flunixin meglumine (0.25-1.1 mg/kg IV BID or TID) is the most commonly employed NSAID, providing potent analgesia and antipyresis while mitigating the downstream effects of the cyclooxygenase-2 (COX-2) pathway activated by endotoxin. Careful monitoring for potential adverse effects, including right dorsal colitis and renal papillary necrosis, is essential, particularly in dehydrated patients. For horses with severe endotoxemia or uncontrolled pain, a low-dose, continuous-rate infusion of flunixin may be considered.
• Polymyxin B: This cationic antimicrobial peptide binds to and neutralizes lipid A of LPS. It can be administered at a dose of 6000-10,000 IU/kg IV in isotonic fluids, typically diluted and given over 30-60 minutes, and is most effective when given early in the course of endotoxemia. Its potential for nephrotoxicity must be weighed, and concurrent fluid therapy helps mitigate this risk.
• Antimicrobial Therapy: The use of systemic antibacterial drugs in ECoV cases is controversial and should be reserved for horses with confirmed or strong clinical suspicion of secondary bacterial translocation or sepsis (e.g., profound leukopenia, degenerative left shift, persistent fever despite antipyretics, positive blood culture). If indicated, a broad-spectrum, gram-negative and anaerobic spectrum regimen is typically chosen, such as a combination of a third-generation cephalosporin (e.g., ceftiofur) or an aminoglycoside (e.g., gentamicin, with careful therapeutic drug monitoring) with metronidazole. Empirical antimicrobial use in all ECoV cases is discouraged due to the potential for disruption of the already compromised gut microbiome and the selection of resistant organisms.

Management of Hyperammonemia and Encephalopathy: A rare but devastating complication of ECoV infection is hyperammonemia-associated encephalopathy, which carries a grave prognosis [20, 21, 28]. The condition is believed to arise from hepatic dysfunction secondary to hypoperfusion or from the direct production of ammonia by urease-producing bacteria translocating across the damaged gut barrier. Affected horses present with rapidly progressive signs of central nervous system dysfunction, including ataxia, head-pressing, circling, blindness, seizures, and coma. Serum ammonia levels can be dramatically elevated (often exceeding 600 μmol/L, with normal <60 μmol/L) [21]. Management is an emergency and includes:

• Reduction of ammonia production and absorption: Oral administration of lactulose (dose titrated to produce soft feces) acidifies the colonic lumen, trapping ammonia as ammonium ions and reducing its absorption. Metronidazole is often used to inhibit the growth of urease-producing anaerobic bacteria in the gastrointestinal tract.
• Enhancement of hepatic clearance: Administration of sodium benzoate or sodium phenylbutyrate can provide an alternative pathway for nitrogen excretion. However, these are not widely available in equine practice.
• Supportive care for neurologic signs: Seizures should be controlled with benzodiazepines (e.g., diazepam) or, in refractory cases, phenobarbital. Affected horses must be kept in a well-padded, quiet, and dark stall to minimize stimulation.

Nutritional Support: Anorexia is a cardinal and near-universal sign of ECoV infection, with rates reported as high as 98.9% in some outbreaks [1]. Prolonged anorexia can lead to significant weight loss, negative nitrogen balance, and gastrointestinal stasis. While most horses will regain appetite within 3-5 days as their fever resolves, nutritional support should be considered for those with protracted illness. Offering small amounts of highly palatable, high-energy feeds (e.g., alfalfa hay, grass hay, soaked hay cubes) is recommended. In severely affected horses with persistent anorexia or those requiring intensive care, enteral nutrition via a nasogastric tube with a commercial equine recovery diet should be considered. Parenteral nutrition is rarely necessary but can be employed in the most critical patients.

Isolation, Biosecurity, and Population-Level Management

ECoV is highly contagious, with a suspected fecal-oral route of transmission [20, 38, 42]. The virus is excreted in high quantities in the feces of both clinically and subclinically infected horses, and shedding can persist for up to 11 days or longer in some cases [37, 42]. Inhalation of aerosolized virus, possibly from contaminated bedding, is also a likely route of infection [42]. Therefore, the immediate implementation of stringent infection control measures is paramount to preventing widespread outbreaks within a facility.

Isolation and Cohort Management: Any horse with clinical signs consistent with ECoV (fever, anorexia, lethargy) should be immediately isolated in a separate airspace and strict barrier nursing protocols instituted. Ideally, isolation should be maintained until the horse is clinically normal and has two consecutive negative fecal PCR tests performed 24-48 hours apart. However, this can be logistically challenging. As a minimum, isolation for 10-14 days beyond the resolution of clinical signs is recommended. Movement of personnel and equipment between affected and unaffected areas must be controlled. Dedicated coveralls, gloves, and footbaths should be used for all personnel entering the isolation area [55]. Outbreak investigations have identified significant risk factors for infection, including being housed in a stall adjacent to a positive horse, being fed rationed hay (as opposed to ad libitum), and being fed alfalfa hay [29]. These findings underscore the importance of environmental contamination and perhaps the role of hay as a fomite or source of exposure. During an outbreak, all horses on the premises should be treated as potentially exposed. A cohort system should be established, separating healthy, exposed horses from sick horses. Any new horse entering the facility should be quarantined for a minimum of 21 days.

Environmental Cleaning and Disinfection: Coronaviruses are enveloped viruses and are generally susceptible to most common disinfectants, including accelerated hydrogen peroxide (e.g., Virkon® S), 10% bleach solutions (sodium hypochlorite), and quaternary ammonium compounds. After removal of all organic material (feces, bedding), the environment, including stall walls, floors, feed buckets, waterers, and aisleways, should be thoroughly cleaned and disinfected. Feces should be managed carefully; composting may reduce viral load, but disposal in a dedicated pit is safest. Manure removal and disposal should be the last task performed each day to avoid cross-contamination.

Role of Subclinical Shedders: A significant challenge to outbreak management is the presence of subclinically infected horses. Studies from Japan and the United States have demonstrated that a substantial proportion (63-80%) of horses that become infected with ECoV during an outbreak remain asymptomatic [34, 44]. These animals excrete high levels of virus in their feces and are critical to the propagation of the outbreak. Diagnostic screening of all apparently healthy contact horses via fecal PCR is therefore recommended to identify these silent shedders and enable their isolation. The duration of shedding can be longer in certain breeds, such as non-Thoroughbreds, which may have implications for control strategies [34].

Prognosis and Prognostic Indicators

The prognosis for the vast majority of horses with ECoV infection is excellent. The disease is typically self-limiting, with most horses recovering fully within 3-7 days of the onset of clinical signs with minimal to moderate supportive care [1, 20, 34, 44, 46, 50]. Reported morbidity rates can vary widely, from 10% to 83%, but overall case fatality rates in large outbreak settings are generally low, often cited at 0-7% [29, 44].

However, a subset of cases can develop severe, life-threatening disease. The case fatality rate has been reported to be significantly higher in certain populations, particularly in miniature horses. In one outbreak among miniature horses and a donkey, the case fatality rate reached 27% (4/15), with deaths attributed to severe acute colitis, endotoxemia, and hyperammonemic encephalopathy [21]. Similarly, sporadic cases and smaller outbreaks have reported deaths due to septicemia secondary to gastrointestinal translocation [44, 46].

Key prognostic indicators include:

• Fecal Viral Load: A strong and statistically significant association exists between higher fecal viral loads (measured as ECoV genome equivalents per gram of feces) and a poor outcome [21]. Horses that are non-survivors have been shown to have significantly higher viral loads at the time of diagnosis compared to survivors. This makes quantitative PCR a potentially valuable prognostic tool in the clinical setting.
• Hyperammonemia: The development of severe hyperammonemia (>60 μmol/L) is a grave sign, indicative of impending encephalopathy. This complication is difficult to treat and carries a very poor prognosis for survival [21, 28].
• Severe Leukopenia and Neutropenia: While leukopenia is a common hematologic abnormality in ECoV infection, horses with the most profound neutropenia (e.g., <1500 cells/μL) are at higher risk for secondary bacterial infections and endotoxemic complications [46, 48].
• Cardiac Arrhythmias: The occurrence of transient ventricular tachycardia or other significant cardiac dysrhythmias, as reported in some cases [47], suggests myocardial inflammation or electrolyte disturbances and warrants a guarded prognosis.
• Secondary Complications: The development of severe colic requiring surgical intervention (e.g., small colon impaction, cecal distension), hyperammonemic encephalopathy, or profound endotoxemic shock are all indicators of a poor prognosis [21, 28, 29, 46].

In summary, while ECoV is generally a disease of low mortality, its potential for severe complications, especially in specific high-risk populations (e.g., miniature horses, those with high co-morbidity) or when associated with high initial viral loads, demands a high index of suspicion, early diagnostic testing, and immediate institution of aggressive supportive care and strict biosecurity measures.

Immunoprophylaxis and Future Therapeutic Directions

Currently, there is no commercially available vaccine specifically for ECoV. Research into vaccination strategies has focused on the closely related bovine coronavirus (BCoV). Studies have demonstrated that administration of a modified-live BCoV vaccine to horses is safe and can induce measurable, albeit often low-titer, neutralizing antibody responses against ECoV in a subset of vaccinated individuals [7, 17]. However, the effectiveness of this cross-reactive immunity in preventing infection or reducing clinical signs upon ECoV challenge remains uncertain. An elegant study using equine intestinal enteroids (organoids) has provided a powerful new in vitro tool for studying virus-host interactions and could be instrumental in screening potential antiviral compounds and vaccine candidates [2].

Advances in structural biology, particularly the recent cryo-electron microscopy (cryo-EM) resolution of the stabilized ECoV spike (S) protein in its prefusion conformation, represent a major breakthrough. This work, guided by artificial intelligence (AI) tools, has identified key structural determinants for stabilization that are likely applicable across the Embecovirus subgenus [12]. This provides a rational template for the design of stabilized, structure-based subunit vaccines against ECoV, which could be far more immunogenic than traditional inactivated or modified-live vaccines. The development of such a vaccine would be a transformative step in the management of this disease.

Looking further ahead, the potential for antiviral therapy remains an active area of research. Studies on related nidoviruses have shown that zinc ionophores, which increase intracellular Zn²⁺ concentrations, can potently inhibit the activity of the viral RNA-dependent RNA polymerase (RdRp) [27]. While this has been demonstrated in cell culture for SARS-CoV and equine arteritis virus, its applicability to ECoV in vivo remains to be proven. The development of safe and effective equine-specific antiviral agents would offer a much-needed adjunct to supportive care for severe cases.

Prevention, Biosecurity, and Vaccine Development

The strategic mitigation of equine coronavirus (ECoV) hinges upon a rigorous, multi-layered framework that integrates proactive biosecurity protocols with the eventual development of effective immunoprophylactic agents. As a pathogen for which no licensed vaccine currently exists [16, 20, 28], the primary defense against ECoV rests entirely on the vigilant implementation of containment strategies designed to interrupt the fecal-oral transmission cycle, a pathway that has been experimentally validated and repeatedly observed in outbreak settings [38, 42]. The World Organisation for Animal Health (WOAH) principles for managing emerging infectious diseases, emphasizing early detection, isolation, and hygiene, are directly applicable here given the virus's demonstrated capacity for rapid dissemination within naive populations and its documented, albeit low, case fatality rate that can reach 7–27% in some outbreaks [17]. The development of effective prevention protocols requires a deep understanding of the virus's epidemiological behavior, its environmental stability, and the immunological correlates of protection, which together inform the design of practical biosecurity measures and the rational targeting of nascent vaccine platforms.

Biosecurity: Epidemiological Foundations and Implementation

The cornerstone of ECoV prevention is a comprehensive biosecurity program that addresses the specific mechanisms of viral transmission and persistence. Experimental inoculation studies have unequivocally demonstrated that the fecal-oral route is the predominant mode of transmission. In a landmark study by Schaefer et al. [38], four adult horses experimentally infected via intragastric inoculation with ECoV-containing feces not only became infected but also transmitted the virus to four naïve contact horses that were merely exposed to their feces daily. This finding, coupled with the observation that infected horses shed a large quantity of virus in their feces for more than nine days post-inoculation regardless of clinical status [42], underscores the critical importance of managing manure and preventing direct contact between horses. The virus is also detectable in nasal secretions, raising the possibility of respiratory transmission, though the primary site of replication is the intestinal tract [5, 42]. This dual shedding pattern mandates that biosecurity protocols target both aerosolized and contaminated environmental sources.

Several epidemiological risk factors have been identified that directly inform biosecurity recommendations. A retrospective cohort study of an outbreak on a large farm in North Carolina identified that being primarily stalled conferred a staggering odds ratio of 167.1 for testing PCR-positive compared to horses on pasture [29]. This is likely because stalled horses are in closer, more sustained contact with contaminated bedding, feed, and water sources. The same study identified that housing next to a positive horse (OR 7.5) and being fed rationed hay versus ad libitum (OR 1,558) were significant risk factors, suggesting that stress and nutritional management play a role in susceptibility [29]. The extreme odds ratio for rationed hay likely reflects increased competition and potential for oral inoculation from a shared, contaminated source. Therefore, biosecurity protocols must prioritize hygiene in stable environments, including the frequent removal and composting of manure, disinfection of feed and water troughs, and the use of individual water sources where possible. Quarantine of new arrivals or horses returning from events for a minimum of 14–21 days is essential, as subclinical shedding is common; in one outbreak in Tokyo, 63% of infected horses were subclinical but still capable of transmitting the virus [34].

The role of fomites and human vectors cannot be overstated. Outbreak investigations have shown that farm personnel can transmit ECoV between premises, as evidenced by a Quarter Horse mare stabled at a separate location from a main outbreak farm testing positive, with the likely vector being an employee [26]. This necessitates stringent personal protective equipment (PPE) protocols, including dedicated boots and coveralls for personnel working with sick or quarantined horses, and the use of footbaths containing effective disinfectants. Coronaviruses, being enveloped, are generally susceptible to common disinfectants such as 0.1% sodium hypochlorite (dilute bleach), 70% ethanol, and accelerated hydrogen peroxide products. However, in the presence of organic matter, efficacy is reduced, making pre-cleaning of surfaces a mandatory step. The prolonged shedding of virus in feces for up to nine days or more [42] and the potential for environmental contamination mean that shared equipment, trailers, and even pasture land should be considered contaminated until proven otherwise. Diagnostic testing using quantitative PCR (qPCR) on fecal samples is the gold standard for confirming infection and should be employed strategically to monitor the effectiveness of biosecurity interventions [28, 40]. The use of syndromic surveillance schemes, as demonstrated in the Swiss outbreak, allows for rapid coordinated action and containment, highlighting the value of veterinary infrastructure in early outbreak detection [32].

For breeding farms, the implications of ECoV are particularly severe. Although foals appear to be less susceptible to clinical disease than adults in some regions like Japan [57], outbreaks in miniature horse populations have been associated with high case fatality rates (27%) due to hyperammonemia and endotoxemia [21]. Consequently, biosecurity protocols on breeding premises must be especially rigorous. Isolating pregnant mares and foals from adult populations during the winter and spring months, when outbreaks are most common in Japan and the USA [33, 44], is a prudent measure. Given that seroprevalence can be high in some populations, up to 94.6% in riding stables in Japan [31] and 82.8% in adult horses in the Netherlands [36], understanding the herd immune status can guide management decisions. Horses that have recovered from infection likely have some degree of protective immunity, though the duration is not well-defined. The detection of salivary antibodies that correlate with recent exposure [31] could theoretically be used for non-invasive surveillance to identify recently infected groups and target biosecurity resources.

Vaccine Development: Current Landscape and Future Horizons

The absence of a commercially available, ECoV-specific vaccine represents the most critical gap in the control of this pathogen [16, 20, 28]. The development of such a vaccine is challenged by the virus's genetic diversity, the need to induce robust mucosal immunity in the gastrointestinal tract, and the limited understanding of the precise immunological correlates of protection. However, substantial progress has been made, leveraging both the close antigenic relationship of ECoV to bovine coronavirus (BCoV) and cutting-edge structural biology techniques that are paving the way for next-generation vaccine antigens.

A logical and well-explored avenue has been the repurposing of modified-live BCoV vaccines, given that ECoV and BCoV belong to the same Betacoronavirus 1 species within the subgenus Embecovirus and share significant antigenic homology [7, 11, 18]. Early serological studies demonstrated that BCoV and ECoV exhibit cross-neutralization, although titers are generally higher against the homologous virus [41]. Nemoto et al. [7] showed that horses inoculated with a BCoV vaccine developed neutralizing antibodies against ECoV, albeit at lower titers than against BCoV. This was followed by a more comprehensive safety and immunogenicity study by Prutton et al. [17], who administered a modified-live BCoV vaccine via oral, intranasal, or intrarectal routes to healthy adult horses. The results were promising in terms of safety, with only transient and self-limiting changes in fecal character observed, and no systemic adverse effects. Furthermore, the vaccine was shed to a minimal degree (only 2 of 12 vaccinated horses tested qPCR-positive for BCoV in nasal secretions post-vaccination, and none in feces), indicating a low risk of environmental contamination [17]. However, the humoral immune response was modest; only 27% of vaccinated horses seroconverted to BCoV, and the antibody response to ECoV itself was not explicitly measured in that study. This suggests that while the BCoV vaccine is safe, its ability to induce a robust, protective immune response against ECoV may be suboptimal, possibly due to the lack of an appropriate delivery system to stimulate strong mucosal immunity or because of antigenic mismatch.

The path to a more effective vaccine is illuminated by recent advances in structural biology and molecular evolution. A landmark study by Melchers et al. [12] used an artificial intelligence tool (ReCaP) to stabilize the prefusion conformation of the human coronavirus OC43 spike protein, and critically, these stabilizing substitutions were transferable to the ECoV spike. This work provided the very first cryo-EM structure of the ECoV spike protein, revealing that stabilization was achieved by arresting the fusion peptide and locking the S1B receptor-binding domain in a ‘down’ state through improved polar interactions [12]. This is a fundamental breakthrough for vaccine design because the metastable prefusion conformation of coronavirus spike proteins is the target of potently neutralizing antibodies. A stabilized version of this antigen forms the basis of many successful COVID-19 vaccines. The ability to now produce a stable, recombinant ECoV spike protein that maintains this critical conformation provides a clear platform for the development of subunit vaccines, virus-like particle (VLP) vaccines, or vector-based approaches. Given that the ECoV spike and nucleocapsid genes are highly conserved among circulating strains [1, 9, 10], a vaccine based on a stabilized spike from a single strain may offer broad protection.

Nevertheless, a successful vaccine strategy must contend with the evolutionary dynamics of ECoV. Zehr et al. [3] identified evidence of intra-host evolution (quasispecies development) specifically within the nucleocapsid (N) gene and detected 12 putative recombination events within the ECoV genome, primarily in ORF1ab. The identification of a recombinant ECoV strain in donkeys in China, with a recombination region around the NS2 gene [4], further underscores the virus's capacity for genetic plasticity. While the spike gene appears relatively stable, these evolutionary forces could potentially generate antigenic variants that escape vaccine-induced immunity. This risk highlights the need for continuous genomic surveillance, as exemplified by the 2025 Obihiro outbreak which was caused by a virus phylogenetically distinct from previous outbreaks at the same racecourse [1]. The use of multiple, high-quality genomes for vaccine target selection, as contributed by Zehr et al. [3], is essential to ensure that vaccine antigens remain representative of circulating field strains.

Passive immunotherapy represents a complementary strategy that could be deployed rapidly in the face of an outbreak for therapeutic or prophylactic use, especially in high-value animals or those at high risk of severe disease. This approach is supported by a rich body of work in equine medicine, where horses are frequently used as bioreactors to produce polyclonal antibodies against viral pathogens, including SARS-CoV and SARS-CoV-2. For example, equine F(ab’)2 fragments derived from horses immunized with SARS-CoV-2 spike protein have shown potent neutralizing activity against all variants of concern, including Omicron subvariants [56, 59]. Similarly, equine immune antibodies against MERS-CoV have been shown to reduce viral titers in mouse models [58]. The concept is directly applicable to ECoV. Hyperimmunization of donor horses with a stabilized ECoV spike antigen (such as that developed by Melchers et al. [12]) could generate high-titer neutralizing antiserum. This antiserum, or its purified F(ab’)2 fragments, could then be administered to exposed horses to provide immediate, passive immunity, buying time for the animal's own immune response to develop or for biosecurity measures to take effect. The safety of such equine-derived products has been established in preclinical and clinical settings for other coronaviruses [60], and their production is a mature technology.

The immunological goal of any ECoV vaccine must be to elicit strong, durable immunity at the intestinal mucosa, where the virus initially establishes infection. The development of enteroids, three-dimensional cultures of equine intestinal epithelium that support ECoV replication, provides an invaluable in vitro tool for studying host-pathogen interactions and for screening potential vaccine candidates and adjuvants [2]. These models allow researchers to directly assess the ability of vaccine-induced antibodies to neutralize viral entry and reduce replication in the target cell type. They can also be used to evaluate the immunogenicity of different antigen formulations and delivery systems, such as oral or intranasal live-attenuated vectors that are more likely to induce secretory IgA antibodies at the mucosal surface. While the BCoV vaccine data [17] suggests that systemic humoral immunity alone may not be fully protective, a vaccine that generates both systemic IgG and mucosal IgA is likely to be the most efficacious. The identification of specific serological markers of protection, such as neutralizing antibody titers against the S1 domain of the spike protein [36], will be crucial for defining vaccine efficacy endpoints in clinical trials. In conclusion, the convergence of advanced antigen design, improved in vitro models, and a deeper understanding of ECoV molecular epidemiology provides a robust foundation for the accelerated development of an ECoV vaccine, which, combined with rigorous biosecurity, remains the ultimate goal for controlling this emerging equine pathogen.

References

[1] Fukumoto N, Bannai H, Kawanishi N, Shibata M, Kishi D, Kambayashi Y, et al.. The first outbreak of equine coronavirus infection in 13 years among draft horses at Obihiro Racecourse in Japan in 2025. Journal of Veterinary Medical Science. 2025. DOI: https://doi.org/10.1292/jvms.25-0271

[2] Kambayashi Y, Nemoto M, Ochi A, Kishi D, Ueno T, Tsujimura K, et al.. Equine coronavirus infection and replication in equine intestinal enteroids. Veterinary Research. 2024. DOI: https://doi.org/10.1186/s13567-024-01381-z

[3] Zehr JD, Pond SKK, Shank SD, McQueary H, Grenier JK, Whittaker GR, et al.. Positive selection, genetic recombination, and intra-host evolution in novel equine coronavirus genomes and other members of the Embecovirus subgenus. Microbiology spectrum. 2024. DOI: https://doi.org/10.1128/spectrum.00867-24

[4] Qi P, Gao X, Ji J, Zhang Y, Yang S, Cheng K, et al.. Identification of a recombinant equine coronavirus in donkey, China. Emerging Microbes and Infections. 2022. DOI: https://doi.org/10.1080/22221751.2022.2056522

[5] Kambayashi Y, Kishi D, Ueno T, Ohta M, Bannai H, Tsujimura K, et al.. Distribution of equine coronavirus RNA in the intestinal and respiratory tracts of experimentally infected horses. Archives of Virology. 2022. DOI: https://doi.org/10.1007/s00705-022-05488-6

[6] Nemoto M, Schofield W, Cullinane A. The First Detection of Equine Coronavirus in Adult Horses and Foals in Ireland. Viruses. 2019. DOI: https://doi.org/10.3390/v11100946

[7] Nemoto M, Kanno T, Bannai H, Tsujimura K, Yamanaka T, Kokado H. Antibody response to equine coronavirus in horses inoculated with a bovine coronavirus vaccine. Journal of Veterinary Medical Science. 2017. DOI: https://doi.org/10.1292/jvms.17-0414

[8] Nemoto M, Oue Y, Murakami S, Kanno T, Bannai H, Tsujimura K, et al.. Complete genome analysis of equine coronavirus isolated in Japan. Archives of Virology. 2015. DOI: https://doi.org/10.1007/s00705-015-2565-1

[9] Oue Y, Morita Y, Kondo T, Nemoto M. Epidemic of equine coronavirus at Obihiro Racecourse, Hokkaido, Japan in 2012.. Journal of Veterinary Medical Science. 2013. DOI: https://doi.org/10.1292/JVMS.13-0056

[10] Oue Y, Ishihara R, Edamatsu H, Morita Y, Yoshida M, Yoshima M, et al.. Isolation of an equine coronavirus from adult horses with pyrogenic and enteric disease and its antigenic and genomic characterization in comparison with the NC99 strain. Veterinary Microbiology. 2011. DOI: https://doi.org/10.1016/j.vetmic.2011.01.004

[11] Zhang J, Guy, J, Snijder E, Denniston DA, Timoney P, Balasuriya U. Genomic characterization of equine coronavirus. Virology. 2007. DOI: https://doi.org/10.1016/j.virol.2007.06.035

[12] Melchers JM, Juraszek J, Hulswit R, Overveld Dv, Le L, Kuppeveld FVv, et al.. AI-guided prefusion stabilization of the human coronavirus OC43 spike protein enables universal embecovirus antigen design. PLoS Pathogens. 2026. DOI: https://doi.org/10.1371/journal.ppat.1013998

[13] Haake CJE, Cook SE, Pusterla N, Murphy B. Coronavirus Infections in Companion Animals: Virology, Epidemiology, Clinical and Pathologic Features. Viruses. 2020. DOI: https://doi.org/10.3390/v12091023

[14] Stummer M, Frisch V, Glitz F, Hinney B, Spergser J, Krücken J, et al.. Presence of Equine and Bovine Coronaviruses, Endoparasites, and Bacteria in Fecal Samples of Horses with Colic. Pathogens. 2023. DOI: https://doi.org/10.3390/pathogens12081043

[15] Zappulli V, Ferro S, Bonsembiante F, Brocca G, Calore A, Cavicchioli L, et al.. Pathology of Coronavirus Infections: A Review of Lesions in Animals in the One-Health Perspective. Animals. 2020. DOI: https://doi.org/10.3390/ani10122377

[16] Pusterla N. Equine Coronaviruses.. The Veterinary clinics of North America. Equine practice. 2022. DOI: https://doi.org/10.1016/j.cveq.2022.11.008

[17] Prutton J, Barnum S, Pusterla N. Evaluation of safety, humoral immune response and faecal shedding in horses inoculated with a modified‐live bovine coronavirus vaccination. Equine Veterinary Education. 2019. DOI: https://doi.org/10.1111/eve.13175

[18] Ghaly ANM. Challenges and Risks Facing Animal Wealth and Human Health Due to Coronavirus Outbreak. . 2020. DOI: https://doi.org/10.33552/AAHDS.2020.02.000529

[19] Fouché N, Remy-Wohlfender F, Blau D, Franzen J, Gurtner C, Seuberlich T, et al.. Characterization of an outbreak of equine coronavirus infection in adult horses in Switzerland.. Schweizer Archiv für Tierheilkunde. 2022. DOI: https://doi.org/10.17236/sat00372

[20] Pusterla N, Vin R, Leutenegger C, Mittel L, Divers T. Equine coronavirus: An emerging enteric virus of adult horses. Equine Veterinary Education. 2015. DOI: https://doi.org/10.1111/eve.12453

[21] Fielding CL, Higgins JK, Higgins JC, McIntosh S, Scott E, Giannitti F, et al.. Disease Associated with Equine Coronavirus Infection and High Case Fatality Rate. Journal of Veterinary Internal Medicine. 2014. DOI: https://doi.org/10.1111/jvim.12480

[22] Pusterla N, Mapes S, Wademan C, White A, Ball R, Sapp K, et al.. Emerging outbreaks associated with equine coronavirus in adult horses. Journal of Equine Veterinary Science. 2012. DOI: https://doi.org/10.1016/j.jevs.2012.08.088

[23] Suzuki K, Matsui Y, Miura Y, Sentsui H. Equine coronavirus induces apoptosis in cultured cells. Veterinary Microbiology. 2007. DOI: https://doi.org/10.1016/j.vetmic.2007.11.034

[24] Xu Z, Kang X, Han P, Du P, Li L, Zheng A, et al.. Binding and structural basis of equine ACE2 to RBDs from SARS-CoV, SARS-CoV-2 and related coronaviruses. Nature Communications. 2022. DOI: https://doi.org/10.1038/s41467-022-31276-6

[25] Grunewald ME, Fehr AR, Athmer J, Perlman S. The coronavirus nucleocapsid protein is ADP-ribosylated. Virology. 2017. DOI: https://doi.org/10.1016/j.virol.2017.11.020

[26] Goodrich EL, Mittel L, Glaser A, Ness S, Radcliffe R, Divers T. Novel findings from a beta coronavirus outbreak on an American Miniature Horse breeding farm in upstate New York. Equine Veterinary Education. 2018. DOI: https://doi.org/10.1111/eve.12938

[27] Velthuis AJTt, Worm SVDvd, Sims A, Baric R, Snijder E, Hemert MVv. Zn2+ Inhibits Coronavirus and Arterivirus RNA Polymerase Activity In Vitro and Zinc Ionophores Block the Replication of These Viruses in Cell Culture. PLoS Pathogens. 2010. DOI: https://doi.org/10.1371/journal.ppat.1001176

[28] Horner S, Agne G, Byrne D, Bain M, Lynch B, Gow W, et al.. Case series report: Equine coronavirus in Australia.. Australian Veterinary Journal. 2024. DOI: https://doi.org/10.1111/avj.13392

[29] Hepworth‐Warren K, Erwin SJ, Moore C, Talbot J, Young KA, Neault MJ, et al.. Risk factors associated with an outbreak of equine coronavirus at a large farm in North Carolina. Frontiers in Veterinary Science. 2023. DOI: https://doi.org/10.3389/fvets.2023.1060759

[30] Pusterla N, Lawton K, Barnum S. Investigation of the seroprevalence to equine coronavirus and SARS-CoV-2 in healthy adult horses recently imported to the United States. Veterinary Quarterly. 2023. DOI: https://doi.org/10.1080/01652176.2023.2288876

[31] Bannai H, Kambayashi Y, Ohta M, Nemoto M, Tsujimura K. Prevalence of serum and salivary virus-neutralizing antibodies against equine coronavirus in four riding stables in Japan. Journal of Equine Science. 2023. DOI: https://doi.org/10.1294/jes.34.13

[32] Hierweger MM, Remy-Wohlfender F, Franzen J, Koch M, Blau D, Schoster A, et al.. Outbreak of equine coronavirus disease in adult horses, Switzerland 2021.. Transboundary and Emerging Diseases. 2022. DOI: https://doi.org/10.1111/tbed.14501

[33] Kambayashi Y, Nemoto M, Tsujimura K, Ohta M, Bannai H. Serosurveillance of equine coronavirus infection among Thoroughbreds in Japan.. Equine Veterinary Journal. 2022. DOI: https://doi.org/10.1111/evj.13857

[34] Kambayashi Y, Bannai H, Tsujimura K, Hirama A, Ohta M, Nemoto M. Outbreak of equine coronavirus infection among riding horses in Tokyo, Japan.. Comparative Immunology, Microbiology & Infectious Diseases. 2021. DOI: https://doi.org/10.1016/j.cimid.2021.101668

[35] Schvartz G, Tirosh-Levy S, Barnum S, David D, Sol A, Pusterla N, et al.. Seroprevalence and Risk Factors for Exposure to Equine Coronavirus in Apparently Healthy Horses in Israel. Animals. 2021. DOI: https://doi.org/10.3390/ani11030894

[36] Zhao S, Smits C, Schuurman N, Barnum S, Pusterla N, Kuppeveld F, et al.. Development and Validation of a S1 Protein-Based ELISA for the Specific Detection of Antibodies against Equine Coronavirus. Viruses. 2019. DOI: https://doi.org/10.3390/v11121109

[37] Bryan J, Marr C, Mackenzie C, Mair T, Fletcher A, Cash R, et al.. Detection of equine coronavirus in horses in the United Kingdom. The Veterinary Record. 2018. DOI: https://doi.org/10.1136/vr.105098

[38] Schaefer E, Harms C, Viner M, Barnum S, Pusterla N. Investigation of an experimental infection model of equine coronavirus in adult horses. Journal of Veterinary Internal Medicine. 2018. DOI: https://doi.org/10.1111/jvim.15318

[39] Kooijman LJ, James K, Mapes S, Theelen M, Pusterla N. Seroprevalence and Risk Factors For Infection With Equine Coronavirus in Healthy Horses in the USA. The Veterinary Journal. 2017. DOI: https://doi.org/10.1111/evj.17_12732

[40] Pusterla N, James K, Mapes S, Bain F. Frequency of molecular detection of equine coronavirus in faeces and nasal secretions in 277 horses with acute onset of fever. The Veterinary Record. 2019. DOI: https://doi.org/10.1136/vr.104919

[41] Oue Y, Kanno T. Serological Survey of Equine Coronavirus Infection in Horses in Hokkaido, Japan. Journal of the Japan Veterinary Medical Association. 2017. DOI: https://doi.org/10.12935/JVMA.70.224

[42] Nemoto M, Oue Y, Morita Y, Kanno T, Kinoshita Y, Niwa H, et al.. Experimental inoculation of equine coronavirus into Japanese draft horses. Archives of Virology. 2014. DOI: https://doi.org/10.1007/s00705-014-2205-1

[43] Miszczak F, Tesson V, Kin N, Dina J, Balasuriya U, Pronost S, et al.. First detection of equine coronavirus (ECoV) in Europe. Veterinary Microbiology. 2014. DOI: https://doi.org/10.1016/j.vetmic.2014.03.031

[44] Pusterla N, Acvim D, Mapes S, Wademan C, White A, Ball R, et al.. Emerging outbreaks associated with equine coronavirus in adult horses. Veterinary Microbiology. 2012. DOI: https://doi.org/10.1016/j.vetmic.2012.10.014

[45] Hemida M, Chu D, Perera RA, Ko R, So RT, Ng BC, et al.. Coronavirus infections in horses in Saudi Arabia and Oman. Transboundary and Emerging Diseases. 2017. DOI: https://doi.org/10.1111/tbed.12630

[46] Berryhill E, Magdesian K, Aleman MR, Pusterla N. Clinical presentation, diagnostic findings, and outcome of adult horses with equine coronavirus infection at a veterinary teaching hospital: 33 cases (2012–2018). The Veterinary Journal. 2019. DOI: https://doi.org/10.1016/j.tvjl.2019.05.001

[47] Mattei D, Kopper J, Sanz M. Equine Coronavirus-Associated Colitis in Horses: A Retrospective Study. Journal of Equine Veterinary Science. 2020. DOI: https://doi.org/10.1016/j.jevs.2019.102906

[48] Manship AJ, Blikslager A, Elfenbein J. Disease features of equine coronavirus and enteric salmonellosis are similar in horses. Journal of Veterinary Internal Medicine. 2019. DOI: https://doi.org/10.1111/jvim.15386

[49] Niebuhr T, Ohnesorge B. Das Equine Coronavirus (ECoV) – ein aktueller Überblick. pferde spiegel. 2020. DOI: https://doi.org/10.1055/a-1127-4178

[50] Pusterla N, Vin R, Leutenegger C, Mittel L, Divers T. Equine Coronavirus Infection. Emerging and Re-emerging Infectious Diseases of Livestock. 2016. DOI: https://doi.org/10.1007/978-3-319-47426-7_4

[51] Pinn-Woodcock T, Aprea M, Lejeune M, Tomlinson J. Molecular detection of pathogens in an equine fever diagnostic panel: 2019–2023. Equine Veterinary Journal. 2025. DOI: https://doi.org/10.1111/evj.14527

[52] Sanz M, Kwon S, Pusterla N, Gold J, Bain F, Evermann J. Evaluation of equine coronavirus fecal shedding among hospitalized horses. Journal of Veterinary Internal Medicine. 2019. DOI: https://doi.org/10.1111/jvim.15449

[53] Nemoto M, Morita Y, Niwa H, Bannai H, Tsujimura K, Yamanaka T, et al.. Rapid detection of equine coronavirus by reverse transcription loop-mediated isothermal amplification. Journal of Virological Methods. 2015. DOI: https://doi.org/10.1016/j.jviromet.2015.02.001

[54] Miszczak F, Kin N, Tesson V, Vabret A. Real-Time RT-PCR Detection of Equine Coronavirus. Animal Coronaviruses. 2015. DOI: https://doi.org/10.1007/978-1-4939-3414-0_8

[55] DeNotta SL, Mittel L. Equine Coronavirus: An Infectious Disease Affecting Florida Horses. EDIS. 2023. DOI: https://doi.org/10.32473/edis-vm250-2023

[56] Luczkowiak J, Radreau P, Nguyen L, Labiod N, Lasala F, Veas F, et al.. Potent Neutralizing Activity of Polyclonal Equine Antibodies Against Severe Acute Respiratory Syndrome Coronavirus 2 Variants of Concern. Journal of Infectious Diseases. 2022. DOI: https://doi.org/10.1093/infdis/jiac331

[57] Nemoto M, Oue Y, Higuchi T, Kinoshita Y, Bannai H, Tsujimura K, et al.. Low prevalence of equine coronavirus in foals in the largest thoroughbred horse breeding region of Japan, 2012–2014. Acta Veterinaria Scandinavica. 2015. DOI: https://doi.org/10.1186/s13028-015-0149-4

[58] Zhao Y, Wang C, Qiu B, Li C, Wang H, Jin H, et al.. Passive immunotherapy for Middle East Respiratory Syndrome coronavirus infection with equine immunoglobulin or immunoglobulin fragments in a mouse model. Antiviral Research. 2016. DOI: https://doi.org/10.1016/j.antiviral.2016.11.016

[59] Botosso V, Jorge S, Astray R, Guimarães AMdS, Mathor M, Carneiro PDSd, et al.. Anti-SARS-CoV-2 equine F (Ab′)2 immunoglobulin as a possible therapy for COVID-19. Scientific Reports. 2022. DOI: https://doi.org/10.1038/s41598-022-07793-1

[60] Salinas FJ, Marelli B, Sanguineti S, Goldbaum F, Muñoz L, Etchevers L, et al.. Non-clinical safety assessment and in vivo biodistribution of CoviFab, an RBD-specific F(ab′)2 fragment derived from equine polyclonal antibodies. Toxicology and Applied Pharmacology. 2021. DOI: https://doi.org/10.1016/j.taap.2021.115796