Equine Parvovirus-Hepatitis

Overview and Taxonomy of Equine Parvovirus-Hepatitis

Equine Parvovirus-Hepatitis (EqPV-H) represents a seminal discovery in equine virology, fundamentally altering our understanding of a historically enigmatic and devastating condition known as Theiler’s disease, or equine serum hepatitis. The identification of this virus in 2018 [16] resolved a nearly century-old mystery first described by Arnold Theiler in 1918, who documented acute, often fatal hepatic necrosis in horses following the administration of equine-origin biological products [6, 9]. The initial detection of EqPV-H in the serum and liver of a horse that succumbed to serum hepatitis in Nebraska, USA, after receiving tetanus antitoxin, marked the beginning of an intensive global research effort that has since revealed a pathogen of considerable complexity, clinical significance, and worldwide distribution [16]. This chapter provides a comprehensive overview of the virus, its taxonomic placement within the Parvoviridae family, its genomic architecture, and its phylogenetic relationships, drawing upon the rapidly expanding body of literature that has emerged over the past several years.

Discovery and Initial Characterization

The seminal report by Divers et al. (2018) utilized viral metagenomics and high-throughput sequencing to identify a novel parvovirus in the liver and serum of a horse that died from Theiler’s disease [16]. Critically, the identical viral sequence was detected in the lot of tetanus antitoxin that had been administered to the horse 65 days prior to death, establishing a direct iatrogenic link between the contaminated biological product and the fatal outcome [16]. Experimental inoculation of two additional horses with this contaminated antitoxin confirmed the virus’s pathogenicity, resulting in viremia, seroconversion, and acute hepatitis confirmed by clinical, biochemical, and histopathological evaluation [16]. This foundational study not only identified the etiological agent but also demonstrated that EqPV-H was endemic in the equine population, with 13% of 100 clinically normal adult horses found to be viremic and 15% seropositive [16]. The subsequent confirmation of EqPV-H in 18 consecutive cases of serum hepatitis in a prospective study, while other candidate viruses (equine pegivirus, Theiler’s disease-associated virus, and non-primate hepacivirus) were inconsistently present, provided compelling evidence that EqPV-H is the primary cause of Theiler’s disease [18]. Since these initial reports, the virus has been detected in horses presenting with the full clinical spectrum of liver disease, from subclinical hepatitis to fulminant hepatic necrosis, as well as in a substantial proportion of apparently healthy horses worldwide [6, 7, 20].

Taxonomic Classification within the Family Parvoviridae

EqPV-H is a non-enveloped, single-stranded DNA virus classified within the family Parvoviridae, subfamily Parvovirinae, and genus Copiparvovirus [16, 17]. The genus Copiparvovirus is a relatively recently established lineage within the subfamily, distinct from the well-known Erythroparvovirus (which includes human parvovirus B19) and Protoparvovirus (which includes feline panleukopenia virus and canine parvovirus). The complete genome of EqPV-H is approximately 5.4–5.5 kilobases in length, containing two major open reading frames (ORFs). The left ORF encodes the non-structural protein 1 (NS1), a multifunctional nuclear phosphoprotein essential for viral replication, while the right ORF encodes the viral capsid proteins VP1 and VP2 [11, 17]. The phylogenetic placement of EqPV-H within Copiparvovirus is based on phylogenetic analysis of the NS1 gene, which is the most highly conserved region of the genome and is therefore the standard target for molecular detection and taxonomic assignment [11, 16].

The NS1 protein sequence of EqPV-H shares less than 50% identity with its nearest phylogenetic relatives in the Copiparvovirus genus, justifying its classification as a distinct species within this group [16]. This substantial divergence from other known parvoviruses underscores the unique evolutionary trajectory of EqPV-H and its specific adaptation to the equine host. Interestingly, the genus Copiparvovirus also encompasses other recently identified equine parvoviruses, including equine parvovirus-cerebrospinal fluid (EqPV-CSF), first identified in the CSF of a horse with neurological signs, and equine copivirus (EqCoPV), detected in respiratory samples [19, 21, 22]. While EqPV-CSF and EqCoPV are genetically distinct from EqPV-H, with sequence identities in the NS1 region often falling below the species demarcation threshold, they share a common ancestral lineage within the Copiparvovirus clade [21, 22]. This expanding diversity of equine copiparvoviruses suggests that horses may harbor a complex virome within this genus, the clinical significance of which remains to be fully elucidated.

Virological and Genomic Architecture

The genomic organization of EqPV-H follows the canonical parvovirus structure, with the NS1 protein playing a central role in viral DNA replication, transcriptional regulation, and cytotoxicity. The NS1 gene is highly conserved among EqPV-H strains globally, with nucleotide identities typically exceeding 97% across diverse geographical isolates [11, 13, 17]. This high degree of conservation makes the NS1 region an ideal target for molecular diagnostic assays, including quantitative PCR (qPCR) and nested PCR, which are the primary tools for detecting active infection [1, 2, 16]. In contrast, the VP1/VP2 capsid gene exhibits significantly greater genetic variability, particularly within a hypervariable region (HVR) that has been identified in the VP1 coding sequence [5, 11]. This HVR is of particular interest because sequence variants within this region can be tracked over time within an individual host, providing a unique window into viral population dynamics and evolution during long-term persistent infections [5].

The existence of a hypervariable region in the capsid gene is a hallmark of many persistent viral infections, facilitating immune evasion and allowing the virus to establish chronic infection. A landmark longitudinal study tracking EqPV-H infection in a single horse over 16 years revealed a complex pattern of viral evolution within the HVR, characterized by an initial sequence variant bottleneck followed by the emergence, dominance, and replacement of distinct viral variants over time [5]. This finding provides the first temporal description of EqPV-H capsid evolution in an individual animal and supports the hypothesis that EqPV-H can establish long-term, dynamic persistent infections, a notion further supported by the detection of chronic viremia in asymptomatic horses [5, 9]. The VP1 protein also contains a unique N-terminal region (VP1u) that is not present in VP2 and is thought to be involved in host cell receptor binding and viral entry, though the specific cellular receptor for EqPV-H remains unidentified.

Phylogenetic Diversity and Global Distribution

Phylogenetic analyses of EqPV-H sequences from around the world have revealed a remarkable degree of genetic stability, with strains from North America, Europe, Asia, South America, and Australia clustering closely together in phylogenetic trees based on the NS1 gene [1, 2, 8, 10, 11, 13, 17]. This high sequence similarity, often exceeding 94% nucleotide identity even between strains separated by vast geographical distances, suggests that EqPV-H is a well-adapted, stably evolving virus that has circulated in the global equine population for an extended period [8, 11, 13]. However, subtle patterns of geographical clustering have emerged from more detailed analyses. For instance, strains from Australia and Argentina have shown a tendency to cluster together in phylogenetic trees, suggesting a potential common origin or recent introduction [1, 2]. In contrast, strains from France and other European countries appear more broadly distributed, indicating a longer history of circulation or multiple independent introduction events [1, 13].

A particularly important discovery has been the identification of natural recombination events between different EqPV-H strains. Lu et al. (2020) provided strong evidence for recombination between Chinese and American strains within the VP protein-encoding region of the genome [11]. Recombination is a powerful evolutionary force in single-stranded DNA viruses, capable of generating novel genetic combinations and potentially altering viral virulence, tissue tropism, or transmissibility. The detection of such events in EqPV-H indicates that the virus is not merely evolving through the accumulation of point mutations but can also undergo more substantial genomic rearrangements [11]. The presence of recombination, combined with the hypervariability of the capsid gene, suggests that EqPV-H has a greater capacity for genetic diversification than was initially appreciated, which has important implications for diagnostic test design and vaccine development. The global phylogeny of EqPV-H also shows that the virus is not confined to a specific equine breed or discipline, with infections documented in Thoroughbreds, Quarter Horses, Tennessee Walking Horses, and various mixed-breed populations across multiple continents [4, 7, 10, 15].

The Epidemiological Landscape and Risk Factors

The taxonomy and phylogeny of EqPV-H have direct implications for understanding its epidemiology. The high global prevalence of EqPV-H DNA in clinically healthy horses, ranging from approximately 3% to 20% in various cross-sectional surveys [1, 2, 10, 12, 15, 17], and seroprevalence estimates of 15% to 35% [10, 15, 16], indicates that the virus circulates widely and that most infections are subclinical. However, the virus is strongly associated with the development of severe, often fatal hepatitis in a subset of infected animals [6, 7, 18, 20]. This discrepancy between high prevalence and relatively low incidence of clinical disease suggests that host factors, viral factors, or environmental triggers play a critical role in determining disease outcome. Large-scale epidemiological studies have begun to identify these risk factors. For example, a study of 1,195 horses in the southern United States found that EqPV-H DNA prevalence was highest in older horses and that male horses had 1.62 times the odds of infection compared to females [4]. Similarly, Austrian [10] and German [15] studies have demonstrated that increasing age is a significant risk factor for active infection. Breed-specific associations have also been identified, with Tennessee Walking Horses showing higher odds of EqPV-H positivity [4].

The phylogenetic stability of EqPV-H, despite its high prevalence and capacity for persistence, is somewhat paradoxical for a virus that relies on a DNA genome and a proofreading host polymerase for replication. However, the presence of the hypervariable region in the capsid gene, combined with the potential for recombination, may provide the virus with sufficient genetic plasticity to evade host immune responses over the long term while maintaining a highly conserved NS1 core that is essential for replication fitness [5, 11]. The widespread circulation of EqPV-H in healthy horses, coupled with its ability to contaminate commercial equine serum products [3, 14, 16, 18], underscores the need for stringent screening of equine-origin biologicals. The United States Department of Agriculture (USDA) Center for Veterinary Biologics has established regulatory requirements mandating that all commercially licensed equine plasma and serum products be tested and found negative for EqPV-H [3, 23]. This international biosecurity concern, consistent with the principles of the World Organisation for Animal Health (WOAH) regarding the safety of veterinary biologics, highlights the practical importance of understanding the taxonomic identity and global distribution of this virus.

In summary, EqPV-H is a highly prevalent, globally distributed, and phylogenetically distinct member of the genus Copiparvovirus that has been definitively linked to the etiology of Theiler’s disease. Its genomic architecture, characterized by a conserved NS1 gene and a hypervariable capsid region, provides the molecular basis for both its diagnostic detection and its capacity for persistent infection and evolutionary adaptation. The ongoing characterization of EqPV-H diversity, including the documentation of natural recombination events and the long-term dynamics of within-host viral populations, continues to refine our understanding of this emerging equine pathogen.

Molecular Pathogenesis of Equine Parvovirus-Hepatitis

The molecular pathogenesis of equine parvovirus-hepatitis (EqPV-H) is a rapidly evolving narrative, defined by the virus’s unique genomic architecture, its sophisticated strategies for cellular entry and replication, and a delicate balance between subclinical persistence and fulminant hepatic necrosis. Unlike many viral pathogens, EqPV-H operates within a paradoxical landscape: it is both a ubiquitous, often innocuous commensal and a potent trigger of Theiler’s disease, a frequently fatal acute hepatitis. Understanding this duality requires a deep dissection of the molecular events at the host-pathogen interface, from the initial engagement of the hepatocyte to the systemic dysregulation that culminates in liver failure.

Genomic Architecture and Evolutionary Dynamics

EqPV-H is a member of the genus Copiparvovirus within the family Parvoviridae, possessing a linear, single-stranded DNA genome of approximately 5.5–6.0 kb [11, 16]. The genome is organized into two major open reading frames (ORFs): the non-structural (NS) region, encoding the multidomain nuclear phosphoprotein NS1, and the structural (VP) region, encoding the capsid proteins VP1 and VP2. The NS1 protein is the molecular workhorse of the virus, essential for genome replication, transcriptional regulation, and, critically, the induction of cellular cytotoxicity [11, 16]. The NS1 sequence exhibits remarkable conservation across global isolates, with nucleotide identities ranging from 97.1–99.9% [11, 17]. This genetic stability contrasts sharply with the VP1/VP2 capsid region, which harbors a hypervariable region (HVR) that serves as a critical nexus for host immune evasion and within-host population dynamics [5, 11].

The molecular evolution of EqPV-H is not static. Deep sequencing analysis of serial serum samples from a single persistently infected horse over 16 years has illuminated the quasispecies nature of the virus [5]. This study documented a definitive sequence variant bottleneck upon initial infection, followed by a complex and dynamic population structure characterized by the emergence, dominance, and recession of distinct capsid variants [5]. This pattern of continuous antigenic drift within the HVR strongly suggests that EqPV-H employs a strategy of immune evasion through rapid diversification, allowing it to persist in the face of a developing humoral immune response. Furthermore, natural recombination events have been detected within the VP protein region, specifically between Chinese and American strains, indicating that co-infection with genetically divergent viruses can generate novel recombinant progeny, further fueling the virus’s evolutionary potential [11, 25].

Viral Entry, Hepatotropism, and Cellular Tropism

The liver is the primary target organ for EqPV-H, a tropism confirmed by the detection of viral DNA and nucleic acid in hepatocytes of infected horses [24, 27, 28, 30]. The molecular basis for this hepatotropism, however, remains incompletely defined. Parvoviruses typically gain entry into permissive cells by binding to specific cell surface receptors. While the exact receptor for EqPV-H is unknown, it is hypothesized to be a glycosphingolipid or globoside, analogous to the P-antigen (globoside) used by human parvovirus B19, which is also expressed on hepatocytes and pronormoblasts [33, 34]. The virus’s ability to infect and replicate within hepatocytes is a central tenet of its pathogenesis.

Beyond hepatocytes, in situ hybridization (ISH) studies have localized EqPV-H nucleic acid to a broader cellular spectrum within the liver. In cases of fulminant Theiler’s disease, viral DNA has been detected not only in the cytoplasm and nuclei of degenerating hepatocytes but also, notably, within bile duct epithelium and Kupffer cells [30]. This suggests that the virus may utilize the biliary tree as a site of replication or as a route for fecal-oral shedding, a hypothesis supported by the detection of EqPV-H DNA in feces [25, 29]. The infection of Kupffer cells, the resident hepatic macrophages, introduces a critical immunopathological dimension; these cells are key orchestrators of the hepatic inflammatory response, and their infection could directly trigger the cytokine storms and lymphocytic infiltration characteristic of severe disease [7, 24].

NS1-Mediated Pathogenesis and Cytopathology

The non-structural protein NS1 is the principal driver of EqPV-H-induced cellular injury. In a manner analogous to other parvoviruses, NS1 possesses endonuclease activity that is critical for viral DNA replication. However, this endonuclease activity can also inflict catastrophic damage on the host cell genome, leading to DNA damage responses, cell cycle arrest, and ultimately, apoptosis or necrosis [33, 34]. The presence of individual hepatocyte necrosis, which is the most common histologic feature observed in EqPV-H-positive liver samples, is a direct consequence of this NS1-mediated cytotoxicity [7].

The molecular pathogenesis unfolds in a temporal cascade. Experimental infection studies have established a close temporal association between peak viremia, the onset of hepatitis, and seroconversion [6, 24]. As the viral load in the serum rises, NS1 accumulates in infected hepatocytes, triggering widespread single-cell death. This is manifest histologically as random, scattered individual hepatocyte necrosis, distinct from the classical centrilobular necrosis seen in toxin-induced or ischemic liver injury [7]. Interestingly, while centrilobular necrosis is also present in many cases, it is individual hepatocyte death and lobular/periportal infiltrates that are most consistently associated with EqPV-H infection [7]. The extent of NS1-mediated damage is directly correlated with viral load; samples with high viral DNA titers show a strong positive association with centrilobular necrosis, portal inflammation, and individual hepatocyte death [7]. However, the virus is not always directly cytolytic. In cases of persistent infection with low-level replication, NS1-induced damage may be subclinical, allowing the virus to maintain a foothold in the host without triggering overt disease [9, 31].

Immunopathogenesis: The Double-Edged Sword of the Host Response

The host’s immune response to EqPV-H is the primary determinant of clinical outcome. The virus’s ability to persist for years in immunocompetent horses points to a sophisticated capacity to subvert or evade sterilizing immunity [5, 9]. The hypervariable region of the capsid protein is the frontline of this evasion strategy. By continuously generating novel antigenic variants, the virus can stay one step ahead of the neutralizing antibody response, a phenomenon elegantly documented in a 16-year longitudinal study of a single horse [5]. This dynamic evolution explains why many horses remain viremic for extended periods despite high titers of anti-VP1 antibodies [10, 14, 15].

Paradoxically, the onset of acute, severe hepatitis is not a direct consequence of massive viral cytolysis alone. The landmark experimental transmission studies revealed that hepatitis becomes clinically and biochemically apparent temporally associated with the decline of viremia and the rise of seroconversion [6, 24]. This observation strongly suggests that the acute liver failure in Theiler’s disease is an immune-mediated pathology. The influx of lymphocytic infiltrates, which are prominent in active lesions, is indicative of a robust adaptive immune response that, in its attempt to clear the virus, inadvertently destroys large swaths of hepatocytes [24, 28]. The ISH detection of viral nucleic acid in necrotic hepatocytes surrounded by inflammatory cells [28] provides a vivid histologic correlate of this immune-mediated destruction: cytotoxic T lymphocytes targeting and killing infected cells. Furthermore, the presence of portal edema, a negative predictor of high viral load [7], may reflect a shift from a cytotoxic to a more humoral inflammatory response, potentially leading to acute, but not necessarily fulminant, injury.

Molecular Basis of Persistent and Chronic Infection

The establishment and maintenance of persistent infection is a hallmark of EqPV-H pathogenesis. The molecular mechanisms enabling this are multifactorial. First, as discussed, the quasispecies diversification of the capsid provides a shield against neutralizing antibodies [5]. Second, the virus may establish a state of immunological tolerance or exhaustion. The high prevalence of EqPV-H DNA in asymptomatic horses (up to 19.3% in some surveys [4]) and the lack of significant elevation in liver enzymes in most chronically viremic horses [12, 17] suggest that the virus can replicate in the liver without triggering a fulminant destructive immune response. This may be due to the immunomodulatory effects of chronic NS1 exposure, which can induce regulatory T cell responses or suppress the interferon signaling pathway, as seen with other chronic hepatotropic viruses [32]. Third, the virus may persist in a low-level replication state in hepatocytes, a phenomenon supported by the detection of low viral loads in archived FFPE liver tissue from horses with cirrhosis and other chronic hepatopathies [31]. This latent or smoldering infection can be reactivated under conditions of immunosuppression, such as neoplastic disease or stress, leading to the emergence of more virulent variants or simply a tipping of the host-pathogen balance toward pathology [31]. The documented ability of EqPV-H to establish chronic infection, with viremia persisting for years without overt signs, is a critical component of its molecular pathogenesis and explains its wide dissemination in the global equine population [9, 15].

Host, Vector, and Iatrogenic Factors in Viral Dissemination

Molecular pathogenesis cannot be divorced from transmission biology. The virus is shed in a variety of bodily secretions, including oral, nasal, and fecal matter [24, 25, 29]. The molecular detection of EqPV-H DNA in semen provides evidence for venereal transmission, adding another route for natural spread [29]. Experimentally, oral inoculation with viremic serum established infection in one horse, demonstrating the potential for fecal-oral transmission, but this route appears inefficient compared to the nasal route, which proved highly effective in a larger cohort study [24, 26]. The prolonged eclipse phase after nasal inoculation (6–12 weeks) before detectable viremia suggests that the virus replicates locally in the upper respiratory tract or lymphatic tissues before establishing a systemic infection [26].

The most significant and well-documented transmission route, however, is iatrogenic, via contaminated equine-origin biological products. The demonstration that commercial tetanus antitoxin, equine plasma, and allogeneic stem cell therapies can harbor infectious EqPV-H at high titers (up to 10⁵ copies/mL) has profound implications for the pathogenesis of the disease [14, 16, 18]. Injection of these contaminated products delivers a direct, high-dose bolus of the virus into the bloodstream (intravenous, intramuscular, or intra-articular), bypassing the natural mucosal barriers. This likely leads to a much higher initial viral load in the liver, overwhelming the host’s early containment mechanisms and rapidly triggering the explosive cytopathic and immunopathological cascade that defines Theiler’s disease [18, 24]. The molecular fingerprint of these iatrogenic outbreaks is often a high degree of sequence similarity between the virus in the product and the infected horses, confirming the direct link [18, 20]. This iatrogenic route not only explains the focal, explosive nature of many outbreaks but also underscores the critical importance of molecular screening in equine biologics to prevent these catastrophic events [3, 23].

Epidemiology and Global Distribution of Equine Parvovirus-Hepatitis

Since its inaugural identification in 2018 from a fatal case of Theiler’s disease in a horse in Nebraska, USA [16], equine parvovirus-hepatitis (EqPV-H) has emerged as a globally distributed pathogen of significant veterinary concern. The virus, a member of the genus Copiparvovirus within the family Parvoviridae, has been documented across North and South America, Europe, Asia, and Oceania, revealing a complex epidemiological landscape characterized by widespread subclinical infection, distinct transmission dynamics, and notable genetic conservation interspersed with pockets of regional diversity. The cumulative evidence from over half a decade of research paints a picture of a virus that is both ubiquitous and enigmatic, with prevalence rates varying dramatically based on geographic region, sampling population, and diagnostic methodology.

Global Prevalence and Geographic Distribution

The global footprint of EqPV-H is now substantial, with molecular evidence of infection reported from every continent where systematic surveillance has been conducted. The earliest studies following its discovery focused on North America, where the initial survey of 100 clinically normal adult horses in the USA revealed a DNA prevalence of 13% and a seroprevalence of 15% [16]. This foundational work established the paradigm that EqPV-H infection is far more common than the rare, fulminant Theiler’s disease it can cause. Subsequent large-scale studies in the United States have confirmed and extended these findings. A landmark molecular survey of 1,195 horses across Alabama, Georgia, and Texas reported an EqPV-H DNA prevalence of 19.3% (231/1,195), making it the most common hepatotropic virus in that cohort, significantly outpacing equine hepacivirus (EqHV) at 5.6% and pegiviruses at 1.8% [4]. This study also identified that positive horses exhibited significantly higher viral loads compared to those infected with other agents, suggesting a robust replicative capacity within the equine host.

In Europe, EqPV-H has been detected in multiple countries with varying frequencies. A comprehensive cross-sectional study in Austria examined 259 horses and 13 donkeys, revealing a DNA prevalence of 8.9% and a seroprevalence of 30.1% among horses, while all donkeys tested negative [10]. This study highlighted the age-dependent nature of infection, with a significantly higher probability of active infection in horses aged 16–31 years compared to younger cohorts (OR = 8.19) [10]. In Germany, a targeted investigation of 392 Thoroughbred broodmares and stallions found a DNA prevalence of 7% and a seroprevalence of 35%, further cementing the notion that a substantial proportion of the equine population has been exposed to the virus [15]. Longitudinal surveillance of 124 horses in Germany over five years (2013–2018) provided critical evidence for chronic infection, demonstrating that EqPV-H viremia can persist in asymptomatic horses without biochemical or pathological signs of liver disease [9]. The first documented outbreak of EqPV-H-associated Theiler’s disease in Europe occurred on a stud farm in Slovenia in 2018–2019, where liver tissue from all four affected horses tested positive by PCR and in situ hybridization, with three having a history of tetanus antitoxin administration [27].

South America has also proven to be a significant endemic region. Brazil was the first country in South America to report EqPV-H circulation, with a retrospective study (2013–2016) detecting viral DNA in 12.5% (12/96) of horses from 46.6% (7/15) of evaluated farms [8]. More recent and expansive data from the northern region of Rio Grande do Sul state, Brazil, using 1,000 serum samples organized into 200 pools, reported a stunning 34.5% pool-level positivity, with positive samples found in 65% (26/40) of municipalities [36]. This suggests the virus is not merely present but is hyperendemic in certain Brazilian equine populations. Argentina provided the first evidence of EqPV-H in commercial equine serum batches, with 60% (3/5) of tested batches containing viral DNA, alongside a 1.71% prevalence in individual donor horses [2]. This finding underscores a critical biosecurity risk and echoes earlier global surveys, such as the detection of EqPV-H DNA in 11 of 18 commercial serum samples from the USA, Canada, New Zealand, Italy, and Germany [14].

Asia has yielded some of the highest reported prevalence rates. In China, initial studies reported a DNA prevalence of 11.9% (17/143) in racehorses across five farms, with a remarkably high co-infection rate of 58.8% with equine flaviviruses [17]. A subsequent broader survey of 225 horses found an EqPV-H viremia rate of 7.6% [21]. South Korea has contributed extensive epidemiological data. A study of 321 clinically healthy horses found a 4.4% DNA prevalence, with significant associations with sex and race performance [12]. In contrast, a study of 160 serum samples reported a substantially higher prevalence of 10.6%, alongside a 35.3% co-infection rate with EqHV [25]. The first clinical case of EqPV-H-associated Theiler’s disease in Asia was documented in a 14-year-old Thoroughbred mare in Korea, imported from the USA, further confirming the transcontinental movement of the virus [30].

Australia and France were the subjects of a seminal comparative study. Testing sera from 188 Australian and 256 French horses, researchers found an EqPV-H DNA prevalence of 3.2% and 4.7%, respectively [1, 13]. While these prevalence rates were statistically similar, the phylogenetic architecture differed markedly: Australian strains were genomically clustered, suggesting a more recent or isolated introduction, whereas French strains were more broadly distributed, indicative of a longer or more diverse endemic presence [1]. Oceania has thus entered the global map of EqPV-H distribution, although remaining a region of relatively lower reported prevalence.

Transmission Dynamics and Iatrogenic Risk

A defining feature of EqPV-H epidemiology is its dual transmission paradigm: efficient iatrogenic spread via contaminated equine-origin biological products, alongside robust horizontal transmission through natural routes. The original discovery of the virus was inextricably linked to the administration of tetanus antitoxin [16]. A prospective study of 18 consecutive cases of equine serum hepatitis in the USA found that all cases were EqPV-H positive, and that the same lot of tetanus antitoxin used in 10 of 12 TAT-associated cases was also EqPV-H positive [18]. This iatrogenic risk is not historical. Regulatory intervention in the USA, specifically governmental requirements that all equine biologics be free of EqPV-H, has demonstrably reduced contamination. Testing of 227 equine biologic product lots revealed that the fraction of serials with EqPV-H detected dropped from 39.6% prior to regulation to 6.8% after implementation [3]. Despite this progress, the presence of the virus in commercial serum batches in Argentina [2] and other nations indicates that global regulatory harmonization is incomplete.

Natural transmission pathways are now well characterized. The virus is shed in oral, nasal, and fecal secretions [24, 25, 29]. Experimental inoculation has definitively demonstrated that intranasal inoculation is an efficient route of infection, producing viremia within 6–12 weeks and subsequent seroconversion [26]. Oral inoculation, in contrast, appears less efficient under experimental conditions, though natural transmission through close contact has been observed. Following the death of an EqPV-H-positive mare with Theiler’s disease, in-contact foals in the same paddock developed high viral loads, providing clear evidence of horizontal transmission without any iatrogenic intervention [37]. Furthermore, a large outbreak on a farm in Ontario, Canada, where no biologic products had been administered, demonstrated that EqPV-H DNA was present in 61.8% (34/55) of horses, including animals with confirmed Theiler’s disease, subclinical liver disease, and clinically normal in-contact horses [35]. This study firmly established that the virus can propagate autonomously within a herd, independent of human medical intervention. Field surveillance in Korea has also identified viral shedding in semen (11.1% of tested samples), suggesting a potential venereal transmission route [29].

Risk Factors and Host Susceptibility

Demographic analysis from multiple large-scale studies has begun to delineate the risk factors for EqPV-H infection. The most consistently identified factor is age. In Austria, horses aged 16–31 years had 8.19 times the odds of active infection compared to 1–8-year-old horses [10]. Similarly, in German Thoroughbreds, age, particularly the 11–15-year-old group, was a significant risk factor [15]. A study in the southern United States confirmed that EqPV-H-positive horses were significantly older than their negative counterparts [4]. This age association likely reflects cumulative exposure over a lifetime, the potential for persistent infection, or age-related immunological senescence.

Sex is another modulatory factor. In the large US survey, male horses had 1.62 times the odds of infection compared to females [4]. A study of clinically healthy horses in South Korea also found that EqPV-H infection was significantly associated with sex, with males being at higher risk [12]. Breed-specific susceptibility has emerged as a notable finding. In the US, Tennessee Walking Horses had higher odds of EqPV-H positivity (OR = 2.46), while Quarter Horses (OR = 4.16) and Thoroughbreds (OR = 9.64) showed increased odds for EqHV, but not necessarily EqPV-H [4]. This suggests that breed-related genetic factors or management practices may influence susceptibility to specific hepatotropic viruses. The association with performance has also been noted, with EqPV-H infection linked to race performance in Korean horses, although the directionality of this relationship remains unclear [12].

Genetic Diversity and Global Viral Dynamics

One of the most striking features of EqPV-H is its remarkable genetic stability, particularly in the non-structural protein 1 (NS1) gene, which is often used for phylogenetic characterization. Global isolates from the USA, China, Germany, Austria, Brazil, Argentina, Korea, and Australia exhibit nucleotide identities of 97.1–99.9% in the NS gene and 95.2–100% in the VP gene [11]. This low genetic variability suggests a relatively recent emergence or a highly constrained evolutionary trajectory. Brazilian isolates, for instance, showed nucleotide identity higher than 94% with previous isolates from North America and Asia [8]. Austrian variants demonstrated high similarity to sequences worldwide [10], and Korean isolates shared approximately 98.7–100% similarity with each other and with strains from the USA, Germany, and China [12].

However, this picture of uniformity is nuanced by evidence of recombination and the presence of a hypervariable region in the capsid gene. Strong evidence for natural recombination events between Chinese and American strains has been documented, specifically within the VP protein, indicating that co-infection with different strains can lead to the emergence of novel genetic variants [11]. Korean strains also showed evidence of a natural recombination event between Chinese and Korean strains [25]. The hypervariable region in the capsid gene provides a mechanism for within-host evolution. A landmark longitudinal study tracking EqPV-H in a single horse over 16 years revealed a complex viral population dynamic, with sequence variants undergoing patterns of emergence, dominance, recession, and replacement following an initial bottleneck [5]. This indicates that while global consensus sequences are stable, intra-host quasispecies diversity may be substantial, potentially influencing immune evasion and persistence. The analysis of complete coding sequences of four Austrian variants by next-generation sequencing confirmed their close relation to global sequences, reinforcing the concept of a single, globally circulating viral clade with minor regional variations [10].

The Enigmatic Epidemiology of Subclinical Infection

The most critical epidemiological insight regarding EqPV-H is the stark disconnect between the high prevalence of infection and the rarity of clinical disease. Across all continents, the vast majority of EqPV-H DNA-positive horses are completely asymptomatic. In the initial US study, 13 of 100 clinically normal horses were viremic [16]. In Brazil, altered serum biochemical parameters suggesting subclinical hepatopathy were noted in only 3 of 12 positive horses, with the majority showing no clinical or laboratory signs [8]. Similarly, in Korea, liver-specific biochemical analytes were within normal ranges in both EqPV-H-positive and control horses [12]. In the Austrian study, glutamate dehydrogenase, gamma-glutamyl transferase, bile acids, and albumin concentrations were not significantly different between horses with active infection and PCR-negative controls [10].

This suggests that EqPV-H, like many parvoviruses, has evolved a strategy of long-term, often life-long persistence with minimal disruption to the host, except under specific triggering conditions. The factors that precipitate the transition from subclinical infection to fulminant Theiler’s disease remain poorly understood but likely involve a combination of high viral load, host immune status, co-infections, genetic predisposition, and perhaps the introduction of specific viral strains via biological products. The fact that Theiler’s disease can occur spontaneously without biologic administration [20, 35] indicates that the virus itself, under the right conditions, is fully capable of inducing severe pathology. The observation that EqPV-H is common in a variety of liver pathologies, not just Theiler’s disease, with detection in 48% of archived liver samples exhibiting diverse histologic findings [7], suggests that its pathogenic role may be broader than currently appreciated. The World Organisation for Animal Health (WOAH) recognizes the importance of monitoring emerging equine pathogens, and the epidemiological profile of EqPV-H, with its high subclinical carriage rate and potential for iatrogenic transmission, fits squarely within the mandate for enhanced biosecurity and surveillance in the international movement of equine biological products.

Gaps in the Global Picture

Despite the rapid accumulation of data, significant gaps remain in the global epidemiological map. Africa, for instance, has no published reports of EqPV-H surveillance, despite its large and diverse equid populations. Central Asia and the Middle East are similarly underrepresented. While cross-species detection has been documented in donkeys [9], the prevalence in other equids such as mules, zebras, and Przewalski’s horses remains almost entirely unknown. The role of arthropod vectors as mechanical vectors, while ruled out in one small experimental study [24], warrants further investigation given the seasonal pattern of some outbreaks. Furthermore, most prevalence studies are cross-sectional; comprehensive longitudinal cohort studies that track individual animals over years are rare, limiting our understanding of the natural history of infection, the duration of viremia, and the frequency of clearance versus lifelong persistence. The development of standardized, internationally validated diagnostic assays, including digital PCR to overcome the issue of PCR inhibitors in biological products [3], will be crucial for generating comparable data across future surveillance efforts. As equine biologics continue to be traded globally, the epidemiological risk posed by EqPV-H demands ongoing vigilance from regulatory bodies, veterinary practitioners, and the racing and breeding industries alike.

Clinical Manifestations and Association with Theiler's Disease

Equine parvovirus-hepatitis (EqPV-H) has emerged from the shadows of veterinary virology to assume a position of paramount importance in the understanding of equine hepatitis, particularly its most devastating manifestation: Theiler's disease. The clinical spectrum of EqPV-H infection is remarkably heterogeneous, ranging from completely subclinical viremia in apparently healthy horses to a rapidly progressive, fulminant hepatic necrosis that is frequently fatal. This disparity between the high prevalence of viral DNA in asymptomatic populations and the relatively low incidence of clinical Theiler's disease has been a central puzzle in equine hepatology, but meticulous experimental and epidemiological investigations have now solidified the causal link between EqPV-H and this historically enigmatic condition.

The Historical Paradigm of Theiler's Disease and the Discovery of EqPV-H

Theiler's disease, also known as equine serum hepatitis or idiopathic acute hepatic necrosis, was first described by Arnold Theiler in 1918 as a fatal hepatopathy occurring in horses following the administration of equine-origin biological products. For a century, the etiological agent remained elusive, with various candidate viruses, including equine pegivirus, Theiler's disease-associated virus (TDAV), and non-primate hepacivirus (NPHV), being proposed but ultimately failing to fulfill Koch's postulates consistently. The landmark discovery by Divers et al. in 2018 fundamentally altered this landscape. In a seminal investigation, a novel parvovirus was identified in the serum and liver of a horse that died 65 days after treatment with equine-origin tetanus antitoxin in Nebraska [16]. Critically, the same virus was detected in the incriminated antitoxin lot, and experimental inoculation of two horses with this contaminated product resulted in viremia, seroconversion, and clinicopathologically confirmed acute hepatitis [16]. This study not only identified EqPV-H but provided the first experimental evidence of causality, a finding that has been replicated and extended across the globe.

The association between EqPV-H and Theiler's disease is now supported by an overwhelming body of evidence. In a prospective study of 18 consecutive cases of equine serum hepatitis enrolled from US referral hospitals between 2014 and 2018, Tomlinson et al. detected EqPV-H DNA in the serum, liver, or both of all 18 cases [18]. Twelve of these horses had received tetanus antitoxin 4 to 12.7 weeks prior to the onset of hepatic failure, three had received commercial equine plasma, and three had undergone allogenic stem cell therapy [18]. The tetanus antitoxin from the same lot number was available for testing in 10 of the 12 TAT-associated cases, and all 10 samples were EqPV-H positive [18]. In contrast, TDAV was absent from all samples, and NPHV was detected in only two of 14 serum samples, confirming that EqPV-H is the consistent and likely causative agent. This complete penetrance of EqPV-H in Theiler's disease cases has been corroborated by subsequent studies across multiple continents, including Europe [27], Asia [30], and Australia [1].

The Clinical Spectrum of EqPV-H Infection

Acute Fulminant Hepatitis (Theiler's Disease)

The archetypal clinical manifestation of EqPV-H infection is Theiler's disease, characterized by the acute onset of severe hepatic failure. Affected horses typically present with icterus, lethargy, inappetence, and neurological abnormalities consistent with hepatic encephalopathy [6, 20]. The disease course is rapidly progressive, with many affected animals succumbing within days to weeks of clinical onset. In the European outbreak reported by Vengust et al., four horses on a stud farm in Slovenia developed fatal Theiler's disease between 2018 and 2019. Three of these horses had a history of tetanus antitoxin administration 7 to 11 weeks prior to disease onset, and liver tissue from all four horses tested positive for EqPV-H by PCR [27]. In situ hybridization (ISH) revealed a widespread distribution of viral nucleic acid in hepatocytes in one case, with a more sporadic distribution in the remaining three [27]. This marked the first documentation of EqPV-H-associated Theiler's disease in Europe and was the first use of ISH to visualize viral nucleic acid in the liver tissues of affected horses, providing direct evidence of hepatocyte infection.

The histopathological hallmarks of acute EqPV-H-induced Theiler's disease are fulminant hepatic necrosis, collapse of the lobular architecture, and extensive lymphocytic infiltration [6, 24]. In a comprehensive histopathological study of 98 archived FFPE liver samples, Jager et al. found that EqPV-H was detected in 48% of samples, with the most common histologic features in positive samples including individual hepatocyte death (85%), lobular infiltrates (80%), portal infiltrates (74%), and ductular reaction (70%) [7]. Crucially, centrilobular necrosis, portal infiltrate, and individual hepatocyte death were positively associated with high viral load, directly linking viral replication with tissue destruction [7]. Neutrophil infiltrates, bridging fibrosis, and portal edema were negatively associated with high viral load, suggesting that these features may represent chronic or resolving phases of infection rather than acute active disease [7]. This study was instrumental in demonstrating that EqPV-H is not restricted to the classical picture of Theiler's disease but is common in a variety of liver pathologies, warranting its consideration as a differential diagnosis in all cases of hepatitis.

Subclinical Hepatitis and the Carrier State

Paradoxically, the vast majority of EqPV-H infections are subclinical. The prevalence of EqPV-H DNA in apparently healthy horse populations is remarkably high, ranging from 3.2% in Australian and French horses [1] to 19.3% in the Southern United States [4], with studies in Brazil reporting up to 34.5% pool-level positivity in healthy horses from the northern region of Rio Grande do Sul [36]. This suggests that EqPV-H, like many parvoviruses, is capable of establishing persistent or latent infections that do not uniformly result in clinical disease.

The subclinical nature of most infections has been confirmed by serial evaluation of liver-associated biochemistry parameters. Badenhorst et al., in a cross-sectional study of Austrian horses, found that liver-associated plasma parameters (GLDH, GGT, bile acids, and albumin) were not significantly different between horses with active EqPV-H infection (PCR-positive) and PCR-negative controls [10]. Similarly, studies in South Korea and China have reported that EqPV-H-positive horses generally have AST and GGT values within normal reference intervals [12, 17]. However, subtle hepatopathy can be detected in a subset of infected animals. In the Brazilian study by Moraes et al., altered serum biochemical parameters suggestive of subclinical hepatopathy were observed in 3 of 12 EqPV-H-positive horses, although the majority of infected animals presented no clinical or laboratory signs of infection [8].

The concept of a chronic carrier state is further supported by longitudinal data. Reinecke et al. followed a cohort of 124 German horses over five years and provided evidence that EqPV-H viremia can become chronic in infected horses that do not show biochemical or pathological signs of liver disease [9]. This ability to establish persistent infection is a hallmark of viruses within the Parvoviridae family and has profound implications for disease transmission and control. The work of Scupham in a single horse over 16 years is particularly illuminating; using high-throughput sequencing to track capsid gene variants, this study demonstrated that EqPV-H infection results in a sequence variant bottleneck, followed by the evolution of a complex viral population characterized by patterns of emergence, dominance, recession, and replacement [5]. This suggests that EqPV-H is capable of evading host immune responses over extended periods, maintaining a dynamic infection within the host.

The Clinical Paradox: Bridging Subclinical Infection and Fulminant Disease

The critical question of what precipitates the transition from subclinical viremia to fulminant hepatitis in a minority of infected horses remains incompletely understood. Evidence from experimental infections provides important clues. Tomlinson et al. experimentally inoculated horses with EqPV-H and found that 8 of 10 horses developed hepatitis, but only one horse showed clinical signs of liver failure [24]. Crucially, the onset of hepatitis was temporally associated with seroconversion and a decline in viremia, suggesting that the liver injury is, at least in part, immune-mediated rather than purely cytopathic [24]. Liver histology and ISH in these experimental animals showed lymphocytic infiltrates and necrotic EqPV-H-infected hepatocytes, a pattern consistent with immune-mediated destruction [24]. Host genetic factors, including major histocompatibility complex (MHC) haplotype, may therefore play a significant role in determining disease outcome, analogous to the situation with hepatitis B and C viruses in humans.

Co-infections with other hepatotropic viruses may also influence disease progression. EqPV-H frequently co-infects horses with equine hepacivirus (EqHV) and equine pegivirus, with co-infection rates reported as high as 58.8% in some Chinese cohorts [17]. The clinical significance of these co-infections is an active area of investigation. In a Korean study, Yoon et al. found that AST was significantly elevated in horses viremic for EqPV-H or EqHV, and 43.5% of viremic horses had at least two liver-specific parameters outside reference intervals [25]. The synergistic or antagonistic effects of these concurrent infections on liver pathology have yet to be fully elucidated, but the high prevalence of co-infection suggests that EqPV-H-positive horses are frequently exposed to multiple hepatotropic agents.

Clinicopathological Correlates and Diagnostic Markers

The diagnosis of EqPV-H-associated Theiler's disease relies on a combination of clinical presentation, serum biochemistry, histopathology, and molecular detection of viral DNA. Horses with acute disease exhibit dramatic elevations in liver-specific enzymes, including AST, GGT, GLDH, and sorbitol dehydrogenase (SDH). Bile acids are typically elevated, and albumin concentrations may be decreased due to impaired hepatic synthetic function [6, 25]. In the Ontario outbreak reported by Baird et al., EqPV-H DNA was detected in the serum of 61.8% of horses on the affected farm, with viral loads ranging from less than 3.75 × 10³ copies/mL to 3.64 × 10⁷ copies/mL [35]. EqPV-H DNA was present in the serum of three horses with a confirmed diagnosis of Theiler's disease, five horses with subclinical liver disease (defined as elevated liver enzymes without clinical signs), and in clinically normal in-contact horses [35]. This highlights the critical diagnostic utility of molecular testing, as clinical signs alone are insufficient to identify all infected horses or predict disease severity.

The use of in situ hybridization has been a transformative tool in confirming the hepatotropism of EqPV-H and its association with pathological changes. In the first Asian case of EqPV-H-associated Theiler's disease, Yoon et al. used ISH to demonstrate EqPV-H DNA not only in hepatocytes but also in bile duct epithelium and Kupffer cells, expanding our understanding of the cellular tropism of the virus [30]. This broad distribution of viral nucleic acid may contribute to the cholestatic features sometimes observed in affected horses. In the New York racetrack study, Jager et al. found that 31 of 42 PCR-positive liver samples had positive viral nucleic acid hybridization in hepatocytes, with 11 samples showing positive hybridization in necrotic hepatocytes associated with inflammatory cells, confirming active hepatitis [28]. Both individual hepatocyte necrosis and hepatitis were positively associated with EqPV-H detection (p < 0.0001 and p = 0.0005, respectively) [28], providing a direct histopathological link between viral presence and tissue damage in a large cohort of horses.

The Unique Case of Racehorses and "High GGT Syndrome"

A particularly intriguing area of investigation involves the relationship between EqPV-H and the phenomenon of elevated gamma-glutamyl transferase (GGT) activity in racehorses. Serum GGT activity is positively correlated with cumulative days in training in Thoroughbred racehorses, and when GGT exceeds 100 IU/L, it has been associated with poor performance. The etiopathogenesis of this "high GGT syndrome" has been elusive, but the high prevalence of EqPV-H in horses suggested a possible infectious cause. However, two rigorous case-control studies have largely ruled out a direct role for EqPV-H in this condition. Mann et al. found no differences in the frequency of detection of EqPV-H DNA or copy numbers between racehorses with high GGT and controls, concluding that viral hepatitis was not a cause for this syndrome [38]. Similarly, Ramsay et al. examined 802 prerace blood samples from Thoroughbreds and found that while EqPV-H infection was present in 2.9% of horses, the relative risk of having concurrently increased liver enzyme activity was 0.916 (95% CI 0.564–1.266, p = 0.7), indicating no significant association [39]. Interestingly, infection with equine pegivirus (PgV E) was actually associated with a reduced risk of increased liver enzyme activity (RR = 0.820, 95% CI 0.662–0.978, p = 0.03), suggesting a potentially protective effect [39]. These findings underscore that the pathophysiology of EqPV-H infection is distinct from the metabolic and oxidative stress-related hepatopathy observed in intensely trained racehorses.

The Iatrogenic Transmission Paradigm and Clinical Implications

The association between EqPV-H and the administration of equine-origin biological products remains the most clinically and economically significant aspect of the disease. The discovery of EqPV-H in commercial equine serum pools has been documented at alarming frequencies. Meister et al. detected EqPV-H DNA in 11 of 18 commercial serum samples from the USA, Canada, New Zealand, Italy, and Germany, with viral loads up to 10⁵ copies/mL [14]. Three of five commercial horse serum batches from Argentina also contained EqPV-H DNA [2]. This iatrogenic transmission has been demonstrated experimentally, with Tomlinson et al. confirming transmission via allogeneic stem cell therapy for orthopedic injuries [24]. The recognition of this risk has prompted regulatory action; as Scupham and Tong reported, federal requirements now mandate that all equine biologics be free of EqPV-H in the United States, a measure that has already reduced the fraction of product serials with detected virus from 39.6% prior to regulation to 6.8% after implementation [3].

However, the disease is not exclusively iatrogenic. Theiler's disease can occur in the absence of any biologic product administration, as demonstrated by the Ontario outbreak described by Baird et al., where no horse had received an equine-origin biologic product in the preceding six months [35]. Similarly, Tomlinson et al. prospectively studied 10 cases of naturally occurring Theiler's disease from six separate properties where no biologic product had been administered within four months; nine of these 10 cases (90%) were EqPV-H positive, and 54% of in-contact horses were also viremic [20]. Hepatitis was significantly associated with EqPV-H infection (p = 0.036) in this cohort [20]. These non-biologic-associated cases highlight the existence of natural transmission routes, including horizontal transmission via oral, nasal, and fecal shedding, as well as potential venereal transmission [24-26, 29].

Epidemiologic and Host Risk Factors

The clinical expression of EqPV-H infection is influenced by host factors, most notably age. Multiple epidemiologic studies have identified advanced age as a significant risk factor for active infection. Badenhorst et al. found that horses aged 16 to 31 years had an 8.19-fold higher odds of active EqPV-H infection compared to horses aged 1 to 8 years (p = 0.002), and a 2.96-fold higher odds compared to horses aged 9 to 15 years (p = 0.03) [10]. Similarly, Meister et al. identified age, particularly in the group of 11- to 15-year-old Thoroughbreds, as a potential risk factor [15]. Barua et al., in the largest US molecular survey to date (1,195 horses), confirmed that EqPV-H-positive horses were significantly older and also identified that male horses had 1.62 times the odds of infection compared to females [4]. Breed-specific associations were also uncovered in this study: Tennessee Walking Horses had higher odds of EqPV-H positivity (OR = 2.46), while Quarter Horses (OR = 4.16) and Thoroughbreds (OR = 9.64) showed increased odds of testing positive for EqHV [4]. These demographic and breed associations provide important insights for targeted surveillance and risk assessment.

The Global Perspective: A Ubiquitous Pathogen

EqPV-H has been identified in horses across the globe, including North America [4, 16, 18], South America [2, 8, 36], Europe [1, 10, 13, 27], Asia [11, 17, 30], and Australia [1, 13]. The molecular characterization of these isolates reveals a remarkably conserved virus. Lu et al. found nucleotide identities of 97.1–99.9% for the NS1 gene and 95.2–100% for the VP gene across strains from China and the USA [11]. This high degree of genetic conservation suggests that EqPV-H is a stable virus with low evolutionary rate, which has implications for diagnostic assay design and vaccine development. The worldwide distribution underscores the importance of EqPV-H as a pathogen of global economic and welfare significance, and the World Organisation for Animal Health (WOAH) has emphasized the need for surveillance and biosecurity measures to prevent iatrogenic transmission through contaminated biological products.

In summary, EqPV-H is a widely distributed, genetically stable parvovirus that is the primary etiological agent of Theiler's disease. Its clinical manifestations span from completely asymptomatic viremia, through subclinical hepatitis detectable only by biochemical and histopathological examination, to acute, life-threatening fulminant hepatic necrosis. The interplay between viral factors, host genetics, and environmental triggers (including iatrogenic exposure) determines the clinical outcome. The consistent detection of EqPV-H in cases of Theiler's disease globally, the experimental reproduction of the disease, and the demonstration of viral nucleic acid within necrotic hepatocytes provide irrefutable evidence of causation. The high prevalence of subclinical infection in healthy horse populations, particularly in older animals, presents a major challenge for disease control and underscores the critical importance of screening equine-origin biological products to prevent iatrogenic outbreaks.

Diagnostic Approaches for Equine Parvovirus-Hepatitis

The accurate and timely diagnosis of equine parvovirus-hepatitis (EqPV-H) infection is a multifaceted endeavor, requiring a sophisticated integration of molecular virology, serology, histopathology, and clinical biochemistry. Since its initial identification in 2018 [16], the diagnostic landscape has evolved rapidly, driven by the need to detect both active viremia and past exposure, differentiate subclinical carriage from fulminant hepatic necrosis, and ensure the safety of equine-origin biological products. A comprehensive diagnostic strategy must account for the virus's unique biology, its tendency to establish persistent infections with low-level replication, and the critical distinction between mere nucleic acid detection and the presence of infectious, pathogenic virus.

Molecular Detection of Viral Nucleic Acid

The cornerstone of EqPV-H diagnosis is the direct detection of viral DNA in clinical specimens. Several polymerase-chain-reaction (PCR) based platforms have been developed and validated, each with specific strengths and limitations that dictate their application in clinical, research, and regulatory settings.

Quantitative Real-Time PCR (qPCR)

Quantitative real-time PCR (qPCR) targeting highly conserved regions of the EqPV-H genome, most commonly the non-structural protein 1 (NS1) gene, is the most widely used method for initial screening and viral load quantification [2, 4, 14]. This technique offers high sensitivity and the ability to monitor viral kinetics over time. Studies have employed various qPCR protocols to document the presence of EqPV-H DNA in diverse matrices, including serum, plasma, liver tissue, oral and nasal secretions, and feces [1, 24, 25, 29]. The assay is capable of detecting a wide dynamic range of viral loads, from as low as 3.75 × 10³ copies/mL in subclinical carriers to over 10⁷ copies/mL in horses with acute Theiler's disease [14, 35]. The quantification cycle (Cq) value provides a reliable, albeit indirect, measure of viremia magnitude, which has been positively correlated with the severity of histopathological lesions in the liver [7].

However, the utility of qPCR is not without significant challenges. A critical issue, particularly relevant to the screening of equine biologics, is the susceptibility of the reaction to co-extracted PCR inhibitors. Source [3] explicitly demonstrates that the initial qPCR test proved "sensitive to co-extracted PCR inhibitors in template nucleic acids, causing false-negative results." This phenomenon is particularly pronounced in complex matrices like serum products and formalin-fixed, paraffin-embedded (FFPE) liver tissues, where nucleic acid quality is often compromised [7]. The presence of inhibitors can lead to an underestimation of viral prevalence or, worse, the release of contaminated biological products into the market. Consequently, reliance on qPCR alone, especially for regulatory compliance, is now considered insufficient.

Nested PCR (nPCR)

To overcome the limitations of sensitivity and inhibitor tolerance in certain sample types, nested PCR (nPCR) has been employed as a confirmatory or secondary screening tool. This technique uses two successive rounds of amplification with two sets of primers, significantly enhancing the limit of detection. Several studies have utilized NS1-specific nPCR to identify EqPV-H DNA in samples that were initially indeterminate or negative by qPCR. For instance, in a survey of Argentine equine sera, 9 of 51 serum pools were indeterminate by qPCR (Cq values above the limit of detection), but subsequent NS1 nPCR detected EqPV-H DNA in 8 of these 9 pools, revealing a prevalence of 15.7% that qPCR alone would have missed [2]. Similarly, nPCR has been instrumental in detecting viral DNA in FFPE liver samples where DNA fragmentation is common [8, 28, 31]. While nPCR is more labor-intensive and carries a higher risk of amplicon contamination, its superior analytical sensitivity makes it invaluable for diagnostic confirmation in low-viremia states and for phylogenetic sequencing of difficult samples (e.g., archival tissues) [8, 21].

Digital PCR (dPCR)

The advent of digital PCR (dPCR) represents a paradigm shift in EqPV-H diagnostics, offering a solution to the inhibitor problem inherent to qPCR. By partitioning the sample into thousands of individual nanoliter-sized reactions, dPCR provides an absolute, endpoint quantification of target DNA that is largely independent of amplification efficiency. Source [3] provides compelling evidence for this assertion, showing that dPCR is "a more robust test" compared to qPCR. In a direct comparison of 227 equine biologic product lots, the qPCR method detected EqPV-H in 39.6% of lots prior to regulatory screening, while dPCR revealed a much higher prevalence. The fraction of serials with EqPV-H detected dropped to only 6.8% after implementation of qPCR-based regulatory screening, a figure that is likely a gross underestimate of the true contamination rate [3]. The authors of source [3] explicitly conclude that adopting dPCR testing is an "opportunity to further decrease the prevalence of EqPV-H in equine biologics." The robustness of dPCR against inhibitors makes it the current gold standard for testing high-value samples, particularly commercial serum pools and antitoxins, where false negatives carry profound clinical and economic consequences. Furthermore, dPCR has been successfully used to quantify viral loads in FFPE liver tissues with a range of histopathologies, providing insights into chronic infection states [31].

Serological Assays: Detecting Past Exposure and Seroconversion

While PCR detects active infection (viremia), serological assays are essential for documenting past exposure and understanding population-level epidemiology. The primary serological tool developed for EqPV-H is the luciferase immunoprecipitation system (LIPS), which detects antibodies against the viral capsid protein VP1 [10, 15]. This assay is highly specific and sensitive, and its application has revealed a striking dichotomy between DNA prevalence and seroprevalence. In various healthy horse populations, seroprevalence (e.g., 30.1% in Austria, 35% in Germany) is consistently three to four times higher than DNA prevalence, indicating that a large proportion of the equine population has been infected and subsequently cleared the virus [10, 15]. This pattern is critical for clinicians: a positive antibody test alone does not indicate active disease, but it does confirm historical exposure.

Serology is also invaluable for documenting seroconversion in experimental infection studies. For example, in the seminal experimental transmission study by Divers et al. [16], horses inoculated with EqPV-H-contaminated antitoxin seroconverted temporally with a decline in viremia and the onset of hepatitis. Similarly, in nasal transmission experiments, horses became seropositive within 10 to 19 weeks post-inoculation, coinciding with detectable viremia [26]. This temporal link between seroconversion and hepatitis onset is a critical diagnostic clue: the detection of anti-VP1 antibodies in a horse with acute liver disease, especially if IgM-specific serology is used, can help confirm recent infection as the likely cause. Currently, an IgM-specific LIPS assay for acute infection is not widely standardized, but the detection of rising IgG titers between paired serum samples (acute and convalescent) can provide definitive retrospective diagnosis. It is noteworthy that commercially available equine serum pools are often positive for both EqPV-H DNA and anti-VP1 antibodies, highlighting that a significant proportion of donor horses are actively infected or were recently exposed [14].

Histopathology and In Situ Hybridization (ISH)

For ante-mortem and post-mortem investigation of liver disease, histopathology combined with in situ hybridization (ISH) provides the most definitive evidence linking EqPV-H infection to hepatic pathology. Microscopic examination of liver biopsies or necropsy specimens can reveal a spectrum of EqPV-H-associated lesions. The most common findings in EqPV-H-positive samples include individual hepatocyte death (apoptosis/necrosis), lobular and portal lymphocytic infiltrates, ductular reaction, and centrilobular necrosis [7, 24, 27]. A critical finding from source [7] is that EqPV-H is significantly associated with a wide variety of hepatic lesions beyond classical Theiler's disease, including chronic hepatitis and fibrosis. Specifically, high viral load (assessed by PCR or ISH) is positively associated with centrilobular necrosis, portal inflammation, and individual hepatocyte death [7].

ISH offers a distinct advantage over PCR on tissue extracts: it allows for the spatial visualization of viral nucleic acid within specific cell types. Using a probe targeting EqPV-H, Tomlinson et al. [24] and Jager et al. [7] have demonstrated that viral RNA is localized primarily within hepatocytes, and particularly within necrotic hepatocytes surrounded by inflammatory cells. In an outbreak of Theiler's disease in Europe, ISH revealed a "widespread distribution of viral nucleic acid in hepatocytes" in one case and a more sporadic distribution in others [27]. This technique has also been used to identify EqPV-H in bile duct epithelium and Kupffer cells [30], expanding our understanding of viral tropism. For a clinician, the presence of ISH-positive hepatocytes in a liver biopsy from a horse with elevated liver enzymes is strong evidence that EqPV-H is the causative agent. This is particularly important in cases of chronic hepatitis where the virus may be present at low levels, and PCR on tissue might be negative due to sampling error or inhibitor effects.

Biochemical Markers and Their Clinical Context

No diagnosis of EqPV-H-related disease is complete without an assessment of liver biochemistry. While EqPV-H is frequently detected in clinically normal horses, its presence is often associated with subclinical hepatocellular injury. The most sensitive biomarkers for EqPV-H-associated hepatopathy include serum gamma-glutamyl transferase (GGT), aspartate aminotransferase (AST), glutamate dehydrogenase (GLDH), and bile acids [6, 10, 25, 35].

A key diagnostic challenge is that a significant proportion of EqPV-H viremic horses (often >50%) have liver enzyme values within the reference interval [10, 12, 17, 37]. This is a critical observation for the clinician: a normal biochemistry panel does not rule out EqPV-H infection. Conversely, in horses with Theiler's disease, these parameters are dramatically elevated, often by one to two orders of magnitude [18, 27, 35]. The specific pattern of injury is typically hepatocellular rather than cholestatic. For example, Yoon et al. [25] found that AST was significantly elevated in viremic horses compared to non-viremic controls, and 43.5% of viremic horses had at least two liver-specific parameters outside the reference range. However, the relationship between EqPV-H and enzyme elevation is not straightforward. In racehorses with high GGT syndrome, a multifactorial metabolic disorder, EqPV-H was not identified as a significant causative factor [38, 39]. This highlights that while EqPV-H can cause hepatitis, the presence of elevated GGT in a racehorse should prompt consideration of other metabolic causes alongside viral testing. For regulatory and clinical screening, the combination of qPCR/dPCR and a liver biochemistry panel (GGT, AST, GLDH, bile acids) is recommended. A horse that is PCR-positive and has elevated liver enzymes should be considered a case of active EqPV-H hepatitis, while a PCR-positive horse with normal enzymes is likely a subclinical carrier or in the early stages of infection [14, 40].

Diagnostic Application to Biological Product Safety

A unique and critical application of EqPV-H diagnostics is the mandatory screening of equine-origin biological products. Following the discovery of EqPV-H in a contaminated tetanus antitoxin that caused an outbreak of Theiler's disease [16], regulatory bodies, including the United States Department of Agriculture (USDA) Center for Veterinary Biologics, have mandated that all licensed equine biologics (serum, plasma, antitoxins, stem cells) be tested and found negative for EqPV-H [3, 23]. This has necessitated the development of highly robust diagnostic protocols. Given the extreme sensitivity of dPCR to inhibitors and its ability to provide absolute quantification, it is rapidly becoming the recommended platform for this application [3]. The diagnostic strategy for biologics must also consider sample type. While serum is the primary matrix, testing the final product (e.g., reconstituted antitoxin) is essential, as the viral genome has been recovered in live commercial sera from around the world [2, 14]. Furthermore, source [23] emphasizes that continued surveillance is necessary, and that regulatory standards are likely to be adopted globally. For manufacturers, the diagnostic pipeline must include not only high-sensitivity PCR or dPCR but also consideration of viral inactivation steps in the manufacturing process, as EqPV-H is a non-enveloped, resilient virus.

Emerging Diagnostic Considerations and Differential Detection

The diagnostic landscape is further complicated by the presence of other equine parvoviruses (EqPV-CSF, EqCoPV) and flaviviruses (EqHV, EPgV) that can co-circulate and cause similar clinical syndromes. While EqPV-H is strongly associated with Theiler's disease, its detection alone does not exclude co-infection, which is common [4, 8, 11, 17]. For example, in horses, the copiparvovirus lineage includes both EqPV-H and EqCoPV, and both can be found in sera and feces [21, 22]. A purely EqPV-H-specific qPCR might miss a co-infection with EqCoPV. Therefore, a comprehensive diagnostic workup for acute hepatitis in horses should include panels that detect EqPV-H, EqHV, and potentially other hepatotropic viruses. Source [32] even reports cases of chronic hepatitis linked to persistent EqHV infection, highlighting that a negative EqPV-H PCR does not rule out a viral cause for chronic liver disease. Additionally, emerging evidence suggests the possibility of cross-species infection, with EqPV-H DNA detected in donkeys [9], indicating that diagnostic algorithms should be considered for other equids.

Finally, the detection of viral shedding in extrabodily fluids has opened new avenues for non-invasive diagnosis. Natural shedding of EqPV-H has been documented in oral and nasal swabs, feces, and semen [25, 26, 29, 37]. For clinicians, this means that a nasal swab or fecal sample could theoretically serve as a screening tool, especially in outbreak situations where repeated serum collection may be impractical. However, the sensitivity of these sample types is lower than that of serum, and negative results from swabs do not rule out infection [19]. In experimental settings, oral inoculation was unsuccessful in establishing infection, while intranasal inoculation was highly effective, supporting the hypothesis that nasal shedding is a primary transmission route [26]. For biosecurity, detecting shed virus in the environment (e.g., via PCR of stable surfaces or communal water sources) is an area of diagnostic interest that aligns with USDA and WOAH guidelines for managing contagious pathogens. The development of rapid, point-of-care antigen tests for EqPV-H remains an unmet need, but the current molecular arsenal, led by dPCR and ISH, provides a robust framework for accurate diagnosis, epidemiological surveillance, and product safety oversight.

Transmission Routes and Risk Factors for Equine Parvovirus-Hepatitis

The elucidation of transmission pathways and the identification of host-specific risk factors are paramount for developing effective biosecurity protocols and mitigating the impact of equine parvovirus-hepatitis (EqPV-H). Since its initial identification in a fatal case of Theiler’s disease following the administration of a contaminated biological product, understanding how this virus moves within and between equine populations has been a central focus of research. The evidence accumulated to date points to a complex epidemiology characterized by multiple, overlapping transmission routes, with iatrogenic spread through contaminated biologics representing the most dramatic and well-documented pathway, while horizontal, natural transmission appears to be the primary driver of endemic circulation among seemingly healthy horses. A nuanced understanding of these routes, coupled with an appreciation of risk factors such as age, sex, and breed, is essential for guiding veterinary practice and regulatory policy.

Iatrogenic Transmission: The Paradigm of Biological Products

The initial discovery of EqPV-H was inextricably linked to the administration of equine-origin tetanus antitoxin, establishing iatrogenic transmission as a foundational mechanism for the most severe clinical expression of the disease. This route is historically associated with outbreaks of equine serum hepatitis, or Theiler’s disease, which have been documented for over a century following the injection of blood-derived products. The index report from the United States identified the virus in the liver and serum of a horse that succumbed to hepatitis 65 days after receiving an EqPV-H-contaminated tetanus antitoxin, and subsequent experimental inoculation of horses with the same contaminated product confirmed its pathogenicity and transmissibility [16]. A prospective study of 18 consecutive cases of serum hepatitis unequivocally demonstrated EqPV-H DNA in all cases, with the implicated commercial equine-origin plasma, allogeneic stem cells, or tetanus antitoxin of the same lot consistently testing positive for the virus [18].

The scope of this contamination is alarmingly broad. Investigations into commercial equine serum pools, products used widely in research, cell culture, and the manufacturing of veterinary biologics, have revealed a high prevalence of EqPV-H DNA. A study of 18 commercial serum samples from the USA, Canada, New Zealand, Italy, and Germany detected viral DNA in 11 samples, with viral loads reaching up to 10⁵ copies/mL [14]. This problem extends to commercial serum batches in Argentina, where three out of five tested batches (60%) were found to contain EqPV-H DNA, underscoring the global nature of this risk [2]. The recognition of this threat has prompted regulatory action; in the United States, the Center for Veterinary Biologics now mandates that all licensed equine biologics containing plasma or serum must test negative for EqPV-H [3]. Implementation of quantitative PCR-based screening has demonstrably reduced the proportion of contaminated product serials from 39.6% prior to regulation to 6.8% after its enforcement, although the adoption of more robust digital PCR methods is recommended to further close the gap and eliminate residual false-negative results from PCR inhibitors [3]. The inclusion of EqPV-H in quality control protocols is now considered essential not only for commercial sera but also for equine plasma used in transfusions and stem cell therapies [6, 23].

Horizontal Transmission: The Natural Route of Endemic Spread

Beyond the iatrogenic pathway, a compelling body of evidence now confirms that EqPV-H is capable of efficient horizontal transmission between horses in the absence of any biological product administration. This discovery is critical, as it explains the high global seroprevalence of the virus observed in seemingly healthy horses with no history of injections [4, 10, 15].

Oral-Nasal and Fecal-Oral Routes The strongest experimental evidence for a natural transmission route comes from a landmark study demonstrating that EqPV-H can be transmitted via the nasal mucosa. In a prospective experimental inoculation, horses that received EqPV-H directly into the nasal passages became viremic and seroconverted, while those inoculated orally did not become infected within the initial 8-week observation period [26]. This study also provided a compelling demonstration of horizontal transmission: after the first cohort of intranasally inoculated horses became viremic and were co-housed with a second cohort, the second group subsequently became viremic at 19 to 22 weeks post-inoculation, strongly suggesting transmission via contact or fomites [26]. Supporting this, the virus has been detected in oral and nasal secretions of naturally infected horses. A field study in Korea identified EqPV-H DNA in 2.9% of oral swabs and 2.9% of nasal swabs from a cohort of horses, providing direct molecular evidence for viral shedding from these mucosal surfaces [29]. This finding was corroborated in a clinical case of Theiler’s disease in Asia, where PCR of both nasal and oral swabs from the affected horse was positive [30].

The fecal-oral route also represents a plausible mechanism for environmental contamination and spread. EqPV-H DNA has been detected in fecal samples from naturally infected horses, with a prevalence of 5.3% reported in a Korean study [25]. Furthermore, experimental infections have demonstrated that EqPV-H is shed in feces, and importantly, oral inoculation with viremic serum has been shown to lead to infection in a recipient horse, confirming that the virus can be infectious upon ingestion [24]. The prolonged eclipse phase, the time between infection and detectable viremia, observed after nasal inoculation (6 to 12 weeks) has significant implications for biosecurity, as it suggests that an infected horse could be shedding virus and exposing others for weeks before clinical signs or positive PCR results are apparent [26].

Venereal and Vertical Transmission Evidence is emerging for additional horizontal routes, including venereal transmission. The detection of EqPV-H DNA in 11.1% of semen samples collected from stallions in Korea marks the first report of viral shedding in reproductive fluids [29]. While the infectivity of the virus in semen has not yet been experimentally confirmed, this finding raises the possibility of sexual transmission, a route that could contribute to the high prevalence seen in breeding herds. Conversely, vertical transmission from mare to foal has been investigated but not conclusively demonstrated. One experimental study attempted to detect EqPV-H in foals born to viremic mares but found no evidence of in utero infection, although the authors noted the limited sample size [24]. A field investigation into a non-biologic outbreak of Theiler’s disease, however, documented that foals in close contact with an infected mare became viremic, with these foals exhibiting higher viral loads than the adult mares [37]. While this suggests a strong horizontal dynamic rather than true vertical transmission, it underscores the risk of infection for young, immunologically naive horses.

Arthropod Vector-Borne Transmission: An Unresolved Hypothesis

The possibility of mechanical or biological transmission via arthropod vectors remains an open and important question. Given that EqPV-H DNA is found at high loads in the blood of infected horses, blood-feeding insects such as horse flies, stable flies, and mosquitoes are biologically plausible vectors. The first experimental attempt to demonstrate mechanical transmission via horse fly bites, however, was unable to confirm this route, again potentially due to an insufficient sample size [24]. The global distribution of EqPV-H, including its presence in regions with different endemic insect populations, suggests that while vector transmission might augment spread, it is unlikely to be the primary or sole natural route [1, 14]. This area of investigation is critical, as the confirmation of insect vector involvement would fundamentally alter prevention strategies.

Host-Specific Risk Factors: Age, Sex, Breed, and Viral Load Dynamics

The probability of EqPV-H infection is not uniform across the equine population; distinct host factors significantly modulate the risk of both exposure and active viremia.

Age as a Dominant Risk Factor Increasing age is consistently identified as one of the strongest and most reproducible risk factors for active EqPV-H infection. A cross-sectional study of Austrian horses found that the odds of being PCR-positive for EqPV-H were 8.19 times higher in horses aged 16-31 years compared to horses aged 1-8 years [10]. Similarly, a study of Thoroughbred broodmares and stallions in Germany identified horses aged 11-15 years as having the highest probability of active infection [15]. In the United States, a large molecular survey (n=1195) confirmed that EqPV-H-positive horses were significantly older than their negative counterparts [4]. This age association is often attributed to a cumulative lifetime exposure risk; older horses simply have had more opportunities for horizontal contact with viremic individuals. Additionally, age-related immunosenescence might lead to a greater likelihood of persistent, detectable viremia in geriatric horses compared to younger adults who may clear the virus more efficiently.

Sex and Breed Associations Evidence for a sex predilection is emerging, though findings are not yet universal across all studies. A large-scale study in the southern United States reported that male horses had 1.62 times the odds of being EqPV-H-positive compared to females [4]. This association was also noted in a study of clinically healthy horses in South Korea, which found that sex was significantly associated with infection (p = 0.006), with males showing a higher prevalence [12]. The biological basis for this male bias is not yet understood but could involve behavioral factors (e.g., more frequent or aggressive social interactions) or subtle underlying immunological differences. Breed-specific susceptibilities are also being identified. The same US study found that Tennessee Walking Horses had significantly higher odds of EqPV-H positivity (Odds Ratio = 2.46) compared to other breeds, while Quarter Horses and Thoroughbreds were at higher risk for EqHV, but not necessarily EqPV-H [4]. This suggests that breed-specific management practices, genetic factors, or geographic clustering may influence exposure dynamics.

Viral Load and Host Immunity The relationship between infection, viral load, and disease is nuanced. While EqPV-H is the etiologic agent of Theiler’s disease, most infections are subclinical. The host's immune response is a critical risk factor for disease outcome. The onset of hepatitis in experimental infections is temporally associated with seroconversion and a decline in viremia, suggesting that immune-mediated tissue damage is a key component of pathogenesis [6, 24]. Furthermore, an unprecedented longitudinal study of a single infected horse tracked the evolution of a hypervariable region in the capsid gene over 16 years, revealing a complex pattern of viral population dynamics with emerging, dominant, and receding variant strains [5]. This not only demonstrates the potential for lifelong persistence but also underscores the challenge this immune evasion mechanism poses for vaccine development. The risk of developing significant clinical hepatitis appears to be linked not just to the presence of the virus, but to the interplay between high viral loads, host genetics, and the immune response, as evidenced by the association of high viral load with centrilobular necrosis and portal infiltrates in natural infections [7, 28].

Geospatial Endemicity and Co-Infection Dynamics

EqPV-H has a truly global distribution, with serologic and molecular evidence of infection reported across North America [4, 16], South America [2, 8, 36], Europe [1, 10, 15], Asia [11, 12, 17, 25], and Australia [1, 13]. The prevalence of DNA in clinically healthy horses ranges from approximately 4% to 20%, while seroprevalence, indicating past exposure, can exceed 30% [4, 10, 15]. The prevalence in horses with clinical Theiler’s disease is dramatically higher, often approaching 100% in case series, reinforcing the causal link [18, 20].

Co-infection with other hepatotropic viruses, particularly equine hepacivirus (EqHV, also known as non-primate hepacivirus), is common and may represent a significant risk modifier. Co-infection rates are notably high, with 35.3% of EqPV-H positive horses in one Korean study also harboring EqHV [25], and a 58.8% co-infection rate reported in Chinese horses [17]. While EqPV-H is the primary suspect for acute fulminant Theiler’s disease, some cases of chronic hepatitis have been attributed to persistent EqHV infection, and the simultaneous presence of both viruses could theoretically synergize to promote more severe or progressive liver pathology [32]. The geopolitical context is also a risk factor, as the movement of horses for trade and breeding can introduce novel strains into naive populations. For instance, natural recombination events between Chinese and American EqPV-H strains have been documented in China, highlighting how international transport facilitates viral evolution and the potential introduction of more pathogenic variants [11, 41].

Prevention and Control Strategies for Equine Parvovirus-Hepatitis

The prevention and control of Equine Parvovirus-Hepatitis (EqPV-H) represent a multifaceted challenge that intersects with regulatory oversight of biological products, biosecurity protocols on breeding and training operations, and a fundamental understanding of viral transmission dynamics. Given that EqPV-H is the primary etiological agent of Theiler’s disease, a frequently fatal, acute hepatic necrosis, the imperative for robust, evidence-based control measures is profound. The strategies must address both the iatrogenic transmission pathway, which is the most clearly documented route of infection, and the more enigmatic natural horizontal transmission routes that sustain viral circulation within equine populations globally.

Regulatory Control of Equine Biological Products

The most direct and impactful intervention for preventing EqPV-H-associated disease is the rigorous screening and regulation of all equine-origin biological products. The seminal discovery of EqPV-H was made in a horse that died following administration of contaminated tetanus antitoxin [16], and subsequent investigations have consistently identified contaminated biologics, including tetanus antitoxin, commercial equine plasma, and allogeneic stem cell therapies, as potent sources of iatrogenic infection [6, 18, 24]. The United States Department of Agriculture (USDA) Center for Veterinary Biologics has established a critical regulatory precedent by mandating that all commercially licensed equine biologics derived from plasma or serum must test negative for EqPV-H [23]. This regulatory action has demonstrably reduced the prevalence of contaminated products; one study documented a decline in the fraction of equine biologic serials containing EqPV-H from 39.6% prior to regulation to 6.8% following implementation [3].

However, the efficacy of such screening programs is contingent upon the diagnostic methodology employed. The initial quantitative real-time PCR (qPCR) tests, while widely used, are susceptible to false-negative results due to co-extracted PCR inhibitors present in biological matrices [3]. Digital PCR (dPCR) has emerged as a more robust and reliable platform for detecting EqPV-H in these complex samples, offering greater tolerance to inhibitors and providing absolute quantification without the need for standard curves [3]. The adoption of dPCR for routine batch-release testing represents a significant opportunity to further decrease the residual risk of contaminated products entering the market. This is particularly critical given that EqPV-H, as a non-enveloped parvovirus, is resistant to the solvent/detergent inactivation methods commonly used in the manufacture of plasma derivatives [42]. Therefore, reliance on viral inactivation is insufficient; the cornerstone of prevention must be the exclusion of viremic donor animals from the production chain.

Biosecurity and Screening of Donor Herds

The high prevalence of subclinical EqPV-H infection in apparently healthy horses worldwide, ranging from 3.2% to 19.3% in various surveys [1, 4, 8, 10, 12, 15, 36], underscores the critical need for stringent biosecurity within donor herds. Horses used for the production of serum, plasma, or stem cell therapies must be subjected to rigorous, repeated screening for EqPV-H DNA using highly sensitive assays. The detection of EqPV-H DNA in commercial serum pools from the USA, Canada, New Zealand, Italy, and Germany [14], as well as in 60% of commercial horse serum batches tested in Argentina [2], highlights the global nature of this contamination risk. A single viremic donor can contaminate an entire production pool, leading to widespread iatrogenic exposure. Consequently, donor herds should be closed populations, with new introductions subjected to strict quarantine and testing protocols. The identification of risk factors for EqPV-H infection can inform targeted screening strategies. Epidemiological studies have consistently identified increasing age as a significant risk factor for active infection, with horses over 15 years old showing markedly higher odds of being EqPV-H DNA-positive compared to younger cohorts [4, 10, 15]. Furthermore, breed-specific predispositions have been observed, with Tennessee Walking Horses, Quarter Horses, and Thoroughbreds demonstrating elevated odds of infection in some populations [4]. Male sex has also been associated with a 1.62-fold increase in the odds of infection [4]. These demographic data can be used to prioritize testing resources and to manage high-risk animals within donor herds more carefully.

Management of Natural Transmission Routes

Beyond iatrogenic transmission, EqPV-H circulates within equine populations through natural horizontal routes, presenting a more complex challenge for on-farm control. The virus has been detected in a variety of bodily secretions and excretions, including oral and nasal fluids, feces, and semen [24, 25, 29, 30]. Experimental evidence has confirmed that intranasal inoculation can establish infection, with a prolonged eclipse phase of 6 to 12 weeks before viremia becomes detectable [26]. This extended period between exposure and detectable infection complicates quarantine protocols, as an exposed animal may appear negative for weeks before becoming infectious. Fecal-oral transmission is strongly supported by the detection of EqPV-H DNA in fecal samples and the demonstration of successful oral inoculation with viremic serum in experimental settings [24, 25]. The presence of viral DNA in semen also raises the possibility of venereal transmission, which could have implications for breeding management [29].

Given these transmission pathways, general biosecurity measures are paramount. Cohorting of horses by age and health status, minimizing the mixing of populations from different sources, and implementing good hygiene practices, including the disinfection of shared equipment and water sources, are foundational strategies. The resistance of non-enveloped parvoviruses to many common disinfectants necessitates the use of appropriate virucidal agents, such as accelerated hydrogen peroxide or bleach-based compounds, for environmental decontamination. The identification of EqPV-H in horses with no history of biologic administration and in outbreaks on farms where no biologics were used [20, 35] confirms that natural transmission alone can sustain infection and cause clinical disease. In one notable outbreak, 61.8% of horses on a farm were found to be EqPV-H DNA-positive, with transmission occurring in the absence of any equine-origin biologic product administration [35]. This highlights the potential for rapid spread within a herd once the virus is introduced.

Surveillance and Risk-Based Control

A comprehensive control strategy must be underpinned by robust surveillance to understand the local epidemiology and to identify high-risk populations. The global distribution of EqPV-H is now well-documented, with reports from North America, Europe, Asia, South America, and Australia [1, 2, 4, 8, 10, 11, 13-15, 17, 25, 30, 36]. The genetic diversity of the virus is relatively low, with high nucleotide identity (>94%) among strains from disparate geographical regions [8, 11, 17], suggesting a single, globally circulating serotype. This genetic stability is favorable for the development of diagnostic assays and potentially for future vaccine development. However, evidence of natural recombination events between Chinese and American strains has been documented, indicating that the virus is capable of genetic evolution [11]. Continued molecular surveillance is essential to monitor for the emergence of novel variants that might evade detection or alter pathogenic potential.

The World Organisation for Animal Health (WOAH) does not currently list EqPV-H as a notifiable disease, but given its economic impact on the equine industry through mortality, morbidity, and the contamination of valuable biological products, consideration of its inclusion in surveillance frameworks is warranted. For individual operations, a risk-based approach is recommended. High-risk populations include geriatric horses, breeding stock, and horses that are frequently transported or commingled. The association of EqPV-H infection with subclinical hepatitis and elevated liver enzymes in a significant proportion of infected horses [7, 28] suggests that routine monitoring of liver biochemistry in high-value or high-risk animals could serve as an adjunct to molecular testing. The finding that EqPV-H is common in a variety of liver pathologies beyond classic Theiler’s disease [7] further emphasizes the need for clinicians to maintain a high index of suspicion and to include EqPV-H testing in the diagnostic workup of any equine hepatitis case. Ultimately, the most effective control will integrate regulatory enforcement of biologic purity, stringent donor management, enhanced biosecurity to interrupt natural transmission, and ongoing surveillance to inform adaptive management strategies.

References

[1] Fortier C, El-Hage C, Normand C, Hue E, Sutton G, Marcillaud-Pitel C, et al.. Detection of Equine Parvovirus-Hepatitis Virus and Equine Hepacivirus in Archived Sera from Horses in France and Australia. Viruses. 2024. DOI: https://doi.org/10.3390/v16060862

[2] Olguin-Perglione C, Politzki R, Álvarez I, Ruiz V. First report of Equine Parvovirus-Hepatitis (EqPV-H) in Argentina.. The Veterinary Journal. 2024. DOI: https://doi.org/10.1016/j.tvjl.2024.106204

[3] Scupham A, Tong C. Detection of equine parvovirus-hepatitis and efficacy of governmental regulation for equine biologics purity. Journal of Veterinary Diagnostic Investigation. 2024. DOI: https://doi.org/10.1177/10406387241292343

[4] Barua S, Tarannum A, Huber L, Easterwood LA, Velayudhan B, Silveira BPd, et al.. Epidemiology and risk factors of equine parvovirus-hepatitis, hepacivirus, Pegivirus caballi, and Pegivirus equi in horses from the Southern United States.. Veterinary Microbiology. 2025. DOI: https://doi.org/10.1016/j.vetmic.2025.110831

[5] Scupham A. Equine Parvovirus-Hepatitis Population Dynamics in a Single Horse over 16 Years. Viruses. 2025. DOI: https://doi.org/10.3390/v17070947

[6] Ramsauer A, Badenhorst M, Cavalleri JV. Equine parvovirus hepatitis. Equine Veterinary Journal. 2021. DOI: https://doi.org/10.1111/evj.13477

[7] Jager MC, Choi E, Tomlinson J, Walle GRVd. Naturally acquired equine parvovirus-hepatitis is associated with a wide range of hepatic lesions in horses. Veterinary Pathology-Supplement. 2023. DOI: https://doi.org/10.1177/03009858231214024

[8] Moraes Md, Salgado CRS, Godoi TLOS, Almeida FQd, Chalhoub FLL, Filippis ABDd, et al.. Equine parvovirus-hepatitis is detected in South America, Brazil.. Transboundary and Emerging Diseases. 2021. DOI: https://doi.org/10.1111/tbed.14226

[9] Reinecke B, Klöhn M, Brüggemann Y, Kinast V, Todt D, Stang A, et al.. Clinical Course of Infection and Cross-Species Detection of Equine Parvovirus-Hepatitis. Viruses. 2021. DOI: https://doi.org/10.3390/v13081454

[10] Badenhorst M, Heus Pd, Auer A, Tegtmeyer B, Stang A, Dimmel K, et al.. Active equine parvovirus‐hepatitis infection is most frequently detected in Austrian horses of advanced age. Equine Veterinary Journal. 2020. DOI: https://doi.org/10.1111/evj.13444

[11] Lu G, Wu L, Ou J, Li S. Equine Parvovirus-Hepatitis in China: Characterization of Its Genetic Diversity and Evidence for Natural Recombination Events Between the Chinese and American Strains. Frontiers in Veterinary Science. 2020. DOI: https://doi.org/10.3389/fvets.2020.00121

[12] Lee S, Park D, Lee I. Molecular Prevalence of Equine Parvovirus-Hepatitis in the Sera of Clinically Healthy Horses in South Korea. Veterinary Sciences. 2021. DOI: https://doi.org/10.3390/vetsci8110282

[13] Fortier C, El-Hage C, Hue E, Sutton G, Pitel C, Jeffers K, et al.. Hepatitis viruses: prevalence of equine parvovirus‐hepatitis virus and equine hepacivirus in France and Australia. Equine Veterinary Journal. 2021. DOI: https://doi.org/10.1111/evj.103_13495

[14] Meister T, Tegtmeyer B, Postel A, Cavalleri JV, Todt D, Stang A, et al.. Equine Parvovirus-Hepatitis Frequently Detectable in Commercial Equine Serum Pools. Viruses. 2019. DOI: https://doi.org/10.3390/v11050461

[15] Meister T, Tegtmeyer B, Brüggemann Y, Sieme H, Feige K, Todt D, et al.. Characterization of Equine Parvovirus in Thoroughbred Breeding Horses from Germany. Viruses. 2019. DOI: https://doi.org/10.3390/v11100965

[16] Divers T, Tennant B, Kumar A, McDonough S, Cullen J, Bhuva NP, et al.. New Parvovirus Associated with Serum Hepatitis in Horses after Inoculation of Common Biological Product. Emerging Infectious Diseases. 2018. DOI: https://doi.org/10.3201/eid2402.171031

[17] Lu G, Sun L, Ou J, Xu H, Wu L, Li S. Identification and genetic characterization of a novel parvovirus associated with serum hepatitis in horses in China. Emerging Microbes and Infections. 2018. DOI: https://doi.org/10.1038/s41426-018-0174-2

[18] Tomlinson J, Kapoor A, Kumar A, Tennant B, Laverack M, Beard L, et al.. Viral testing of 18 consecutive cases of equine serum hepatitis: A prospective study (2014‐2018). Journal of Veterinary Internal Medicine. 2018. DOI: https://doi.org/10.1111/jvim.15368

[19] Pusterla N, James K, Barnum S, Delwart E. Investigation of Three Newly Identified Equine Parvoviruses in Blood and Nasal Fluid Samples of Clinically Healthy Horses and Horses with Acute Onset of Respiratory Disease. Animals. 2021. DOI: https://doi.org/10.3390/ani11103006

[20] Tomlinson J, Tennant B, Struzyna A, Mrad D, Browne NS, Whelchel D, et al.. Viral testing of 10 cases of Theiler's disease and 37 in‐contact horses in the absence of equine biologic product administration: A prospective study (2014‐2018). Journal of Veterinary Internal Medicine. 2018. DOI: https://doi.org/10.1111/jvim.15362

[21] Ou J, Li J, Wang X, Zhong L, Xu L, Xie J, et al.. Genetic characterization of three recently discovered parvoviruses circulating in equines in China. Frontiers in Veterinary Science. 2022. DOI: https://doi.org/10.3389/fvets.2022.1033107

[22] Yoon J, Park T, Kim A, Song H, Park B, Ahn H, et al.. First Detection and Genetic Characterization of New Equine Parvovirus Species Circulating among Horses in Korea. Veterinary Sciences. 2021. DOI: https://doi.org/10.3390/vetsci8110268

[23] Ruiz V, Alvarez I. Parvovirus Equino-Hepatitis (EqPV-H): un nuevo virus contaminante de productos biológicos de origen veterinario. Revista Veterinaria. 2023. DOI: https://doi.org/10.30972/vet.3427058

[24] Tomlinson J, Jager MC, Struzyna A, Laverack M, Fortier L, Dubovi E, et al.. Tropism, pathology, and transmission of equine parvovirus-hepatitis. Emerging Microbes and Infections. 2020. DOI: https://doi.org/10.1080/22221751.2020.1741326

[25] Yoon J, Park T, Kim A, Song H, Park B, Ahn H, et al.. First report of equine parvovirus-hepatitis and equine hepacivirus co-infection in horses in Korea.. Transboundary and Emerging Diseases. 2021. DOI: https://doi.org/10.1111/tbed.14425

[26] Tomlinson J, Walle GRVd. Nasal transmission of equine parvovirus hepatitis. Journal of Veterinary Internal Medicine. 2022. DOI: https://doi.org/10.1111/jvim.16569

[27] Vengust M, Jager MC, Zalig V, Cociancich V, Laverack M, Renshaw R, et al.. First report of Equine Parvovirus-Hepatitis-associated Theiler’s disease in Europe. Equine Veterinary Journal. 2020. DOI: https://doi.org/10.1111/evj.13254

[28] Jager MC, Tomlinson J, Henry C, Fahey MJ, Walle GRVd. Prevalence and pathology of equine parvovirus-hepatitis in racehorses from New York racetracks. Virology Journal. 2022. DOI: https://doi.org/10.1186/s12985-022-01901-3

[29] Yoon J, Park T, Kim A, Park J, Park B, Ahn H, et al.. Molecular surveillance of equine parvovirus-hepatitis from oral, nasal, vaginal, and semen specimens collected from horses living in Korea.. Transboundary and Emerging Diseases. 2022. DOI: https://doi.org/10.1111/tbed.14746

[30] Yoon J, Park T, Kim A, Park J, Park B, Ahn H, et al.. First Clinical Case of Equine Parvovirus-Hepatitis-Related Theiler’s Disease in Asia. Viruses. 2021. DOI: https://doi.org/10.3390/v13101917

[31] Zehetner V, Cavalleri JV, Klang A, Hofer M, Preining I, Steinborn R, et al.. Equine Parvovirus-Hepatitis Screening in Horses and Donkeys with Histopathologic Liver Abnormalities. Viruses. 2021. DOI: https://doi.org/10.3390/v13081599

[32] Jager MC, Luethy D, Shallop S, Cathcart J, Divers T, Tan J, et al.. Chronic hepatitis in horses with persistent equine hepacivirus infection. Equine Veterinary Journal. 2025. DOI: https://doi.org/10.1111/evj.70124

[33] Bihari C, Rastogi A, Saxena P, Rangegowda D, Chowdhury A, Gupta N, et al.. Parvovirus B19 Associated Hepatitis. Hepatitis Research and Treatment. 2013. DOI: https://doi.org/10.1155/2013/472027

[34] Alves A, Langella B, Lima MMdS, Coelho WLdCNP, Garcia RdCNC, Cardoso C, et al.. Evaluation of Molecular Test for the Discrimination of “Naked” DNA from Infectious Parvovirus B19 Particles in Serum and Bone Marrow Samples. Viruses. 2022. DOI: https://doi.org/10.3390/v14040843

[35] Baird J, Tegtmeyer B, Arroyo L, Stang A, Brüggemann Y, Hazlett M, et al.. The association of Equine Parvovirus-Hepatitis (EqPV-H) with cases of non-biologic-associated Theiler's disease on a farm in Ontario, Canada.. Veterinary Microbiology. 2020. DOI: https://doi.org/10.1016/j.vetmic.2019.108575

[36] Picetti TS, Figueiredo AS, Segalin JT, Mostardeiro NL, Almeida GDd, Frandoloso R, et al.. Molecular prevalence of equine parvovirus hepatitis in healthy horses from the Northern region of the state of Rio Grande do Sul, Brazil. Veterinary research communications. 2026. DOI: https://doi.org/10.1007/s11259-026-11251-y

[37] Meister T, Arroyo L, Shanahan RA, Papapetrou MA, Reinecke BM, Brüggemann Y, et al.. Infection of young foals with Equine Parvovirus-Hepatitis following a fatal non-biologic case of Theiler's disease.. Veterinary Microbiology. 2022. DOI: https://doi.org/10.1016/j.vetmic.2022.109557

[38] Mann S, Ramsay JD, Wakshlag J, Stokol T, Reed S, Divers T. Investigating the pathogenesis of high serum gamma-glutamyl transferase activity in Thoroughbred racehorses: a series of case-control studies.. Equine Veterinary Journal. 2021. DOI: https://doi.org/10.1111/evj.13435

[39] Ramsay JD, Evanoff RM, Mealey R, Simpson EL. The prevalence of elevated gamma-glutamyltransferase and sorbitol dehydrogenase activity in racing Thoroughbreds and their associations with viral infection.. Equine Veterinary Journal. 2019. DOI: https://doi.org/10.1111/evj.13092

[40] Tomlinson J, Walle GRVd, Divers T. What Do We Know About Hepatitis Viruses in Horses?. The Veterinary clinics of North America. Equine practice. 2019. DOI: https://doi.org/10.1016/j.cveq.2019.03.001

[41] Xie J, Tong P, Zhang A, Song X, Zhang L, Shaya N, et al.. An emerging equine parvovirus circulates in thoroughbred horses in north-Xinjiang, China, 2018.. Transboundary and Emerging Diseases. 2019. DOI: https://doi.org/10.1111/tbed.13443

[42] Alves A, Magaldi M, Menezes AD, Lopes JIF, Silva CAdC, Oliveira JMd, et al.. Incidence and estimated risk of residual transmission of hepatitis a virus and parvovirus B19 by blood transfusion in the state of Rio De Janeiro – Brazil: a retrospective study. Virology Journal. 2025. DOI: https://doi.org/10.1186/s12985-025-02627-8