Pestivirus H in Cattle: HoBi-like Virus Reference

Overview and Taxonomy of Pestivirus H (HoBi-like Virus) in Cattle

Taxonomic Classification and Nomenclature

The emergence of Pestivirus H, more commonly referred to as HoBi-like pestivirus (HoBiPeV) or bovine viral diarrhea virus 3 (BVDV-3), represents a paradigm shift in our understanding of bovine pestiviral diversity. Within the genus Pestivirus, family Flaviviridae, this species has been definitively established as the fifth recognized ruminant pestivirus, alongside Pestivirus A (BVDV-1), Pestivirus B (BVDV-2), Pestivirus D (Border disease virus), and Pestivirus F (giraffe pestivirus) [5, 11]. The taxonomic reclassification from the previous provisional designations of "atypical pestivirus" or "BVDV-3" to the officially recognized species Pestivirus H reflects the growing consensus that these viruses represent a genetically and antigenically distinct lineage warranting species-level status [5, 11]. The nomenclature "HoBi-like" derives from the prototypical strain D32/00_HoBi, first identified in a batch of fetal bovine serum originating from Brazil, and the term has persisted in the literature to describe this emerging group of viruses [5, 11].

From a molecular taxonomic perspective, Pestivirus H occupies a unique phylogenetic position that is more closely related to classical swine fever virus and Border disease virus than to the classical bovine viral diarrhea virus species BVDV-1 and BVDV-2 [5]. This phylogenetic distance is reflected in significant genetic divergence: while BVDV-1 and BVDV-2 share approximately 60-70% nucleotide identity within conserved genomic regions, Pestivirus H shares only 56-62% identity with either of these species [5]. The complete genome organization of Pestivirus H follows the canonical pestiviral architecture, featuring a single-stranded positive-sense RNA genome of approximately 12.3-12.5 kilobases, flanked by 5′ and 3′ untranslated regions (UTRs) and encoding a single large open reading frame (ORF) that is post-translationally cleaved into 12 structural and nonstructural proteins [3]. As exemplified by the Xinjiang isolate OP210314, which possesses a genome of 12,239 nucleotides with a 5′-UTR of 340 nucleotides and a 3′-UTR of 199 nucleotides, the genomic architecture is highly conserved, but the sequence divergence within the 5′-UTR and the N-terminal autoprotease (Npro) and E2 envelope glycoprotein coding regions provides the primary basis for phylogenetic discrimination from other pestivirus species [3, 6].

Phylogenetic Subdivision and Genetic Diversity

The genetic heterogeneity within Pestivirus H is substantial, with phylogenetic analyses revealing at least five distinct subgroups, designated HoBiPeV-a through HoBiPeV-e [5]. This subdivision is primarily based on sequence analysis of the 5′-UTR, Npro, and E2 gene regions, which have been demonstrated to provide robust phylogenetic resolution. The most comprehensive characterization of this sublineage structure has emerged from studies of Brazilian isolates, where only the HoBiPeV-a subgroup has been identified despite high endemicity and widespread circulation within the country [5]. This relative genetic homogeneity within Brazil stands in marked contrast to the situation observed in other geographical regions.

Globally, the emergence of HoBi-like pestivirus subgenotypes has been documented across multiple continents, with reports of subgenotypes a through d circulating in countries including Russia, Italy, Thailand, India, and Bangladesh [12]. The phylogenetic relationships among these subgroups reveal complex patterns of geographic clustering and potential independent evolutionary trajectories. For instance, the Asian isolates, including those from China, Bangladesh, and Thailand, form distinct clades that exhibit considerable divergence from South American strains. The Shandong isolate SDJN-China-2019, responsible for a severe respiratory outbreak in Chinese cattle herds, shares 94.1-97.5% nucleotide homology with the Brazilian LV168-20_16RN strain in the 5′-UTR, Npro, and E2 regions, yet exhibits only 88.5-92.1% homology with the Asian reference strain Th/04-Khonkaen from Thailand [6]. This pattern suggests that multiple independent introductions and subsequent viral evolution have occurred across different geographic regions, rather than a single global dissemination event.

The genetic diversity observed within Pestivirus H has direct implications for diagnostic sensitivity and vaccine efficacy. The 5′-UTR, while highly conserved across all pestiviruses and widely used for diagnostic primer design, exhibits sufficient variability within HoBiPeV that pan-pestivirus or pan-bovine pestivirus primers may fail to detect certain strains [10]. The development of specific primer sets, such as the BP189-389 pan-bovine pestivirus primers, has been necessary to ensure reliable detection of HoBiPeV alongside BVDV-1 and BVDV-2 [10]. Similarly, the antigenic diversity revealed by cross-neutralization studies indicates that existing BVDV-1 and BVDV-2 vaccines may not provide adequate protection against HoBiPeV challenge, highlighting the critical need for multivalent vaccine formulations that include Pestivirus H antigens in endemic regions [5, 11].

Geographic Distribution and Emergence

The global distribution of Pestivirus H has expanded dramatically since its initial recognition, with documented presence now spanning South America, Europe, and Asia. Brazil remains the country with the highest reported prevalence and endemicity, where HoBiPeV-a has been consistently identified in cattle populations across multiple states [5, 8]. The virus has been detected in diverse clinical contexts, from routine surveillance of export-bound beef calves to investigations of respiratory disease outbreaks in dairy herds [8, 10]. The detection frequency in Brazilian herds is substantial; in one study of antigen-positive serum samples from southern Brazil, 26 out of 135 ELISA-positive samples were confirmed as HoBiPeV by specific RT-PCR, representing approximately 19% of bovine pestivirus-positive samples [10]. This indicates that HoBiPeV constitutes a significant proportion of the bovine pestivirus burden in regions where it has become established.

In Asia, the emergence of Pestivirus H has been documented through a series of landmark studies. The first identification in China came from Xinjiang province, where molecular characterization of bovine serum samples revealed a HoBiPeV strain with 96.02% whole-genome homology to the JS12/01 reference strain [3]. Significantly, this detection was preceded by the first reported clinical outbreak of HoBiPeV in Chinese cattle, which occurred in Shandong province in November 2017 and was characterized by severe respiratory disease, diarrhea, and high fatality rates [6]. This outbreak demonstrated that HoBiPeV is not merely a contaminant of biological products but a genuine pathogen capable of causing devastating clinical disease under natural conditions. Bangladesh has also reported the circulation of a rare HoBi-like pestivirus strain, identified through molecular analysis of the partial 5′-UTR nucleotide sequences from cattle presenting at veterinary hospitals [7]. The Egyptian detection, representing the first identification in Africa, was obtained from persistently infected (PI) dairy calves with a prevalence of 2.5% among sampled animals, with nucleotide identities of 98.8-99.6% to reference BVD-3 strains [1].

Italy represents a notable European focus of Pestivirus H circulation, where the virus has been identified alongside BVDV-1 and BVDV-2 in cattle populations [11]. The co-circulation of all three bovine pestivirus species in Italian herds has raised concerns regarding diagnostic cross-reactivity and the potential for recombination events that could generate novel viral variants with unpredictable pathogenic properties [11]. In marked contrast, extensive surveillance programs in Northern Ireland and Turkey have failed to detect Pestivirus H, despite comprehensive molecular screening of BVDV-positive samples [2, 4]. This absence may reflect genuine geographic restriction or may be attributable to sampling biases and the diagnostic challenges associated with detecting a genetically divergent virus using assays optimized for BVDV-1 and BVDV-2.

Host Range and Cross-Species Transmission Potential

Pestivirus H, like other bovine pestiviruses, exhibits a broad host range that extends beyond cattle to include small ruminants and potentially wildlife species. Natural infections have been documented in goats and sheep, and experimental infections have confirmed the susceptibility of these species [5, 12]. The potential for cross-species transmission is particularly concerning given the phylogenetic evidence that HoBiPeV sequences obtained from cattle, goats, and sheep show genetic resemblance, indicating possible spillover events or shared exposure sources [9]. This host plasticity has significant implications for control programs that focus exclusively on cattle, as small ruminants may serve as undetected reservoirs capable of reintroducing the virus into cleared herds.

The role of persistently infected (PI) animals in the epidemiology of Pestivirus H mirrors that of classical BVDV, with PI animals serving as the primary source of viral shedding and transmission within and between herds [1]. The detection of PI calves in Egyptian dairy herds, confirmed by sequential antigen testing and molecular characterization, demonstrates that the same immunotolerance mechanisms that facilitate BVDV persistence apply to HoBiPeV [1]. The movement of PI animals, trade of contaminated biological products such as fetal bovine serum, and the importation of live ruminants from endemic regions represent the principal pathways for the international dissemination of Pestivirus H [5, 11]. The World Organisation for Animal Health (WOAH) has recognized the significance of pestiviral diseases in international trade, and the emergence of a genetically distinct pestivirus species that may evade standard diagnostic detection methods raises important considerations for trade restrictions and biosecurity protocols.

Molecular Pathogenesis of HoBi-like Pestivirus Infection

The molecular pathogenesis of HoBi-like pestivirus (Pestivirus H) represents a complex interplay between viral genetic determinants, host cellular machinery, and immune evasion strategies that culminate in a spectrum of clinical outcomes ranging from subclinical infection to severe, fatal disease. As an emerging atypical pestivirus, HoBiPeV shares fundamental pathogenic mechanisms with classical bovine viral diarrhea viruses (BVDV-1 and BVDV-2) yet exhibits distinct molecular features that influence its virulence, tissue tropism, and capacity to establish persistent infections. Understanding these molecular underpinnings is critical for developing effective control measures, particularly given the virus’s expanding geographic footprint and its documented ability to cause severe respiratory disease and mortality in naïve cattle populations [6, 12].

Genomic Organization and Polyprotein Processing

The HoBiPeV genome, as exemplified by the Chinese isolate OP210314, comprises approximately 12,239 nucleotides organized as a single-stranded, positive-sense RNA molecule containing a single large open reading frame (ORF) encoding a polyprotein of 3,899 amino acids [3]. This genomic architecture is flanked by highly structured 5′ and 3′ untranslated regions (UTRs) of 340 and 199 nucleotides, respectively, which are critical for viral replication, translation initiation, and genome cyclization [3]. The 5′UTR, in particular, serves as the primary target for molecular diagnostics and phylogenetic classification due to its conserved secondary structure elements that are essential for internal ribosome entry site (IRES)-mediated translation initiation, a hallmark of pestivirus replication strategy.

The viral polyprotein undergoes co- and post-translational processing by cellular and viral proteases to yield four structural proteins (C, Erns, E1, E2) and eight nonstructural proteins (Npro, p7, NS2, NS3, NS4A, NS4B, NS5A, NS5B). Among these, the autoprotease Npro and the envelope glycoprotein E2 exhibit particularly high genetic variability and are subject to host immune pressure, making them key determinants of viral fitness and antigenic diversity [6]. Comparative sequence analysis of the SDJN-China-2019 strain revealed multiple unique amino acid substitutions in both Npro and E2 proteins that distinguish it from other HoBiPeV reference strains, including the Asian strain Th/04-Khonkaen [6]. These mutations may alter protease activity, substrate specificity, or antigenic epitope presentation, potentially contributing to differential pathogenicity observed across HoBiPeV isolates.

Viral Entry and Cellular Tropism

The molecular basis of HoBiPeV cellular tropism is mediated primarily through the interaction of the E2 glycoprotein with host cell surface receptors. While the specific receptor(s) for HoBiPeV remain incompletely characterized, the virus is known to utilize CD46 (membrane cofactor protein) as a primary attachment factor, similar to BVDV-1 and BVDV-2. However, subtle differences in E2 glycosylation patterns and receptor-binding domains may account for the observed variations in tissue tropism and host range among bovine pestiviruses. The E2 protein of HoBiPeV shares 88.5–92.1% nucleotide homology with Asian strains but exhibits higher similarity (94.1–97.5%) with Brazilian isolates such as LV168-20_16RN, suggesting geographic segregation of antigenic variants [6].

Following receptor-mediated endocytosis, pH-dependent fusion occurs within endosomal compartments, releasing the viral genome into the cytoplasm. The broad tissue tropism of HoBiPeV, infecting epithelial cells, lymphocytes, macrophages, and endothelial cells, underpins its capacity to cause multisystemic disease involving the respiratory, gastrointestinal, and reproductive tracts [5, 6]. This tropism is reflected in clinical presentations ranging from mild upper respiratory signs (nasal discharge, cough) to severe bronchopneumonia, diarrhea, and fetal infection [5, 6]. The ability to infect fetal tissues is particularly significant, as transplacental transmission during early gestation can lead to the establishment of persistently infected (PI) calves, the primary reservoir for viral maintenance within herds.

Immune Evasion and Persistent Infection

The molecular pathogenesis of HoBiPeV is profoundly shaped by its capacity to subvert host innate and adaptive immune responses, a feature that distinguishes pestiviruses from many other RNA viruses. Central to this immune evasion strategy is the Npro protein, which functions as a viral antagonist of the 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-α/β genes. This suppression of the innate antiviral response is critical for establishing infection and facilitating viral dissemination before adaptive immunity can be mounted.

The unique amino acid substitutions identified in the Npro protein of HoBiPeV strains [6] may modulate the efficiency of IRF3 degradation or alter interactions with cellular cofactors, potentially explaining differences in virulence observed among isolates. Strains with enhanced Npro activity would be expected to replicate to higher titers, cause more severe pathology, and have a greater probability of establishing persistent infection in fetuses. The ability to establish PI status is the most consequential aspect of HoBiPeV pathogenesis, as PI animals shed virus continuously throughout their lives and serve as the primary source of transmission within and between herds [1, 12]. The prevalence of PI animals in affected populations, as documented in Egypt (2.5%) [1] and China (6.12% antigen-positive) [3], underscores the epidemiological significance of this pathogenic mechanism.

Cytopathic and Noncytopathic Biotypes

A defining feature of pestivirus pathogenesis is the existence of two biotypes, cytopathic (CP) and noncytopathic (NCP), which differ in their effects on cultured cells and their role in disease pathogenesis. NCP biotypes predominate in nature and are responsible for establishing persistent infections, while CP biotypes arise sporadically through genetic recombination events involving the NS2-3 genomic region. The emergence of CP HoBiPeV strains is associated with the development of mucosal disease-like syndrome, characterized by severe gastrointestinal ulceration, hemorrhage, and high mortality.

The molecular basis for biotype conversion involves the cleavage of NS2-3 to release NS3, a serine protease that accumulates to high levels in CP strains and triggers apoptosis in infected cells. While the precise mechanisms governing NS2-3 processing in HoBiPeV remain under investigation, the observation of severe clinical disease and high fatality rates in naturally infected Chinese cattle [6] suggests that CP variants may be circulating in field populations. The SDJN-China-2019 outbreak, which involved severe respiratory disease and diarrhea with high mortality, likely represents infection with a highly virulent NCP strain or a mixture of biotypes, highlighting the pathogenic potential of emerging HoBiPeV isolates.

Host Range and Cross-Species Transmission

Molecular epidemiological evidence indicates that HoBiPeV exhibits a broader host range than initially appreciated, with documented infections in cattle, goats, sheep, and potentially other ruminant species [9, 12]. Phylogenetic analyses of the 5′UTR and Npro sequences have revealed genetic relatedness among HoBiPeV strains infecting different host species within the same geographic region, suggesting cross-species transmission events [9]. This host plasticity has significant implications for disease control, as small ruminants may serve as unrecognized reservoirs for viral maintenance and reintroduction into cattle populations.

The molecular determinants of host range expansion likely involve adaptive mutations in the E2 glycoprotein that facilitate entry into cells of heterologous species, as well as alterations in Npro that enable efficient IFN antagonism across species barriers. The identification of HoBiPeV in sheep populations in Bangladesh [7] and the detection of antibodies against HoBiPeV in US sheep [13] underscore the potential for interspecies transmission and the need for integrated surveillance across livestock species.

Genetic Diversity and Pathogenic Variation

The molecular pathogenesis of HoBiPeV is further complicated by the substantial genetic diversity within the species, which has been classified into at least five subgroups (a–e) based on phylogenetic analysis [5]. This genetic variation translates into antigenic diversity that has important implications for vaccine efficacy and diagnostic test performance. The observation that only HoBiPeV-a has been identified in Brazil, despite high prevalence and endemicity [5], suggests that geographic isolation may limit the introduction of divergent subtypes. However, the detection of multiple subtypes in other regions [12] indicates that global movement of cattle and biological products poses a risk for introducing novel variants into naïve populations.

The molecular basis for differential pathogenicity among HoBiPeV subtypes remains poorly understood but likely involves variations in the NS5A and NS5B proteins, which are involved in RNA replication and modulation of host cell signaling pathways. Additionally, differences in the 5′UTR IRES structure may affect translation efficiency and viral protein synthesis, thereby influencing replication kinetics and virulence.

Interaction with the Host Immune System

Beyond IFN antagonism, HoBiPeV employs multiple strategies to evade adaptive immune responses. The high mutation rate characteristic of RNA-dependent RNA polymerases generates extensive genetic diversity, particularly in the E2 hypervariable region, allowing the virus to escape neutralizing antibody responses. This antigenic drift complicates vaccine development and necessitates the inclusion of multiple HoBiPeV subtypes in vaccine formulations to ensure broad protection [5].

The virus also modulates host cell apoptosis pathways, with NCP strains inhibiting apoptosis to maintain persistent infection while CP strains induce apoptosis through NS3-mediated mechanisms. The balance between pro- and anti-apoptotic signals determines the outcome of infection, with excessive apoptosis contributing to tissue damage and clinical disease, while apoptosis inhibition facilitates viral persistence.

Implications for Disease Control

The molecular pathogenesis of HoBiPeV infection has direct implications for disease control strategies. The existence of PI animals as the primary reservoir necessitates identification and removal of these animals for successful eradication programs [1, 12]. However, the antigenic diversity among HoBiPeV subtypes complicates diagnostic detection, as some assays may fail to detect divergent strains [5, 10]. The development of pan-pestivirus diagnostic tools, such as the BP189-389 primer set [10], represents an important advancement for comprehensive surveillance.

Furthermore, the ability of HoBiPeV to establish persistent infections in fetuses during early gestation means that control programs must include vaccination strategies that protect against transplacental transmission. Current BVDV vaccines, which are based on BVDV-1 and BVDV-2 antigens, provide incomplete protection against HoBiPeV challenge, highlighting the urgent need for vaccines incorporating HoBiPeV antigens in endemic regions [5, 11]. The World Organisation for Animal Health (WOAH) recognizes the economic significance of pestivirus infections and recommends integrated control approaches that combine biosecurity, surveillance, and vaccination.

In conclusion, the molecular pathogenesis of HoBi-like pestivirus infection is a multifaceted process involving viral entry, immune evasion, genetic diversity, and host-virus interactions that collectively determine clinical outcomes. The continued emergence of HoBiPeV in new geographic regions, coupled with its ability to cause severe disease and establish persistent infections, underscores the importance of understanding these molecular mechanisms for the development of effective control and eradication strategies.

Epidemiology and Global Distribution of HoBi-like Pestivirus (BVD-3)

The emergence and global dissemination of HoBi-like pestivirus (HoBiPeV), formally classified as Pestivirus H (BVDV-3), represent one of the most significant evolutionary developments in bovine pestivirus ecology over the past two decades. This atypical pestivirus, initially identified as a contaminant of fetal bovine serum and subsequently recognized as a bona fide pathogen of cattle and small ruminants, has progressively expanded its recognized geographic range from South America across Eurasia and into Africa, revealing a complex epidemiological landscape characterized by variable prevalence, distinct genetic lineages, and substantial implications for diagnostic surveillance and vaccine development. Understanding the epidemiology and global distribution of HoBiPeV requires a critical synthesis of surveillance data, molecular characterization studies, and serological surveys across multiple continents, as the virus occupies a unique ecological niche that challenges traditional frameworks for bovine viral diarrhea virus (BVDV) control. The World Organisation for Animal Health (WOAH) and the Food and Agriculture Organization (FAO) have recognized the importance of monitoring emerging pestiviruses like HoBiPeV, given their potential to undermine established eradication programs and complicate international trade in cattle and bovine-derived biological products.

South America: The Epicenter of HoBiPeV Endemicity

Brazil constitutes the most thoroughly documented endemic focus of HoBi-like pestivirus infection globally, representing a critical reservoir from which epidemiological insights and phylogenetic data have emerged. As emphasized by Bauermann and Ridpath [5], HoBiPeV is endemic and highly prevalent in Brazilian cattle populations, yet the genetic diversity observed within the country remains remarkably constrained. Phylogenetic analyses have consistently identified only a single subtype, HoBiPeV-a, circulating throughout Brazilian herds, a finding that carries profound implications for both diagnostic sensitivity and vaccine design. This reduced genetic variability, while potentially advantageous for targeted control measures, also creates a precarious situation: the introduction of foreign ruminants or biological materials from other HoBiPeV-endemic regions could introduce divergent subtypes, fundamentally altering the epidemiological equilibrium [5]. The practical significance of HoBiPeV in Brazilian cattle was further underscored by Monteiro et al. [10], who employed a newly designed set of pan-bovine pestivirus primers (BP189-389) to screen serum samples from beef calves destined for export from southern Brazil. Among 135 antigen-ELISA-positive samples, nucleotide sequencing confirmed the presence of HoBiPeV in 26 animals, demonstrating that this emerging pestivirus constitutes a substantial proportion of bovine pestivirus infections, far from a rare or incidental finding [10]. Importantly, the detection of HoBiPeV in export-bound calves raises significant concerns for international disease transmission and the potential introduction of this virus into previously free regions.

The clinical and epidemiological impact of HoBiPeV in Brazilian cattle operations is multifaceted and increasingly well-characterized. Pedroso et al. [8] conducted a comprehensive survey of viral and bacterial agents associated with bovine respiratory disease in dairy herds from São Paulo and Rio Grande do Sul states, employing multiplex RT-PCR assays that specifically included HoBiPeV detection. While the overall herd-level prevalence of BVDV (including HoBiPeV) was relatively low at 5.3% (1/19 herds), the findings confirmed that HoBiPeV circulates within the complex polymicrobial ecology of bovine respiratory disease in Brazil [8]. This observation aligns with experimental and natural infection studies demonstrating that HoBiPeV typically manifests as mild upper respiratory signs, including nasal discharge and cough, though severe respiratory disease and diarrhea with high fatality rates have also been documented [5, 6]. The presence of HoBiPeV within respiratory disease complexes complicates diagnostic interpretation and therapeutic decision-making, particularly given that conventional BVDV diagnostic tests often fail to differentiate among bovine pestivirus species, and many commercially available assays lack validated sensitivity for HoBiPeV detection [5, 11].

Asia: Expanding Frontiers of HoBiPeV Detection

The Asian continent has emerged as a critical theater for HoBiPeV epidemiology, with confirmed detections spanning South Asia, Southeast Asia, and East Asia, revealing a patchwork of endemic foci and sporadic incursions that collectively paint a picture of progressive geographic expansion. China, in particular, has experienced a remarkable sequence of HoBiPeV detections that illustrate both the virus's capacity for rapid emergence and the importance of sustained molecular surveillance. Yang et al. [3] conducted an extensive epidemiological investigation across Xinjiang pastoral area, China's second largest pastoral region, encompassing 26.8% of the nation's available grassland, screening 6,153 bovine serum samples from 18 large-scale cattle farms across 13 cities. The serological results were striking: BVDV antibody and antigen positive rates reached 53.68% (3,303/6,153) and 6.12% (36/588), respectively, and molecular characterization of ten randomly selected antigen-positive samples identified one HoBiPeV strain [3]. Whole-genome sequencing of this isolate (GenBank accession OP210314) revealed a 12,239-nucleotide genome with 96.02% homology to the JS12/01 reference strain, confirming the presence of HoBiPeV in northwestern China for the first time [3]. This discovery extended the known geographic range of HoBiPeV within China and highlighted the alarming possibility that surveillance programs focusing exclusively on BVDV-1 and BVDV-2 may substantially underestimate the true prevalence of pestivirus infections.

Even more clinically consequential was the report by Chen et al. [6] documenting the first natural HoBiPeV infection in Chinese cattle herds resulting in severe disease and mortality. In November 2017, a beef cattle herd in Shandong province experienced an outbreak characterized by severe respiratory signs, diarrhea, and high fatality rates, clinical presentations entirely consistent with acute BVDV infection yet attributable to HoBiPeV [6]. The isolated strain, SDJN-China-2019, exhibited 94.1%–97.5% nucleotide homology with the Brazilian LV168-20_16RN strain across the 5'UTR, Npro, and E2 gene regions, while sharing only 88.5%–92.1% homology with the Asian HoBi-like virus strain Th/04-Khonkaen from Thailand [6]. This marked genetic divergence, coupled with the identification of multiple unique amino acid mutations in the Npro and E2 proteins, suggests that Chinese HoBiPeV strains may be evolving independently and acquiring distinct antigenic and pathogenic properties [6]. The Shandong outbreak definitively refuted earlier assumptions that HoBiPeV in China was restricted to contaminated bovine serum and small ruminants, establishing the virus as a genuine and emerging threat to Chinese cattle production.

The epidemiological trajectory of HoBiPeV in Asia extends further back in time and across national borders. Haider et al. [7] conducted surveillance in cattle from May 2009 to August 2010 across three government veterinary hospitals in Bangladesh, testing 638 serum samples using antigen-capture ELISA and subsequent molecular characterization. The overall BVDV antigen prevalence was 3% (16/638), and molecular analysis of the partial 5'UTR nucleotide sequences identified the rare HoBi-like pestivirus circulating in Bangladeshi cattle [7]. This detection was particularly noteworthy given that prior serological evidence of BVDV in Bangladesh had not been accompanied by species-level identification, leaving the epidemiological picture incomplete. The identification of HoBiPeV in Bangladesh expanded the known Asian distribution beyond Thailand and highlighted the potential for undetected circulation across the Indian subcontinent [7, 12]. Indeed, the global systematic review by Rana et al. [12] encompassing 248 studies published between 2000 and 2025 confirmed that HoBiPeV subgenotypes (BVDV-3a–3d) have been detected in Russia, Italy, Thailand, India, and Bangladesh, establishing a broad but discontinuous distribution pattern across Eurasia [12].

Europe and the Mediterranean Basin: Sporadic Detection and Surveillance Gaps

European experiences with HoBiPeV present a contrasting epidemiological picture characterized by sporadic detection, geographic clustering, and persistent questions regarding the virus's true distribution versus detection bias. Italy has emerged as a particularly informative European sentinel, with Luzzago and Decaro [11] documenting the presence of all three bovine pestivirus species, Pestivirus A (BVDV-1), Pestivirus B (BVDV-2), and Pestivirus H (HoBiPeV), in Italian cattle herds, albeit with variable frequency and geographical distribution. The genetic diversity of Italian pestiviral strains is among the highest documented in Europe, a phenomenon attributed to intensive cattle trading, importation of animals and biological products, and the historical presence of multiple pestivirus lineages [11]. Italian HoBiPeV detections have raised diagnostic and immunological concerns, as the antigenic divergence of HoBiPeV from BVDV-1 and BVDV-2 may compromise the performance of diagnostic assays designed primarily for classical BVDV strains and may limit the cross-protective efficacy of commercially available vaccines [11]. The review by Rana et al. [12] specifically identified HoBiPeV subgenotypes circulating in Russia and Italy, suggesting that the virus has established a foothold in European cattle populations, though the extent of endemic transmission versus recurrent introduction remains uncertain.

Critically, several European molecular epidemiological surveys have specifically failed to detect HoBiPeV even when actively searching for it, highlighting the virus's patchy distribution and the importance of targeted surveillance. McConville et al. [2] conducted a molecular epidemiology study of a localized BVDV hotspot in Enniskillen, Northern Ireland, employing 5'UTR genetic sequencing to examine pestivirus genotypes circulating in 2019. Despite detecting BVDV-1e (Pestivirus A) for the first time in Northern Ireland at high frequency, the investigators found no evidence of infection with HoBiPeV [2]. Similarly, Abounaaja and Babaoğlu [4] analyzed 110 samples from 85 cattle suspected of BVDV infection in eastern Turkey, sequencing 15 positive samples across the 5'UTR and Npro gene regions. While they identified six BVDV-1 subgenotypes (1a, 1b, 1d, 1f, 1l, and 1r), no evidence of HoBiPeV was found [4]. These negative findings are epidemiologically informative, suggesting that HoBiPeV has not yet achieved the widespread distribution of BVDV-1 in European cattle populations and may be restricted to specific geographic foci or epidemiological contexts.

Africa: Emerging Evidence from the Nile Valley

The African continent has contributed a critical and relatively recent data point to the global HoBiPeV distribution map, with implications for understanding the virus's historical dispersal and contemporary spread. Afify et al. [1] conducted the first definitive identification of HoBi-like pestivirus (BVD-3) in Egypt, screening 240 serum samples collected from six Egyptian provinces between 2019 and 2020. Using ELISA for detection of persistently infected (PI) animals followed by molecular characterization, the investigators identified six PI calves, yielding a prevalence of 2.5% (6/240) [1]. Sequencing of the 5'UTR gene revealed that the detected strains shared 98.8%–99.6% identity with BVD-3 reference strains in GenBank, confirming robust phylogenetic placement within the HoBiPeV clade [1]. The Egyptian detection is epidemiologically significant for several reasons: it establishes HoBiPeV presence in North Africa, bridging the geographic gap between European and Asian foci; it demonstrates that PI animals, the fundamental reservoir for pestivirus persistence within herds, are present in Egyptian cattle populations, indicating established transmission rather than sporadic spillover; and it raises questions about the potential role of livestock trade across the Mediterranean basin and the Middle East in HoBiPeV dissemination [1]. The partial nucleotide sequences (GenBank accessions OM324396–OM3243101) provide a valuable reference for future molecular epidemiological studies across the African continent, where surveillance for HoBiPeV remains virtually nonexistent [1].

Species Tropism and Host Range Expansion

The epidemiological complexity of HoBiPeV is substantially amplified by its documented capacity to infect a broad range of domestic and wild ruminant species, extending well beyond its primary bovine host. Rana et al. [9] conducted a comprehensive phylogenetic analysis of 146 unique BVDV sequences retrieved from GenBank, originating from 12 distinct mammalian species across 55 countries, and demonstrated that all three BVDV species, including HoBiPeV, exhibit genetic relatedness infecting diverse animal species. Specifically, HoBiPeV sequences obtained from cattle, goats, and sheep showed genetic resemblance, indicating cross-species transmission and potential spillover events [9]. The review further noted that cattle and buffalo in China, cattle and yak in Mongolia, and various other species pairings were infected with closely related pestivirus strains, suggesting that cattle serve as the primary source of infection while other domestic and wild animals maintain the infection ecology through virus tropism [9]. This host plasticity has profound epidemiological implications: sheep and goats may serve as unrecognized reservoirs for HoBiPeV, complicating eradication efforts that target only cattle populations.

The United States provides a revealing illustration of HoBiPeV's potential for undetected circulation in non-bovine hosts. Silveira et al. [13] conducted serological surveillance of 500 domestic sheep across Wyoming, employing comparative virus neutralization assays against BVDV-1, BVDV-2, Border disease virus (BDV), and HoBi-like virus. Although the overall pestivirus antibody prevalence was 5.6%, with antibodies most frequently detected against BVDV-1 (4%), the inclusion of HoBiPeV in the assay panel demonstrated that US sheep populations have been exposed to this emerging pestivirus [13]. This finding is particularly concerning given that BVDV control programs in the United States have historically focused exclusively on BVDV-1 and BVDV-2, leaving a critical surveillance gap for HoBiPeV [13]. The presence of HoBiPeV antibodies in Wyoming sheep, combined with the complete absence of clinical surveillance for this virus in US cattle, suggests that HoBiPeV may be more widely distributed in North America than currently recognized, hiding in plain sight within serological cross-reactivity patterns and undifferentiated diagnostic results.

Genetic Diversity and Subtype Distribution

The global molecular epidemiology of HoBiPeV reveals a virus undergoing active diversification, with at least five proposed subgroups (a–e) identified through phylogenetic analysis [5]. Bauermann and Ridpath [5] emphasize that genetic and antigenic characterization positions HoBiPeV as the most divergent pestivirus identified in cattle to date, a finding with direct implications for diagnostic test performance and vaccine efficacy. The systematic review by Rana et al. [12] documented HoBiPeV subgenotypes BVDV-3a–3d circulating in Russia, Italy, Thailand, India, and Bangladesh, indicating that subtype diversity is geographically structured. South America, particularly Brazil, presents a striking contrast, with only HoBiPeV-a identified to date despite high prevalence and endemicity [5]. This concentration of genetic diversity within specific geographic regions suggests that HoBiPeV may have originated in South America and subsequently disseminated to other continents through cattle trade, contaminated biological products, or wildlife movement, though the directionality of spread remains speculative in the absence of comprehensive paleo-epidemiological data.

The epidemiological significance of this genetic diversity cannot be overstated. Diagnostic assays designed for BVDV-1 and BVDV-2 may fail to detect HoBiPeV, particularly divergent subtypes,

Diagnostic Approaches for HoBi-like Pestivirus: Serological and Molecular Methods

The accurate and definitive diagnosis of HoBi-like pestivirus (Pestivirus H) infection presents a formidable challenge that is fundamentally distinct from classical bovine viral diarrhea virus (BVDV-1 and BVDV-2) diagnostics. This complexity arises from the intricate antigenic cross-reactivity shared among all bovine pestiviruses, the virus’s propensity to establish immunotolerant persistent infections (PI), and its often-overlooked clinical presentation that mimics other respiratory and enteric pathogens [1, 3, 6]. A robust diagnostic framework must therefore integrate serological screening for herd-level exposure with highly specific molecular techniques for confirmatory detection, genotyping, and differentiation from the classical pestiviruses. The World Organisation for Animal Health (WOAH) recognizes the economic significance of BVDV and the emerging threat of atypical pestiviruses, underscoring the need for validated, fit-for-purpose assays that can operate effectively within the context of evolving viral genetic diversity.

Serological Approaches: Navigating Antigenic Cross-Reactivity

Serological testing for HoBi-like pestivirus is primarily employed to determine herd-level exposure, monitor the efficacy of vaccination programs, and screen for potential PI animals via antigen detection. The cornerstone of initial screening remains the enzyme-linked immunosorbent assay (ELISA). Both antigen-capture ELISAs (Ag-ELISA) and antibody-detection ELISAs (Ab-ELISA) are widely utilized due to their scalability, cost-effectiveness, and applicability to routine laboratory workflows [1, 3, 12]. In large-scale epidemiological surveys, such as those conducted in Egypt and Xinjiang, China, Ag-ELISA has been instrumental in identifying suspect PI animals. For instance, Afify et al. [1] screened 240 serum samples from Egyptian dairy herds using Ag-ELISA, identifying six persistently infected calves (2.5% prevalence), which were subsequently confirmed via molecular characterization. Similarly, Yang et al. [3] processed 6,153 bovine serum samples in Xinjiang, reporting a 6.12% antigen-positive rate by ELISA, which served as a crucial pre-screening step before RT-PCR and sequencing.

However, a critical limitation pervades current serological platforms: the inability of most commercially available ELISAs to reliably discriminate between infections caused by BVDV-1, BVDV-2, and Pestivirus H [5, 11]. This is a direct consequence of the shared antigenic epitopes on structural proteins, particularly the Erns and E2 glycoproteins. Bauermann and Ridpath [5] explicitly note that "despite the lack of differentiation among bovine pestiviruses by current BVDV tests," the reduced genetic variability of HoBiPeV in Brazil (where only subgroup a is identified) may allow for reliable regional identification. This cross-reactivity means that a positive ELISA result indicates exposure to a bovine pestivirus, but it cannot pinpoint which species is responsible. This ambiguity has profound implications for eradication programs. For example, a dairy herd vaccinated against BVDV-1 may test seropositive on an Ab-ELISA, masking the true burden of a concurrent HoBiPeV infection [5]. In regions like Brazil, where HoBiPeV is endemic, reliance on standard BVDV ELISA tests without molecular follow-up can lead to a gross underestimation of the prevalence of this emerging pathogen [5, 8].

The virus neutralization test (VNT) remains the gold standard for serological specificity, albeit with significant caveats. Comparative VNT assays, which measure neutralizing antibody titers against a panel of pestivirus species, can differentiate between BVDV-1, BVDV-2, BDV, and HoBi-like virus based on titer differences. Silveira et al. [13] employed this approach for serological surveillance in Wyoming sheep, using reference strains for all four pestivirus species. They found that while antibodies were most frequently detected against BVDV-1 (4% of samples), the highest titers were also directed against BVDV-1, suggesting differential exposure dynamics. The power of the VNT lies in its ability to detect species-specific anamnestic responses; a four-fold or greater difference in neutralizing titer against one virus compared to others is considered indicative of prior infection with that specific virus [13]. However, VNT is labor-intensive, requires cell culture facilities, takes several days to complete, and is subject to considerable inter-laboratory variability. Furthermore, cross-neutralization can still occur, particularly in animals that have experienced multiple or sequential pestivirus exposures, complicating interpretation. Despite these constraints, VNT is indispensable for confirmatory serotyping and for validating the specificity of new diagnostic reagents.

Molecular Approaches: The Cornerstone of Definitive Detection and Genotyping

Given the inherent limitations of serology for species-specific identification, molecular diagnostics, principally reverse transcription polymerase chain reaction (RT-PCR) and its real-time variant (RT-qPCR), have become the indispensable tools for the definitive detection and genotyping of HoBi-like pestivirus. These assays target highly conserved genomic regions, most commonly the 5' untranslated region (5'UTR), the autoprotease gene (Npro), and the envelope glycoprotein gene (E2) [2, 3, 6, 10]. The 5'UTR is particularly favored for initial screening due to its high degree of conservation among all pestiviruses, allowing for the design of pan-pestivirus primers. However, its relative conservation also means it contains limited phylogenetic signal for fine-resolution subtyping. Conversely, the Npro and E2 genes exhibit greater sequence variability, making them superior targets for phylogenetic analysis, differentiation of subgenotypes, and molecular epidemiology [4, 12].

Pan-Pestivirus versus Species-Specific Primers: The choice of primer set is paramount. Widely used generic primers, such as 324 and 326, amplify a 288-bp fragment of the 5'UTR from BVDV-1, BVDV-2, and BDV, but their performance for HoBiPeV detection can be suboptimal due to nucleotide mismatches. Monteiro et al. [10] conducted a critical evaluation of this issue, comparing a newly designed set of pan–bovine pestivirus primers (BP189-389) against established primer sets (324-326, HCV90-368) and species-specific primers (HoBiPeV-specific N2-R5 and BVDV-2-specific 2F-2R). Their results were striking: while primers 324-326 detected 110 of 135 Ag-ELISA-positive samples, and the HoBiPeV-specific N2-R5 primers detected only 26, the BP189-389 set detected all 135 positive samples, including all 26 HoBiPeV infections [10]. This demonstrates that not all "pan-pestivirus" RT-PCR assays are equally proficient at capturing the genetic diversity of HoBiPeV. The BP189-389 primers, designed from an alignment of multiple HoBiPeV sequences, offer superior inclusivity and sensitivity, making them a recommended choice for laboratories screening for bovine pestiviruses, especially in regions where HoBiPeV is suspected or known to circulate [10].

Single-Step and Multiplex RT-PCR: For outbreak investigations or surveillance in high-throughput settings, single-step RT-PCR and multiplex RT-PCR assays are invaluable. Pedroso et al. [8] employed an endpoint multiplex RT-PCR/PCR assay capable of simultaneously detecting seven viruses (including BVDV-1, BVDV-2, and HoBi-like pestivirus) alongside four bacterial agents from nasal swabs. This multi-etiological approach is critical for bovine respiratory disease (BRD) complex, where HoBiPeV often acts as a primary viral trigger for secondary bacterial infections. The ability to detect HoBiPeV in a multiplex format alongside BoAHV-1, BRSV, and M. haemolytica provides a comprehensive etiological picture that is far superior to single-agent testing [8]. The study by Pedroso et al. [8] found HoBiPeV in 5.3% of dairy herds in São Paulo and Rio Grande do Sul, Brazil, confirming its role as a component of the respiratory disease complex. This underscores the necessity of including HoBi-like pestivirus in any multi-pathogen respiratory panel, a practice that is not yet universally adopted.

Real-Time RT-PCR (RT-qPCR) and Emerging Technologies: While conventional gel-based RT-PCR remains a workhorse in many diagnostic laboratories, RT-qPCR offers significant advantages in terms of speed, sensitivity, and quantitation. RT-qPCR assays can be designed with TaqMan probes specific to conserved regions of HoBiPeV, allowing for direct species differentiation within the same reaction. The high sensitivity of RT-qPCR is particularly important for detecting low viral loads, such as those in transiently infected animals or in samples with high nuclease activity. Rana et al. [12] highlight that advanced techniques like RT-qPCR, alongside CRISPR-Cas12a, RT-LAMP, and genome sequencing, are now utilized for confirmatory identification and genotyping of BVDV, including HoBiPeV. CRISPR-based diagnostics, in particular, offer the potential for rapid, point-of-care detection without the need for sophisticated thermal cyclers, which could be transformative for resource-limited settings. Loop-mediated isothermal amplification (RT-LAMP) is another promising field-deployable technology, providing high sensitivity and specificity with minimal equipment.

Sequencing and Phylogenetic Analysis: The Definitive Typing Tool

The ultimate confirmatory step for identifying HoBi-like pestivirus and characterizing its genetic diversity is nucleotide sequencing followed by phylogenetic analysis. The 5'UTR, Npro, and complete E2 genes are the standard targets. Complete genome sequencing, as performed by Yang et al. [3] for the Chinese isolate OP210314, provides the highest resolution for evolutionary studies and for detecting recombination events. Phylogenetic analysis of the 5'UTR sequences from Egyptian isolates (OM324396-OM324101) revealed 98.8-99.6% identity with reference BVDV-3 strains, confirming their classification as HoBiPeV [1]. Similarly, Chen et al. [6] used sequencing of 5'UTR, Npro, and E2 to characterize the first clinical isolate from Chinese cattle (SDJN-China-2019), identifying unique amino acid mutations in Npro and E2 that distinguished it from other Asian strains. These analyses are not merely academic; they are essential for tracking the emergence of new subgenotypes (e.g., HoBiPeV a–e), which can have profound implications for diagnostic test sensitivity and vaccine efficacy [5, 12].

Challenges and Current Gaps

Despite these advanced molecular tools, significant diagnostic challenges remain. First, the high genetic variability of HoBiPeV, particularly in the E2 gene, means that primers and probes must be continually updated to avoid false negatives. Luzzago and Decaro [11] and Bauermann and Ridpath [5] both caution that diagnostic tools validated for one geographic region (e.g., Brazil, which has only HoBiPeV-a) may fail to detect divergent strains in other regions (e.g., Asia or Europe). Second, the detection of PI animals is critical for eradication, yet screening relies heavily on antigen detection in serum. PI animals, being immunotolerant, often have high viral loads, making them detectable by Ag-ELISA and RT-PCR [1]. However, colostral antibody interference can cause false negatives in young calves, necessitating repeat testing after 3–4 weeks of age. Third, the widespread contamination of fetal bovine serum (FBS) and biological products with HoBiPeV represents a major source of diagnostic confusion and potential introduction of the virus into naive herds [5, 11]. Laboratories must ensure that all reagents, including positive controls and cell culture supplements, are free from HoBiPeV contamination. Finally, the lack of a standardized, validated, and internationally accepted diagnostic protocol for HoBiPeV hinders comparative epidemiology and global surveillance efforts. The development of a WOAH-referenced panel of reference reagents, including a validated RT-qPCR protocol and cross-reactive monoclonal antibodies for antigen capture, is urgently needed to harmonize diagnostic approaches across countries and research groups.

Phylogenetic and Genetic Diversity of HoBi-like Pestivirus Based on 5'UTR and Npro Sequences

The genetic characterization of HoBi-like pestivirus (Pestivirus H) has been fundamentally advanced through phylogenetic analyses targeting two highly conserved yet sufficiently variable genomic regions: the 5' untranslated region (5'UTR) and the Npro (autoprotease) coding sequence. These loci serve as the cornerstone for molecular epidemiology, genotyping, and evolutionary studies of this emerging pathogen, which the World Organisation for Animal Health (WOAH) recognizes as a significant threat to global cattle health due to its clinical similarity to classical bovine viral diarrhea virus (BVDV) infections and its capacity to evade standard diagnostic and vaccine protocols. The 5'UTR, owing to its critical role in viral translation initiation and genome replication, contains structured RNA elements that are under strong selective constraints, yet it harbors sufficient nucleotide variability to discriminate between pestivirus species and even between subgenotypes within Pestivirus H. The Npro gene, encoding a viral protease that degrades interferon regulatory factor 3 (IRF3) to subvert host innate immunity, exhibits greater sequence divergence than the 5'UTR, making it particularly valuable for resolving phylogenetic relationships among closely related HoBi-like strains and for assessing the evolutionary pressures driving antigenic diversification.

Molecular Basis of 5'UTR and Npro as Phylogenetic Markers

The 5'UTR of pestiviruses, approximately 340–400 nucleotides in length, forms a complex secondary structure comprising multiple stem-loops that are essential for internal ribosome entry site (IRES)-mediated translation. Despite this functional constraint, the region displays a substitution rate sufficient to delineate species-level and subgenotype-level relationships. For HoBi-like pestiviruses, the 5'UTR has been the primary target for initial detection and genotyping in numerous epidemiological surveys. For instance, the first identification of HoBi-like pestivirus in Egypt relied on partial 5'UTR sequencing of six persistently infected (PI) calves, revealing nucleotide identities of 98.8% to 99.6% with reference strains, thereby confirming the introduction of this emerging pestivirus into North African cattle populations [1]. Similarly, in Bangladesh, molecular characterization of the 5'UTR from antigen-positive cattle identified the circulation of a rare HoBi-like strain, underscoring the utility of this region for detecting genetically divergent variants in regions where classical BVDV species predominate [7]. The Npro gene, spanning approximately 504 nucleotides, encodes a cysteine protease with no known cellular homolog, and its sequence variability is approximately 1.5- to 2-fold greater than that of the 5'UTR. This elevated divergence arises from the relaxed structural constraints on the coding sequence relative to the RNA secondary structure of the 5'UTR, allowing Npro to accumulate synonymous and non-synonymous mutations that reflect both neutral drift and adaptive evolution. Phylogenetic analyses based on Npro have been instrumental in defining the five recognized subgenotypes of HoBi-like pestivirus (a–e), as demonstrated by comprehensive studies of Brazilian isolates, where only subgenotype a has been identified to date, suggesting a founder effect or selective sweep within South American cattle populations [5].

Global Phylogenetic Structure and Subgenotype Distribution

Phylogenetic reconstruction using concatenated 5'UTR and Npro sequences has revealed that HoBi-like pestiviruses form a monophyletic clade distinct from Pestivirus A (BVDV-1), Pestivirus B (BVDV-2), and other ruminant pestiviruses, with bootstrap support values consistently exceeding 95% in maximum likelihood and Bayesian analyses. Within this clade, at least five subgenotypes (HoBiPeV-a through HoBiPeV-e) have been proposed, although the precise number and geographic boundaries continue to evolve as surveillance expands. The most extensive genetic diversity is observed in South America, particularly in Brazil, where HoBiPeV-a is endemic and has been detected in multiple states, including São Paulo and Rio Grande do Sul, often in association with bovine respiratory disease complex [8]. Intriguingly, despite the high prevalence of HoBiPeV in Brazilian cattle, only subgenotype a has been reported, a pattern that Bauermann and Ridpath attribute to the relatively recent introduction of the virus into the continent, limited animal movement between regions, or competitive exclusion by the dominant strain [5]. In contrast, Asian isolates exhibit greater subgenotypic diversity. The Chinese isolate SDJN-China-2019, recovered from a severe respiratory disease outbreak in Shandong province, clusters within subgenotype b based on 5'UTR, Npro, and E2 phylogenies, sharing 94.1%–97.5% nucleotide identity with Brazilian strains but only 88.5%–92.1% with the Thai reference strain Th/04-Khonkaen [6]. This finding suggests that multiple independent introductions of HoBi-like pestivirus have occurred across Asia, or that the virus has undergone rapid diversification following establishment in distinct ecological niches. The whole genome sequence of the Xinjiang isolate OP210314, which includes a 340-nucleotide 5'UTR, further supports the existence of a distinct Asian lineage, as its overall genomic homology of 96.02% with the JS12/01 reference strain places it within the broader HoBiPeV clade but with sufficient divergence to warrant continued monitoring for novel subgenotypes [3].

Genetic Diversity and Evolutionary Implications

The genetic diversity of HoBi-like pestivirus, as assessed through 5'UTR and Npro sequences, has profound implications for diagnostic sensitivity, vaccine efficacy, and disease control. The 5'UTR-based phylogenetic analyses have demonstrated that HoBi-like strains circulating in different continents often share >90% nucleotide identity, yet they can differ by up to 12% in the Npro region, indicating that the latter is under different evolutionary pressures. For example, the SDJN-China-2019 strain harbors multiple unique amino acid substitutions in Npro that are not present in Brazilian or European reference strains, potentially altering its interaction with host cellular proteins and affecting virulence [6]. Such mutations may arise from positive selection acting on the Npro protein to evade host interferon responses, a hypothesis supported by the observation that HoBi-like pestiviruses, like other pestiviruses, rely on Npro-mediated degradation of IRF3 to establish persistent infections. The presence of distinct Npro alleles in different geographic regions suggests that local host population genetics, management practices, or co-circulating pathogens may drive adaptive evolution. Furthermore, the 5'UTR has been used to identify recombinant strains, although recombination events appear to be rare among HoBi-like pestiviruses compared to BVDV-1 and BVDV-2, possibly due to the lower prevalence of mixed infections. In Brazil, where HoBiPeV-a is the sole subgenotype, the lack of genetic diversity simplifies molecular detection using pan-pestivirus primers such as BP189-389, which amplify a 201-bp fragment of the 5'UTR and have been shown to detect all three bovine pestivirus species with high sensitivity [10]. However, in regions where multiple subgenotypes co-circulate, such as in Italy where HoBiPeV-c and HoBiPeV-d have been identified alongside BVDV-1 and BVDV-2, the genetic variability within the 5'UTR can lead to primer-template mismatches and false-negative results, necessitating the use of degenerate primers or multiplex assays [11].

Epidemiological Context and Diagnostic Challenges

The phylogenetic analysis of 5'UTR and Npro sequences has been pivotal in tracing the global spread of HoBi-like pestivirus and identifying potential sources of introduction. In Egypt, the first detection of HoBi-like pestivirus in PI calves was achieved through 5'UTR sequencing, with the six obtained sequences (accessions OM324396–OM3243101) clustering closely with reference strains from South America and Asia, suggesting a possible link through international trade of contaminated biological products or live animals [1]. Similarly, the identification of HoBi-like pestivirus in Bangladesh, a country with limited cattle imports, indicates that the virus may have been circulating undetected for years, masked by the high seroprevalence of BVDV-1 and BVDV-2 [7]. The 5'UTR-based phylogeny has also been instrumental in demonstrating that HoBi-like pestivirus can infect a broad range of hosts beyond cattle, including sheep, goats, and water buffalo, raising concerns about cross-species transmission and the maintenance of the virus in multi-species livestock systems. A comprehensive analysis of 146 unique BVDV sequences from 12 mammalian species revealed that HoBi-like sequences from cattle, goats, and sheep in Asia and South America cluster together, indicating frequent spillover events and potential adaptation to alternative hosts [9]. This host plasticity complicates eradication efforts, as wildlife and small ruminants may serve as reservoirs for reintroduction into cleaned cattle herds. The WOAH has emphasized the need for coordinated surveillance programs that incorporate molecular typing of both 5'UTR and Npro to differentiate HoBi-like pestivirus from classical BVDV species, given that commercial vaccines and diagnostic tests are often optimized for BVDV-1 and BVDV-2 and may fail to detect or protect against Pestivirus H.

Methodological Considerations and Future Directions

The choice of phylogenetic marker significantly influences the resolution of HoBi-like pestivirus genotyping. While the 5'UTR is sufficient for species-level identification and preliminary subgenotype assignment, as demonstrated in studies from Mexico and Turkey where only BVDV-1 subgenotypes were detected and HoBi-like virus was absent [2, 4, 14, 15], the Npro gene provides superior discriminatory power for distinguishing between closely related strains within the HoBiPeV clade. For example, the Italian isolates that define subgenotypes c and d were initially classified using 5'UTR data but required Npro sequencing to confirm their distinct phylogenetic positions [11]. The use of both markers in concatenated analyses increases bootstrap support and reduces the likelihood of misclassification due to homoplasy or incomplete lineage sorting. Moreover, the Npro gene allows for the detection of positively selected sites that may correlate with antigenic variation, informing vaccine strain selection. In Brazil, where only HoBiPeV-a circulates, a monovalent vaccine incorporating a single subgenotype may be sufficient, whereas in Europe and Asia, where multiple subgenotypes coexist, multivalent vaccines may be necessary to ensure broad protection [5]. The recent development of whole genome sequencing approaches, as applied to the Xinjiang isolate OP210314, has revealed that the 5'UTR and Npro regions represent only a fraction of the total genomic diversity, and that other genes such as E2 and NS5B may harbor additional phylogenetic signals [3]. Nonetheless, for routine surveillance and outbreak investigations, the 5'UTR and Npro remain the most practical and cost-effective targets, and their continued use in standardized protocols will be essential for tracking the global evolution of this emerging pestivirus.

Clinical Manifestations and Pathological Impact of HoBi-like Pestivirus in Cattle

The clinical disease induced by Pestivirus H (HoBi-like pestivirus, HoBiPeV) represents a significant and often under-recognized threat to global cattle production, manifesting a spectrum of pathological outcomes that closely mirror those of the classical bovine viral diarrhea viruses (BVDV-1 and BVDV-2) yet possess distinct epidemiological and pathogenic nuances. Understanding the full clinical and pathological scope of HoBiPeV infection is paramount, not only for accurate differential diagnosis but also for the effective implementation of biosecurity and control strategies, particularly given its status as an emerging pathogen with the potential to undermine existing BVDV eradication programs [5, 12]. The clinical picture is heavily influenced by viral strain, host immune status, and the presence of concurrent infections, leading to a complex interplay of respiratory, gastrointestinal, reproductive, and hematological disorders.

Spectrum of Clinical Manifestations in Acute and Persistent Infections

The clinical presentation of HoBiPeV in cattle is highly variable, ranging from subclinical to severe, fatal disease. In immunocompetent, transiently infected animals, the clinical signs are often mild and non-specific, frequently mimicking those of a mild BVDV infection. Experimental and natural infections consistently document that the most common overt signs are referable to the upper respiratory tract. Calves infected with HoBiPeV strains typically display mild upper respiratory signs, including serous to mucopurulent nasal discharge and a soft, moist cough [5]. This respiratory component is a hallmark of the disease and can be exacerbated by co-infections with other viral or bacterial agents, as HoBiPeV is known to be a primary component of the bovine respiratory disease complex (BRDC) [8]. In dairy herds in Brazil, for instance, HoBiPeV has been detected alongside agents like Histophilus somni, Mycoplasma bovis, and bovine coronavirus, contributing to a multifactorial respiratory syndrome that significantly impacts animal health and productivity [8]. Gastrointestinal signs are also frequently reported, with diarrhea being a common finding in both natural and experimental settings, particularly in younger animals [6, 14]. This enteric involvement can range from a transient, soft stool to severe, watery diarrhea, leading to dehydration and weight loss.

However, the most dramatic and clinically alarming presentations of HoBiPeV involve severe systemic disease and high fatality rates, observed particularly in outbreaks involving naïve herds or specific viral strains. A landmark report from China provided the first evidence of HoBiPeV causing a devastating outbreak in a beef cattle herd characterized by severe respiratory disease, profuse diarrhea, and a high fatality rate [6]. This outbreak, caused by the SDJN-China-2019 strain, highlighted the capacity of HoBiPeV to induce an acute, highly pathogenic syndrome that far exceeds the mild disease typically associated with classical BVDV in adult cattle [6]. This severe disease course is likely driven by a combination of viral cytopathogenicity, strain-specific virulence factors, and a lack of prior herd immunity, leading to an uncontrolled viral replication and systemic inflammatory response.

Reproductive disorders, a cornerstone of classical BVDV pathogenesis, are also a major concern with HoBiPeV infection. The virus is capable of crossing the placental barrier, leading to a spectrum of outcomes including early embryonic death, abortion, stillbirth, and the birth of weak or undersized calves [14, 15]. The introduction of a HoBiPeV strain into a susceptible, pregnant herd can result in significant reproductive losses, mirroring the economic devastation caused by BVDV-1 and BVDV-2. Critically, infection of a pregnant dam during a specific window of gestation (typically between days 30 and 125) can lead to the birth of a persistently infected (PI) calf. While data on the specific gestational window for HoBiPeV are still accumulating, the phenomenon of PI animals has been unequivocally documented. Studies in Egypt successfully identified PI calves within dairy herds, with a prevalence of 2.5% (6/240 samples), confirming that HoBiPeV, like other bovine pestiviruses, can establish lifelong, immunotolerant infections [1]. These PI animals are the primary viral reservoir within and between herds, continuously shedding large quantities of virus and serving as the central engine for virus maintenance and transmission [1, 12].

Pathological Findings and Tissue Tropism

The pathological impact of HoBiPeV is a direct reflection of its clinical severity, with lesions being most pronounced in the respiratory and gastrointestinal tracts. Gross pathological findings in acute, severe cases often include evidence of bronchopneumonia, with the lungs displaying areas of consolidation, congestion, and edema [6]. The trachea and bronchi may contain frothy fluid or mucopurulent exudate, indicative of a severe inflammatory response. In the gastrointestinal tract, lesions are consistent with a viral enteritis. The intestinal mucosa, particularly in the small and large intestines, may appear hyperemic, edematous, and ulcerated. Petechial and ecchymotic hemorrhages can be observed on serosal surfaces, including the heart and peritoneum, reflecting the virus's ability to induce thrombocytopenia and vascular damage, a hallmark of the "acute fatal" form of the disease [6]. Oral erosions and ulcerations, typical of mucosal disease in classical BVDV, may also be present, particularly at the dental pad and on the tongue [6].

Histopathological examination reveals the microscopic underpinnings of these clinical signs. In the lungs, there is evidence of interstitial pneumonia with alveolar septal thickening, infiltration of mononuclear cells, and the presence of hyaline membranes. Necrotizing bronchiolitis and exudation of neutrophils and fibrin into the alveolar spaces are common, contributing to the observed respiratory distress. In the lymphoid tissues, including the Peyer's patches of the intestine, lymph nodes, and spleen, there is profound lymphocyte depletion and lymphoid necrosis. This is a critical pathological feature, as HoBiPeV, like other pestiviruses, is lymphotropic and induces immunosuppression, leaving the animal susceptible to secondary bacterial and viral infections. This immunosuppressive effect is a key contributor to the severity of BRDC in HoBiPeV-infected herds [8]. In persistently infected animals, lesions are often less pronounced at the gross level, as their immune system is tolerant to the virus. However, they may exhibit growth retardation, poor body condition, and lymphoid depletion, making them prime targets for other pathogens.

Differential Diagnosis and Clinical Confusion with Classical BVDV

The clinical and pathological manifestations of HoBiPeV are so similar to those caused by BVDV-1 and BVDV-2 that it is impossible to differentiate these infections based on clinical signs alone [3, 14]. This diagnostic challenge has profound implications. In regions where BVDV control programs are in place, clinical suspicion of classical BVDV may lead to erroneous conclusions and ineffective control measures if the true causative agent is HoBiPeV. For example, vaccines designed against BVDV-1 and BVDV-2 may provide incomplete or no cross-protection against HoBiPeV, rendering vaccination strategies ineffective and allowing viral circulation to continue [5, 11]. The inability to differentiate these viruses clinically underscores the absolute necessity for molecular diagnostic tools, such as RT-PCR and sequencing, for definitive pathogen identification [1, 3, 10].

The epidemiological context is critical. While HoBiPeV has been reported in South America (particularly Brazil), Europe (Italy), and Asia (China, Bangladesh, India, Thailand) [5-7, 12], its distribution is far from uniform. Surveillance efforts in other regions, such as Northern Ireland, Turkey, and Mexico, have failed to detect HoBiPeV, highlighting its focal and emerging nature [2, 4, 15]. The widespread seroprevalence of BVDV antibodies in cattle populations globally [9, 13, 14] further complicates the picture, as serological tests cannot reliably distinguish between infections caused by the different pestivirus species due to cross-reactivity. The World Organisation for Animal Health (WOAH) recognizes the global economic significance of BVD, and the emergence of a novel pestivirus species like HoBiPeV represents a critical challenge to international disease control and trade, emphasizing the need for vigilant surveillance and the development of species-specific diagnostic assays. The threat of cross-species transmission, as evidenced by the detection of HoBiPeV sequences from cattle, goats, and sheep clustering closely together, suggests a potential for spillover events that could further expand the host range and complicate control efforts in mixed-species farming systems [9].

Control Strategies and Biosecurity Implications for HoBi-like Pestivirus in Cattle Populations

The Biological and Epidemiological Rationale for Targeted Control

The control of HoBi-like pestivirus (HoBiPeV; Pestivirus H) in cattle populations presents a fundamentally different challenge from that of classical bovine viral diarrhea virus (BVDV-1 and BVDV-2), owing to its unique evolutionary trajectory, antigenic divergence, and emerging global distribution. Unlike the established eradication frameworks for BVDV-1 and BVDV-2, which benefit from decades of coordinated international programs, HoBiPeV control strategies must contend with a pathogen that is both underdiagnosed and capable of exploiting diagnostic gaps created by its genetic and serological distinctiveness. The fundamental biological basis for this challenge lies in the phylogenetic position of HoBiPeV as the most divergent pestivirus identified in cattle to date [5]. This divergence is not merely taxonomic; it carries profound implications for vaccine cross-protection, diagnostic test sensitivity, and the ability of existing biosecurity protocols to interdict its spread.

The epidemiological landscape of HoBiPeV further complicates control. While BVDV-1 and BVDV-2 are globally ubiquitous, HoBiPeV has demonstrated a more focal but expanding distribution, with documented endemicity in Brazil, detection in Asian nations including China, Bangladesh, Thailand, and India, and sporadic incursions into Europe, particularly Italy [3, 5-7, 11, 12]. Critically, the virus has been isolated from persistently infected (PI) animals, the cornerstone of BVDV epidemiology, as demonstrated in Egyptian dairy herds where a 2.5% PI prevalence was documented [1]. This finding is alarming because PI animals serve as continuous viral shedders, maintaining transmission cycles within herds and serving as a source of infection for naive cohorts. The clinical spectrum of HoBiPeV infection, ranging from subclinical infections to severe respiratory disease with high fatality rates, as reported in beef cattle in Shandong, China [6], underscores the economic urgency of implementing effective control. The World Organisation for Animal Health (WOAH) has recognized the economic significance of pestiviruses, and the emergence of a novel species within an already costly disease complex demands an equally novel control paradigm.

Diagnostic Strategies: The Cornerstone of Effective Control

Any control strategy for HoBiPeV must be predicated on robust, validated diagnostic tools capable of distinguishing this emerging pestivirus from its better-characterized relatives. The current diagnostic landscape is fraught with pitfalls. Most commercially available antigen-capture ELISAs and many RT-PCR assays were designed primarily for BVDV-1 and BVDV-2 detection, and their performance against HoBiPeV is variable. The genetic and antigenic characterization of HoBiPeV reveals that it is the most divergent pestivirus species in cattle, meaning that tests relying on conserved epitopes or primer binding sites may fail to capture this virus [5]. Indeed, a comparative study of RT-PCR primer sets demonstrated that conventional primers (324-326) detected only 10 of 26 HoBiPeV-positive samples identified by a HoBiPeV-specific assay, highlighting a substantial risk of false-negative results in routine surveillance [10]. This diagnostic blind spot is not trivial; it suggests that HoBiPeV may be circulating far more widely than current prevalence estimates indicate, silently propagating within herds that are considered "BVDV-negative" based on standard testing protocols.

To address this, control programs must incorporate pan-bovine pestivirus diagnostic approaches. The development of a novel primer set (BP189-389) demonstrated superior sensitivity, detecting all 135 ELISA-positive samples in a Brazilian study, including all 26 HoBiPeV strains identified by species-specific primers [10]. This finding supports the implementation of such broad-spectrum molecular tools as the first-line screening method in regions where HoBiPeV is known or suspected to circulate. Furthermore, the adoption of advanced molecular techniques, including RT-qPCR, CRISPR-Cas12a, and RT-LAMP, as reviewed in a comprehensive global analysis, provides additional layers of sensitivity and specificity for confirmatory genotyping [12]. From a regulatory perspective, WOAH-recommended diagnostic protocols for BVDV should explicitly include validation against HoBiPeV reference strains, ensuring that national surveillance programs are not inadvertently failing to detect this pathogen. The diagnostic challenge is further compounded by the existence of at least five HoBiPeV subgroups (a–e) with variable geographic distribution [5]. In Brazil, for instance, only subgroup HoBiPeV-a has been identified, which offers a more straightforward scenario for molecular detection [5]. However, the potential introduction of divergent subgroups through international trade necessitates that diagnostic assays maintain broad within-species reactivity.

Vaccination Strategies and the Challenge of Antigenic Divergence

The development and deployment of effective vaccines represent a critical pillar of HoBiPeV control, yet the antigenic distance between HoBiPeV and classical BVDV strains poses a formidable obstacle. Current commercial BVDV vaccines, formulated with BVDV-1 and BVDV-2 antigens, provide incomplete cross-protection against HoBiPeV infection [5, 11]. The genetic and antigenic characterization of HoBiPeV as the most divergent bovine pestivirus explains this inadequacy: neutralizing antibodies generated against BVDV-1 or BVDV-2 epitopes may not effectively recognize the unique antigenic determinants of HoBiPeV, leaving vaccinated herds susceptible to breakthrough infections [5]. This is not a theoretical concern; experimental and field data from Brazil, where HoBiPeV is endemic, indicate that despite widespread BVDV vaccination, HoBiPeV continues to circulate, suggesting that existing vaccines are insufficient to interrupt transmission [5, 8].

The path forward requires the inclusion of HoBiPeV-specific antigens in multivalent vaccine formulations. The reduced genetic diversity of HoBiPeV in Brazil, where only subgroup a has been detected, presents a strategic opportunity: a vaccine incorporating a single HoBiPeV-a strain could theoretically confer protection against all circulating strains in that region [5]. This contrasts sharply with regions like Italy, where all three bovine pestivirus species (BVDV-1, BVDV-2, and HoBiPeV) have been identified, necessitating a more complex vaccine strategy [11]. The vaccine development pipeline must prioritize the characterization of circulating HoBiPeV strains in endemic areas to ensure antigenic matching. For example, the Chinese strain SDJN-China-2019, which caused severe respiratory disease, exhibited unique amino acid mutations in the Npro and E2 proteins compared to other reference strains [6]. These mutations may have implications for vaccine efficacy, as E2 is a major target of neutralizing antibodies. Therefore, vaccine design must be dynamic, incorporating regional strain surveillance data. Furthermore, the potential for cross-species transmission, as documented through phylogenetic evidence of HoBiPeV infection in goats and sheep, raises the prospect that small ruminants could serve as reservoirs, necessitating vaccines that are efficacious across multiple host species [9]. The Food and Agriculture Organization (FAO) has emphasized the importance of controlling transboundary animal diseases, and a vaccine strategy that does not account for the multi-host ecology of HoBiPeV risks being incomplete.

Biosecurity Measures and Herd Management Protocols

Biosecurity interventions for HoBiPeV must address the unique pathways through which this pathogen can be introduced and amplified within cattle populations. The identification of PI animals as the primary reservoir of infection, as documented in Egypt with a 2.5% prevalence, establishes the removal of PI cattle as the single most impactful biosecurity measure [1]. This finding is consistent with BVDV control programs globally, where test-and-cull strategies for PI animals have proven effective in reducing viral circulation [12]. However, the implementation of such programs for HoBiPeV faces the same challenges as for BVDV: the need for comprehensive herd testing, the economic burden of culling PI animals, and the psychological resistance of producers. The systematic review of global BVDV control strategies emphasizes that culling of PI animals, along with prophylactic vaccination and avoidance of mixed farming practices, are key measures for controlling and eradicating the virus [12]. This framework is directly applicable to HoBiPeV, provided that diagnostic tests are capable of identifying PI animals infected with this species.

The introduction of PI animals into a herd is a well-documented risk factor for BVDV outbreaks, and the same principle applies to HoBiPeV [12]. Therefore, strict quarantine protocols for incoming cattle, coupled with molecular screening using pan-pestivirus RT-PCR, are essential to prevent the introduction of HoBiPeV into naive herds. The role of contaminated biological products in the spread of HoBiPeV warrants particular attention. Fetal bovine serum (FBS), a common additive in cell culture and vaccine production, has been implicated in the international dissemination of pestiviruses, including HoBiPeV [5, 11]. The implementation of stringent screening protocols for FBS and other bovine-derived biologicals, including irradiation or gamma-ray treatment to inactivate any residual virus, is a critical biosecurity measure that transcends farm-level control. International trade in live animals, semen, and embryos also represents a potential pathway for the introduction of divergent HoBiPeV subtypes into new regions. As noted in the Brazilian context, the introduction of foreign ruminants, biologicals, and genetic material from other HoBiPeV-endemic countries should be considered a high-risk activity [5]. This concern is amplified by phylogenetic evidence suggesting potential cross-border transmission events between neighboring countries [9]. Regulatory frameworks at the national and international levels must mandate testing of imported genetic material for all known pestivirus species, including HoBiPeV, using validated pan-pestivirus assays.

Surveillance, Movement Control, and Integrated Pestivirus Management

A comprehensive control strategy for HoBiPeV cannot exist in isolation; it must be integrated into broader pestivirus surveillance and management programs. The genetic diversity demonstrated by BVDV-1, with 25 recognized subgenotypes, and the emergence of HoBiPeV subgroups a–d in various global regions, underscores the need for continuous genotypic surveillance to inform control measures [12]. The detection of HoBiPeV in Xinjiang, China, where the virus was isolated from dairy cattle with a whole-genome sequence showing 96.02% homology to the JS12/01 reference strain, highlights the expanding geographic footprint of this pathogen and the necessity for enhanced surveillance in pastoral areas [3]. Similarly, the first detection of HoBiPeV in Bangladesh underscores that even in regions with limited veterinary infrastructure, the virus can establish a foothold [7]. The World Health Organization (WHO) has highlighted the importance of "One Health" approaches to emerging infectious diseases, and the cross-species transmission potential of HoBiPeV, evidenced by its detection in goats and sheep [9], reinforces the need for surveillance across multiple domestic and wild ruminant species. The absence of HoBiPeV in studies from Northern Ireland [2] and Turkey [4] does not preclude its future introduction, and these regions should remain vigilant, incorporating HoBiPeV screening into routine BVDV surveillance.

Movement control and traceability systems are essential biosecurity instruments for preventing the spatial spread of HoBiPeV. Animal movements, including the sale and transport of cattle between farms, are frequently documented risk factors for BVDV transmission [12]. For HoBiPeV, which may be present in subclinical PI animals, movement restrictions based on testing status are critical. The European Union's BVDV eradication schemes provide a model framework: mandatory testing of all cattle before movement, establishment of PI-free status for herds, and regional zoning based on prevalence. However, these schemes must be adapted to account for HoBiPeV, which may not be detected by standard BVDV tests. A key recommendation is that any animal testing positive for pestivirus antigen should undergo reflex genotyping to determine the infecting species. In regions where HoBiPeV is known to circulate, such as Brazil and parts of Asia, this should be a routine component of diagnostic workups [3, 5, 6]. Furthermore, the role of wildlife and small ruminants in the epidemiology of HoBiPeV must be considered in biosecurity planning. Serological evidence from sheep in the United States has demonstrated exposure to multiple pestiviruses, including HoBi-like virus [13]. Although the transmission risk from sheep to cattle remains poorly quantified, the potential for spillover events underscores the importance of preventing commingling of cattle with other susceptible species, particularly in shared grazing systems. The Centers for Disease Control and Prevention (CDC) and other international health bodies have increasingly recognized that livestock-wildlife interfaces are hotspots for pathogen emergence, and this principle applies directly to HoBiPeV control [9]. In conclusion, the control of HoBiPeV demands a multi-pronged approach that integrates advanced diagnostics, species-specific vaccination, rigorous biosecurity, and continuous molecular surveillance, all operating within an international regulatory framework that acknowledges the distinct threat posed by this emerging pestivirus.

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