Border Disease Virus

Overview and Taxonomy of Border Disease Virus

Historical Context and Nomenclature

Border disease virus (BDV) represents a globally significant pestivirus of the family Flaviviridae, first recognized as the etiological agent of a congenital disease syndrome in lambs originating from the border region of England and Wales in 1959 [2, 28]. The earliest descriptions of this condition, characterized by tremors, abnormal fleece development ("hairy shaker" lambs), and poor growth, predated the molecular era, and the causative agent was initially designated simply as "border disease virus" based on geographic association and the unique clinical presentation in ovine neonates [2]. For decades, the taxonomic classification of pestiviruses relied heavily upon the host species from which a given isolate was derived; accordingly, BDV was considered the prototypical pestivirus of sheep and goats, whereas bovine viral diarrhea virus (BVDV) was associated with cattle and classical swine fever virus (CSFV) with swine [26, 28]. This host-centric system, while pragmatically useful, rapidly proved inadequate as molecular characterization tools, particularly reverse transcription PCR and comparative sequencing of the 5′ untranslated region (5′-UTR) and the Npro gene, revealed that BDV not only infects a broad range of ungulates but also that many pestivirus isolates from cattle, pigs, and wildlife cluster phylogenetically with canonical ovine BDV strains [13, 26, 28].

The International Committee on Taxonomy of Viruses (ICTV) has since implemented a systematic nomenclature for the genus Pestivirus, replacing the traditional species names with a standardized alphanumeric format: pestivirus A through pestivirus K and beyond [7, 28]. Under this system, border disease virus is formally designated Pestivirus D. This reclassification reflects a fundamental shift away from host-based naming and toward a phylogenetically robust framework that acknowledges the extensive interspecies transmission and genetic plasticity that characterize this viral group [7, 28]. Nevertheless, the historical name "border disease virus" remains in widespread use in the veterinary literature and diagnostic communities, and both terms, BDV and Pestivirus D, are employed interchangeably depending on context.

Phylogenetic Placement and Genetic Heterogeneity

BDV is a positive-sense, single-stranded RNA virus with a genome of approximately 12.3 kb, comprising a single open reading frame flanked by highly structured 5′ and 3′ untranslated regions [16, 28]. The 5′-UTR, along with the Npro gene encoding a nonstructural autoprotease, serves as the primary target for genotyping due to its relatively conserved nature within species yet sufficient variability to discriminate among genotypes and subgenotypes [1, 3, 23]. Phylogenetic analyses conducted over the past two decades have delineated at least eight distinct BDV genotypes (BDV-1 through BDV-8), with numerous subgenotypes recognized within each major clade [3, 13, 15]. The distribution of these genotypes is non-random and exhibits pronounced geographic structure. BDV-1 and BDV-2 are predominantly associated with European small ruminant populations, whereas BDV-3, BDV-4, and more recently described genotypes such as BDV-5 through BDV-8 have been identified across Asia, the Middle East, and North Africa, reflecting independent evolutionary trajectories shaped by regional livestock movements and interspecies transmission events [1, 3, 16, 23].

The type species within Pestivirus D exhibits a mean evolutionary rate estimated at approximately (2.9 \times 10^{-3}) substitutions per site per year, based on Bayesian coalescent analysis of 5′-UTR sequences from Pyrenean chamois [15]. This rate is comparable to that of other RNA viruses and underpins the capacity of BDV to generate substantial genetic diversity over relatively short time scales. Notably, the BDV-4 clade has emerged as a particularly dynamic lineage; within this group, a distinct subclade designated BDV-4a has been associated with epizootic outbreaks in Pyrenean chamois (Rupicapra pyrenaica pyrenaica) in Spain, France, and Andorra, with estimated divergence from ovine ancestors dating to the early 1990s [15, 27]. The dramatic population-level impacts of BDV-4 on chamois, including localized population declines of more than 50%, underscore the capacity of this virus to cross species barriers and cause emergent disease in naive wildlife populations [15, 17].

Classification Challenges and the Problem of Host Range

One of the most salient features of BDV taxonomy is the blurring of boundaries between pestivirus species due to frequent interspecies transmission. BVDV and BDV are antigenically and genetically closely related; indeed, the nucleocapsid protein and envelope glycoproteins share sufficient amino acid homology that serological cross-reactivity is the rule rather than the exception [3, 5, 14]. This homology poses significant diagnostic and epidemiological challenges. Monoclonal antibody panels, while useful for initial antigenic characterization, often fail to provide unequivocal species discrimination, particularly when applied to field isolates from atypical hosts such as cattle or pigs [5, 19]. Molecular characterization through sequencing of the 5′-UTR or Npro region remains the gold standard for definitive taxonomic assignment, but even these regions can exhibit mosaic patterns or ambiguous clustering for certain isolates, suggesting the possibility of historical recombination events [13, 19].

The capacity of BDV to infect swine further complicates classification. Pigs are susceptible to infection with BDV, BVDV, and CSFV, and the resulting antibody responses are cross-reactive in many serological assays [2, 5, 6]. This has direct implications for classical swine fever (CSF) surveillance: BDV-infected pigs can generate false-positive reactions in CSFV-specific serological tests, potentially triggering costly and disruptive control measures in naive populations [2, 6]. Recent investigations have identified BDV subgenotype 1b in pigs in England and BDV genotype 3 (strain Gifhorn) originally isolated from pigs in Germany, confirming that this virus circulates in swine populations in the absence of classical swine fever [2, 22]. The European Food Safety Authority and the World Organisation for Animal Health (WOAH) have emphasized that such spillover events must be accounted for in the design of surveillance programs for CSFV and BVDV, as failure to do so may compromise the integrity of eradication campaigns [3, 14, 20].

The Concept of Persistently Infected Animals and Its Taxonomic Implications

A defining biological feature of BDV infection, with profound implications for both taxonomy and epidemiology, is the establishment of persistent infection (PI) following in utero exposure before the development of immunological competence (typically before 60–80 days of gestation in sheep and goats) [1, 3, 9, 18]. PI animals are immunotolerant to the infecting viral strain, shed virus continuously throughout life, and serve as the primary reservoir for maintaining BDV within populations [3, 16]. The detection of PI individuals is a hallmark of field investigations; studies in Greece [1], Iraq [9], China [16, 23], and Turkey [21] have all identified PI sheep and goats through concurrent detection of viral RNA and absence of specific antibodies. The PI state is not limited to small ruminants: experimental and field data have demonstrated that cattle can also become persistently infected with BDV, with the same consequences for virus shedding and transmission [18, 26]. In Switzerland, the identification of BDV genotype BDSwiss in a persistently infected calf led to transmission of the virus to contact heifers and subsequent establishment of PI fetuses, directly mimicking the epidemiology of BVDV [18]. This phenomenon has forced a reevaluation of the assumption that BDV is of minor importance in cattle; instead, the virus must be considered a potential threat to BVDV eradication programs, as PI cattle infected with BDV are indistinguishable from those infected with BVDV in many antigen-detection assays [14, 18, 26].

Geographic and Host Range Distribution

BDV exhibits a truly global distribution, with molecular and serological evidence of infection reported across Europe, Asia, Africa, and North America [3, 4, 9-11, 25]. The primary hosts remain domestic sheep and goats, but the virus has been identified in cattle, pigs, wild boar (Sus scrofa), chamois, bison, and even in the sheep ked (Melophagus ovinus), a blood-feeding ectoparasite that may serve as a mechanical vector [8, 15, 26, 27]. The first detection of BDV in wild boar in Turkey [7] and the identification of BDV in Melophagus ovinus in Xinjiang, China [8], highlight the breadth of ecological niches this virus can occupy. The presence of BDV in ectoparasites raises questions about potential non-venereal, non-respiratory transmission routes, although the epidemiological significance of such vectors remains to be fully determined.

The economic toll exacted by BDV is substantial: reproductive losses, abortion, birth of PI lambs, and poor growth performance contribute to significant financial burdens for sheep and goat producers worldwide [3, 12, 24]. The World Organisation for Animal Health (WOAH) includes border disease in its list of notifiable diseases for small ruminants, and the Food and Agriculture Organization (FAO) has identified BDV as a pathogen that undermines food security in resource-limited settings where diagnostic capacity is constrained [4]. The integration of BDV surveillance into broader pestivirus monitoring programs is now recognized as essential given the potential for this virus to undermine costly BVDV and CSFV eradication schemes through cross-species transmission and serological interference [14, 20, 26].

Summary of Taxonomic Framework

In summary, Border disease virus (Pestivirus D) is a member of the genus Pestivirus within the family Flaviviridae, comprising at least eight recognized genotypes with complex geographical and host-specific patterns. Its taxonomy has evolved from a simplistic host-based system to a robust phylogenetic classification that acknowledges the virus’s remarkable capacity for interspecies transmission and persistent infection. The continued discovery of novel BDV genotypes in both domestic and wildlife hosts, coupled with the detection of BDV in vectors such as sheep keds, underscores the dynamic nature of this pathogen. Genotyping efforts targeting the 5′-UTR and Npro regions remain central to taxonomic assignment, but the molecular diversity of BDV demands ongoing surveillance and revision of classification schemes as new isolates are characterized. The clear phylogenetic and antigenic overlap with other pestiviruses, particularly BVDV and CSFV, necessitates that BDV be considered an integral component of any comprehensive pestivirus control strategy, not merely a pathogen of minor or localized significance.

Molecular Pathogenesis and Persistent Infection Mechanisms

Border disease virus (BDV), taxonomically classified as Pestivirus D within the family Flaviviridae, is a prototypical pestivirus that exhibits a sophisticated molecular strategy for establishing lifelong, persistent infections in its natural ovine and caprine hosts [3, 4, 28]. The virus’s capacity for persistence is not merely a clinical curiosity but the cornerstone of its epidemiology, enabling its maintenance within populations and driving substantial economic losses globally through reproductive failure, neonatal mortality, and the production of poorly viable offspring [1, 9, 12, 19]. Understanding the molecular underpinnings of this persistence, from the structure of the viral genome to the subversion of host innate immunity and the induction of immunotolerance, is critical for devising effective control strategies, particularly given the virus’s propensity for interspecies transmission, which complicates eradication programs for bovine viral diarrhea virus (BVDV) and classical swine fever virus (CSFV) [3, 14, 18, 20].

Genomic Architecture and the Molecular Basis of Biotype

The BDV genome is a single-stranded, positive-sense RNA molecule of approximately 12.2–12.3 kilobases, characterized by a single open reading frame (ORF) flanked by highly structured 5′ and 3′ untranslated regions (UTRs) [16, 28]. The 5′-UTR contains an internal ribosome entry site (IRES) essential for cap-independent translation initiation, a feature conserved across the Flaviviridae. Translation of the ORF yields a single polyprotein, which is co- and post-translationally cleaved by viral and host proteases into four structural proteins (C, Eʳⁿˢ, E1, E2) and eight nonstructural proteins (Nᵖʳᵒ, p7, NS2, NS3, NS4A, NS4B, NS5A, NS5B) [28]. The nonstructural protein Nᵖʳᵒ is a singularly important virulence factor: it is a viral autoprotease that, in addition to cleaving itself from the nascent polyprotein, acts as a potent antagonist of the host type I interferon (IFN) system. Nᵖʳᵒ achieves this by targeting interferon regulatory factor 3 (IRF3) for proteasomal degradation, thereby blocking the transcriptional induction of IFN-β and establishing a permissive environment for viral replication from the earliest stages of infection [28]. This direct countermeasure against innate immunity is a key enabler of viral spread before the adaptive immune response is mounted.

A hallmark of BDV pathogenesis, shared with BVDV, is the existence of two viral biotypes: noncytopathic (ncp) and cytopathic (cp). The ncp biotype is the predominant form found in nature and is uniquely capable of establishing persistent infection. It replicates in cultured cells without causing detectable cell death. In contrast, the cp biotype arises from the ncp biotype through specific genetic rearrangements, most commonly duplications or insertions of cellular ubiquitin sequences or other pestiviral sequences within the NS2-3 gene region [28]. The cp biotype is associated with the induction of apoptosis in infected cells and is responsible for the acute, often fatal, form of mucosal disease-like pathology in sheep, analogous to its role in cattle. Critically, cp virus can only emerge in an animal already persistently infected with an ncp strain; the cp variant is the trigger for clinical disease but cannot, by itself, initiate a new persistent infection [28]. This dichotomy underscores the molecular evolution within a single host, where the ncp biotype maintains the carrier state and the cp biotype causes terminal illness.

Induction and Maintenance of Persistent Infection: Immunotolerance

The central paradigm of BDV persistence is the establishment of specific immunotolerance following in utero infection. The virus exploits the developmental window of the fetal immune system before it is immunocompetent, typically before day 60 of gestation in sheep. If a pregnant ewe is infected with ncp BDV during this critical period, the virus crosses the placenta and infects the fetus [12, 17, 18, 23]. Because the fetal immune system recognizes the viral proteins as "self," it fails to mount a neutralizing antibody response. Consequently, the offspring is born persistently infected (PI), lifelong viremic, and seronegative for anti-pestivirus antibodies [1, 9, 16].

The PI animal is the central epidemiological unit of BDV, serving as a continuous source of viral shedding via all secretions and excretions throughout its life. These animals often appear clinically normal at birth but later exhibit poor growth, ill-thrift, and a reduced lifespan, acting as "Trojan horses" within flocks [1, 16]. Diagnostic identification of PI animals relies on the detection of viral antigen or RNA in the absence of specific antibodies; the classic profile is antigen-ELISA or RT-PCR positive and antibody-ELISA negative [1, 9, 16]. For example, in a Greek study, one animal displaying this precise diagnostic profile was identified as PI, confirming its role as a likely reservoir for the BDV-4 lineage on that farm [1]. Similarly, studies in Algeria and Iraq have reported detection of PI animals, albeit at low prevalence (0–1.4%), indicating that even a small number of PI carriers can sustain endemicity [9, 10]. The molecular mechanism maintaining this state involves continuous viral replication in the face of a compromised adaptive immune system. The virus replicates primarily in lymphoid tissues, but its presence is tolerated, and no inflammatory response is mounted against it. The Nᵖʳᵒ-mediated blockade of IFN induction is particularly critical in the fetus, as it prevents a crucial innate antiviral barrier during the establishment of tolerance [28].

Cellular and Molecular Pathogenesis: Neuropathology and Oxidative Stress

Despite the quiet nature of the carrier state, BDV infection can cause profound pathological changes, particularly in the central nervous system (CNS) of developing fetuses and neonates. The condition "hypomyelinogenesis," characterized by a deficiency in myelin production in the brain and spinal cord, leads to the characteristic tremors and ataxia seen in newborn lambs ("hairy shaker" lambs). At the molecular level, this has been linked to dysregulation of key proteins. A seminal study demonstrated significantly increased expression of neuronal nitric oxide synthase (nNOS), A Disintegrin And Metalloprotease with Thrombospondin type I repeats-13 (ADAMTS-13), and neurofilament (NF) in the brains of BDV-infected small ruminants [29]. The upregulation of nNOS and subsequent nitric oxide (NO) production are hypothesized to contribute to oxidative stress and neuronal damage, while elevated ADAMTS-13, a protease involved in cleaving von Willebrand factor, may play a role in regulating the CNS microenvironment during neurodegeneration [29]. The correlation between NF expression and disease severity suggests that these molecular markers are not merely bystanders but active participants in the neurodegenerative cascade.

Further evidence of systemic pathogenesis comes from analyses of the acute-phase response and oxidative stress biomarkers. In a study on goats, abortion associated with BDV infection was correlated with significantly elevated plasma levels of malondialdehyde (MDA) and nitric oxide (NO), coupled with decreased erythrocyte superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) activities [24]. Concurrently, serum amyloid A (SAA) and haptoglobin (Hp) levels were markedly higher in aborting animals. These findings indicate that BDV infection induces a state of heightened oxidative stress and a pronounced systemic inflammatory response, which directly correlates with the severity of reproductive pathology [24]. The virus’s ability to induce apoptosis in infected cells, particularly in the placenta and fetal tissues, is another key pathogenic mechanism. While the ncp biotype initially avoids cytopathology, the emergence of a cp biotype or the overwhelming innate immune signaling can trigger caspase-dependent apoptotic pathways, leading to tissue damage and abortion [12, 28].

Interspecies Transmission and Persistent Infection in Non-Ovine Hosts

While sheep are the primary reservoir, BDV displays a remarkably broad host tropism, causing persistent infections in cattle, goats, pigs, and wildlife, a feature that significantly complicates control efforts [2, 3, 13, 26]. The molecular basis for this broad tropism lies in the conservation of the pestiviral receptor CD46 across cloven-hoofed animals, which interacts with the E2 glycoprotein to mediate viral entry [28]. The E2 glycoprotein is also a major target for neutralizing antibodies, and cross-reactivity between BDV, BVDV, and CSFV antibodies poses a major diagnostic challenge in BVDV and CSFV eradication programs [5, 6]. Notably, research has identified a common conformational epitope on E2 that is shared between CSFV and BDV, explaining the observed serological cross-reactivity and underscoring the need for differential diagnostic assays in pigs [5].

In cattle, BDV can establish persistent infections identical to those in sheep. Experimentally, co-housing of seronegative heifers in early pregnancy with a PI calf led to seroconversion in all heifers and, critically, the birth of PI fetuses in three of them [18]. This demonstrates that PI cattle can efficiently transmit BDV to other cattle, creating a self-sustaining cycle independent of the ovine reservoir. The risk is particularly acute for BVDV-free herds. In Switzerland, where a robust BVDV eradication program is in place, serological surveillance revealed that 6.7% of seropositive cattle had significantly higher neutralizing antibody titers against BDV than BVDV, with common housing of cattle and sheep being the most significant risk factor [14]. This finding indicates that BDV from small ruminants is a tangible threat to the integrity of BVDV eradication efforts, as it can cause false-positive serological results and even induce fetal infection in cattle [14, 20, 26].

BDV’s ability to infect pigs, as demonstrated by several outbreaks, is also of major concern. An outbreak in England linked to a mixed pig and sheep holding resulted in poor growth and anaemia in pigs, with the virus (subgenotype 1b) detected via panviral microarray and confirmed by immunohistochemistry [2]. The genetic proximity of BDV to CSFV, coupled with its ability to replicate in swine, can lead to misdiagnosis of classical swine fever, a notifiable disease to the World Organisation for Animal Health (WOAH), creating significant economic and trade repercussions [3, 6]. Furthermore, BDV has been documented in wildlife, notably in Pyrenean chamois (Rupicapra pyrenaica pyrenaica), where a specific BDV-4a lineage jumped from sheep in the early 1990s. This spillover event triggered a founder effect and resulted in massive population declines, with phylogeographic analyses demonstrating a westward and southward spread of the virus at a rate of ~13 km/year [15, 17, 27]. The virus’s ability to cause high mortality and reduce population viability in a naive wildlife species highlights its immense pathogenic potential [17]. Additionally, BDV RNA has been identified in Melophagus ovinus (sheep ked), raising the possibility of a mechanical or biological vector that could facilitate inter-herd and interspecies transmission [8].

In summary, the molecular pathogenesis of BDV is a masterpiece of viral adaptation. It uses the Nᵖʳᵒ protein to disable the innate immune response, exploits the developmental window of the fetal immune system to guarantee lifelong persistence, and relies on a plastic genome capable of generating highly pathogenic cytopathic variants. Its broad host tropism, mediated by conserved entry receptors, allows it to colonize multiple livestock and wildlife species, creating complex multi-host reservoirs that directly undermine national eradication programs for other economically critical pestiviruses [14, 18, 20, 26]. This molecular versatility makes BDV a persistent and formidable challenge to animal health authorities worldwide.

Genetic Diversity and Global Distribution of BDV Genotypes

Border disease virus (BDV), classified as Pestivirus D within the family Flaviviridae, exhibits a remarkable degree of genetic heterogeneity that has profound implications for its epidemiology, host range, and control. The genetic diversity of BDV is manifested in at least seven distinct genotypes (BDV-1 through BDV-7), with several subgenotypes further complicating the phylogenetic landscape. This diversity arises from the error-prone nature of the viral RNA-dependent RNA polymerase, coupled with frequent interspecies transmission events that create opportunities for viral evolution and adaptation in novel hosts [3, 28]. Understanding the global distribution and genetic relationships among BDV strains is not merely an academic exercise; it is essential for designing effective diagnostic assays, predicting cross-species transmission risk, and implementing surveillance programs that can differentiate BDV from other pestiviruses such as bovine viral diarrhea virus (BVDV) and classical swine fever virus (CSFV) [3, 14, 28].

Genotypic Classification and Molecular Basis of Heterogeneity

The phylogenetic classification of BDV genotypes has historically relied on analysis of the highly conserved 5′-untranslated region (5′-UTR) and the N-terminal protease (Npro) gene, both of which provide sufficient resolution for genotypic discrimination while maintaining adequate conservation for robust primer design [3, 15, 16]. The 5′-UTR, in particular, serves as the primary target for molecular epidemiological studies due to its secondary structure conservation and the availability of extensive reference sequences from diverse geographic regions. More recently, complete genome sequencing has allowed for more refined phylogenetic analyses, confirming the existence of multiple genotypes and revealing previously unrecognized subgenotypic diversity [13, 22, 31].

The accepted genotype nomenclature currently comprises seven distinct clades. BDV-1 is the most widely distributed genotype globally, having been identified in Europe, Asia, and the Americas [3, 7, 11]. Within BDV-1, subgenotypes 1a and 1b have been characterized, with BDV-1b strains associated with interspecies transmission events, including the cross-species spillover from sheep to pigs documented in the United Kingdom [2]. BDV-2 is primarily associated with European isolates, while BDV-3 has emerged as a significant genotype in China, where it has been identified in both sheep and goats [16, 23, 31]. BDV-4 is notably associated with the Pyrenean chamois (Rupicapra pyrenaica pyrenaica) epizootics in Spain and France, although it also circulates in domestic sheep populations [15, 17, 27]. The remaining genotypes (BDV-5, BDV-6, and BDV-7) have been described more recently from limited geographic areas, reflecting ongoing surveillance efforts and the continued discovery of novel genetic variants [3].

The evolutionary rate of BDV has been estimated at approximately 2.9 × 10⁻³ substitutions per site per year based on Bayesian Markov Chain Monte Carlo analysis of 5′-UTR sequences from Pyrenean chamois [15]. This rate, while consistent with other RNA viruses, places BDV in a position of moderate evolutionary dynamism that allows for measurable genetic drift over relatively short time scales. Phylogeographic reconstructions suggest that BDV was introduced into chamois populations from domestic sheep in the early 1990s, with subsequent spatial spread occurring at a rate of approximately 13.1 km/year across the Pyrenees [15]. This founder effect, whereby a limited subset of ovine BDV-4 strains gave rise to the entire chamois-associated clade, demonstrates how interspecies transmission can fundamentally reshape viral population structure and generate new epidemiological dynamics [15, 17].

Global Distribution Patterns and Regional Genotype Predominance

The distribution of BDV genotypes exhibits clear geographic structuring, with certain genotypes predominating in specific regions while others demonstrate broader dissemination. BDV-1 has been reported across an extraordinarily wide geographic range, encompassing Europe, Asia, and the Americas. In Turkey, BDV-1 was identified in a wild boar (Sus scrofa ferus) from the Burdur region, representing the first molecular evidence of BDV in Turkish wildlife and highlighting the potential role of wild suids as sentinels for pestivirus circulation [7]. Similarly, BDV-1 has been detected in cattle in Mexico, where border disease is classified as an exotic disease, raising questions about the origin of these strains and the potential for undetected circulation in the region [11]. The presence of BDV-1 in Mexican cattle, despite the absence of reported clinical disease, suggests that the virus may have been introduced through imported livestock or contaminated biological products and may be circulating at low levels that evade routine surveillance [11].

BDV-2 has been predominantly documented in European sheep populations, although its precise geographic boundaries remain incompletely defined. The genotype appears to be less widely distributed than BDV-1, possibly reflecting more recent emergence or more restricted host range [3]. In contrast, BDV-3 has emerged as the dominant genotype in China, where it has been isolated from both sheep and goats with diarrhea and persistent infections [16, 23, 31]. The Chinese BDV-3 strains, including JSLS12-01 isolated from sheep in Jiangsu Province, share high sequence homology with the German strain Gifhorn, which was originally isolated from pigs [16, 22]. This genetic relatedness across vast geographic distances and different host species suggests that BDV-3 may possess particular adaptive advantages that facilitate long-distance dissemination and cross-species transmission.

BDV-4 has been the focus of intensive study due to its association with mass mortality events in Pyrenean chamois. The genotype was first detected in chamois in Spain during the 2001 epizootic, which caused approximately 3,000 deaths in northeastern Spain between 2005 and 2007 [27]. Phylogenetic analysis has demonstrated that all chamois isolates form a highly significant monophyletic clade within the BDV-4a ovine group, indicating a single introduction event from domestic sheep [15]. The virus subsequently underwent rapid spatial expansion, spreading westward for more than 125 km and southward for approximately 50 km [15]. Population viability analysis has confirmed that BDV-4 infection doubles the risk of extinction for chamois populations over a 50-year period, underscoring the devastating impact of this genotype on wildlife [17]. More recently, BDV-4 has been identified in sheep in Greece, representing the first molecular detection of BDV in that country and suggesting that this genotype may be more widespread in European small ruminant populations than previously recognized [1].

Emerging Genotypes and the Role of Wildlife Reservoirs

The detection of BDV in wildlife species has expanded our understanding of viral genetic diversity and revealed novel transmission pathways. In addition to the well-characterized chamois epizootics, BDV has been identified in wild boar in Turkey, providing the first molecular evidence of BDV infection in this species in the region [7]. The wild boar isolate, TR/Burdur/13/Boar3, clustered with BDV-1, a genotype commonly found in domestic sheep and goats in Europe and Asia. The presence of BDV in wild boar raises important questions about the role of suids as potential reservoirs and bridging hosts for pestiviruses. Wild boar populations often exhibit overlapping habitat use with domestic sheep and goats, creating opportunities for bidirectional virus transmission. Furthermore, wild boar may serve as sentinel species, providing early warning of pestivirus circulation in regions where surveillance of domestic livestock is limited.

The detection of BDV in Melophagus ovinus (sheep ked) in Xinjiang, China, represents a novel dimension in our understanding of BDV ecology and potential transmission mechanisms [8]. The BDV-3 strain identified in sheep keds shared high nucleotide identity with a goat-derived strain from Anhui Province, suggesting that ectoparasites may play a role in virus dissemination within and between flocks [8]. While the primary mode of BDV transmission is horizontal through direct contact and vertical from persistently infected dams to offspring, the presence of infectious virus in blood-feeding ectoparasites introduces the possibility of mechanical vector-borne transmission. However, further experimental studies are needed to confirm whether sheep keds can transmit BDV to naïve hosts or whether the detection of viral RNA merely reflects ingestion of infected blood.

The genetic relationship between BDV and other pestiviruses, including the Tunisian sheep-like virus (TSV) and Bungowannah virus, adds another layer of complexity to the taxonomic and epidemiological picture. TSV, which is more closely related to CSFV than to BDV, can infect pigs and produce cross-reactive antibodies that interfere with CSFV serological diagnosis [6]. This antigenic cross-reactivity underscores the importance of genotyping for accurate diagnosis and highlights the potential for novel pestiviruses to complicate established surveillance programs. The close phylogenetic relationship between TSV and CSFV, combined with the ability of TSV to replicate in porcine cells, suggests that the genetic diversity of pestiviruses may be greater than currently recognized and that additional genotypes may await discovery in under-sampled regions [6].

Cross-Species Transmission and Implications for Eradication Programs

The interspecies transmission of BDV between sheep, cattle, and pigs is a well-documented phenomenon with significant implications for pestivirus control programs [3, 13, 26]. In Switzerland, three BDV strains of the same subgenotype (BDSwiss) were isolated from sheep, cattle, and pigs, providing direct evidence of cross-species transmission within a limited geographic area [13]. The complete genome sequences of these strains confirmed their genetic identity, ruling out the possibility of independent introductions from different sources. This finding has practical consequences for the Swiss BVD eradication program, which relies on serological surveillance to detect ongoing virus circulation. Because BDV and BVDV induce cross-reactive antibodies, BDV infection in cattle can produce false-positive results in BVDV serological tests, potentially leading to unnecessary culling of valuable breeding stock [14, 26].

The impact of BDV on BVD eradication programs has been quantified in Switzerland, where 6.7% of seropositive cattle samples showed significantly higher neutralizing antibody titers against BDV than BVDV [14]. These BDV-reactive samples originated from 65 farms across 15 cantons, with the highest prevalence observed in Central Switzerland. Logistic regression analysis identified co-housing of cattle with sheep as the most significant risk factor for BDV infection in cattle [14]. This finding emphasizes the need for integrated pestivirus control strategies that account for infection in multiple host species, particularly in mixed farming operations where sheep and cattle share housing or grazing areas [14, 20, 26].

The transmission of BDV from persistently infected cattle to naïve pregnant heifers has been experimentally demonstrated, confirming that cattle can serve as maintenance hosts for BDV and that the virus can establish persistent infection in bovine fetuses [18]. In one study, co-housing of seronegative heifers with a persistently infected BDV calf resulted in seroconversion in all heifers and persistent fetal infection in three out of six animals [18]. This finding challenges the assumption that BDV is primarily a pathogen of small ruminants and suggests that cattle may play a more significant role in BDV epidemiology than previously appreciated. The recognition that BDV can cause persistent infection in cattle has direct implications for BVD eradication programs, as PI cattle shedding BDV could perpetuate pestivirus circulation even after BVDV has been eliminated from a herd [18, 26].

The detection of BDV in pigs, both in Europe and Asia, further complicates the epidemiological picture. In the United Kingdom, BDV subgenotype 1b was identified in pigs experiencing poor growth and anemia on a mixed pig and sheep holding [2]. The virus was detected in multiple tissues by pan-viral microarray, sequencing, and immunohistochemistry, confirming systemic infection [2]. Similarly, BDV genotype 3 strain Gifhorn was originally isolated from pigs, and its complete genome sequence has been determined [22]. The presence of BDV in swine herds poses a particular challenge for classical swine fever (CSF) surveillance, as BDV infection can induce cross-reactive antibodies that interfere with CSFV diagnostic assays [5, 6]. The identification of a common conformational epitope on the E2 glycoprotein of CSFV and BDV provides a molecular explanation for this serological cross-reactivity and highlights the need for differential diagnostic tests that can distinguish between these closely related pestiviruses [5].

Regional Epidemiological Patterns and Unexplored Geographic Niches

Despite decades of research, significant gaps remain in our understanding of BDV distribution and genetic diversity. In the Middle East and North Africa, serological surveys have demonstrated widespread BDV circulation, but molecular characterization of circulating strains remains limited. In Algeria, a seroprevalence of 68.2% was documented in sheep, with BDV-specific neutralizing antibodies identified in 144 of 197 sera tested [10]. However, no persistently infected animals were detected, and no virus isolates were characterized at the molecular level [10]. Similarly, in Iraq, seroprevalence rates of 46.9% in sheep and 16% in goats were reported, but molecular data are lacking [9]. These serological findings suggest that BDV is endemic across North Africa and the Middle East, yet the genotypes circulating in these regions remain unknown, representing a critical gap in global BDV molecular epidemiology.

In Turkey, serological surveys have consistently demonstrated high BDV seroprevalence in sheep, with rates ranging from 45.2% in the Van province to 74.57% in the Kars District [21, 30]. The detection of BDV antigen and nucleic acid in seronegative animals confirms the presence of persistently infected sheep that serve as reservoirs for virus maintenance [21]. However, molecular characterization of Turkish BDV strains has only recently begun, with the identification of BDV-1 in a single wild boar sample [

Epidemiology and Interspecies Transmission Dynamics

The epidemiology of Border disease virus (BDV) is characterized by a complex interplay between primary small ruminant hosts, secondary spillover into cattle and swine, and the emergence of wildlife reservoirs, all underpinned by a remarkable genotypic diversity that reflects both ancient divergence and contemporary cross-species movement. As a member of the genus Pestivirus (family Flaviviridae), BDV is antigenically and structurally related to bovine viral diarrhea virus (BVDV) and classical swine fever virus (CSFV), a proximity that not only complicates serological diagnostics but also facilitates periodic host jumps that challenge national eradication campaigns [3, 28]. The virus is now recognized as endemic in sheep populations across Europe, the Americas, Asia, the Middle East, and North Africa, with prevalence rates varying dramatically by region, husbandry system, and surveillance intensity. Understanding the full spatial and temporal dynamics of BDV requires integrating data from serosurveys, molecular typing, phylogenetic phylogeography, and experimental transmission studies, all of which reveal a pathogen that is far more ecologically and economically significant than its “sheep-specific” label would suggest.

Global Distribution and Genotypic Diversity

BDV is currently classified into at least four major genotypes (BDV-1 through BDV-4), with several subgenotypes recognized within each clade. Genotype 1 (BDV-1) is the most widely distributed, having been identified in sheep, cattle, pigs, and wild boar across Europe, Turkey, the United Kingdom, and Mexico [3, 11, 13]. Genotype 2 (BDV-2) appears more restricted to European sheep populations, while genotype 3 (BDV-3) has emerged as the predominant lineage in China, isolated from both sheep and goats as well as from pigs in Germany (strain Gifhorn) [16, 22, 23]. Genotype 4 (BDV-4) is of particular interest because of its association with high-mortality outbreaks in Pyrenean chamois (Rupicapra pyrenaica pyrenaica) and its documented introduction from domestic sheep into a naïve wildlife population in the early 1990s [15, 17, 27]. The molecular epidemiology of BDV is further complicated by the existence of multiple subgenotypes (e.g., BDV-1a, -1b, -4a) that often coexist within the same geographic region, as demonstrated by the detection of BDV-1b in pigs in England [2], BDV-4 in sheep and chamois in Spain and southern Europe [15, 19], and BDV-3 in both ovine and caprine hosts in eastern China [23]. The 5′ untranslated region (5′UTR) and the Npro gene are the most commonly used targets for phylogenetic classification, providing robust resolution at the genotype and subgenotype levels [16, 21]. The evolutionary rate of BDV has been estimated at approximately 2.9 × 10⁻³ substitutions per site per year (95% HPD: 1.5–4.6 × 10⁻³) based on phylogeographic analysis of chamois strains, indicating a relatively slow but steady molecular clock that allows reconstruction of historic transmission events [15].

Prevalence in Primary Hosts: Sheep and Goats

Sheep and goats serve as the principal reservoir and amplifier hosts for BDV, with persistently infected (PI) animals acting as the primary source of viral shedding and maintenance within populations. Seroprevalence among adult sheep varies widely: a study in the Mosul region of Iraq reported 46.9% seropositivity in sheep and 16% in goats, with PI animals detected in 1.4% of sheep tested by antigen ELISA [9]. In Algeria, a comprehensive survey of 56 flocks across nine departments found that 98% of flocks had at least one seropositive animal, with a true within-flock seroprevalence of 68.2% (95% CI: 60.2–76.3) [10]. Similar high rates have been documented in Turkey, where seropositivity reached 45.2% in sheep from Van province and 74.6% in the Kars district [21, 30]. In Greece, while BDV had not been reported since 1974, a targeted survey in 2023 using p80 antibody ELISA and RT-PCR identified seropositive sheep in 4 of 24 flocks, with one animal exhibiting the diagnostic profile of a PI animal, confirming that the virus circulates undetected even in countries with no recent history of clinical disease [1]. The detection of BDV in goat herds in China was initially linked to outbreaks of diarrhea with morbidity of 28–37% and mortality of 10–15%, underscoring the clinical impact of the virus in goats, which had previously been considered less susceptible than sheep [23]. In Japan, serosurveillance in northern prefectures revealed a BDV seroprevalence of 17.6% in sheep, with no reported clinical cases, suggesting subclinical circulation [25]. These data collectively illustrate that BDV is far more widespread than clinical outbreaks would indicate, and that PI lambs, which remain viremic and seronegative for life, are the silent drivers of endemicity [16].

Interspecies Transmission to Cattle

Cattle are susceptible to BDV infection through direct contact with PI sheep or, less commonly, through contaminated fomites or semen. The clinical and pathological outcomes of BDV infection in cattle closely resemble those caused by BVDV, including transient viremia, leukopenia, reproductive failure, and the potential to generate PI calves if a susceptible dam is infected during early gestation [26]. Experimental co-housing of seronegative heifers in early pregnancy with a PI BDV calf resulted in seroconversion in all animals and persistent fetal infection in 50% of them, confirming that BDV can establish PI status in cattle exactly as BVDV does [18]. Conversely, artificial insemination with BDV-infected semen led to seroconversion in cows but did not produce PI fetuses, suggesting that the route of exposure (systemic vs. local) may influence transplacental transmission efficiency [32]. The significance of BDV in cattle is magnified in countries that have implemented BVDV eradication programs. In Switzerland, for example, after the elimination of BVDV PI cattle, serological surveillance using a cross-neutralization test revealed that 6.7% of seropositive cattle (104 of 1,555 samples) had significantly higher neutralizing antibody titers against BDV than against BVDV. Logistic regression identified common housing of cattle and sheep as the single most important risk factor (odds ratio >10), indicating that spillover from sheep reservoirs is now the primary source of pestivirus exposure in the Swiss cattle population [14]. These findings are echoed in Austria, where the first documented BDV outbreak in a sheep flock was recognized as a high-risk factor for adjacent BVDV-free cattle herds, prompting calls for a revision of the national BVDV program to include BDV monitoring [20]. In Mexico, BDV was first identified in cattle during a pestivirus surveillance program that had previously recorded 67.1% BVDV seroprevalence; phylogenetic analysis of the 5′UTR classified the Mexican strains as BDV-1, confirming that BDV circulates in cattle even in regions where it is officially considered an exotic disease [11]. The food and agriculture organization (FAO) and the world organisation for animal health (WOAH) have recognized that pestivirus cross-infection undermines the specificity of serological surveillance, and the Swiss experience serves as a template for how national programs can adapt.

Transmission to Swine and Implications for CSFV Surveillance

Pigs are incidental hosts for BDV but play a disproportionate role as potential confounders in classical swine fever (CSF) surveillance. Natural BDV infection in pigs was first reported in the United Kingdom in 1992 and has since been documented in Spain, Japan, and elsewhere [2]. A persistent problem of poor growth and anemia in growing pigs on a mixed pig-and-sheep holding in England was traced to BDV subgenotype 1b via panviral microarray, sequencing, and immunohistochemistry [2]. The complete genome sequence of the BDV-3 strain Gifhorn, originally isolated from pigs in Germany, provided the reference for understanding the genetic basis of swine adaptation [22]. In Switzerland, three BDV strains of the same subgenotype (BDSwiss) were isolated from sheep, cattle, and pigs, providing unequivocal evidence of a common source and cross-species transmission [13]. The primary concern for swine health authorities is the serological cross-reactivity between BDV and CSFV. A study mapping the glycoprotein E2 epitopes identified a common conformational epitope shared between CSFV and BDV within domain B/C, meaning that antibodies elicited by BDV infection in pigs can be misidentified as CSFV antibodies in routine ELISA-based surveillance [5]. This cross-reactivity is not merely theoretical; experimental infection of pigs with BDV and with the related Tunisian sheep-like virus (TSV) produced antibodies that were reactive in CSFV-specific assays, interfering with serological diagnosis [6]. The presence of BDV in wild boar further complicates matters. In Turkey, BDV genotype 1 was detected in wild boar (Sus scrofa ferus) for the first time, highlighting the potential for spillback from wild suids to domestic pigs and the need to include BDV in differential diagnostics for CSFV surveillance in free-ranging populations [7]. The FAO and WOAH guidelines for CSF surveillance recommend considering pestivirus cross-reactivity, but BDV is rarely included in routine molecular monitoring in swine.

Wildlife Reservoirs: Chamois, Wild Boar, and Beyond

Perhaps the most dramatic illustration of BDV’s interspecies transmission capacity is the emergence of BDV-4 as a lethal pathogen in Pyrenean chamois. Starting in 2001 and continuing through 2005–2007, epizootics in northeastern Spain and adjacent areas of France and Andorra caused the death of approximately 3,000 chamois, with mortality rates far exceeding those seen in domestic sheep [27]. Phylogeographic analysis using Bayesian Markov Chain Monte Carlo methods traced the origin of the chamois clade to BDV-4a strains circulating in domestic sheep in the early 1990s, with a founder effect that established a novel, highly virulent lineage in the wildlife population [15]. The virus spread westward at an average rate of 13.1 km/year (95% HPD: 5.2–21.4 km/year), reaching distances of over 125 km, and exhibited strong spatial clustering such that strains from a single locality segregated together in phylogenetic reconstructions [15]. Population viability analysis demonstrated that BDV-4 infection doubled the probability of extinction of a virtual chamois population over 50 years, confirming that this pestivirus can act as a primary driver of population decline in naïve wildlife species [17]. Wild boar have also been identified as BDV hosts, as evidenced by the detection of BDV-1 in a wild boar in Turkey [7]. The role of arthropod vectors in BDV transmission is less clear, but the detection of BDV-3 RNA in Melophagus ovinus (sheep ked) collected from sheep in Xinjiang, China, raises the possibility that ectoparasites may serve as mechanical vectors, particularly in extensive grazing systems where sheep density is high [8]. While ked-borne transmission has not been experimentally proven, the close phylogenetic relationship between the China/BDV/2018 strain from keds and a goat-derived strain from Anhui (AH12-02) suggests a shared epidemiological cycle [8].

Risk Factors and Epidemiological Drivers

The maintenance and spread of BDV are governed by a set of well-characterized risk factors. Flock-level risk factors identified in Algeria include climate (humid vs. arid), landscape type (mountainous vs. plains), flock management practices (communal grazing, lack of quarantine for introduced animals), and the presence of other ruminant species on the farm (especially cattle) [10]. In Switzerland, housing sheep and cattle together was the strongest predictor of BDV seropositivity in cattle [14]. The presence of a PI animal within a flock is the single greatest risk factor for within-herd transmission, as PI sheep shed virus continuously in nasal secretions, saliva, urine, and feces, contaminating feed and water sources. In the Greek study, the identification of a PI sheep in one of the four seropositive flocks provided direct evidence that PI animals act as the long-term sources of infection even in flocks with low seroprevalence [1]. Abortion storms, as documented in Iran where BDV RNA was detected in 9% of aborted ovine fetuses [12] and in goats in China where BDV infection was associated with increased malondialdehyde and acute-phase proteins [24], further contribute to viral dissemination through fetal fluids and tissues. The economic burden of BDV, encompassing abortion, stillbirth, poor growth of PI lambs, and interference with BVDV and CSFV eradication, cannot be overstated. The world organisation for animal health (WOAH) lists border disease as a notifiable disease in some member countries, but surveillance remains fragmented, and many infections go undiagnosed [1, 3]. The epidemiological picture that emerges is one of a virus that is endemic in small ruminant populations across four continents, repeatedly spilling over into cattle and pigs, and occasionally establishing highly pathogenic lineages in wildlife. Control efforts must address the PI animal reservoir, enforce biosecurity at the livestock-wildlife interface, and incorporate BDV-specific diagnostics into existing pestivirus monitoring frameworks to prevent the virus from undermining hard-won progress in BVD and CSF eradication.

Clinical Manifestations and Pathological Features in Small Ruminants and Pigs

Border disease virus (BDV) induces a spectrum of clinical and pathological outcomes that are profoundly influenced by the gestational stage at the time of infection, the viral genotype, and the host species. In small ruminants, the disease is classically characterized by reproductive failure and congenital malformations, while in pigs, the clinical picture is more variable, often subclinical, but can manifest as a wasting syndrome with significant economic implications. The pathogenesis is rooted in the virus’s non-cytopathic (NCP) biotype, which allows for the establishment of persistent infection (PI) following in utero infection before immunocompetence, typically before day 80 of gestation in sheep. These PI animals are the cornerstone of BDV epidemiology, serving as lifelong shedders and reservoirs for transmission to naïve cohorts and other species [1, 3, 16].

Clinical Manifestations in Sheep and Goats

The clinical presentation of BDV in sheep and goats ranges from inapparent to severe, with the most devastating outcomes observed in fetal and neonatal infections. Acute postnatal infections in immunocompetent adult sheep are typically subclinical or result in a transient, mild febrile response, as demonstrated experimentally with the BDV-3 strain JSLS12-01, where infected sheep exhibited only short-term pyrexia lasting approximately five days and transient depression [16]. However, the true economic and welfare burden of BDV lies in its impact on reproduction and the development of PI lambs.

Reproductive Failure and Congenital Disease: The hallmark of BDV infection in naïve pregnant ewes and does is reproductive dysfunction, which includes early embryonic death, abortion, stillbirth, and the birth of weak, non-viable lambs. A study in Iran identified BDV RNA in 9% of aborted ovine fetuses, directly implicating the virus as a significant cause of ovine abortion in the region [12]. The pathophysiological basis for abortion is multifaceted, involving placental damage, fetal viremia, and disruption of fetal endocrine and immune functions. In aborting goats, infection is associated with a pronounced systemic inflammatory response, evidenced by significantly elevated serum levels of acute phase proteins such as haptoglobin (Hp) and serum amyloid A (SAA), alongside increased oxidative stress markers like malondialdehyde (MDA) and nitric oxide (NO) [24]. These biomarkers indicate that abortion is not merely a consequence of direct viral cytopathology but involves a severe maternal systemic disturbance.

The most characteristic clinical manifestation of in utero BDV infection is the birth of lambs with a "hairy shaker" or "fuzzy lamb" syndrome. This phenotype is defined by a combination of neurological deficits and integumentary abnormalities. Affected lambs exhibit a coarse, hairy, and often pigmented fleece, contrasting with the normal woolly coat of their breed [4, 28]. The neurological component, which gives the disease its name, is a generalized tremor (hypomyelinogenesis), ataxia, and hypermetria. These lambs often have difficulty standing and suckling, leading to poor growth and high mortality. The neuropathology is characterized by hypomyelinogenesis and a spongiform change in the white matter of the cerebellum, brainstem, and spinal cord. This is not a direct result of oligodendrocyte destruction but rather a failure of myelin maturation. Molecular studies have revealed that BDV infection upregulates the expression of neuronal nitric oxide synthase (nNOS) and neurofilament (NF) proteins in the brain, with the severity of these molecular changes correlating directly with the degree of neuropathology [29]. The increased nNOS suggests that the L-arginine-NO synthase pathway is activated, potentially contributing to neuronal damage and dysmyelination. Furthermore, elevated expression of ADAMTS-13 (A Disintegrin And Metalloprotease with Thrombospondin type I repeats-13) in the CNS of infected animals points to a role for this protease in modulating the brain microenvironment during neurodegeneration, although its precise function in BDV pathogenesis remains under investigation [29].

Persistent Infection and Wasting: PI lambs, which are born viremic and seronegative due to immune tolerance, are the primary viral reservoir. While some PI lambs may appear clinically normal at birth, they invariably suffer from poor growth performance, chronic ill-thrift, and increased susceptibility to secondary infections. This was starkly illustrated in a Greek study where a PI animal was identified within a seropositive flock, highlighting its role as a source of ongoing viral circulation [1]. In a Chinese investigation, a PI sheep was identified that remained viremic and antibody-negative throughout a six-month fattening period, demonstrating the long-term nature of this carrier state [16]. The economic impact of PI animals is substantial; they are unthrifty, have reduced life expectancy, and are a constant source of infection for herdmates [3, 9]. In goat herds, BDV infection has been associated with severe, unremitting diarrhea, with morbidity rates of 28-37% and mortality reaching 10-15% in some outbreaks in eastern China [23]. This enteric form of the disease underscores the virus's ability to cause severe clinical disease beyond the classic reproductive and neurological syndromes.

Clinical Manifestations in Pigs

Pigs are considered incidental hosts for BDV, with infections typically arising from spillover events from infected small ruminants, often on mixed-species farms [2, 3, 13]. The clinical spectrum in swine is highly variable, ranging from completely inapparent infections to severe systemic disease, a disparity that is likely influenced by viral strain, pig age, and immune status.

Subclinical and Mild Infections: Many BDV infections in pigs are subclinical, as evidenced by serological surveys in Japan and Spain, where antibodies were detected in healthy animals with no history of clinical disease [25]. Experimental infections of pigs with the Tunisian sheep-like virus (TSV), a close relative of BDV, and the BDV strain Frijters resulted in seroconversion without overt clinical signs, although the TSV showed a notable ability to replicate in the porcine host [6]. This subclinical carriage is a significant concern for disease surveillance, as it can lead to false-positive results in serological tests for classical swine fever (CSF), a WOAH-listed disease, due to antigenic cross-reactivity between BDV and CSFV [5, 6].

Wasting Syndrome and Anemia: A more severe clinical presentation in pigs has been documented, characterized by poor growth, ill-thrift, and anemia. A landmark investigation in England described a persistent problem in a small proportion of growing pigs on a mixed pig and sheep farm. The affected pigs exhibited poor growth rates and anemia, leading to a diagnostic investigation that ultimately identified BDV sub-genotype 1b via pan-viral microarray and specific RT-PCR [2]. This case highlights the potential for BDV to cause a chronic, debilitating condition in pigs that may be misdiagnosed as a nutritional or management problem. The pathogenesis of this wasting syndrome is likely multifactorial, involving virus-induced damage to the intestinal epithelium leading to malabsorption, as well as a chronic inflammatory state that diverts energy away from growth.

Reproductive and Hemorrhagic Disease: Historically, the first natural BDV infection in pigs in the UK was reported in 1992 in animals presenting with hemorrhagic lesions, a finding that raised initial concerns about a possible CSFV incursion [2]. While such hemorrhagic presentations appear to be rare, they underscore the potential for BDV to cause severe, acute disease in pigs. Furthermore, the potential for reproductive impact cannot be ignored. Although experimental insemination of cows with BDV-infected semen led to seroconversion but not fetal infection [32], the situation in pigs may differ. Given that BDV can cross the placenta in sheep and cattle [18], it is biologically plausible that infection of pregnant sows could lead to transplacental transmission, potentially resulting in abortion, stillbirth, or the birth of PI piglets Mendelian. The detection of BDV in wild boar in Turkey [7] further complicates the epidemiological picture, suggesting that wildlife can serve as a reservoir and potentially introduce the virus into domestic swine herds.

Pathological Features in Small Ruminants and Pigs

The pathological findings in BDV-infected animals mirror the clinical syndromes and provide critical insights into the mechanisms of disease.

Gross and Histopathological Lesions in Small Ruminants: In aborted fetuses and neonatal lambs, the most striking gross lesions are found in the central nervous system. Cerebellar hypoplasia and hydranencephaly are classic findings, reflecting the virus's predilection for rapidly dividing neuroblasts [28, 29]. Histologically, the hallmark is hypomyelinogenesis, characterized by a paucity of myelin sheaths in the white matter tracts of the cerebellum, cerebrum, and spinal cord. This is accompanied by a spongiform change (status spongiosus) due to vacuolation of the neuropil. As previously noted, these lesions are associated with increased expression of nNOS and NF, indicating active neurodegeneration [29]. In PI lambs that survive to adulthood, gross lesions may be absent, but histopathological examination of lymphoid tissues often reveals lymphoid depletion in the spleen and lymph nodes, contributing to their immunocompromised state. In goats with diarrhea, gross lesions may include catarrhal to hemorrhagic enteritis, with histopathological evidence of villous atrophy and crypt hyperplasia in the small intestine [23].

Pathological Features in Pigs: Pathological descriptions of BDV in pigs are less common but are emerging. In the English outbreak of wasting and anemia, affected pigs showed no specific gross lesions at necropsy, but immunohistochemistry (IHC) revealed the presence of BDV antigen in multiple tissues, including the thyroid gland, pancreas, and lymphoid tissues [2]. This widespread distribution of viral antigen in the absence of overt inflammation suggests a non-cytopathic infection that may disrupt endocrine function, contributing to poor growth. The detection of BDV RNA in the thyroid is particularly intriguing, as it could lead to metabolic dysregulation. In contrast, the hemorrhagic form of the disease, though rare, would be expected to show petechial and ecchymotic hemorrhages on serosal surfaces and in lymph nodes, similar to lesions seen in acute CSF, further complicating differential diagnosis [2]. The potential for BDV to cause persistent infection in pigs, analogous to that seen in ruminants, remains a critical knowledge gap. If PI piglets are born, they would represent a significant and unrecognized source of viral maintenance within swine herds, posing a risk to both pig health and the integrity of CSFV eradication programs.

Diagnostic Strategies: Serological and Molecular Detection Methods

The accurate diagnosis of Border disease virus (BDV) infection is a multifaceted challenge, fundamentally complicated by the virus's genetic heterogeneity, its broad host tropism, and its profound antigenic cross-reactivity with other pestiviruses, particularly Bovine viral diarrhea virus 1 and 2 (BVDV-1, BVDV-2) and Classical swine fever virus (CSFV) [3, 5, 6, 14, 28]. This cross-reactivity is not merely a laboratory nuisance but a genuine biological consequence of shared viral ancestry and conserved structural epitopes, particularly within the major envelope glycoprotein E2 [5]. The serological and molecular toolkits available to veterinary diagnosticians must, therefore, be deployed with a clear understanding of these limitations, aiming not only for detection but also for definitive differentiation. The diagnostic strategy must be tailored to the epidemiological context, whether one is screening for acute infection, identifying the critical persistently infected (PI) animals that serve as reservoirs, or attempting to differentiate BDV from BVDV or CSFV in the context of national eradication programs [1, 2, 14, 18, 20].

### Serological Detection: Navigating Cross-Reactivity

Serological assays are indispensable for flock-level surveillance, determining the prevalence of past or current infection, and for verifying the absence of viral circulation following an eradication program [9, 14, 21, 30]. The foundation of current serological screening is the enzyme-linked immunosorbent assay (ELISA) targeting antibodies (Ab-ELISA) against the highly conserved non-structural protein p80 (NS3), an immunodominant antigen across all pestiviruses [1, 10]. While highly sensitive and suitable for high-throughput screening, the p80-targeted Ab-ELISA is incapable of differentiating between BDV, BVDV, or CSFV infections [14]. As demonstrated by Feknous et al. [10], a seroprevalence of 68.2% in Algerian sheep, as measured by a p80-blocking ELISA, only provided a broad pestivirus prevalence, necessitating further characterization via virus neutralization test (VNT) to confirm BDV specificity.

The gold standard for serological differentiation remains the virus neutralization test (VNT). This assay measures the titer of serum antibodies capable of neutralizing the infectivity of specific, well-characterized pestivirus strains (e.g., BVDV-1a, BVDV-1h, BDV strains like BDSwiss) [10, 14]. A four-fold or greater difference in neutralizing antibody titer against one virus compared to the others is generally considered evidence of the specific infecting species. Kaiser et al. [14] systematically adapted this approach for the Swiss BVD eradication program, demonstrating that 6.7% of seropositive cattle had substantially higher titers against BDV than BVDV, revealing significant BDV spillover from sheep. This technique, however, is labor-intensive, time-consuming, and requires cell culture facilities and high-containment laboratory conditions for CSFV strains, limiting its use to reference laboratories.

The biological mechanism driving the need for VNT lies in the epitope structure of the E2 glycoprotein. Huang et al. [5] elegantly demonstrated that a common conformational epitope exists on domain B/C of the E2 protein, shared between CSFV and BDV, explaining widespread cross-neutralization. The presence of such common epitopes, alongside species-specific ones, means that polyclonal sera from infected animals often contain a mix of cross-reactive and specific antibodies. The VNT’s ability to detect functional neutralizing antibodies makes it the most reliable, albeit resource-intensive, method for specific serodiagnosis, crucial for distinguishing BDV from the economically devastating CSFV in pigs [2, 6] and for assessing the impact of BDV on BVDV eradication campaigns [14, 26].

### Molecular Detection: Direct Pathogen Identification and Genotyping

Reverse transcription polymerase chain reaction (RT-PCR) has revolutionized the direct detection of BDV, offering high sensitivity, rapid turnaround, and the capacity for genotyping. The vast majority of molecular diagnostic efforts target the 5' untranslated region (5'-UTR) of the viral genome [1, 7, 8, 12, 15, 21, 23]. This region is highly conserved, allowing for the design of pan-pestivirus primers that can detect a wide array of BDV genotypes, as well as BVDV and CSFV [1, 23]. Pan-pestivirus PCR is often the first-line molecular screen, as demonstrated by Bouzalas et al. [1] in Greece, where it was used initially to detect viral RNA in two sheep from seropositive flocks. Following detection, amplicon sequencing and phylogenetic analysis of the 5'-UTR are the standard methods for definitive genotyping, allowing researchers to classify a strain as BDV-1, BDV-3, BDV-4, or other genogroups [1, 11, 16, 19]. Indeed, this approach identified the first BDV in Mexican cattle (BDV-1) [11], a BDV-4 strain in Greek sheep linked to Pyrenean chamois [1, 15], and BDV-3 in Chinese sheep and goats [16, 23].

Beyond the 5'-UTR, the Npro gene (encoding the N-terminal autoprotease) is another popular target for genotyping, as it offers even greater phylogenetic resolution than the 5'-UTR, especially for distinguishing closely related strains within a single genotype [13, 16, 23]. For definitive characterization of novel or highly divergent strains, such as the Swiss BDSwiss subgenotype found in sheep, cattle, and pigs, complete genome sequencing is the ultimate molecular tool [13, 22, 31]. This approach, typically achieved through next-generation sequencing or primer walking, provides the full evolutionary context of a virus and is critical for understanding viral emergence and interspecies transmission events [13].

The most critical application of molecular detection is the identification of persistently infected (PI) animals. PI animals are the key epidemiological drivers of BDV, shedding large quantities of virus for life [1, 9, 16, 18]. The diagnostic profile of a PI animal is defined by a double-positive result: detection of viral antigen or RNA (via RT-PCR or Ag-ELISA) coupled with the absence of specific antibodies (seronegative) [1, 16, 21]. As shown in studies from China, Iraq, and Greece, the discovery of a single PI animal in a flock suggests a source of continuing infection and highlights the need for their removal to control the disease [1, 9, 16]. Yilmaz et al. [21] in Turkey demonstrated this by finding that 1 of 117 seronegative sheep was both antigen-positive and RT-PCR-positive, confirming the PI status. The detection of viral nucleic acid in aborted fetuses via RT-PCR, as shown by Mokhtari and Manshoori [12] in Iran, is a direct molecular confirmation of vertical transmission and a cause of reproductive loss.

### Antigen Detection and in situ Methods

Alongside RNA detection, antigen-capture ELISA (Ag-ELISA) provides a rapid, cost-effective method for detecting the p80/NS3 protein in serum or tissue homogenates [1, 9, 10, 21]. While sensitive for PI animals with high viral loads, Ag-ELISA can be less sensitive than RT-PCR for transiently infected animals with lower viremia. It serves as a valuable point-of-care or field-deployable tool for screening large numbers of animals, with positive results typically confirmed by RT-PCR [1]. For definitive visualization of virus in tissues, immunohistochemistry (IHC) is employed, using monoclonal antibodies against pestiviral antigens (e.g., E2 or NS3) on formalin-fixed, paraffin-embedded tissue sections [2, 13, 18, 32]. IHC is particularly powerful for diagnosing fetal infection and confirming the presence of viral antigen in specific target organs like the placenta, brain, and lymphoid tissue, thereby linking molecular detection with pathological lesions [13, 18, 29]. This technique was crucial in demonstrating BDV transmission from a PI calf to the placentas and fetuses of co-housed heifers [18].

Prevention and Control Strategies for Border Disease Virus

The implementation of effective prevention and control strategies for Border Disease Virus (BDV) necessitates a paradigm shift away from species-specific management toward a multi-host, ecosystem-based approach. BDV is not merely a pathogen of sheep and goats; its ability to infect cattle, pigs, and a range of wild ungulates, including chamois and wild boar, renders it a formidable threat to livestock health, wildlife conservation, and national eradication programs for economically critical pestiviruses such as Bovine Viral Diarrhea Virus (BVDV) and Classical Swine Fever Virus (CSFV) [3, 13, 26]. The biological mechanisms underpinning BDV persistence, particularly the establishment of persistently infected (PI) animals, coupled with extensive antigenic cross-reactivity, demand that control strategies be robust, diagnostic-driven, and integrated across species boundaries. A failure to address BDV reservoirs in sheep populations, for instance, has been demonstrated to directly undermine serological surveillance for BVDV in cattle, a challenge recognized by veterinary authorities worldwide and one that has implications for the sanitary standards set by the World Organisation for Animal Health (WOAH) [14, 18].

Surveillance, Diagnosis, and the Critical Role of Persistently Infected Animals

The cornerstone of any successful BDV control program is the systematic identification and removal of PI animals. These individuals, resulting from in utero infection before the development of immunological competence, are life-long shedders of the virus and serve as the primary reservoirs for maintaining infection within and across herds [1, 9, 10, 16]. Serological surveys using antibody-detection ELISAs (Ab-ELISA) provide a valuable first-line tool for establishing the prevalence of exposure within a flock. For example, studies in Algeria and Iraq have reported alarmingly high flock-level seroprevalences of 98% and individual seroprevalences of 46.9% in sheep, respectively, underscoring the endemic nature of BDV in many regions [9, 10]. However, serology alone is insufficient. The true control target is the PI animal, which is typically seronegative but antigen-positive. Therefore, a two-tiered diagnostic approach is mandatory: initial screening with Ab-ELISA, followed by antigen-capture ELISAs (Ag-ELISA) or reverse-transcription PCR (RT-PCR) on seronegative animals.

The detection of PI sheep in Greece, Iraq, and China, with prevalence rates ranging from 1.4% to upwards of 5% in some cohorts, confirms that these animals are the linchpin of BDV epidemiology [1, 9, 16]. The failure to identify even a single PI animal can lead to the rapid re-infection of a cleaned herd. Furthermore, molecular diagnostics targeting the 5’-untranslated region (5’UTR) or the Npro gene are essential not only for detection but also for genotyping, which reveals the circulating strains (e.g., BDV-1, BDV-3, BDV-4) and informs epidemiological tracing [1, 3, 7, 11]. The emergence of BDV-4 in Pyrenean chamois, which originated from sheep and spread with an estimated epidemic diffusion rate of approximately 13.1 km/year, highlights how molecular surveillance can track viral incursions into wildlife and inform spatial control measures [15, 17]. Control programs must therefore mandate the use of RT-PCR or pooled antigen testing on all young stock, lambs, and newly introduced animals, as PI animals are most efficiently identified at birth or during the early fattening period.

Biosecurity, Herd Management, and the Interspecies Transmission Axis

Biosecurity measures for BDV must extend beyond the boundaries of a single species, given the frequent and documented interspecies transmission between sheep, cattle, and pigs [2, 3, 13]. A key risk factor identified in multiple epidemiological studies is the co-mingling of sheep and cattle. In Switzerland, common housing of these two species was identified as the most significant risk factor for BDV seropositivity in cattle, with 6.7% of seropositive cattle showing significantly higher neutralizing antibody titers against BDV than BVDV [14]. This finding aligns with observations from Austria, where a BDV outbreak in a sheep flock was identified as a high-risk factor for virus introduction into BVDV-free cattle herds [20]. The biological mechanism is clear: a PI sheep or lamb can efficiently transmit the virus to naive pregnant cattle via direct contact or contaminated fomites, leading to transient infection, seroconversion, and, critically, the birth of PI calves [18]. In a landmark experimental study, co-housing of seronegative heifers in early pregnancy with a PI calf persistently infected with BDV resulted in the birth of PI fetuses in 50% of the animals, demonstrating that cattle-to-cattle transmission of BDV is not only possible but a significant threat to BVDV eradication schemes [18].

Consequently, prevention strategies must enforce strict physical and spatial segregation of sheep and cattle, particularly during the breeding and lambing seasons. The risk is bidirectional: while cattle are dead-end hosts for BDV in terms of species maintenance, they can act as amplifying hosts if a PI animal is established. Furthermore, the role of swine as sentinel or spillover hosts cannot be ignored. Investigations in England and elsewhere have identified BDV in pigs presenting with poor growth and anemia, with phylogenetic analysis clustering the isolates with BDV-1b [2]. Given that BDV is antigenically related to CSFV, its presence in swine herds can generate false-positive results in serological surveillance for Classical Swine Fever, a WOAH-listed notifiable disease [5, 6]. This diagnostic confusion can trigger costly and unnecessary quarantine measures. Control programs in mixed farming operations should, therefore, include pigs in their pestivirus surveillance protocols, especially where they have contact with small ruminants.

Broader Ecological and Vector-Borne Considerations

A comprehensive control strategy must also acknowledge the role of wildlife reservoirs and potential arthropod vectors. The emergence of BDV-4 as a driver of population decline in Pyrenean chamois, with an estimated doubling of extinction risk over 50 years, demonstrates that this virus is not just a production disease but a conservation threat [15, 17]. Wild ungulates, including chamois and wild boar, can maintain BDV independently of domestic livestock, serving as a sylvatic reservoir that can re-infect cleared flocks [7, 27]. This necessitates the implementation of surveillance at the wildlife-livestock interface, particularly in mountainous or border regions. The detection of BDV genotype 1 in a wild boar in Turkey, where the animal was sympatric with domestic ruminants, further underscores this risk [7].

More recently, the detection of BDV RNA in the sheep ked (Melophagus ovinus) in Xinjiang, China, has opened a new frontier in our understanding of BDV ecology [8]. While the identification of BDV genotype 3 in this hematophagous ectoparasite does not prove vector competence, it suggests a potential mechanical or biological transmission route that could facilitate within-flock spread and complicate control efforts. Although the USDA and WOAH do not currently classify BDV as a vector-borne disease, the presence of the virus in M. ovinus warrants further investigation and may eventually necessitate the inclusion of ectoparasite control in integrated pest management and biosecurity plans. For now, standard biosecurity, including quarantine of new arrivals, avoidance of shared grazing with wildlife, and rigorous cleaning and disinfection of facilities, remains the bedrock of prevention. Vaccination, while theoretically possible, is not widely practiced for BDV due to the high cost and the absence of commercial vaccines in many regions, and the risk that vaccination could mask serological surveillance efforts crucial for identifying PI animals.

Integration with National Eradication Programs

Perhaps the most critical strategic imperative is the integration of BDV control into existing national BVDV and CSFV eradication campaigns. As a matter of policy, veterinary authorities must recognize that BDV is a legitimate source of cross-reactivity that can confound serological surveillance. In Switzerland, the application of a differential serum neutralization test (SNT) using BVDV-1a, BVDV-1h, and BDV (BDSwiss-a) strains revealed that a substantial proportion of seropositive cattle had actually been infected with BDV, not BVDV [14]. This finding has profound implications for the epidemiological interpretation of surveillance data. Programs that rely solely on pan-pestivirus antibody ELISAs without follow-up differentiation risk misclassifying BDV-infected farms as BVDV-positive, leading to erroneous conclusions about the success of eradication.

Therefore, the prevention of BDV is inextricably linked to the successful eradication of BVDV and the maintenance of CSFV-free status. This requires: (1) mandatory reporting of BDV outbreaks, (2) inclusion of sheep and goat flocks in biosecurity zones around BVDV-free cattle herds, (3) implementation of differential diagnostics in national reference laboratories, and (4) economic support for farmers to cull PI animals. The WOAH Terrestrial Code, while focused on BVDV and CSFV, provides a framework for compartmentalization and zoning that can be adapted for BDV. The United States Department of Agriculture (USDA) and the European Food Safety Authority (EFSA) have both acknowledged the spillover risk from small ruminants to cattle. Ultimately, the failure to control BDV in sheep populations will perpetuate a continuous source of infection for cattle, undermining the multi-billion dollar investments made in BVDV eradication and posing a persistent threat to global livestock health security.

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