Lumpy Skin Disease Virus Epidemiology

Overview and Taxonomy of Lumpy Skin Disease Virus (LSDV) and Its Global Epidemiological Significance

Lumpy skin disease virus (LSDV) is the etiological agent of lumpy skin disease (LSD), a highly contagious, transboundary viral disease of bovines that has emerged as one of the most economically significant threats to global livestock production over the past decade. The virus belongs to the family Poxviridae, subfamily Chordopoxvirinae, genus Capripoxvirus (CaPV), a genus that also encompasses sheeppox virus (SPPV) and goatpox virus (GTPV) [1, 4, 35]. These three viruses are antigenically and morphologically indistinguishable, sharing a high degree of genetic homology, yet they exhibit distinct host preferences: LSDV primarily infects cattle (Bos taurus and Bos indicus) and Asian water buffalo (Bubalus bubalis), while SPPV and GTPV are largely restricted to sheep and goats, respectively [39, 40, 53]. The classification of LSDV within the Capripoxvirus genus is unequivocally confirmed through molecular analyses targeting key genetic loci, including the RNA polymerase subunit 30 kDa (RPO30) gene, the G protein-coupled chemokine receptor (GPCR) gene, the extracellular enveloped virus (EEV) glycoprotein gene, and the B22R gene, which collectively provide species-differentiating signatures [1, 6, 7]. The World Organisation for Animal Health (WOAH) classifies LSD as a notifiable disease due to its rapid transboundary spread and severe economic consequences, underscoring the critical importance of understanding its virological and taxonomic foundations [43, 44].

Taxonomic Classification and Genomic Architecture

LSDV possesses a large, double-stranded linear DNA genome of approximately 151 kilobase pairs (kbp), encoding an estimated 156 open reading frames (ORFs) [42, 43]. The genome is flanked by inverted terminal repeat (ITR) regions, which are characteristically variable and contain genes implicated in host range determination, immune evasion, and virulence [2, 43, 45]. These ITR regions have been identified as hotspots for genomic variability and recombination, encoding proteins such as LSDV001/156 and LSDV004 which modulate host interferon responses and apoptosis pathways [43, 47, 51]. The central core of the genome is highly conserved, encoding essential replication and structural proteins, while the termini exhibit greater plasticity, facilitating adaptation to selective pressures [2, 18].

Within the Capripoxvirus genus, LSDV can be reliably differentiated from SPPV and GTPV through analysis of specific genetic markers. For instance, a consistent 21-nucleotide deletion in the RPO30 gene is a hallmark of SPPV, while LSDV and GTPV retain a full-length sequence [39]. Furthermore, LSDV possesses unique amino acid signatures within the RPO30 protein (e.g., specific residues at positions 1–10 and 23–31) that enable species-level genotyping [7, 40]. The EEV glycoprotein gene and the GPCR gene also contain discriminative nucleotide motifs, with the GPCR gene notably exhibiting a 12-nucleotide insertion in LSDV strains from the 2019 Indian and Bangladeshi outbreaks, a feature absent in ancestral Neethling-like strains [33]. These molecular distinctions are not merely academic; they are essential for accurate diagnosis, phylogenetic tracing, and the development of DIVA (Differentiating Infected from Vaccinated Animals) strategies [29, 48].

Phylogenetic Structure and Global Lineage Diversity

Phylogenetic analyses based on whole-genome sequences and multi-gene concatenated datasets have resolved LSDV into two major clades, designated Clade 1 and Clade 2, with further subdivision into subclusters that reflect the virus’s complex evolutionary history and geographic expansion [2, 14, 46]. Clade 1 encompasses the classical, wild-type field strains that originated in Africa. Within Clade 1, two principal subclusters are recognized: Clade 1.1, which includes the Neethling vaccine-like strains and the original Neethling field isolate from South Africa, and Clade 1.2, which comprises the Kenyan sheep and goat pox (KSGP)-like field strains [1, 16, 33]. Until recently, strains belonging to Clade 1.2 were considered the predominant circulating lineage across the Middle East, South Asia, and parts of Eastern Europe [14, 52]. However, a paradigm shift occurred following the emergence of novel recombinant strains.

Clade 2 represents a distinct evolutionary branch composed entirely of recombinant viruses. The first reported recombinant LSDV strain, LSDV/Russia/Saratov/2017, was identified in Russia and was found to be a mosaic of the live-attenuated Neethling vaccine strain and a KSGP-like field strain [18]. Subsequent surveillance revealed that this recombinant event was not isolated; multiple unique recombinant lineages, including LSDV/Russia/Udmurtya/2019, LSDV/KZ-Kostanay/Kazakhstan/2018, and LSDV/GD01/China/2020, were detected across Russia, Kazakhstan, China, and Vietnam [5, 18, 23]. These recombinants exhibited consistent patterns of targeted selection, suggesting that recombination with the Neethling vaccine backbone conferred a selective advantage in the field [18, 27]. Importantly, from 2020 onward, the recombinant lineage has converged into a single dominant cluster, designated Cluster 2.5, which is now the principal lineage circulating in Southeast Asia, including China, Thailand, Vietnam, and South Korea [2, 5, 20]. The global epidemiological picture is further complicated by the detection of recombinant-like vaccine-derived strains in outbreaks in Europe and the Middle East, demonstrating the ongoing risks associated with the use of live-attenuated homologous vaccines in the face of active viral circulation [14, 20].

Epidemiological Significance and Global Dissemination

The taxonomic and phylogenetic characterization of LSDV is inextricably linked to its epidemiological trajectory. The virus was first described in Zambia in 1929, and for over 60 years, its distribution was restricted to sub-Saharan Africa [12, 13, 26]. The first documented intercontinental spread occurred in 1988–1989, when LSDV was introduced into Egypt and subsequently into the Middle East [12, 34]. The contemporary pandemic initiated in 2012, when LSDV underwent a dramatic expansion across the Middle East, reaching Israel, Turkey, Iran, and Iraq, characterized by an effective reproductive number (Rₜ) as high as 22.2 in Israel during September 2013 [31]. Concurrently, the virus infiltrated the Balkan region of southeastern Europe, affecting Greece, Bulgaria, Serbia, and Albania from 2015 to 2017 [14, 21]. The rapidity of this spread, traversing over 9000 km from the initial Russian incursion to the Far East by 2020, is unprecedented for a poxvirus and highlights the influence of anthropogenic movement, vector ecology, and viral genomics [23, 31].

Molecular epidemiology has been instrumental in tracing these dispersal pathways. Whole-genome sequencing and phylogeographic reconstructions have confirmed that the 2015–2017 Balkan outbreak was caused by a homogeneous Clade 1.2 lineage, with intense intermixing among affected countries, indicating a single introduction followed by rapid regional circulation [14]. In contrast, the concurrent spread into Central Asia and the Indian subcontinent involved distinct lineages. LSDV strains from the 2019 Indian and Bangladeshi outbreaks clustered with KSGP-like field strains (Clade 1.2.1), exhibiting 99.7–100% identity to the historical Kenyan NI-2490 strain, suggesting a direct introduction from Africa or the Middle East [22, 25, 33]. Subsequent outbreaks in India (2022–2023) and Bhutan (2023) have shown a shift toward Neethling Warmbaths (NW)-like LSDVs (Clade 1.2.2), indicating ongoing viral evolution and replacement of circulating lineages [1, 6, 46].

The emergence of recombinant isolates has profound implications for LSDV epidemiology. The Cluster 2.5 recombinant lineage is now dominant in Southeast Asia, where it has supplanted earlier wild-type strains and continues to spread in a monophyletic pattern [5, 49]. This lineage is characterized by a KSGP-like backbone with numerous genomic regions derived from the Neethling vaccine, raising concerns about vaccine safety and the potential for increased virulence [18, 42]. Experimental infections with the recombinant strain in Kazakhstan demonstrated rapid onset of severe clinical signs (fever, nodulosis, lymphadenopathy by day 5 post-infection) and aggressive pathology, including extensive necrosis, thrombosis, and systemic tissue damage [41]. Such findings underscore the need for continuous genomic surveillance to monitor the phenotypic consequences of recombination.

Expanding Host Range and Implications for Non-Bovine Species

Historically, LSDV was considered host-specific to cattle. However, the past decade has witnessed a concerning expansion in its host range. Clinical LSD has been documented in Asian water buffaloes (Bubalus bubalis), which are now recognized as competent hosts capable of transmitting the virus to cattle [34, 36]. In Bhutan, a massive outbreak in 2023 affected approximately 3179 yaks (Bos grunniens), with a case fatality rate (CFR) of 38.66%, far exceeding the CFR of 9.92% observed in cattle during the same outbreak [1]. Similarly, LSDV was isolated from diseased yaks in the Himalayan state of Sikkim, India (2022–2023), where identical viral sequences were found in yaks and co-located cattle, confirming spillover events [6]. Molecular characterization of the yak isolates in both Bhutan and India grouped them within Clade 1.2.2 and Clade 1.2.1, respectively, indicating that no host-specific viral adaptation has yet occurred [1, 6].

The virus has also been documented in non-bovine wildlife, including giraffes (Giraffa camelopardalis) in Vietnamese zoos, and in semi-domesticated bovine species such as Bos frontalis (mithun) in Northeast India [15, 46]. These findings are of considerable concern for conservation and animal health, as wildlife reservoirs could serve as persistent sources of infection and complicate eradication efforts [15, 28]. The susceptibility of multiple ruminant species to LSDV underscores the need for a One Health approach, integrating veterinary and wildlife surveillance.

Transmission Dynamics and Vector Involvement

LSDV transmission is primarily mechanical via arthropod vectors, with blood-feeding insects serving as the principal agents of short-distance spread within and between herds [19, 21, 27]. The most competent vectors identified to date include stable flies (Stomoxys calcitrans), mosquitoes (Aedes aegypti), and hard ticks of the genera Rhipicephalus and Amblyomma [19, 27]. Experimental evidence also implicates the synanthropic house fly (Musca domestica) as a potential vector, although its clinical significance requires further validation [19]. The role of vectors in the epidemiology of LSDV is supported by distinct seasonal patterns: outbreaks predominantly occur during warm, humid months when vector populations peak, corresponding to summer and early autumn in temperate regions [23, 26, 31].

Long-distance dispersal, however, is almost exclusively attributable to anthropogenic movement of infected animals. Incursions into LSDV-free areas have been repeatedly linked to the transport of cattle from endemic zones, often through legal and illegal trade networks [27, 31]. Epidemiological modeling studies have identified animal density, herd size, transhumance practices, and shared communal water sources as significant risk factors for LSDV introduction and amplification [3, 9, 11, 24]. Importantly, within an infected herd, direct contact transmission is considered inefficient; the virus is shed in high concentrations in skin nodules, saliva, nasal discharges, and semen, but infection typically requires mechanical transfer via vectors or fomites [4, 21, 37]. The demonstration that peripheral blood mononuclear cells (PBMCs) can harbor LSDV and transmit it to permissive cells via direct contact suggests a potential role for cell-associated viremia in dissemination within the host [50].

Economic and Regulatory Significance

The global epidemiological significance of LSDV is inextricably tied to its profound economic impact. The disease directly reduces milk yield (up to 50–80% in lactating cows), causes weight loss, hides damage, permanent infertility, abortion, and mortality, particularly in young animals [17, 26, 30, 38]. Indirect losses arise from trade restrictions, quarantine costs, and reduced market access. The WOAH has designated LSD as a notifiable disease, and its incursion into a country triggers immediate trade embargoes on live cattle and bovine products, as observed during the Balkan outbreaks [26, 44]. The rapid spread of LSDV through Asia has threatened the livelihoods of millions of smallholder farmers, who constitute the backbone of livestock production in the region [17, 32, 37]. In countries such as Bangladesh, Pakistan, and India, the 2020–2022 outbreaks resulted in catastrophic losses, with reported morbidity rates of 10–61% and mortality rates of 1–40% depending on breed, age, and management factors [8, 9, 17, 22].

Genomic Evolution and Future Emergence Risks

The evolutionary trajectory of LSDV is characterized by a paradox: despite possessing a DNA genome that generally evolves slowly, the virus has demonstrated remarkable genomic plasticity through homologous recombination in the presence of live-attenuated vaccines [18, 27]. This has given rise to virulent recombinant strains that challenge diagnostic capabilities, vaccine efficacy, and control programs. The World Health Organization (WHO), in collaboration with FAO and WOAH, has emphasized the need for enhanced genomic surveillance as a core component of early warning systems for emerging infectious diseases. The ongoing circulation of multiple lineages (Clade 1.2.1, Clade 1.2.2, and Cluster 2.5) in overlapping geographic regions raises the prospect of further recombination events and the emergence of strains with altered virulence or host range [5, 10, 16].

In summary, LSDV occupies a unique niche among poxviruses as an emerging pathogen that has transitioned from a localized African disease to a global threat within a single decade. Its taxonomic classification within the Capripoxvirus genus provides the framework for molecular detection and phylogenetic characterization, while the recent discovery of recombinant lineages and expansion into new host species underscores the urgency of continued virological and epidemiological research. The interconnectedness of global livestock trade, vector ecology, and animal movement ensures that LSDV will remain a focus of international veterinary public health efforts for the foreseeable future.

Molecular Pathogenesis of LSDV: Host-Virus Interactions, Clinical Manifestations, and Virulence Determinants

Viral Genome Architecture and Structural Basis of Pathogenesis

The molecular pathogenesis of Lumpy Skin Disease Virus (LSDV) is rooted in its complex genomic architecture and the sophisticated arsenal of immune-modulatory proteins it encodes. The LSDV genome, a linear double-stranded DNA molecule of approximately 151 kilobase pairs, contains 156 predicted open reading frames (ORFs), the functions of which remain largely uncharacterized [42, 43]. The virus belongs to the genus Capripoxvirus within the family Poxviridae, sharing antigenic and structural similarity with sheeppox virus (SPPV) and goatpox virus (GTPV), yet exhibiting a distinct host tropism primarily for bovines [4, 35, 40]. The genome is characterized by central core regions encoding conserved replication and structural proteins, flanked by highly variable inverted terminal repeat (ITR) regions that are enriched with genes involved in host-range determination and immune evasion [2, 43]. These ITR-encoded proteins, including LSDV001/156 and several ORFs of unknown function, are now recognized as critical determinants of viral virulence and host interaction [43, 47]. The structural organization of the genome, with its capacity for recombination and the acquisition of novel genetic elements, underpins the virus's ability to adapt to new ecological niches and host species, driving its ongoing global expansion.

Molecular Mechanisms of Host-Virus Interactions

Immune Evasion Strategies: Subversion of Interferon Signaling

LSDV has evolved a multifaceted strategy to dismantle the host innate immune response, particularly the type I interferon (IFN) pathway, which is the first line of antiviral defense. The virus encodes at least two distinct proteins, LSDV001/156 and ORF142, that act as potent negative regulators of IFN-β production, effectively silencing the antiviral state. LSDV001/156, a late-expressed protein located within the ITR region, functions by directly interacting with interferon regulatory factor 3 (IRF3). This interaction physically disrupts the dimerization and subsequent nuclear translocation of IRF3, a master transcription factor required for IFN-β gene expression [43]. By preventing IRF3 activation, LSDV001/156 effectively cripples the host's capacity to mount a robust antiviral response, facilitating viral replication and dissemination. Critically, deletion of the LSDV001/156 gene results in a mutant virus that exhibits significantly reduced replication and virulence in cattle, confirming this protein's indispensable role in pathogenesis [43].

In a parallel immune evasion mechanism, the LSDV ORF142 protein targets the same pathway but at a different molecular node. ORF142, identified as an early gene product, inhibits IFN-β production by two complementary mechanisms. First, it directly interacts with IRF3, specifically binding to its C-terminal domain (amino acids 381–417), and interferes with the recruitment of IRF3 to TANK-binding kinase 1 (TBK1) in a dose-dependent manner [45]. This impedes the phosphorylation and activation of IRF3 by TBK1, preventing its translocation to the nucleus. Second, ORF142 subverts the cyclic GMP-AMP synthase (cGAS)/stimulator of interferon genes (STING) pathway, a critical cytosolic DNA-sensing mechanism. ORF142 achieves this by interacting with STING and promoting its autophagic degradation via the cargo receptor NBR1, thereby depleting STING protein levels and terminating downstream signaling [55]. This dual blockade, at the level of the sensor (STING) and the transcription factor (IRF3), demonstrates a highly refined, multi-layered immune evasion strategy that allows LSDV to replicate unabated in the face of host antiviral defenses.

Modulation of Inflammatory Responses: A Double-Edged Sword

While LSDV actively suppresses interferon-mediated antiviral signaling, it simultaneously promotes a robust, and often pathological, inflammatory response. This paradoxical effect is central to the clinical manifestations of the disease. The LSDV001 protein, distinct from LSDV001/156, has been identified as a positive regulator of inflammation. Specifically, LSDV001 potentiates interleukin-1β (IL-1β) and tumor necrosis factor-alpha (TNF-α) signaling by interacting with transforming growth factor-β-activated kinase 1 (TAK1) and the adaptor proteins TAB2 and TAB3. This interaction promotes the assembly of the TAK1-TAB2/3 complex, which in turn activates the IKK complex, leading to the nuclear translocation of NF-κB and the transcriptional upregulation of a cascade of pro-inflammatory cytokines [47]. The resultant cytokine storm contributes to the severe dermal inflammation, edema, and necrosis characteristic of LSD. Infection with a LSDV001-deficient virus results in smaller skin nodules and reduced inflammation, directly linking this viral protein to the tissue damage observed in natural infections [47]. The LSDV087 protein adds another layer of complexity by positively regulating the innate immune response through a different mechanism. LSDV087 interacts with the adaptor protein MITA/STING, inhibits its K48-linked polyubiquitination (a signal for proteasomal degradation), and promotes its oligomerization, thereby enhancing downstream signaling and the expression of type I IFNs and inflammatory genes [56]. The net effect of these competing viral activities, suppression of antiviral IFN and promotion of pathological inflammation, defines the unique pathogenic profile of LSDV, where viral replication proceeds efficiently in an environment of dysregulated host immunity.

Clinical Manifestations: A Consequence of Molecular Pathogenesis

The clinical signs of LSD arise directly from the molecular interactions between the virus and the host's immune and vascular systems. Following an incubation period of 2–5 weeks in natural infections, or 7–14 days under experimental conditions, the disease manifests with pyrexia, depression, and generalized lymphadenopathy [4, 22, 30]. The hallmark of LSD is the development of firm, circumscribed skin nodules, ranging from a few (mild form) to numerous lesions covering the entire body (severe form) [4, 30]. These nodules are not merely cosmetic; they result from the underlying pathological processes of vasculitis, thrombosis, and infarction driven by viral replication in endothelial cells and dermal macrophages [22]. Histologically, affected skin shows transmural, hemorrhagic, necrotizing, and proliferative dermatitis, with a granulomatous reaction extending from the dermis into the hypodermis [22]. The viral antigen is detected within macrophages, epithelial cells, and vascular smooth muscle cells, underscoring the direct cytopathic effect of LSDV on these cell types [22]. The severe edema and necrosis can lead to sloughing of skin, creating deep, painful ulcers that are prone to secondary bacterial infections [54]. Systemic involvement, including lesions on mucous membranes of the respiratory, urogenital, and gastrointestinal tracts, leads to additional clinical complications such as pneumonia, diarrhea, and reproductive disorders, including abortion and temporary or permanent infertility [4, 30, 32]. The economic impact is profound, driven by a sharp decrease in milk production, damage to hides, reduced growth in beef cattle, and substantial mortality in young animals [26, 30, 38].

Virulence Determinants: Genetic Signatures of Pathogenicity

The virulence of LSDV strains is a polygenic trait, influenced by a combination of genetic determinants that modulate host range, immune evasion, and tissue tropism. Comparative genomic analyses have identified several key regions and genes associated with increased pathogenicity. The LSDV001/156 and LSDV142 genes, as discussed above, are proven virulence factors, with their deletion leading to marked attenuation [43, 45, 55]. The LSDV001 protein, which promotes TAK1-TAB2/3 complex formation, is also a confirmed virulence determinant, directly linking its pro-inflammatory function to disease severity [47]. Furthermore, in silico analysis suggests that the LSDV004 hypothetical protein may function as an anti-apoptotic factor, structurally similar to Bcl-2-like proteins, potentially contributing to virulence by preventing premature host cell death and thereby allowing for prolonged viral replication [51]. The RPO30 gene (ORF036), which encodes a subunit of the viral RNA polymerase, has also been implicated in virulence. Analysis of field isolates from India revealed specific amino acid substitutions within the RPO30 protein, and estimates of evolutionary divergence showed a 9.52% genetic distance between vaccine and field isolates, suggesting that these changes may be linked to the increased virulence observed during recent outbreaks [7]. Similarly, a 12-nucleotide insertion in the G-protein-coupled chemokine receptor (GPCR) gene, a known virulence determinant in poxviruses, was identified in LSDV strains from the initial 2019 outbreaks in India and Bangladesh, further implicating this locus in altering host range or pathogenesis [33].

The emergence of recombinant LSDV strains, particularly those involving a backbone of the Kenyan sheep and goat pox (KSGP) vaccine strain, represents a major evolutionary development with profound implications for virulence. Since 2017, novel recombinant strains have been identified in Russia, Kazakhstan, China, and Vietnam, arising from recombination between live-attenuated Neethling vaccine strains and field viruses [5, 18, 28]. These recombinants, classified within cluster 2.5, have become the dominant lineage in Southeast Asia, displaying their own patterns of monophyletic evolution and exhibiting enhanced fitness and transmissibility compared to classical field strains [5, 18]. The recombination events have targeted specific genomic regions, including those encoding immune evasion proteins, potentially leading to viruses with altered antigenicity and virulence profiles [18]. This ongoing genetic diversification, driven by the co-circulation of vaccine and field strains, presents a significant challenge for disease diagnosis and control, as standard molecular assays and vaccines may not be fully effective against these novel variants [2, 16, 52]. Additionally, the host range of LSDV is expanding, with confirmed infections in yaks (Bos grunniens), water buffalo (Bubalus bubalis), and even captive giraffes, indicating a degree of plasticity in viral virulence determinants that allows for cross-species transmission [1, 6, 15, 34]. The case fatality rate in yaks (38.66% in Bhutan) far exceeds that in cattle, highlighting the existence of host-specific virulence factors and the potential for severe disease in novel hosts [1]. The differential expression of exosomal microRNAs (miRNAs) in infected cattle, including the up- and downregulation of 59 specific miRNAs that target genes involved in viral replication and immune response, further underscores the complex, ongoing molecular dialogue between the virus and its host that determines the ultimate outcome of infection [57].

Transmission Dynamics and Vector-Borne Epidemiology of Lumpy Skin Disease

Lumpy skin disease virus (LSDV) exhibits a complex transmission ecology that integrates mechanical vector-borne dissemination, direct contact pathways, and anthropogenic long-distance dispersal, creating a multifaceted epidemiological paradigm distinct from many other poxviruses. The preponderance of evidence indicates that mechanical transmission by hematophagous arthropods constitutes the principal route for short- and medium-range spread, while the movement of infected livestock underpins transcontinental and inter-regional viral incursions [19, 27]. Understanding the nuanced interplay between these mechanisms is paramount for designing effective surveillance, containment, and eradication strategies.

The Primacy of Mechanical Vector-Borne Transmission

A substantial body of experimental and observational research has established that LSDV is not transmitted through a biological cycle within arthropod vectors but rather through mechanical carriage. In this process, virus-laden blood or tissue fluids are transferred from an infected viremic host to a susceptible animal via contaminated mouthparts of biting flies, mosquitoes, and ticks. The virus does not replicate within the vector, but its persistence on external structures is sufficient to initiate infection if a subsequent blood meal is taken from a naive host within a finite temporal window [19, 27]. This mechanical paradigm is supported by the strong seasonality of LSD outbreaks, which consistently correlate with periods of peak vector abundance in temperate and subtropical zones [26, 31].

The most consistently implicated and experimentally validated vector is the stable fly, Stomoxys calcitrans. Multiple studies have demonstrated its capacity to transmit LSDV under controlled conditions, and field epidemiological investigations routinely identify high stable fly populations as a significant risk factor for outbreak occurrence [19, 21, 27]. The feeding behavior of S. calcitrans, characterized by repeated probing and interrupted meals on multiple hosts, renders it an extraordinarily efficient mechanical vector. Furthermore, the ubiquity of this fly around cattle holdings, its strong association with manure and decaying organic matter, and its ability to travel considerable distances (several kilometers) align perfectly with the observed patterns of LSDV spread within and between farms [19, 21]. Beyond stable flies, mosquitoes, particularly Aedes aegypti, have been shown to transmit LSDV experimentally, and hard ticks of the genera Rhipicephalus and Amblyomma have yielded LSDV DNA upon testing, although their precise epidemiological role as transstadial or transovarial reservoirs remains incompletely defined [4, 19]. More recently, the synanthropic house fly, Musca domestica, has been proposed as a potential mechanical vector, given its close association with cattle and its ability to move between feces, feed, and skin lesions, although clinical transmission studies remain pending [19]. The mechanical nature of LSDV transmission has profound implications for control; it dictates that vector management, through insecticide application, removal of breeding sites, and physical barriers, constitutes a critical pillar of outbreak response, complementing vaccination and movement restrictions.

Seasonal Dynamics and Climatic Drivers

The temporal distribution of LSD outbreaks globally provides unequivocal evidence for the dependence of transmission on vector activity. Analysis of World Organisation for Animal Health (WOAH) data from 2005 to 2020 demonstrates that the vast majority of LSD outbreaks (98.46% since 2012) occurred during the summer months, with July registering the highest number of events [26]. This pattern is mirrored in regional studies from Europe, the Middle East, and Asia, where outbreaks typically commence in late spring, peak in mid-to-late summer, and wane with the onset of colder autumn temperatures that suppress insect activity [23, 31]. A seminal spatial-temporal study of the 2012–2015 Middle Eastern epidemic estimated the effective reproductive number (Rₜ) to average 2.2, but identified super-spreading events in Israel (Rₜ = 22.2) during September 2013, a period coinciding with optimal climatic conditions for vector proliferation [31]. The sharp decline in Rₜ following the implementation of a national vaccination campaign in Israel further underscores the combined importance of vector seasonality and host immunity in modulating transmission dynamics [31].

Ecological niche modeling has identified annual precipitation, mean diurnal temperature range, and land cover as the most important environmental predictors for LSDV occurrence [31]. Regions receiving high rainfall (>1000 mm annually) provide abundant breeding sites for mosquitoes and flies, while moderate temperatures and high humidity extend vector survival and foraging activity. In contrast, transmission in arid or cold environments is markedly suppressed [24, 31]. However, a significant departure from this pattern has emerged from Russian epidemiological data, where outbreaks have been documented under snow cover and at sub-zero temperatures, conditions that preclude any arthropod activity [23]. This observation strongly suggests that alternative transmission routes, such as direct contact through respiratory droplets or contaminated fomites, may play a more prominent role under specific environmental constraints, particularly in housed cattle during winter [21, 23]. This phenomenon, termed "cold-climate transmission," challenges the dogma of strict vector dependence and indicates that LSDV possesses a degree of plasticity in its transmission mechanisms that allows persistence even when the primary vector pathway is suppressed.

Direct and Indirect Contact Transmission

While vector-borne spread is dominant, direct contact transmission does occur, albeit at a lower efficiency. Experimental infections have demonstrated that naive cattle can acquire LSDV after close confinement with infected animals, even in the absence of arthropod vectors, although the attack rate is substantially lower than that observed when vectors are present [19, 27]. The virus is shed in high concentrations from ruptured skin nodules, nasal and ocular discharges, saliva, milk, and semen [4, 12]. Contaminated feeding and watering equipment, bedding, and transport vehicles can therefore serve as fomites, facilitating indirect transmission within and between herds [28, 58]. The World Organisation for Animal Health (WOAH) and the Food and Agriculture Organization of the United Nations (FAO) recognize that while direct and indirect routes are insufficient to sustain an epidemic in the absence of vectors, they are critical for localized amplification, particularly in intensive production systems where animal density is high and biosecurity is poor [28, 44].

One of the most epidemiologically significant indirect pathways is iatrogenic transmission through contaminated needles and veterinary instruments. Reuse of needles during mass vaccination campaigns or treatment procedures has been suggested to contribute to the rapid dissemination of LSDV within herds, analogous to the spread of other blood-borne pathogens [32]. Similarly, the use of contaminated semen for artificial insemination represents a recognized risk, as the virus can be excreted in semen for extended periods following recovery from clinical disease [4, 44]. These considerations highlight the necessity for stringent biosecurity protocols even in settings where vector pressure is low.

Long-Distance Dispersal and Anthropogenic Drivers

The remarkable transcontinental spread of LSDV from its endemic focus in sub-Saharan Africa to the Middle East, Europe, and across South and Southeast Asia within a single decade cannot be explained by vector-borne movement alone. The flight range of mechanical vectors, while considerable (up to 5–10 km for S. calcitrans), is insufficient to account for jumps of hundreds or thousands of kilometers [19, 27]. The consensus from multiple phylogenetic and epidemiological investigations is that long-distance dispersal is overwhelmingly driven by anthropogenic factors, principally the unrestricted movement of live animals for trade [27, 28].

The introduction of LSDV into the Middle East in 2012–2013 was epidemiologically linked to the importation of infected cattle from endemic African countries [31]. The subsequent spread into Turkey, the Balkans, and the Caucasus was facilitated by porous borders, informal livestock trading networks, and a lack of quarantine enforcement [23, 31]. More recently, the incursion of LSDV into South and East Asia (China, Vietnam, Thailand, India, Bangladesh) from 2019 onwards has been traced genetically to introductions of specific clades (e.g., Clade 1.2.2, Cluster 2.5) that mirror known trade corridors and cattle movement patterns [1, 5, 6, 33]. Whole-genome sequencing has been indispensable in reconstructing these dispersal routes, demonstrating that the virus did not spread in a contiguous wave but rather through multiple independent introductions separated by large geographic distances [2, 14, 18].

Additional anthropogenic factors that amplify long-distance risk include the transport of infected animals during the incubation period (when clinical signs are absent), the movement of contaminated vehicles and equipment, and the transhumance practices of pastoralist communities [3, 8, 9, 23]. Studies from Russia have shown that the epidemic trajectory from 2015 to 2020 advanced in a northward and then eastward direction, closely following major highways and railway lines that connect cattle markets, rather than following a purely vector-driven pattern [23]. Mathematical modeling of the Russian outbreak confirmed that the disease formed statistically significant spatio-temporal clusters in the early years (2016–2018) but transitioned to a more diffuse, unpredictable pattern in later years as it became entrenched in the region [23]. These findings underscore that effective long-distance control requires synchronized national and international policies on animal movement certification, mandatory pre- and post-movement quarantine, and traceability systems.

Host Range and Spillover Dynamics

The host range of LSDV is broader than initially appreciated, with significant implications for transmission dynamics and maintenance. While cattle are the primary amplifying host, domestic water buffaloes (Bubalus bubalis) are susceptible and can develop clinical disease, though they are generally considered accidental or less competent hosts [27, 34]. Studies in Egypt have found that buffaloes in contact with infected cattle can seroconvert but rarely exhibit clinical signs or shed detectable virus, suggesting they may serve as sentinel rather than reservoir populations [34, 36]. However, the situation in Asia is different; reports from India and China have documented severe LSD in water buffaloes, with mortality rates approaching those in cattle for certain clades [1, 6].

A critical and emerging dimension is the susceptibility of yak (Bos grunniens). The 2023 outbreak in Bhutan devastated yak populations, with a reported case fatality rate of 38.66%, nearly four times that observed in cattle old the same region [1]. Molecular characterization of LSDV strains from yaks and sympatric cattle in Sikkim, India, revealed identical sequences, providing strong evidence of spillover from cattle [6]. This pattern indicates that yaks act as dead-end hosts with high susceptibility but likely do not sustain independent transmission cycles. Nonetheless, the high mortality in this species represents a severe threat to the livelihoods of high-altitude pastoral communities and a significant animal welfare concern. Furthermore, LSDV has been isolated from a giraffe (Giraffa camelopardalis) in a Vietnamese zoo, and antibodies have been detected in other wild ruminants such as impala and springbok [15, 28]. The role of wildlife in the maintenance and spread of LSDV remains poorly understood but warrants ongoing surveillance, particularly as the virus invades new ecosystems.

Risk Factors for Transmission and Amplification

A multitude of observational studies have consistently identified a hierarchy of risk factors that modulate the probability of LSDV transmission and establishment at the animal, herd, and landscape levels. At the individual animal level, female cattle, particularly those that are lactating or pregnant, have been found at higher risk in several studies, likely due to increased physiological stress and, in the case of lactating cows, their attraction for blood-feeding insects [9, 25, 59]. Younger animals, particularly calves, often exhibit higher morbidity and mortality rates, reflecting an underdeveloped immune system [32, 59]. Breed susceptibility is also noted, with crossbred and exotic breeds showing higher clinical attack rates compared to indigenous Bos indicus cattle in some settings, though this is variable [8, 17, 59].

At the herd level, large herd size is a robust predictor of LSDV presence. Studies in Ivory Coast reported a prevalence of 76.51% in herds larger than 50 cattle compared to 34.72% in smaller herds [3]. Similarly, transhumant or pastoral management systems that involve movement of herds over long distances and mixing at communal grazing and watering points significantly elevate risk, a pattern documented in Pakistan, Bangladesh, and across Africa [3, 8, 9, 24, 59]. The absence of vector control measures, such as the use of acaricides or insecticides, is consistently associated with higher odds of infection, providing a direct intervention target [9, 17]. At the landscape level, proximity to water bodies, high annual rainfall, and bush or forest cover that harbors vectors are all positive predictors [24, 31, 58, 59].

Communal water sources deserve special mention; they act as congregation points for multiple herds, facilitating both direct contact and vector-borne transmission among animals that would otherwise not interact. Studies from Uganda and Bangladesh have identified drinking from communal dams or ponds as a significant risk factor [11, 24]. The introduction of new animals into a herd without quarantine is another powerful amplifier, as it allows the entry of an infected but asymptomatic animal into a naive population [11, 29]. These risk factors are not independent but interact synergistically, creating high-risk "hotspots" where transmission probability is multiplicatively increased.

Spatio-Temporal Epidemiology and Global Dispersal Patterns of LSDV Clades (1.2, 2.5, and Emerging Lineages)

The global dispersal of Lumpy Skin Disease Virus (LSDV) represents one of the most significant transboundary animal disease expansions of the 21st century, transitioning from an African-endemic pathogen to a pan-Eurasian threat within a single decade. This unprecedented geographic invasion has been accompanied by a parallel diversification of viral lineages, driven by both gradual evolutionary drift and punctuated recombination events. Understanding the spatio-temporal dynamics of the major LSDV clades, particularly the wild-type Clade 1.2, the recombinant Clade 2.5, and the emerging mosaic lineages, is essential for predicting future incursions, designing surveillance strategies, and implementing targeted control measures. The World Organisation for Animal Health (WOAH) classifies LSD as a notifiable disease, underscoring its economic significance and the imperative for global epidemiological monitoring.

The Foundational Phylogenetic Framework: Clade 1.2 as the Global Dispersal Lineage

The phylogenetic architecture of LSDV has been refined considerably through whole-genome sequencing (WGS) initiatives, which have superseded the limited resolution afforded by single-gene analyses. Breman et al. [2] demonstrated that single-gene phylogenies based on RPO30, GPCR, EEV, or B22R lack the discriminatory power to resolve fine-scale spatio-temporal patterns, and they strongly advocate for WGS-based approaches whenever feasible. Within the broader LSDV phylogeny, the previously defined cluster 1.2 has emerged as the dominant lineage responsible for the vast majority of outbreaks across Eurasia and the Indian subcontinent [2]. This clade encompasses strains that are genetically distinct from the historical African vaccine strains (Clade 1.1) and the recombinant lineages (Clade 2).

The westward and northward expansion of Clade 1.2 from its African origins into the Middle East and Europe has been meticulously documented. Alkhamis and VanderWaal [31] employed ecological niche modeling and time-dependent estimation of the effective reproductive number (R-TD) to characterize the 2012–2015 Middle Eastern epidemic. Their analysis identified Israel and Turkey as the most suitable locations for LSDV persistence, with an average monthly R-TD of 2.2 (95% CI: 1.2–3.5). Critically, they estimated a super-spreading event in Israel during September 2013 (R-TD = 22.2), which was abruptly curtailed by a successful vaccination campaign. This work highlights the critical role of anthropogenic factors, particularly livestock movement and vaccination policy, in modulating the spatio-temporal dynamics of Clade 1.2.

The southeastern European (SEE) outbreak of 2015–2017 provided a unique opportunity for fine-grained phylogeographic analysis. Borm et al. [14] conducted the first comprehensive WGS sampling of a time-constrained LSDV outbreak, analyzing curated public genomes alongside their own data. Their haplotype network analysis revealed intense genetic intermixing between affected countries, indicating that the virus moved freely across national borders within the region. Importantly, they formally assessed the temporal signal in the LSDV genome and found that, despite the slow molecular evolution characteristic of poxviruses, a measurable clock signal exists when recombination events are accounted for. Their time-scaled continuous phylogeographic reconstruction confirmed the origin of the SEE outbreak from a Kenyan-like ancestor and traced its dissemination through the Balkans. This study underscores a fundamental challenge in LSDV phylogeography: the limited genetic variation of its DNA genome constrains the precision of molecular clock calibrations, yet careful analytical approaches can still yield meaningful insights into dispersal pathways.

The Emergence and Dominance of Clade 2.5: A Recombinant Lineage Redefining Asian Epidemiology

The most dramatic evolutionary development in recent LSDV history has been the emergence and subsequent dominance of recombinant strains belonging to Clade 2.5. These strains are not the product of gradual mutation but rather of homologous recombination between field viruses and live-attenuated vaccine strains, a phenomenon that has fundamentally altered the epidemiological landscape of Asia. Krotova et al. [18] provided the foundational genomic characterization of these novel recombinants, sequencing the complete genomes of two Russian strains (Khabarovsk/2020 and Tomsk/2020) and comparing them with five other recombinant isolates from Russia, Kazakhstan, and China. Their analysis revealed a striking pattern: strains isolated prior to 2020 exhibited unique, individualistic combinations of open reading frames (ORFs) inherited from both Neethling vaccine and Kenyan vaccine parental strains. However, from 2020 onwards, all circulating recombinant strains in Russia and Southeast Asia belonged to a single, monophyletic lineage, with LSDV/GD01/China/2020 serving as the founding representative. This transition from genetically unique recombinants to a clonal lineage indicates a selective sweep, where a particularly fit recombinant variant outcompeted its predecessors and established itself as the dominant circulating strain.

The epidemiological consequences of this evolutionary shift have been profound. Sprygin et al. [5] conducted WGS of 11 LSDV isolates from Russia and Mongolia collected between 2021 and 2023, specifically searching for evidence of ongoing recombination. Their findings were unequivocal: no new mosaic variants were detected, and Clade 2.5 had become the exclusive lineage circulating in the region, exhibiting its own patterns of monophyletic evolution. This suggests that the recombination-driven diversification phase has given way to a stabilization phase, where the established Clade 2.5 lineage is now undergoing adaptive evolution through conventional mutation and selection. The authors emphasize that this lineage is "now the dominant lineage currently on the rise in the region" [5], with a geographic footprint extending across China, Thailand, Vietnam, and South Korea.

The spatial progression of Clade 2.5 across Asia has been meticulously mapped. Following its initial detection in China in 2019, the lineage spread southward into Vietnam and Thailand. Singhla et al. [20] characterized LSDV isolates from six outbreaks in northern Thailand during July–September 2021, finding that 24 of 26 isolates clustered with Asian sequences from China, Hong Kong, and Vietnam. Notably, two isolates from vaccinated animals clustered with Neethling-derived vaccine strains, highlighting the diagnostic challenge posed by vaccine-like viruses in the field. The expansion continued into the Korean peninsula, with South Korea reporting incursions linked to the same lineage. The vector-borne transmission dynamics, combined with the movement of infected cattle, facilitated this rapid southward and eastward progression, covering thousands of kilometers in less than five years.

Clade 1.2.1 and 1.2.2: Regional Diversification within the Indian Subcontinent

While Clade 2.5 dominated Southeast Asia, the Indian subcontinent experienced a parallel but distinct epidemiological trajectory driven by sub-lineages of Clade 1.2. The initial introduction of LSDV into India occurred in August 2019 in Odisha State, and genetic characterization of the earliest isolates revealed a close relationship with historical Kenyan NI-2490/Kenya/KSGP-like field strains [33]. Sudhakar et al. [33] demonstrated that the 2019 Indian isolates shared 99.7–100% identity with these African strains across multiple genes, and they identified a distinctive 12-nucleotide insertion in the GPCR gene that became a molecular marker for the Indian outbreak. This finding strongly suggested a single, exotic introduction event rather than multiple independent incursions.

Subsequent years witnessed the diversification of Indian LSDV into at least two distinct sub-clades: 1.2.1 and 1.2.2. The 1.2.2 sub-clade, exemplified by the Neethling Warmbaths (NW)-like viruses, has been associated with outbreaks across northern and central India. Sharma et al. [1] characterized the 2023 Bhutanese outbreak and found that the Bhutanese strain (LSDV_Bhutan_03) clustered within Clade 1.2.2, closely related to recent isolates from cattle and buffalo in India and yaks in China. This finding underscores the transboundary connectivity of the Himalayan region and the role of livestock movement in disseminating specific sub-lineages.

In contrast, the 1.2.1 sub-clade has been identified in northeastern India, particularly in the Himalayan state of Sikkim. Sudhakar et al. [6] analyzed LSDV strains from yaks (Bos grunniens) and cattle during the 2022–2023 outbreaks and found that all sequences belonged to the 1.2.1 sub-cluster, closely related to variants from India and Tibet but distinct from the 1.2.2 strains dominant elsewhere in India and the 2.5 recombinants in China. The detection of identical sequences in yaks and local cattle, coupled with their close spatial proximity, provided compelling evidence for spillover transmission from cattle to yaks. This finding has significant implications for wildlife epidemiology, as yaks are semi-domesticated bovines that share grazing grounds with cattle in high-altitude ecosystems. The case fatality rate in yaks was alarmingly high at 38.66% compared to 9.92% in cattle [1], suggesting that this species may be particularly vulnerable to LSDV infection.

The Kenyan Sheep and Goat Pox (KSGP) Lineage: A Persistent African Signature

Amidst the focus on Eurasian lineages, it is critical to recognize that the ancestral African diversity of LSDV continues to circulate and, in some regions, remains the dominant field strain. The KSGP-like strains, belonging to Clade 1.1, have been identified in multiple Indian outbreaks. Das et al. [10] reported the first LSD outbreak in the northeastern Indian state of Meghalaya and confirmed the circulation of Kenyan field strain-associated LSDV (subgroup 1.2-related strains). Similarly, Mummidisetty et al. [7] isolated LSDV from Andhra Pradesh and found that their isolates formed a distinct clade with Middle Eastern and African isolates based on the RPO30 gene, with a 9.52% genetic distance from vaccine strains.

The co-circulation of KSGP-like and Clade 1.2 strains in the same geographic region presents a diagnostic and epidemiological challenge. Sprygin et al. [16] developed a real-time PCR assay targeting the LW032 ORF that is specifically capable of detecting KSGP-related isolates and recombinant strains containing the KSGP backbone. This tool is essential for regions where multiple lineages may be present, as it enables rapid differentiation between field strains and vaccine-like viruses. The authors emphasize that if the distribution of KSGP-like and Clade 2.5 lineages encroaches into each other's ranges, it will be impossible to differentiate between them without such fit-for-purpose molecular tools [16].

Phylogeographic Drivers: Vectors, Trade, and Climate

The spatio-temporal patterns of LSDV dispersal cannot be understood without reference to the underlying ecological and anthropogenic drivers. The vector-borne transmission of LSDV, primarily by stable flies (Stomoxys calcitrans), mosquitoes (Aedes aegypti), and ticks (Rhipicephalus and Amblyomma species), imposes a strong seasonal signature on outbreak dynamics [19]. Sprygin et al. [19] documented that long-distance dispersal occurs primarily through the movement of infected animals, while short-distance spread is driven by arthropod vectors. This dual-mode transmission explains the characteristic pattern of LSDV emergence: index cases often appear near livestock markets or along trade routes, followed by rapid local amplification during vector-active seasons.

The Russian experience provides a compelling case study of how climate and geography shape dispersal patterns. Byadovskaya et al. [23] analyzed 471 outbreaks across Russia over a six-year period, spanning a 9,000 km range. They found that outbreaks occurred primarily in smallholder farms between mid-May and mid-November, with a distinct northward movement from 2015 to 2016, followed by an eastward turn through Siberia to the Far East by 2020. Remarkably, outbreaks continued to occur even under snow and freezing temperatures that would normally preclude vector activity, suggesting that alternative transmission mechanisms, possibly direct contact or fomite transmission, may become more important in hemiboreal climates. Mathematical modeling revealed statistically significant annual spatio-temporal clusters in 2016–2018, but this segregation disappeared in 2019–2020, indicating that the virus had become widely established and was no longer constrained by focal transmission dynamics [23].

Machine learning approaches have further refined our understanding of the environmental niche of LSDV. Safavi [60] applied artificial neural networks (ANN) to predict LSDV occurrence based on meteorological and geospatial features, achieving an AUC of 0.97 and F1 score of 0.94. The analysis identified annual precipitation, land cover, mean diurnal temperature range, livestock production system type, and global livestock densities as the most important predictors [31, 60]. These models have practical utility for risk-based surveillance, enabling veterinary authorities to prioritize vaccination and vector control efforts in areas with the highest predicted probability of incursion.

Emerging Lineages and the Threat of Further Diversification

The evolutionary trajectory of LSDV remains dynamic, and the potential for further lineage emergence is a major concern for global animal health. The recombination events that gave rise to Clade 2.5 were not isolated incidents; rather, they represent a recurring phenomenon whenever field and vaccine strains co-circulate. Mazloum et al. [28] reviewed the molecular evolution of LSDV and concluded that the virus population is "prone to molecular evolution, generating novel phylogenetically distinct variants resulting from a diverse range of selective pressures, including recombination between field and homologous vaccine strains in cell culture that produce virulent recombinants." The use of live-attenuated vaccines, while essential for disease control, creates a reservoir of vaccine virus that can recombine with circulating field strains, potentially generating variants with altered virulence, host range, or transmissibility.

The emergence of the GD01-like lineage in China and its subsequent spread across Southeast Asia serves as a cautionary tale. Xie et al. [49] isolated two LSDV strains from different geographical regions in China and performed comparative genomic analyses with other Asian isolates. Their phylogenetic analysis revealed distinct branches of LSDV evolution, signifying the prevalence of multiple lineages across various regions in Asia. Critically, they demonstrated that LSDV exhibits an "open" pan-genome, meaning that the total gene repertoire of the species is not fixed and can expand through horizontal gene transfer or recombination. This genomic plasticity, combined with the ongoing use of live vaccines, creates conditions conducive to the emergence of novel lineages.

The potential for LSDV to adapt to new hosts is another dimension of emerging lineage risk. The detection of LSDV in a giraffe in Vietnam [15], in yaks in Bhutan and India [1, 6], and in water buffaloes across multiple countries [27] indicates that the host range is broader than traditionally recognized. Bianchini et al. [27] conducted a systematic review and identified buffaloes as the main non-cattle hosts, with mechanical transmission via blood-sucking vectors as the primary mode. The spillover into wildlife populations raises the possibility of sylvatic cycles that could serve as persistent reservoirs, complicating eradication efforts.

The African Context: Endemicity and Under-Reporting

While much of the recent literature focuses on the Eurasian expansion, it is essential to recognize that Africa remains the ancestral home of LSDV and continues to harbor significant genetic diversity. The epidemiological situation in Africa is characterized by endemicity, with periodic outbreaks driven by fluctuations in vector populations, herd immunity, and livestock movement. Khafagi et al. [12] reviewed the status of LSD in Egypt, where the disease was first reported in 1988 and has since become endemic. The Egyptian experience illustrates the challenges of controlling a vector-borne disease in a region with limited veterinary infrastructure and transboundary livestock trade.

The Ivory Coast study by Kadja et al. [3] provides a contemporary snapshot of LSD epidemiology in West Africa. Their cross-sectional survey in the Poro Region revealed an overall prevalence of 51.85% among cattle with pox-like lesions, with significant variation across localities (66.67% in M'bengué and 70.87% in Dikodougou). Phylogenetic analysis confirmed that the Ivory Coast strains clustered with other African field strains and were distinct from the Asian recombinants. The identification of herd size and transhumance as major risk factors highlights the importance of livestock management practices in driving transmission dynamics. The authors call for regional surveillance and control measures, recognizing that the porous borders of West Africa facilitate the movement of both animals and vectors.

The under-reporting of LSDV outbreaks in Africa is a persistent challenge that biases our understanding of global dispersal patterns. Alkhamis and VanderWaal [31] explicitly identified areas in the Middle East where under-reporting likely occurred, and similar gaps almost certainly exist across Africa. The WOAH's World Animal Health Information System (WAHIS) provides the most comprehensive global dataset, but its utility is limited by the quality and timeliness of national reporting. Jyoti et al. [26] analyzed WAHIS data from 2005 to 2020 and found that 72.6% of reported outbreaks were from Europe, 14.8% from Asia, and only 12.5% from Africa, a distribution that reflects reporting capacity rather than true disease incidence. This disparity underscores the need for capacity building in veterinary surveillance systems across endemic regions.

Temporal Dynamics and the Role of Vaccination

The temporal patterns of LSDV dispersal are strongly influenced by vaccination campaigns, which can dramatically alter the effective reproductive number and interrupt transmission chains. The Israeli experience of 2013, where a sharp drop in R-TD followed a targeted vaccination campaign [31], demonstrates the potential for rapid control when political will and resources are aligned. Conversely, the Russian experience illustrates the consequences of delayed or incomplete vaccination coverage. Byadovskaya et al. [23] documented a decline in morbidity and mortality from 2015 to 2020, which they attributed to the gradual build-up of herd immunity through vaccination. However, the continued eastward spread of the virus through Siberia suggests that vaccination coverage was insufficient to prevent geographic expansion, even if it reduced within-herd impact.

The emergence of vaccine-derived recombinants has complicated the risk-benefit calculus of vaccination. The very tools used to control LSDV, live-attenuated vaccines, can, under certain conditions, generate new viral variants with unpredictable properties. Krotova et al. [18] demonstrated that the recombinant strains consistently exhibit targeted selection in specific genomic regions, particularly those encoding proteins involved in immune evasion. This suggests that recombination is not a random process but is driven by selective pressures that favor certain genetic combinations. The identification of three highly variable gene regions in the 5′ and 3′ flanking regions of the LSDV genome, which encode immune evasion genes [2], provides a mechanistic basis for this observation. These regions may represent "hotspots" for recombination and adaptive evolution, and they warrant close monitoring in future surveillance efforts.

Molecular Epidemiology and Phylogenetic Analysis: Single-Gene vs. Whole-Genome Sequencing for Outbreak Tracking

The unprecedented transboundary expansion of Lumpy Skin Disease Virus (LSDV) across Africa, the Middle East, Europe, and Asia has fundamentally challenged the capabilities of conventional molecular epidemiological surveillance frameworks. As the virus has traversed thousands of kilometers over the past decade, establishing endemicity in regions as disparate as the Indian subcontinent and Southeast Asia, the need for robust, high-resolution phylogenetic tools has become paramount for effective outbreak tracking and control. The debate between utilizing single-gene sequencing approaches versus whole-genome sequencing (WGS) for LSDV molecular epidemiology is not merely an academic exercise; it has profound implications for the capacity of veterinary services and international organizations, including the World Organisation for Animal Health (WOAH) and the Food and Agriculture Organization (FAO), to trace viral incursions, differentiate field strains from vaccine-derived recombinants, and understand the evolutionary trajectories driving the ongoing pandemic.

The core of the methodological tension lies in the fundamental biology of LSDV. As a member of the Poxviridae family, LSDV possesses a large, double-stranded DNA genome of approximately 151 kilobase pairs encoding 156 predicted open reading frames [42]. For decades, the prevailing wisdom held that poxviruses, given their proofreading DNA polymerases and relatively slow mutation rates compared to RNA viruses, exhibited limited genetic plasticity and were thus amenable to characterization using a handful of conserved genetic markers. This assumption, however, has been systematically dismantled by the emergence of complex recombinant strains and the realization that the virus is undergoing far more dynamic genomic evolution than previously appreciated [2, 14, 18]. The slow molecular evolution of its DNA genome does indeed limit the utility of genetic variation for accurate tracing based on evolutionary analyses, but this limitation has not been fully appreciated in the context of single-gene phylogenies [14].

The Limitations of Single-Gene Phylogenies: Inadequate Resolution and Misleading Clustering

The most commonly employed single-gene targets for LSDV molecular epidemiology include the RNA polymerase 30 kDa subunit (RPO30), the G-protein-coupled chemokine receptor (GPCR) gene, the extracellular enveloped virus (EEV) glycoprotein gene, the P32 gene, and the B22R gene [1, 3, 6, 25, 33, 54]. These markers have been historically favored because they contain sufficient inter-species variation to differentiate LSDV from the closely related sheeppox virus (SPPV) and goatpox virus (GTPV), and they have been instrumental in establishing the initial phylogenetic frameworks for capripoxvirus classification [39, 40]. However, a growing body of evidence demonstrates that reliance on single-gene phylogenies for intra-species outbreak tracking is fundamentally insufficient and can lead to erroneous epidemiological conclusions.

A landmark study by Breman et al. (2023) systematically compared whole-genome-sequence (WGS)-based phylogenies with single-gene-based phylogenies and concluded unequivocally that using only a single gene for sequencing lacks phylogenetic and spatiotemporal resolution [2]. The study demonstrated that when strains were analyzed using only the GPCR or RPO30 genes, viruses belonging to distinct clades within the broader cluster 1.2, which encompasses the vast majority of field strains circulating in Eurasia and South Asia, were often collapsed into poorly resolved, polytomous branches. This lack of resolution is particularly problematic for outbreak tracking because it obscures the subtle genetic distinctions that can differentiate between a newly introduced strain and a locally circulating endemic variant. For example, the Bhutanese LSDV strain LSDV_Bhutan_03, characterized by Sharma et al. (2025), was placed within the broader Clade 1.2 based on the RPO30, GPCR, EEV, and B22R genes, but only WGS was able to refine its position to the more specific Clade 1.2.2 (Neethling Warmbaths-like), revealing its close relationship to recent isolates from India and China [1]. Single-gene analysis would have detected the outbreak but would have been incapable of definitively pinpointing its origin within the highly diversified clade structure.

The biological basis for this resolution deficit is rooted in the uneven distribution of genetic variation across the LSDV genome. Breman et al. (2023) identified three highly variable gene regions, located specifically in the 5′ and 3′ flanking regions (the inverted terminal repeats, or ITRs) [2]. These regions encode genes involved in immune evasion strategies, including the LSDV001, LSDV142, and LSDV156 proteins, which are known to modulate interferon signaling and inflammatory responses [43, 45, 47]. Single-gene approaches that target conserved internal loci like RPO30 or P32 systematically ignore this hypervariable genomic landscape, thereby discarding the very markers that could provide the highest epidemiological resolution. This is not merely a technical limitation; it represents a fundamental misalignment between the regions of the genome under selective pressure (the ITR-encoded immune modulators) and the regions commonly sequenced for phylogenetic inference. As the virus encounters novel host populations and vector ecologies across Asia, it is precisely the variable ITR regions that are expected to bear the signatures of host adaptation and immune selection [5, 18].

The Crucial Issue of Recombination: A Blind Spot for Single-Gene Approaches

Perhaps the most critical failure of single-gene phylogenies is their inability to detect or characterize recombinant strains, which have become a defining feature of the LSDV pandemic. The emergence of mosaic LSDV strains, first identified in Russia in 2017 and subsequently spreading across Kazakhstan, China, and Southeast Asia, represents a paradigm shift in our understanding of LSDV evolution [18]. These recombinants are genetic chimeras, composed of segments from both a live-attenuated Neethling vaccine strain and a Kenyan sheep and goat pox (KSGP)-like field strain. The most prominent of these, belonging to cluster 2.5, have now become the dominant lineage spreading throughout Southeast Asia, including China, Thailand, Vietnam, and South Korea [2, 5, 18].

A single-gene phylogenetic analysis of a recombinant strain will produce a profoundly misleading picture. If the sequenced gene happens to derive from the Neethling vaccine-like parental segment, the strain will falsely cluster with vaccine strains, leading to the erroneous conclusion that a vaccine-derived virus is causing the outbreak. Conversely, if the gene derives from the KSGP-like segment, the strain will cluster with African field strains, obscuring its recombinant nature. Only WGS can resolve this ambiguity by revealing the mosaic pattern of alternating parental segments across the length of the genome. Krotova et al. (2022) provided a comprehensive bioinformatic analysis of all available recombinant LSDV strains and demonstrated that strains isolated prior to 2020 were composed of unique combinations of open reading frames, while from 2020 onwards, all circulating strains in Russia and South-Eastern Asia belonged to a single, established recombinant lineage first represented by LSDV/GD01/China/2020 [18]. This epidemiological transition, from sporadic, unique recombinants to a dominant clonal lineage, could only be documented through comparative WGS. Monitoring this lineage as it continues its eastward expansion is essential for understanding the spatial dynamics of the pandemic, and single-gene markers simply lack the discriminatory power to track the evolution of this lineage over time.

Sprygin et al. (2025) provided a critical update demonstrating that, within the dominant cluster 2.5 lineage, no further recombination signals have been observed, and the lineage is now undergoing monophyletic evolution via its own patterns of genetic drift [5]. This finding underscores a central tenet of molecular epidemiology: even after a recombinant lineage stabilizes, WGS remains indispensable for monitoring within-clade diversification. The emergence of sub-clades within cluster 2.5, each with potentially different transmission dynamics or virulence characteristics, can only be elucidated through genome-wide single nucleotide polymorphism (SNP) analysis.

The Complementary Role of Concatenated Sequences and Targeted Gene Panels

Recognizing the prohibitive cost and technical demands of routine WGS in many endemic regions, particularly across Africa and parts of South Asia where laboratory capacity remains limited, researchers have explored pragmatic compromises. A highly promising approach involves the use of concatenated gene sequences, combining the sequences of multiple selected marker genes into a single phylogenetic analysis. Sudhakar et al. (2025) explicitly demonstrated that concatenated sequence-based analysis could function as a proxy to whole-genome sequence analysis [6]. In their study of LSDV outbreaks in yaks and cattle in the Himalayan state of Sikkim, the concatenation of GPCR, RPO30, EEV, and B22R sequences provided sufficient phylogenetic signal to place the Sikkim strains within the 1.2.1 sub-cluster, revealing their close relationship to circulating variants from India and Tibet and clearly distinguishing them from the dominant 1.2.2 strains in India and the 2.5 recombinants in China. This approach significantly improves resolution compared to any single gene alone because it effectively samples multiple independent genomic loci, increasing the total number of informative characters.

Furthermore, Sudhakar et al. (2025) identified a specific promising target: the C-terminal 717 base pair segment of the B22R gene [6]. This single-gene target, when used in isolation, demonstrated an unexpected capacity for robust phylogenetic classification, successfully differentiating LSDV into well-supported clusters that recapitulated the major clades defined by WGS. The B22R gene encodes a virulence factor involved in immune modulation, and the C-terminal domain appears to be under sufficient selective pressure to accumulate lineage-specific substitutions. This finding provides a potential "best-available" single-gene target for resource-limited settings where multi-gene concatenation or WGS is not feasible. However, it must be emphasized that even this optimized target cannot detect recombination. A strain could perfectly match the B22R sequence of a vaccine strain while the rest of its genome is derived from a field parent, a scenario that has already been documented in Russia and Kazakhstan [18].

Operational Recommendations for Surveillance Networks

For national and international surveillance programs, a tiered sequencing strategy emerges as the most rational and cost-effective approach. For initial outbreak confirmation and genus-level identification, real-time PCR targeting the LSDV126 region, as recommended by WOAH, remains the gold standard [10]. However, for phylogenetic characterization, the selection of sequencing targets must be deliberate.

Level 1 (Routine Surveillance): For monitoring known endemic circulation where the lineage is already characterized (e.g., ongoing Kenyan-like field strain circulation in Uganda or Bangladesh), a standardized panel of four to five marker genes, GPCR, RPO30, EEV, B22R, and the P32 gene, should be amplified and concatenated for phylogenetic analysis. This multi-gene approach provides sufficient resolution for lineage-level assignment (e.g., Clade 1.1 vs. Clade 1.2 vs. Clade 2) and allows for the detection of major shifts in circulating strains.

Level 2 (Epidemiological Investigation of New Incursions): When LSDV emerges in a previously free area, or when an outbreak displays unusual clinical characteristics (e.g., high mortality in vaccinated herds), WGS must be performed on a minimum of two to three representative isolates. WGS is the only method capable of definitively differentiating between a true field virus incursion, a vaccine-derived outbreak, or a mosaic recombinant, the three most common and epidemiologically distinct scenarios. The study by Breman et al. (2023) highlighted the importance of accounting for recombination events before reconstructing global and regional dynamics of LSDV, noting that WGS sampling in endemic areas remains patchy and must be expanded [14].

Level 3 (Phylodynamic and Phylogeographic Analysis): For understanding regional dispersal dynamics and estimating parameters such as the effective reproductive number (Rₜ) and geographic diffusion rates, high-density WGS sampling linked to precise metadata (date of collection, geographic coordinates, clinical severity, vaccination status) is required. The continuous phylogeographic analysis conducted by Van Borm et al. (2023) demonstrated that on a global scale, proper accounting for recombination in the alignment is necessary for accurate time-scaled trees [14]. The fine-grained sampling of LSDV whole genomes from a time-constrained (2015–2017) southeastern European outbreak revealed intense intermixing between countries, a pattern that would have been completely invisible to single-gene analysis [14].

The Path Forward: Integrating Genomics into the Global Control Strategy

The WOAH and the FAO have increasingly advocated for the integration of WGS into transboundary animal disease surveillance. For LSDV, the argument is now unambiguous. The continued circulation of both field strains (Clade 1.2.1 and 1.2.2 in South Asia, Cluster 2.5 in Southeast Asia) and vaccine-like recombinants creates a complex molecular epidemiological landscape that single-gene approaches are fundamentally inadequate to resolve [5, 14, 16]. The development of targeted real-time PCR assays capable of discriminating between KSGP-like strains, Neethling vaccine strains, and the dominant recombinant cluster 2.5 is a valuable stopgap measure, but these tools are designed for detection, not for phylogenetics [16]. They can tell a veterinary authority which lineage is present but cannot reveal the subtle evolutionary relationships necessary for source attribution.

In conclusion, the evidence from the recent pandemic overwhelmingly supports the abandonment of routine single-gene sequencing for LSDV outbreak tracking. The slow molecular evolution of the LSDV genome, combined with the profound impact of recombination, renders single-gene phylogenies insufficient for any meaningful epidemiological inference beyond initial clade assignment. The future of LSDV molecular epidemiology lies in tiered, strategic WGS implementation, supported by fit-for-purpose multiplex PCR panels for initial screening and the routine use of concatenated multi-gene sequences in settings where WGS remains economically unfeasible. Only through a genomic-scale understanding of LSDV evolution can the global veterinary community hope to reconstruct the complex dynamics of transmission, anticipate the emergence of novel variants, and inform the rational deployment of vaccines and biosecurity measures.

Diagnostic Approaches for LSDV: Molecular, Serological, and Differential Detection Strategies

The accurate and timely diagnosis of Lumpy Skin Disease Virus (LSDV) is paramount for effective outbreak management, surveillance, and the implementation of control strategies. The diagnostic landscape for LSDV has evolved considerably, moving from clinical and histopathological observation to a sophisticated array of molecular and serological techniques that offer high sensitivity, specificity, and the capacity for differentiating between wild-type, vaccine, and recombinant strains. This section provides a comprehensive analysis of the contemporary diagnostic approaches for LSDV, emphasizing the biological principles, epidemiological applications, and strategic importance of each method.

Foundational and Clinical Diagnosis

The initial diagnostic suspicion of LSDV infection is frequently based on the highly characteristic clinical presentation, particularly in severe, classical cases. The disease is typified by the sudden onset of pyrexia, lachrymation, nasal discharge, hypersalivation, and pronounced swelling of superficial lymph nodes, followed by the development of circumscribed, firm skin nodules that can cover the entire body [12, 22, 30, 58]. These nodules, which can become necrotic and slough, are pathognomonic in epizootic settings. However, reliance on clinical signs alone is fraught with limitations. Mild or subclinical infections, which are increasingly recognized as a significant component of LSDV epidemiology, often lack the full spectrum of visible nodular lesions, leading to substantial underreporting [12, 36]. Furthermore, conditions such as pseudo-lumpy skin disease (caused by bovine herpesvirus 2), dermatophilosis, insect bites, and urticaria can produce nodular or papular skin lesions that mimic LSD, necessitating laboratory confirmation for definitive diagnosis [26, 35, 58]. Therefore, clinical diagnosis serves as a critical initial screening tool, particularly for alerting veterinary authorities to potential incursions, but it is insufficient for confirmation and must be supported by robust laboratory methods.

Molecular Diagnostic Approaches: The Gold Standard

Molecular techniques, particularly polymerase chain reaction (PCR)-based assays, have become the cornerstone of LSDV diagnosis due to their exceptional sensitivity, specificity, and rapid turnaround time. The World Organisation for Animal Health (WOAH) recommends PCR as a primary test for the detection of LSDV nucleic acid in clinical samples such as skin nodules, scabs, blood, nasal swabs, and semen [12, 44].

Real-Time PCR (qPCR) and Conventional PCR

Real-time PCR (qPCR) is the preferred molecular method for routine diagnosis and surveillance. It offers quantitative data, reduced risk of cross-contamination, and faster results compared to conventional PCR. A widely used WOAH-recommended qPCR targets the LSDV126 gene, which has proven highly effective for detecting the virus across various lineages [10]. Das et al. [10] validated this probe-based qPCR on 56 clinical samples from northeastern India, detecting LSDV in 75% of them, confirming its utility. Similarly, Pham et al. [11] emphasized the importance of a well-optimized real-time PCR for effective LSD surveillance at the household level in Vietnam.

The need for targeted molecular tools has been underscored by the global emergence of genetically distinct LSDV lineages, including recombinant strains. To address this, Sprygin et al. [16] developed a novel qPCR assay targeting the open reading frame LW032, specifically designed to detect LSDV strains belonging to the Kenyan sheep and goat pox (KSGP) group (cluster 1.1) and recombinant strains containing a KSGP backbone. This assay, with an amplification efficiency of 91.08%, provides a critical tool for epidemiological investigations in regions like South Asia where KSGP-like strains circulate, and its use is recommended for accurate detection where multiple lineages co-exist [16].

Conventional PCR assays, while less quantitative, remain valuable for molecular characterization and phylogenetic analysis. They are commonly used to amplify specific target genes for sequencing. The most frequently targeted genes include the P32 (a structural protein gene), RPO30 (encoding the RNA polymerase 30 kDa subunit), and GPCR (G-protein-coupled chemokine receptor) [7, 9, 20, 22, 25]. For instance, Jabbar et al. [9] used a conventional PCR targeting the P32 gene to determine a 36.25% prevalence of LSDV in clinically suspected cattle in Pakistan. In Egypt, El-Nady et al. [29] amplified a 958 bp fragment of the EEV glycoprotein gene, achieving a 95.65% positivity rate among samples. Importantly, these single-gene PCRs are often the first step in molecular epidemiological studies, providing the raw sequence data for phylogenetic clustering and strain identification.

Gene Sequencing and Phylogenetic Analysis

Sequencing of specific viral genes is essential for genotyping, tracing the origin and spread of outbreaks, and understanding viral evolution. A multi-gene approach is generally recommended over reliance on a single locus to enhance phylogenetic resolution. The study by Breman et al. [2] explicitly demonstrated that single-gene phylogenies (using RPO30, GPCR, EEV, or B22R) lack the discriminatory power of whole-genome sequencing (WGS) and can lead to erroneous conclusions about spatiotemporal relationships. They found that while single-gene approaches are "cheap, fast and easy to routinely use," they lack resolution and recommend WGS whenever possible [2].

Despite these limitations, a panel of four target genes, GPCR, RPO30, EEV, and B22R, is widely used for molecular characterization and has proven effective for clustering LSDV strains into major clades. Multiple studies have successfully employed this quartet to characterize field isolates from Bhutan [1], Ivory Coast [3], Sikkim, India [6], and Egypt [34]. Sudhakar et al. [6] further proposed that the C-terminal 717 bp of the B22R gene could serve as a valuable single-gene sequencing target for robust cluster classification, potentially acting as a proxy for more extensive analyses. Analyses of the RPO30 gene have also revealed species-specific signatures that help differentiate LSDV from sheeppox virus (SPPV) and goatpox virus (GTPV), as highlighted by Reddy et al. [39] and Mummidisetty et al. [7], who noted key amino acid substitutions potentially linked to increased virulence in field isolates.

The GPCR gene is another critical target, particularly for identifying recombinant and vaccine-derived strains. Singhla et al. [20] used GPCR sequencing to differentiate between field isolates and Neethling-derived vaccine strains in Thailand. The presence of a 12-nucleotide insertion in the GPCR gene has been identified as a signature for LSDV strains involved in the 2019 outbreaks in India and Bangladesh, suggesting a common exotic source [33]. This gene's variable region provides high-resolution phylogenetic insights, as demonstrated by Bayyappa et al. [46], who used GPCR sequences to map transboundary movement and genomic diversity.

Whole-Genome Sequencing (WGS)

WGS is the most powerful molecular tool for LSDV investigation, providing the ultimate resolution for phylogenetic and evolutionary studies. As the LSDV genome is approximately 151 kbp and evolves slowly, WGS is crucial for capturing the full extent of genetic variation, including recombination events and single nucleotide polymorphisms (SNPs) that might be missed by targeted gene sequencing [2, 14]. The detection and characterization of novel recombinant strains, such as those emerging in Russia, Kazakhstan, China, and Vietnam, has been entirely reliant on WGS. Krotova et al. [18] used WGS to reveal that recombinant strains isolated before 2020 were genetically unique, whereas from 2020 onwards, a single dominant lineage (represented by LSDV/GD01/China/2020) began radiating across southeastern Asia, highlighting the power of WGS to track lineage displacement.

Furthermore, WGS provides the data necessary for robust phylogeographic and phylodynamic analyses. Van Borm et al. [14] demonstrated that WGS from a time-constrained outbreak in southeastern Europe (2015–2017) allowed for the first fine-grained assessment of dispersal dynamics, confirming the importance of accounting for recombination in these analyses. Sprygin et al. [5] used WGS of 11 isolates from Russia and Mongolia to demonstrate that the cluster 2.5 lineage is now dominant in the region, with no further mosaic variants observed, indicating a shift from a dynamic recombination phase to a more stable monophyletic evolution. WGS has also been essential for understanding the expansion of LSDV into new species, such as yaks in Bhutan [1] and giraffes in Vietnam [15]. The comprehensive genomic analysis by Uddin et al. [62] of a Bangladeshi isolate identified 156 protein-coding sequences, numerous SNPs, and small indels, providing critical baseline data for vaccine and diagnostic development.

Serological Diagnostic Approaches

Serological assays are indispensable for detecting prior exposure to LSDV, assessing the prevalence of infection in a population, and monitoring vaccine-induced immunity. They are particularly useful for epidemiological surveys and retrospective studies.

Enzyme-Linked Immunosorbent Assay (ELISA)

ELISA is the most widely used serological technique due to its relative simplicity, high throughput, and ability to process large numbers of samples. Commercial ELISA kits are available and have been validated for the detection of LSDV-specific antibodies. Das et al. [10] used a commercial ELISA on 182 serum samples from Meghalaya, India, reporting an apparent seroprevalence of 11.54% and a true prevalence of 12.3%. This technique is crucial for understanding the true extent of viral circulation, as it can identify animals that have recovered from subclinical or mild infections that were missed by clinical surveillance. The study by Ochwo et al. [24] in Uganda utilized an indirect ELISA to reveal a high herd-level seroprevalence of 72.3%, despite a moderate animal-level seroprevalence of 8.7%, highlighting the endemic nature of LSDV in the region.

A major advancement in serological diagnostics is the development of DIVA (Differentiating Infected from Vaccinated Animals) strategies. This is vital for post-vaccination surveillance, as most vaccines are live-attenuated and induce antibodies indistinguishable from those generated by natural infection. Yuan et al. [48] performed a genome-wide screening using a phage display library and identified the LSDV103 protein as a specific immunogenic target. They developed an indirect ELISA based on a truncated version of this protein (TrLSDV103), which showed 100% diagnostic specificity and 86.67% sensitivity for detecting infected animals, while clearly differentiating them from animals vaccinated with a GTPV-based vaccine. Similarly, Angsujinda et al. [61] evaluated several recombinant extracellular enveloped virion (EEV) proteins and identified F13L as a promising candidate for serological assays, correlating 85.7% and 75% with commercial ELISA results for infected and vaccinated groups, respectively.

Virus Neutralization Test (VNT)

The VNT is considered the gold standard for serological detection due to its high specificity. It measures the ability of serum antibodies to neutralize infectious virus in vitro. However, the VNT is laborious, time-consuming (requiring several days for cell culture), and requires the use of live virus, which necessitates high-level biosafety facilities (BSL-3). Despite these drawbacks, it remains an important confirmatory test and is used for evaluating the functional efficacy of vaccine-induced antibodies. Studies such as those by Kutumbetov et al. [41] and Ahmed et al. [34] have utilized VNT to confirm seroconversion in experimentally infected or vaccinated animals. The presence of neutralizing antibodies is a strong correlate of protection.

Differential Diagnosis and Genotyping

A critical component of LSDV diagnostics is the ability to differentiate LSDV from other closely related capripoxviruses (SPPV and GTPV) and from recombinant versus field strains. This has become increasingly complex with the emergence of vaccine-like recombinants [18, 27, 52].

Differentiating Capripoxviruses

The three capripoxviruses are antigenically indistinguishable by serological methods, making molecular differentiation essential [40, 53]. The RPO30 gene is a key target for species-level differentiation. Reddy et al. [39] demonstrated that a 21-nucleotide deletion in the RPO30 gene is a conserved signature for all SPPV isolates, while LSDV and GTPV have distinct nucleotide signatures. Gelaye et al. [40] developed a low-cost, rapid genotyping method using unlabeled snapback primers and a dsDNA intercalating dye. This assay, which analyzes the melting temperature of specific hairpin structures, could simultaneously detect and correctly genotype all three capripoxviruses (SPPV, GTPV, and LSDV) with 100% sensitivity and specificity, providing a powerful tool for field-based epidemiology. Monoclonal antibodies targeting conserved proteins, such as the ORF123 mAb described by Wang et al. [53], which shows cross-neutralizing activity against all three viruses, further illustrate the antigenic overlap and the need for molecular tools.

Differentiating Wild-Type, Vaccine, and Recombinant Strains

The co-circulation of field and vaccine viruses, coupled with the potential for recombination, has created a diagnostic challenge. The emergence of mosaic recombinants, particularly within the cluster 2.5 lineage, necessitates the use of specific molecular assays capable of distinguishing these strains [16, 18, 52]. Ferdoos et al. [52] employed a combination of real-time PCRs to differentiate between vaccine-, wild-type-, and recombinant LSDV strains during the 2021/22 outbreak in Pakistan. Their analysis demonstrated that the circulating strains belonged to clade 1.2 and were distinct from the vaccine clade 1.1 and the recombinant clade 2. The qPCR developed by Sprygin et al. [16] is specifically designed to detect KSGP-like strains, which are prevalent in India, Pakistan, and Bangladesh, and are distinct from the recombinant strains found in China and Thailand. This assay is vital for monitoring the potential encroachment of different lineages into each other's ranges [16].

Emerging and Novel Diagnostic Technologies

Beyond established methods, innovative diagnostic tools are being developed to enhance field applicability, speed, and sensitivity. Das et al. [10] explored a novel isothermal amplification method called Saltatory Rolling Circle Amplification (SRCA) for LSDV detection. This technique, which can be performed in a simple water bath, yielded results comparable to real-time PCR with a detection limit of 10 copies/µL, making it a highly promising tool for resource-limited field laboratories. Advances in understanding the host's immune response to LSDV are also opening new diagnostic avenues. The identification of differentially expressed exosomal miRNAs in the serum of LSDV-infected cattle, as reported by Truong et al. [57], may provide a new class of biomarkers for early and accurate diagnosis. Furthermore, a detailed understanding of the functions of viral proteins, such as the LSDV001 protein which promotes inflammatory responses [47], and the LSDV087 and ORF142 proteins which modulate the host's interferon response [43, 45, 55], is critical for the rational design of next-generation vaccines and DIVA diagnostic antigens. The in silico characterization of hypothetical proteins like LSDV004 also identifies potential new targets for diagnostic and therapeutic intervention [51].

Epidemiological Risk Factors, Host Susceptibility, and Economic Impact in Cattle and Wild Bovines

The epidemiology of Lumpy Skin Disease Virus (LSDV) is a complex interplay of host factors, environmental determinants, vector ecology, and management practices, all of which converge to dictate the patterns of disease emergence, propagation, and persistence. While LSDV has historically been considered a pathogen of cattle, the expanding host range documented across multiple continents necessitates a re-evaluation of host susceptibility, particularly among wild bovines and other ruminants. The economic ramifications of LSD are profound, extending from direct production losses to the destabilization of regional and international livestock trade. This section provides an exhaustive examination of these interwoven dimensions, drawing upon the most recent and comprehensive epidemiological investigations from the 64 sources provided.

Animal-Level Risk Factors: Breed, Age, Sex, and Physiological Status

The intrinsic susceptibility of individual animals is modulated by a constellation of host-specific attributes. Among the most consistently reported risk factors is breed. Crossbred and exotic breeds of cattle have been repeatedly demonstrated to exhibit higher morbidity rates compared to indigenous, local breeds. In the Swat Valley of Pakistan, morbidity rates were notably higher in crossbred animals (up to 53% in certain age cohorts) relative to indigenous breeds, a finding attributed to reduced natural selection for resistance against locally circulating pathogens [8]. Similarly, in Bangladesh, crossbred cattle were found to be more susceptible than their indigenous counterparts, with local breeds showing a prevalence of 50% compared to lower rates in exotic types [17, 59]. This phenomenon is likely rooted in the genetic architecture of innate immunity; indigenous breeds have co-evolved with endemic pathogens and vector populations over millennia, whereas improved breeds selected for high production often possess a narrower immunological repertoire.

Age is another critical determinant of susceptibility and outcome. The literature consistently identifies young animals, particularly those under two years of age, as being at elevated risk for both severe clinical disease and mortality. In the Jhenaidah district of Bangladesh, cattle aged ≤24 months were significantly more affected by LSD [59]. The case fatality rate (CFR) is disproportionately higher in calves; data from the 2023 Bhutan outbreak documented an overall CFR of 9.92% in cattle, but this figure was considerably higher in younger stock [1]. The mechanistic basis for this age-dependent susceptibility likely involves the maturation of the adaptive immune system. Calves possess a naïve immune system that has not yet developed a robust memory response, and maternal antibodies, if present, may wane before the animal’s own immunity is fully established. Conversely, adult animals, particularly those in endemic areas, may have acquired some degree of protective immunity through prior subclinical or clinical exposure. Yet, this does not confer complete protection, as evidenced by outbreaks affecting all age groups. The rigorous investigation in Uganda revealed that females over 25 months of age had higher odds of seropositivity, suggesting that age-related exposure risk interacts with management practices, such as the longer lifespan of dairy cows kept for repeated lactation cycles [24].

Sex-based differences are also evident, though the data are not entirely uniform. Several studies indicate that female cattle are at higher risk for LSDV infection. In Punjab, Pakistan, multivariable logistic regression identified female gender as a significant risk factor (OR = 1.435; p = 0.016) [9]. This elevated risk in females is frequently linked to the physiological stressors of lactation and pregnancy. Lactating cows experience significant metabolic demand, which can suppress immune function, a state known as periparturient immunosuppression. Data from the first LSD outbreak in Odisha, India, identified lactation and pregnancy status as potent risk factors (OR = 2.86; p = 0.007), underscoring the vulnerability of reproductively active females [25]. This observation is consequential for the dairy industry, where the economic impact is magnified because the most productive segment of the herd, lactating cows, is also the most susceptible. In contrast, some studies, including those from Ivory Coast and Egypt, found no significant association between sex and LSD prevalence, indicating that the influence of sex may be modulated by other factors such as breed, vector pressure, and the specific viral strain involved [3, 29, 36].

Body condition score (BCS) has emerged as a significant predictor of infection. Emaciated animals, reflecting poor nutritional status and underlying disease, were found to be at significantly higher risk in the Pakistani study (OR = 1.573; p = 0.019) [9]. Malnutrition compromises cell-mediated immunity, reducing the animal's capacity to mount an effective antiviral response. This finding has direct implications for management, suggesting that herds maintained on suboptimal diets or those suffering from concurrent parasitic or bacterial infections are more vulnerable to LSDV incursion and severe disease.

Management-Level Risk Factors: Herd Size, Grazing Systems, and Biosecurity Practices

Management practices are perhaps the most modifiable set of risk factors and thus represent critical targets for intervention. Herd size is consistently associated with LSD prevalence. In the Poro Region of Ivory Coast, larger herds (over 50 cattle) had a substantially higher prevalence (76.51%) compared to smaller herds (34.72%) [3]. This relationship is likely multifactorial: larger herds have a greater probability of receiving an infected animal, provide a larger pool of susceptible hosts for vectors to feed upon, and often experience higher animal density, which facilitates both direct and vector-mediated transmission. The ecological niche modeling conducted for the Middle East identified livestock density as a key environmental predictor of LSDV occurrence, corroborating the herd size effect [31]. However, it is noteworthy that in Uganda, herd size was not a statistically significant risk factor for seropositivity in the final multivariable model, suggesting that the effect may be context-dependent and influenced by other management variables [24].

Grazing systems and animal movement patterns are profoundly influential. Transhumant herds, which move seasonally in search of pasture and water, consistently show elevated prevalence. In Ivory Coast, transhumance was associated with significantly higher LSDV PCR positivity (p < 0.001) [3]. The movement of animals across landscapes brings them into contact with new vector populations, potentially infected animals from other regions, and communal resources that may act as fomites. Communal grazing, watering points, and markets were identified as common sources of infection in Pakistan, highlighting how shared resources amplify transmission risk [8]. The Odisha outbreak investigation found that grazing animals were at nearly two-fold higher risk (OR = 1.90; p = 0.023) compared to those kept in confinement or zero-grazing systems [25]. This is consistent with the known biology of LSDV transmission; as animals graze, they are more exposed to biting flies and other vectors that thrive in outdoor environments.

The biosecurity practices, or their absence, at the farm level are decisive. The introduction of new animals without quarantine is a well-established risk factor. In Egypt, univariable logistic regression identified the introduction of new animals without quarantine as a significant predictor of LSD outbreaks [29]. Similarly, in Lang Son Province, Vietnam, the replacement of cattle with new animals from outside was a significant risk factor at the household level (p < 0.01) [11]. The movement of infected but subclinically affected animals is the primary mechanism for long-distance viral dispersal, as asymptomatic carriers can shed virus and be moved across borders or between regions before clinical signs manifest. Once an infected animal enters a naïve herd, the virus can rapidly amplify via vectors.

Non-treatment with acaricides was identified as a risk factor in Punjab, Pakistan (OR = 1.459; p = 0.014) [9]. While acaricides are primarily targeted at ticks, their use likely serves as a proxy for overall vector control management. The reduction of vector populations through chemical or biological means can significantly lower the force of infection, as mechanical transmission by insects is the predominant route of short-distance spread. The use of fly repellents was specifically recommended as a control measure in the Egyptian study [29]. Furthermore, the absence of any biosecurity measures on smallholder farms in Bangladesh was noted, with many farmers unaware of the disease's transmission mechanisms [17]. This knowledge gap translates directly into behavioral risk, as farmers may fail to isolate sick animals, report outbreaks promptly, or implement vector control.

Environmental and Vector-Related Risk Factors

The environmental determinants of LSD epidemiology are intimately tied to the ecology of its arthropod vectors. Seasonal patterns are stark: outbreaks predominate in the summer months, corresponding with peak vector activity. An analysis of global outbreak data from 2005 to 2020 found that 72.6% of outbreaks occurred in Europe, with the highest number (n=778) reported in July, and 1873 outbreaks during the summer season overall [26]. In Russia, outbreaks occurred primarily between mid-May and mid-November, yet notably, some transmission was observed even during periods of snow and freezing temperatures that preclude typical insect activity, raising the possibility of alternative transmission routes or vector overwintering [23]. The spatial analysis of LSD in Egypt highlighted higher prevalence in low-lying areas and beside watercourses, habitats that support mosquito and fly breeding [58]. In Uganda, mean annual precipitation exceeding 1000 mm was a significant risk factor for seropositivity (p < 0.05), as higher rainfall supports vector proliferation [24].

The presence of bush and vegetation around farms was identified as a risk factor in Bangladesh, where 57.6% of affected cattle were on farms with bush cover [59]. Such environments provide resting and breeding sites for vectors, increasing the vector density in close proximity to livestock. Communal water sources (dams, rivers, ponds) also increase risk, as documented in both Vietnam and Bangladesh, because they aggregate animals from multiple herds and provide ideal breeding habitats for mosquitoes and flies [11, 24, 59]. The ecological niche model for the Middle East identified annual precipitation, land cover type, and the mean diurnal temperature range as the most important predictors of LSDV occurrence [31]. This modeling work underscores the role of climate in shaping the geographic limits of the disease, with implications for its potential spread under changing climatic conditions.

Host Susceptibility and the Expanding Host Range: Wild Bovines and Other Ruminants

The species barrier for LSDV is demonstrably porous. While cattle are the primary hosts, domestic water buffalo (Bubalus bubalis) and an increasing number of wild ruminant species are susceptible, raising critical questions about wildlife reservoirs and spillover dynamics. The Bhutan outbreak of 2023 is highly instructive. A total of 3,179 yaks (Bos grunniens) were affected, with a staggering case fatality rate of 38.66%, compared to 9.92% in cattle [1]. This dramatically higher CFR in yaks indicates a heightened species-specific susceptibility. Yaks are high-altitude bovines with a distinctive physiology and immune system adapted to hypoxic, cold environments. Their apparent immunological naivety to LSDV, combined with the stress of recent introduction of the virus, likely explains the catastrophic mortality. Molecular characterization of the virus from both cattle and yaks showed 100% similarity in the RPO30, GPCR, EEV, and B22R genes, confirming a common source and spillover event from cattle [1]. This finding is echoed in the Himalayan state of Sikkim, India, where LSDV-infected yaks displayed clinical signs identical to cattle, with necropsy revealing lesions on vital organs. Sequencing of the viral genes showed that the yak and cattle sequences belonged to the same 1.2.1 sub-cluster, implying spillover from local cattle to yaks [6]. This highlights the risk of LSDV establishment in wild bovine populations, where it could create a sylvatic cycle that complicates eradication efforts.

Beyond yaks, LSDV has been isolated from a giraffe in a Vietnamese zoo, marking the first such report in this species. The isolate was phylogenetically closely related to contemporaneous Vietnamese and Chinese cattle strains, demonstrating cross-species transmission from domestic livestock to captive wildlife [15]. This has profound implications for zoological collections and conservation programs. The potential for spillover into other endangered wild bovids, such as the gaur (Bos gaurus) or banteng (Bos javanicus), is an urgent concern. The role of the African buffalo (Syncerus caffer) in the endemic cycle in Africa has long been hypothesized, but empirical evidence remains limited. The review by Mazloum et al. explicitly notes that host restriction is not limited to livestock, with certain wild ruminants being susceptible, and the consequences for the epidemiology of the disease remain unknown [28]. Phylogenetic evidence from the Bhutanese study explicitly warns of the need for surveillance in both domestic and wild bovines to identify spillover incidences and understand the extent of disease spread [1]. The potential for wildlife to act as maintenance hosts, capable of re-infecting domestic cattle after control measures have been lifted, represents a major gap in our understanding.

Economic Impact on Cattle and Wild Bovine Populations

The economic toll exacted by LSDV is staggering and multifaceted, affecting individual farmers, national economies, and international trade. Direct losses manifest through reduced milk yield, mortality, damage to hides, abortion, and infertility. Milk production losses are frequently cited as the most immediate and visible impact. In Bangladesh, 54.5% of affected cattle showed a reduction in milk yield, and poor growth was noted in 13.6% of beef cattle [17]. The review by Jyoti et al. explicitly links LSD to "significant milk loss, damage of the hides, and reproductive problems such as abortion and infertility in affected animals" [26]. The Egyptian study documented a morbidity rate of 35.53% with 5.32% mortality, translating to direct losses in affected herds [29]. Mortality, though often low (1-5% in typical outbreaks), can spike dramatically, as seen in the 38.66% CFR in yaks in Bhutan, which represents a catastrophic loss to communities that depend on these animals for sustenance and income [1, 37].

Reproductive losses have cascading long-term economic effects. LSDV causes orchitis and temporary or permanent infertility in bulls, and abortion and prolonged ancestrus in cows [4, 30, 32, 35]. The damage to hides is a significant but often overlooked component; the characteristic nodular lesions lead to scarring that renders leather worthless, impacting a secondary industry [12, 26]. In the Poro Region of Ivory Coast, the high prevalence (51.85%) in a region with limited veterinary resources implies substantial, unquantified economic hardship for pastoralist communities [3].

Indirect economic impacts are equally devastating. Restrictions on animal movement and trade, imposed by national veterinary authorities and international bodies such as WOAH and FAO, can cripple livestock markets. During the Russian outbreaks, the disease spread over 9,000 km, with outbreaks concentrated in smallholder farms that lack the capital to absorb such shocks [23]. The costs of control measures, vaccination campaigns, vector control, quarantine enforcement, and diagnostic testing, place a heavy burden on both government budgets and farmers' finances [21]. The review by Hailu et al. emphasizes that LSD causes "several financial problems in livestock industries as a result of significant milk yield loss, infertility, abortion and death" [38]. Modeling studies have shown that the most effective interventions, such as mass vaccination achieving >90% coverage or near-complete culling of infected animals, are often economically unfeasible in resource-poor settings [21]. The emergence of vaccine-like recombinant strains, reported in Russia, China, and Southeast Asia, adds a further layer of complexity, as these strains can cause disease and complicate both diagnosis and vaccination strategies, leading to costly vaccine failures or the need for new, more expensive vaccine formulations [2, 5, 18, 27].

The economic impact on wild bovine populations, while less directly measured in monetary terms, is significant in terms of biodiversity loss and conservation costs. The high mortality in yaks represents a direct threat to the livelihoods of highland pastoralists and the genetic diversity of this species. The spillover into captive wildlife, such as the giraffe, incurs costs for zoos in terms of animal replacement, veterinary care, and enhanced biosecurity [15]. Furthermore, the establishment of LSDV in wild bovine populations could jeopardize the 'disease-free' status of nations, leading to trade restrictions that penalize the entire livestock sector, not just the affected farms [12]. The continuous genetic diversification of LSDV, driven by both drift and recombination, as documented by Bayyappa et al. and Breman et al., suggests that the virus will continue to challenge control efforts, ensuring that its economic impact will persist and potentially worsen in the foreseeable future [2, 46].

Surveillance, Control Strategies, and Vaccination Challenges in Endemic and Emerging Regions

The global expansion of Lumpy Skin Disease Virus (LSDV) from its historical epicenter in sub-Saharan Africa to the Middle East, Europe, and across the vast expanse of Asia represents one of the most significant epizootic events of the 21st century. This transboundary viral pathogen, classified as a notifiable disease by the World Organisation for Animal Health (WOAH), has demonstrated a remarkable capacity for rapid geographic dispersal, driven by a complex interplay of vector-borne transmission, anthropogenic animal movement, and the emergence of novel recombinant strains. The epidemiological landscape of LSDV is now characterized by a dichotomy: deeply entrenched endemicity in Africa and parts of the Middle East, and dynamic, often explosive, emergence in naive populations across Eurasia. Consequently, the strategies for surveillance, control, and vaccination must be context-dependent, tailored to the specific ecological, economic, and infrastructural realities of each region. The challenges are formidable, ranging from the biological constraints of the virus itself, including its slow molecular evolution and capacity for recombination, to the practical limitations of vaccine deployment, diagnostic capacity, and vector management in resource-limited settings.

Surveillance Architectures: From Passive Reporting to Genomic Intelligence

Effective surveillance for LSDV is the cornerstone of any successful control program, yet its implementation varies dramatically across the virus’s range. In endemic African nations such as Ethiopia, Uganda, and Ivory Coast, surveillance often relies on passive clinical reporting, which is hampered by limited veterinary infrastructure, underreporting, and the clinical similarity of LSD to other poxviral diseases [3, 13, 24]. The seroprevalence data from Uganda, for instance, reveals a stark contrast between animal-level (8.7%) and herd-level (72.3%) seropositivity, indicating widespread viral circulation that is often subclinical or unreported [24]. This gap in detection allows the virus to circulate undetected, maintaining a persistent reservoir. In Ivory Coast, a cross-sectional study utilizing syndromic surveillance in the Poro Region found that 51.85% of cattle with pox-like lesions were PCR-positive for LSDV, with prevalence rates soaring to 70.87% in certain localities [3]. These findings underscore that passive surveillance alone is grossly insufficient; active, risk-based sampling is required to capture the true burden of disease.

The situation in emerging regions, particularly South and Southeast Asia, has catalyzed a shift towards more sophisticated molecular surveillance frameworks. The rapid incursion of LSDV into India in 2019, followed by its spread to Bangladesh, Pakistan, Nepal, Bhutan, China, and Southeast Asia, has necessitated the deployment of real-time PCR and whole-genome sequencing (WGS) to track viral incursions and evolution [1, 5, 6, 10, 22]. The experience in Bhutan during the 2023 outbreak is instructive: molecular characterization of the RPO30, GPCR, EEV, and B22R genes from affected cattle and yaks revealed 100% similarity among samples, clustering them with Clade 1.2 field isolates from Sudan, India, and China [1]. Critically, WGS of a representative sample (LSDV_Bhutan_03) placed the virus within the Neethling Warmbaths (NW)-like clade (Clade 1.2.2), closely related to recent isolates from Indian cattle and buffalo and Chinese yaks [1]. This level of phylogenetic resolution is impossible with single-gene sequencing alone. As Breman et al. (2023) have rigorously demonstrated, single-gene phylogenies (e.g., based on GPCR or RPO30) lack the spatiotemporal resolution to differentiate between closely related outbreak strains, and they strongly advocate for generating at least one WGS per outbreak to accurately trace transmission pathways and identify the origin of incursions [2].

The emergence of recombinant LSDV strains has further complicated surveillance efforts. The discovery of mosaic viruses, particularly those in Cluster 2.5 that have become dominant in Southeast Asia, arose from recombination between live-attenuated Neethling vaccine strains and Kenyan sheep and goat pox (KSGP)-like field strains [5, 18]. These recombinants pose a unique diagnostic challenge, as they can contain genetic signatures from both parental lineages. To address this, Sprygin et al. (2023) developed a novel real-time PCR assay targeting the LW032 open reading frame, which is capable of specifically detecting KSGP-related isolates and recombinant strains containing the KSGP backbone [16]. This assay is critical for regions like India and Pakistan, where KSGP-like strains (Cluster 1.1) circulate, and their potential encroachment into areas dominated by Cluster 2.5 recombinants would be undetectable with conventional pan-capripox PCRs [16]. The development of such fit-for-purpose molecular tools represents a crucial evolution in surveillance, moving from simple detection to precise genotyping and lineage discrimination. Furthermore, the application of novel diagnostic platforms, such as the Saltatory Rolling Circle Amplification (SRCA) method developed by Das et al. (2025) for use in Meghalaya, India, offers a rapid, field-deployable alternative to PCR in resource-limited settings, achieving a detection limit of 10 copies/µL [10]. This technology could revolutionize early warning systems in remote, endemic areas.

Control Strategies: The Interplay of Movement Restriction, Vector Management, and Stamping-Out

Control strategies for LSDV must be multi-pronged, targeting the virus at multiple points in its transmission cycle. The primary long-distance dispersal mechanism for LSDV is the movement of infected animals, often through trade [19, 27]. Therefore, strict quarantine measures and movement restrictions are the first line of defense for free areas. The epidemiological investigation in Lang Son Province, Vietnam, identified the introduction of new cattle from outside the herd as a significant risk factor (p<0.01), reinforcing the need for pre-movement testing and quarantine protocols [11]. Similarly, in the Poro Region of Ivory Coast, transhumant herds, those that move seasonally in search of pasture and water, had a significantly higher prevalence of LSDV (p<0.001) compared to sedentary herds [3]. This highlights the challenge of controlling the disease in pastoralist systems where animal movement is integral to the production system.

Vector control is another critical, yet often neglected, component of LSDV management. The virus is mechanically transmitted by a variety of blood-feeding arthropods, with Stomoxys calcitrans (stable flies), Aedes aegypti mosquitoes, and ticks of the genera Rhipicephalus and Amblyomma being the most competent vectors [19, 27]. The strong seasonality of LSD outbreaks, with peaks during summer and wet seasons, directly correlates with vector abundance [26]. In Russia, outbreaks were observed from mid-May through mid-November, but notably, transmission also occurred during periods of snow and freezing temperatures, suggesting that direct contact or other non-vector routes may play a more significant role in colder climates than previously thought [23]. This finding has profound implications for control strategies in temperate regions, where vector control alone may be insufficient. Bianchini et al. (2023) emphasize that controlling insects in animal transport trucks is a highly targeted and effective measure to prevent long-distance spread, as vectors can be transported alongside cattle [27]. On-farm vector management, including the use of acaricides, insecticides, and environmental management to reduce breeding sites (e.g., manure management), is essential to reduce within-herd transmission [9, 17].

The most drastic control measure, stamping-out (culling of infected and in-contact animals), has been employed successfully in some European countries to eradicate incursions. However, its application is fraught with economic, ethical, and logistical difficulties. Modeling studies suggest that culling is only successful when nearly 100% of infected animals are removed, a feat that requires rapid diagnostic capabilities and significant financial compensation for farmers [21]. In the context of developing nations, where cattle represent a primary asset and source of livelihood, mass culling is often socially and economically unacceptable. For instance, in Bangladesh, where the prevalence of LSDV in some districts reaches 27.5% and smallholder farms are most severely affected, culling is not a viable strategy [17]. Instead, the focus must shift to supportive care, biosecurity, and vaccination.

Vaccination Challenges: Efficacy, Safety, and the Specter of Recombination

Vaccination remains the most effective tool for controlling LSDV in both endemic and emerging regions. Homologous live-attenuated vaccines, derived from the Neethling strain, are considered the gold standard, providing robust and long-lasting immunity [42, 44]. The success of these vaccines was dramatically demonstrated in the Balkans, where high-coverage vaccination campaigns (above 90%) successfully eradicated the disease after its introduction in 2015-2017 [21, 44]. However, the deployment of these vaccines is beset by a series of profound challenges that threaten their long-term utility.

Challenge 1: The Recombination Crisis. The most alarming challenge is the documented emergence of virulent recombinant LSDV strains arising from the co-circulation of live-attenuated vaccine strains and wild-type field viruses. The first such recombinant was reported in Russia in 2017, and subsequent analysis revealed that these novel strains were composed of genetic material from both a Neethling vaccine strain and a KSGP-like vaccine strain [18]. This phenomenon is not a rare event; Krotova et al. (2022) demonstrated that strains isolated prior to 2020 were genetically unique, but from 2020 onwards, a single recombinant lineage (Cluster 2.5) became dominant across Russia, China, and Southeast Asia [18]. This suggests that recombination can generate a fitter, more transmissible virus that can outcompete its parental strains. The implications are staggering: the very tool used to control the disease, live-attenuated vaccination, can, under the right conditions, create a new, more dangerous pathogen. This has forced a re-evaluation of vaccination strategies in regions where multiple LSDV lineages co-circulate. In India, for example, the dominant circulating strain is a KSGP-like field virus (Cluster 1.1), while in neighboring China and Southeast Asia, the Cluster 2.5 recombinants are prevalent [5, 6, 16]. The use of a Neethling-based vaccine in India could theoretically recombine with the circulating KSGP-like field strain, potentially generating a new recombinant. This risk is not hypothetical; it is a documented reality.

Challenge 2: Vaccine Safety and Adverse Reactions. Live-attenuated Neethling vaccines, while efficacious, are not without side effects. A significant proportion of vaccinated animals can develop mild to moderate clinical signs, including a transient fever and the formation of a small nodule at the injection site, sometimes referred to as "Neethling disease" [44]. More concerning is the potential for the vaccine virus to revert to virulence, particularly in immunocompromised animals. In Thailand, Singhla et al. (2022) reported that two cows presenting with LSD-like clinical signs after vaccination were found to be infected with a virus that clustered with Neethling-derived vaccine strains, not field isolates [20]. This raises the possibility that the vaccine virus itself caused the clinical disease, either through residual virulence or reversion. This safety concern is a major barrier to vaccine acceptance among farmers, who may perceive the vaccine as causing the disease it is meant to prevent.

Challenge 3: Differentiating Infected from Vaccinated Animals (DIVA). A critical limitation of current live-attenuated vaccines is the inability to serologically distinguish a vaccinated animal from one that has been naturally infected. This complicates surveillance efforts, as seropositivity cannot be used as a reliable indicator of viral circulation in vaccinated populations. The development of DIVA-compatible vaccines and companion diagnostic tests is a high priority. Recent research has identified several promising targets. The LSDV103 protein has been identified as a specific immunogenic antigen that reacts strongly with sera from naturally infected cattle but shows significantly lower reactivity with sera from animals vaccinated with a heterologous goatpox virus vaccine [48]. An indirect ELISA based on the truncated TrLSDV103 protein demonstrated 100% diagnostic specificity and 86.67% sensitivity, making it a strong candidate for DIVA applications [48]. Similarly, the EEV glycoprotein gene has been shown to possess single-nucleotide polymorphisms (SNPs) that can differentiate Egyptian field isolates from the ARRIAH LSD VAC strain, offering another potential DIVA target [29]. The F13L protein, a component of the extracellular enveloped virion, has also shown high correlation with commercial ELISA results, suggesting its utility in serosurveillance [61]. The transition from whole-virus vaccines to subunit, vectored, or gene-deleted marker vaccines is essential for the future of LSDV control.

Challenge 4: Logistical and Economic Barriers to Coverage. Achieving the >90% vaccination coverage required for herd immunity is a monumental task in many endemic and emerging regions. In Bangladesh, a study in the Habiganj district found that only 50% of farmers were vaccinating their cattle, and the prevalence of LSDV in unvaccinated animals (27.5%) was significantly higher than in vaccinated animals (10%) [17]. The reasons for low coverage are multifaceted: cost of the vaccine, lack of cold chain infrastructure, insufficient veterinary personnel, and lack of farmer awareness. In Pakistan, the 2021/22 outbreak caused devastating economic losses, yet the molecular characterization of the virus revealed it was a Clade 1.2 strain, distinct from the vaccine strains in use [52]. This highlights the critical need for vaccine matching; using a heterologous vaccine (e.g., sheep pox or goat pox virus vaccines) may provide only partial protection, as they are less efficacious than homologous LSDV vaccines [42]. The World Organisation for Animal Health (WOAH) and the Food and Agriculture Organization (FAO) have emphasized the need for regional coordination in vaccine procurement and deployment to ensure that the correct vaccine is used and that coverage is sufficient to interrupt transmission.

In conclusion, the control of LSDV in the 21st century is a complex balancing act. Surveillance must evolve from passive reporting to a genomic intelligence system capable of detecting incursions, tracking evolution, and identifying recombinants in real time. Control strategies must be adaptive, integrating movement controls, vector management, and vaccination in a locally appropriate manner. The vaccination enterprise itself is at a crossroads, threatened by the very real risk of recombination and hampered by safety concerns and logistical hurdles. The path forward lies in the development of next-generation vaccines, safer, DIVA-compatible, and genetically stable, coupled with a global commitment to strengthening veterinary services and diagnostic capacity in the regions where LSDV poses the greatest threat.

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