Lumpy Skin Disease Virus

Overview and Taxonomy of Lumpy Skin Disease Virus

Introduction and Historical Context

Lumpy skin disease (LSD) is a devastating, transboundary viral infection of cattle and water buffalo, caused by the lumpy skin disease virus (LSDV). Classified as a notifiable disease by the World Organisation for Animal Health (WOAH), LSD imposes severe economic burdens on the global livestock industry through marked reductions in milk production, infertility, abortion, hide damage, and mortality [1, 4, 5]. First identified in Zambia in 1929, LSD was historically confined to sub-Saharan Africa for decades. However, the pathogen has undergone a dramatic and well-documented geographic expansion since the early 21st century, spreading northward through the Middle East, into Turkey and the Balkans, and subsequently across vast swathes of Asia, including Russia, Kazakhstan, China, India, and nations of Southeast Asia [1, 6, 7]. This relentless incursion into naïve territories, facilitated by climate change, vector movement, and livestock trade, has transformed LSD from a regional concern into a major global veterinary emergency [17, 24].

Taxonomic Classification

LSDV is a member of the genus Capripoxvirus within the subfamily Chordopoxvirinae of the family Poxviridae [1, 5]. The genus Capripoxvirus also includes sheeppox virus (SPPV) and goatpox virus (GTPV), which are the etiological agents of sheeppox and goatpox, respectively. These three viruses are antigenically and genetically closely related, exhibiting a nucleotide sequence similarity of up to 97% [4, 32]. This high degree of homology underpins the cross-protective immunity observed between the viruses, a principle that has been exploited for vaccination. Indeed, live-attenuated vaccines derived from SPPV or GTPV have been employed to protect cattle against LSDV infection, although homologous LSDV-based vaccines are considered more efficacious [1, 4, 21].

Despite their close relationship, LSDV is genetically and biologically distinct from SPPV and GTPV, forming a monophyletic clade within the genus [17]. The differentiation of LSDV from its capripoxvirus relatives is critical for accurate diagnosis, epidemiological surveillance, and the implementation of DIVA (Differentiating Infected from Vaccinated Animals) strategies. Molecular tools targeting specific gene regions, such as the P32 antigen gene, the RPO30 RNA polymerase subunit, and the G protein-coupled chemokine receptor (GPCR) gene, are routinely employed for species-level discrimination and phylogenetic characterization [14, 19, 28, 39]. The GPCR gene, in particular, has proven invaluable for sub-grouping LSDV strains due to its relatively higher genetic variability [27, 39].

Virion Structure and Genome Organization

The LSDV virion is a large, enveloped, brick-shaped or ovoid particle, typical of poxviruses, measuring approximately 320 nm in length and 260 nm in width [17]. The virus exists in two main infectious forms: the intracellular mature virus (IMV), which is released upon cell lysis, and the extracellular enveloped virus (EEV), which is crucial for cell-to-cell spread and long-range dissemination within the host [10, 17]. The EEV possesses an additional outer lipid envelope derived from the host cell Golgi apparatus, and several EEV-specific proteins, such as F13L, A33R, A34R, and B5R, are critical for its formation and infectivity [10].

The viral core, which contains the genome, is flanked by lateral bodies that house viral proteins involved in subverting the host immune response immediately upon entry [17]. The LSDV genome is a single, linear, double-stranded DNA molecule of approximately 151 kilobase pairs (kbp). This large genome encodes an estimated 156 open reading frames (ORFs), though the precise functions of many of these predicted proteins remain poorly characterized [1, 3, 5]. The genome is organized with a central conserved core region, which contains genes essential for viral replication, transcription, and structural morphogenesis. Flanking this core are the two terminal regions, known as the inverted terminal repeats (ITRs), which are highly variable and encode proteins primarily involved in host range determination, virulence, and immune evasion [3, 17, 27]. The ITRs play a disproportionate role in the virus's pathogenic potential, a subject of intense investigation [8, 9, 11, 18].

Phylogenetic Diversity and Genomic Evolution

The rapid global spread of LSDV has been accompanied by significant genomic diversification, leading to the emergence of multiple distinct lineages and subclades. Extensive whole-genome sequencing and phylogenetic analyses have resolved the global population structure of LSDV into two major clusters: cluster 1 and cluster 2 [23, 27].

Cluster 1 largely encompasses classical field strains and vaccine-related strains. Within cluster 1, several key subclades have been identified. Clade 1.1 includes the classical Neethling vaccine strain and its derivatives, which have been used extensively in Europe and Africa for successful control programs [4, 38]. Clade 1.2 is a highly diverse group that has been further subdivided. Clade 1.2.1 comprises strains associated with outbreaks in the Middle East and Europe, such as the Saratov/2017 strain from Russia [2, 23]. Clade 1.2.2 contains the "Kenya-like" or "KSGP-like" strains, which are genetically distinct from the Neethling lineage and are primarily found in East Africa [23, 31]. Clade 1.2.3 represents a more recently identified subgroup from West and Central Africa, highlighting the ongoing diversification of LSDV on its home continent [23]. Importantly, recent outbreaks in the Indian subcontinent and Pakistan have been attributed to field strains belonging to clade 1.2, which are genetically distinct from both the vaccine strains and the recombinant viruses circulating in East Asia [13, 19, 31]. These Indian strains, while part of clade 1.2, possess unique mutations and are evolutionarily conservative within their own distinct haplogroup, indicating regional adaptation [6, 20, 26, 28].

Cluster 2 is a relatively new and evolutionarily dramatic group, consisting almost entirely of recombinant strains. These "vaccine-like" or mosaic strains arose from homologous recombination events between a live-attenuated vaccine strain (typically an Neethling-like or KSGP-like derivative) and a wild-type field virus [7, 21, 23]. The resulting chimeric genomes contain a mixture of genes from both parental lineages. These recombinant viruses, particularly those belonging to clade 2.5, have demonstrated a remarkable fitness advantage and have become the dominant lineage spreading across East and Southeast Asia, including China, Thailand, Vietnam, Malaysia, and Indonesia [5, 7, 12, 16, 22, 38]. The emergence of clade 2.5 underscores a critical consequence of vaccination – the potential for recombination between vaccine and field strains in co-infected animals, creating new pathogens with altered virulence and transmission dynamics [21, 30]. Crucially, recent genomic surveillance indicates that the circulating pool of LSDV in Eastern Eurasia has stabilized, with clade 2.5 strains now undergoing their own independent, monophyletic evolution without the emergence of new, major recombinant forms [7]. This suggests that while recombination was a key driver of initial emergence, the current dominant lineage is now established.

Host Range and Species Susceptibility

LSDV has a primary host range that is largely restricted to bovines. The principal hosts are domestic cattle (Bos taurus and Bos indicus), but it also causes disease in water buffalo (Bubalus bubalis), albeit with reduced susceptibility compared to cattle [24, 35]. The virus has demonstrated an expanding host range in recent years. Significant outbreaks and high mortality have been documented in yaks (Bos grunniens), particularly in the Qinghai-Tibet Plateau, with recombinant strains showing increased pathogenicity in this species [12, 28]. Natural infection has also been confirmed in free-ranging Indian gazelles (Gazella bennettii), indicating a broadening host range among wild ungulates [37]. Most alarmingly, LSDV has been isolated from camels (Camelus dromedarius), with the affected animals displaying characteristic skin nodules and seroconversion, suggesting that this species, previously considered resistant, can be infected [42].

Interestingly, despite its primary tropism for bovids, LSDV can replicate efficiently in a wide range of mammalian and even avian cell lines in vitro, including human A549 cells and murine MC3T3-E1 osteoblastic cells [5, 34]. This broad in vitro tropism contrasts with its restricted in vivo host range. Evidence of LSDV nucleic acid has been detected in the human upper respiratory tract microbiome, but the virus is not considered zoonotic, and these findings require further investigation to elucidate the route of exposure and potential for replication [33]. The pathogenesis involves a complex interplay between the virus and the host immune system. The virus is known to suppress interferon-β production and signaling through multiple viral proteins, including LSDV 001/156, LSDV ORF137, LSDV ORF127, and LSDV ORF142, which target and disable critical molecules like IRF3, TBK1, and STING [3, 9, 11, 18, 25]. This sophisticated immune evasion is a key determinant of its virulence.

Transmission and Vectors

LSDV is primarily transmitted mechanically by arthropod vectors. The disease is not highly contagious through direct contact alone under natural conditions; rather, the mechanical transfer of virus-laden blood by biting flies is the major driver of epizootics [1, 4, 24]. A wide range of blood-feeding insects can serve as vectors, including stable flies (Stomoxys calcitrans), multiple species of mosquitoes (Aedes aegypti, Culex tritaeniorhynchus, Culex quinquefasciatus), and hard ticks (Rhipicephalus annulatus) [15, 29, 36, 41]. LSDV can replicate and disseminate within the mosquito vector, and evidence of transovarial transmission in Ae. aegypti has been demonstrated, suggesting that mosquitoes may act as more than just mechanical vectors, potentially serving as biological reservoirs [15, 29]. Stomoxys flies have been shown to transmit the virus from both clinically ill and, critically, from subclinically infected donor animals, underscoring the importance of silent transmission in the field [36]. Long-distance airborne movement of infected vectors and the transport of infected livestock across borders are the primary routes for inter-regional and international spread [24, 40]. The ability of subclinically infected animals to transmit the virus to naïve cohorts via vectors represents a significant challenge for disease surveillance and control, as these animals exhibit no visible nodules but can still be infectious [30, 36].

Molecular Pathogenesis and Virulence Factors

Lumpy skin disease virus (LSDV), a member of the genus Capripoxvirus within the family Poxviridae, possesses a double-stranded linear DNA genome of approximately 151 kbp encoding 156 predicted open reading frames (ORFs) [1]. Despite its growing global significance, LSDV remains one of the most poorly characterized poxviruses, with the specific functions of the vast majority of its encoded proteins remaining unknown [1, 3]. The molecular pathogenesis of LSDV is a complex, multifactorial process that hinges on a sophisticated arsenal of virulence factors that orchestrate host cell entry, subversion of innate immune signaling, modulation of inflammatory responses, manipulation of cellular metabolism, and systemic dissemination. Understanding these molecular mechanisms is paramount for the rational design of attenuated vaccines and targeted therapeutics, particularly given the virus's rapid transboundary spread across Africa, Europe, the Middle East, and Asia [1, 4].

Genomic Architecture and Evolutionary Plasticity as a Foundation for Virulence

The LSDV genome is characterized by a central conserved core region, flanked by variable inverted terminal repeat (ITR) regions that are hotbeds of genetic innovation and immune evasion [3, 27]. Comparative genomic analyses have revealed that LSDV exhibits an 'open' pan-genome, with significant plasticity driven by recombination and single nucleotide polymorphisms (SNPs), particularly in the ITR regions [5, 27]. The emergence of recombinant strains, such as those belonging to the dominant cluster 2.5 lineage now circulating in Southeast Asia, underscores the virus's capacity to generate novel virulence profiles through genetic mixing, often involving vaccine-derived sequences [7, 21, 47]. For instance, the recombinant LSDV strain isolated from yaks in the Qinghai-Tibet Plateau (LSDV/China/GS/Yak) exhibited high mortality (46.67%) and severe systemic pathogenesis, highlighting that recombination events can directly enhance pathogenicity in new host species [12]. Phylogenetic studies have delineated multiple clades and subclades, with strains from clade 1.2 associated with outbreaks in Eurasia and South Asia, and clade 2.5 recombinants dominating in East and Southeast Asia [7, 27]. The GPCR gene, in particular, serves as a powerful marker for viral subgrouping and has identified distinct variants circulating within countries like Vietnam [39].

The Viral Life Cycle: Entry, Replication, and Egress

LSDV entry into host cells is a dynamic, pH-dependent process. Detailed mechanistic studies using pharmacological inhibitors in MDBK and bovine mammary epithelial cells (BMEC) have demonstrated that LSDV enters via macropinocytosis, a process requiring dynamin and the Na⁺/H⁺ exchanger (NHE) but independent of phosphatidylinositol 3-kinase (PI3K) [48]. This entry mechanism is distinct from classical clathrin- or caveolae-mediated endocytosis, which were shown to be non-essential for LSDV infection [48]. Following entry and uncoating, viral replication occurs in the cytoplasm. The virus produces multiple forms: the intracellular mature virus (IMV) and the extracellular enveloped virus (EEV), the latter of which is critical for cell-to-cell spread and long-distance dissemination within the host [17]. The EEV glycoproteins, including F13L, A33R, A34R, and B5R, are key targets for serological detection and neutralizing antibodies, with F13L showing high conservation across strains and promise for diagnostic assays [10]. Notably, genome sequencing of LSDV from wetland areas in Bangladesh has identified distinct amino acid substitutions in the EEV glycoprotein and DNA-dependent RNA polymerase, which are predicted to alter protein structure and potentially impact viral pathogenesis and fitness [49].

Subversion of the Type I Interferon Response: A Multilayered Strategy

The most extensively characterized virulence mechanism of LSDV is its capacity to antagonize the host type I interferon (IFN-I) system. LSDV encodes a suite of proteins that target multiple nodes of the cGAS-STING-TBK1-IRF3 signaling axis, collectively ensuring a profound suppression of IFN-β production. This is a hallmark of LSDV pathogenesis and a key determinant of virulence.

ITR-Encoded Proteins as IFN Antagonists: The LSDV 001/156 protein, encoded within the ITR region, has been identified as a critical virulence factor that functions as a late-expressed, virion-associated protein [3]. Mechanistically, LSDV 001/156 interacts directly with interferon regulatory factor 3 (IRF3), disrupting its dimerization and subsequent nuclear translocation, thereby potently attenuating IFN production [3]. Deletion of the LSDV 001/156 gene resulted in a mutant virus exhibiting reduced replication and significantly attenuated virulence in cattle, correlating with enhanced IFN responses [3].

Multitargeting of the TBK1-IRF3 Interface: LSDV employs at least three distinct proteins to dismantle theTBK1-IRF3 signaling complex. LSDV ORF142 directly interacts with IRF3 and interferes with its recruitment to TANK-binding kinase 1 (TBK1) in a dose-dependent manner, preventing IRF3 phosphorylation and nuclear translocation [9]. Furthermore, ORF142 has been shown to suppress the cGAS/STING-mediated IFN-I pathway by a second, distinct mechanism: it facilitates the autophagic degradation of STING by recruiting the autophagy receptor NBR1, thereby abrogating the activation of downstream signaling molecules [11]. This dual mechanism, direct inhibition of IRF3 binding and targeting STING for degradation, illustrates the functional redundancy and evolutionary refinement of LSDV's immune evasion arsenal. Another protein, LSDV ORF127, suppresses IFN-I responses by specifically interacting with TBK1 and inhibiting its K63-linked polyubiquitination, a post-translational modification essential for TBK1 activation and downstream signaling [25]. The N-terminal 1–43 amino acids of ORF127 were identified as the critical domain for this interaction [25]. Additionally, the LSDV ORF137 protein inhibits IFN-β and ISG expression by interacting with IRF3 and reducing its phosphorylation, thereby acting as a negative regulator of the IFN pathway [18]. Deletion of ORF137 from the LSDV genome restored IFN-β transcription and impaired viral replication in MDBK cells [18].

Modulation of Inflammatory and Innate Immune Pathways: A Yin-Yang of Virulence

Beyond IFN suppression, LSDV exhibits a sophisticated and seemingly paradoxical regulation of inflammatory signaling. The LSDV001 protein (distinct from 001/156) has been identified as a positive regulator of the inflammatory response. LSDV001 interacts with TAK1 and TAB2/3, promoting the assembly of the TAK1-TAB2/3 complex, which in turn drives IKK-dependent activation of NF-κB and downstream inflammatory cytokines such as IL-1β and TNFα [8]. Infection with an LSDVΔ001 deletion mutant led to smaller skin nodules and reduced inflammation in vivo, demonstrating that excessive inflammation driven by LSDV001 is a key contributor to the characteristic dermal pathology of LSD [8]. This suggests that LSDV actively promotes a hyper-inflammatory state as a virulence mechanism, potentially facilitating viral replication and dissemination through the recruitment of target cells.

Conversely, LSDV also encodes proteins that negatively regulate other branches of innate immunity. For example, LSDV087 has been characterized as an immediate-early protein with dual functionality: it acts as a decapping enzyme to downregulate host gene transcription, and it also positively regulates the cGAS-MITA (STING) pathway by interacting with MITA, inhibiting its K48-linked polyubiquitination, and promoting its oligomerization, thereby enhancing innate immune signaling [46]. This seemingly counterintuitive positive regulation may represent a fine-tuning mechanism to prevent premature host cell death or to maintain an optimal cellular environment for viral replication.

Manipulation of Cellular Signaling and Metabolism

The virus's pathogenic strategy extends to subverting host cellular metabolism. LSDV infection has been shown to upregulate lysophosphatidic acid (LPA), a key metabolite in glycerophospholipid metabolism [45]. Exogenous LPA promotes LSDV replication by activating the MEK/ERK signaling pathway and simultaneously suppressing the host innate immune response [45]. This metabolic hijacking is a novel pathogenic mechanism, and the class IIa histone deacetylase inhibitor TMP269 can suppress LSDV replication by downregulating LPA production, highlighting a potential therapeutic target [45]. Furthermore, LSDV infection induces significant alterations in the expression of serum exosomal miRNAs, with 59 differentially expressed miRNAs identified that target genes involved in viral replication, immune response, and signal transduction [44]. These exosomal miRNAs represent a previously unappreciated layer of host-virus interaction that shapes the molecular pathogenesis of LSD.

Cellular Tropism and Systemic Dissemination

LSDV exhibits a broad cellular tropism in vitro, replicating efficiently in several mammalian cell lines, including bovine kidney cells (MDBK), bovine macrophages, primary fibroblasts, and even human A549 cells [5, 43]. Notably, the murine osteoblastic cell line MC3T3-E1 has been identified as a permissive cell model, expanding the known host range relevant to research [34]. However, in vivo pathogenesis is defined by a more selective tropism for skin, lymph nodes, and systemic organs. In peripheral blood mononuclear cells (PBMCs), LSDV exhibits an abortive infection: active viral transcription is detected, but significant viral DNA replication or production of infectious progeny does not occur [43]. This is a critical observation, as it suggests PBMCs function as Trojan horses rather than productive replication sites. Despite the abortive infection, LSDV-infected PBMCs evade the antiviral response (failing to induce ISGs upon infection with live virus, unlike heat-inactivated virus) and effectively transmit infectious virus to permissive cells through direct contact [43]. This mechanism is central to viral dissemination from the site of inoculation to distant tissues. This cell-mediated dissemination is further supported by the fact that subclinically infected animals, which lack overt skin nodules but harbor viremia, can act as potent sources of transmission via mechanical vectors like Stomoxys calcitrans [36].

Conclusion of Section (Integrated Analysis)

The molecular pathogenesis of LSDV is a paradigm of viral sophistication, characterized by a multi-pronged attack on host innate immunity. The virus encodes a diverse repertoire of virulence factors that function in concert to suppress IFN-I production through disruption of IRF3 dimerization [3], blockade of TBK1 recruitment [9], induction of STING autophagic degradation [11], and inhibition of TBK1 ubiquitination [25]. Simultaneously, it manipulates inflammatory signaling to its advantage, promoting excessive NF-κB-driven inflammation via LSDV001 to generate the pathognomonic skin nodules [8], while also engaging in metabolic reprogramming via LPA upregulation to create a permissive replicative niche [45]. The capacity for genetic recombination, particularly in the ITR regions, provides the evolutionary plasticity to generate novel virulence profiles, as exemplified by the emergence of highly pathogenic recombinant strains in Asia [12, 21]. This intricate web of host-pathogen interactions, from the molecular level of protein-protein interactions to the systemic level of subclinical dissemination via PBMCs [43], defines LSDV as a formidable pathogen whose pathogenesis continues to challenge our control efforts.

Epidemiology and Global Spread

Lumpy skin disease virus (LSDV) has undergone one of the most dramatic global range expansions documented for any arthropod-borne viral pathogen of livestock in the twenty-first century. For decades following its initial description in Zambia in 1929, LSDV was considered an exclusively African pathogen, confined largely to sub-Saharan Africa with sporadic incursions into southern Africa and Madagascar [1, 23]. The epidemiological landscape shifted fundamentally beginning in the late 1980s, when LSDV breached continental boundaries and established endemicity in Egypt [4]. This incursion into North Africa presaged a relentless northward and eastward advance that would, over the subsequent three decades, culminate in the virus establishing a transcontinental presence spanning Africa, the Middle East, Europe, Central Asia, South Asia, and Southeast Asia [1, 6, 7]. The current epizootiological situation represents an unprecedented expansion of LSDV’s ecological niche, driven by a complex interplay of viral genetic plasticity, vector ecology, anthropogenic livestock movements, and climate-mediated environmental changes.

The initial wave of extra-African spread occurred between 1988 and 2012, during which LSDV became enzootic in Egypt and subsequently moved through the Middle East, including Israel, Palestine, Jordan, and Lebanon [4]. The epidemiological dynamics in this period were characterized by relatively slow, punctuated spread, likely limited by the distribution of competent arthropod vectors and the absence of large-scale, long-distance livestock transport networks connecting endemic and naïve regions. However, a second, far more explosive phase of global dissemination began in 2012–2013, when LSDV was detected in Turkey and rapidly spread through the Balkans, reaching Greece, Bulgaria, Serbia, Albania, Montenegro, North Macedonia, and Kosovo by 2016 [23, 27]. The European epizootic was ultimately contained through aggressive vaccination campaigns employing Neethling-based live attenuated vaccines, demonstrating that coordinated control measures could reverse the virus’s advance [4, 38]. Critically, the European experience provided the first large-scale evidence that homologous LSDV vaccines could effectively interrupt transmission when deployed with sufficient coverage, a lesson that would prove invaluable as the virus continued its eastward expansion.

The most consequential epidemiological development occurred in 2015–2016, when LSDV crossed into the Russian Federation, affecting the Southern Federal District, including the Saratov and Astrakhan regions, and subsequently Kazakhstan [2, 51]. From this Central Asian bridgehead, the virus launched a rapid and devastating sweep across the Asian landmass. By 2019, LSDV had been reported in China, initially in the Xinjiang Uygur Autonomous Region, and from there spread inexorably eastward and southward [5, 28]. Whole-genome phylogenetic analyses have meticulously reconstructed this transboundary movement, revealing that the Asian epizootic comprises at least two major genetically distinct lineages: strains belonging to cluster 1.2, which spread through South Asia (including the Indian subcontinent and Pakistan), and recombinant cluster 2.5 strains, which have become dominant throughout Southeast Asia [7, 16, 27]. The emergence and rapid fixation of cluster 2.5 recombinants represents a particularly alarming evolutionary development, as these strains are mosaic viruses containing genomic segments derived from both wild-type field strains and vaccine strains, including KSGP/O240-like and Neethling-like lineages [7, 21, 47]. The recombinant nature of cluster 2.5 strains strongly suggests that the indiscriminate use of live attenuated vaccines in field settings, coupled with incomplete biosecurity, has created conditions favoring genetic recombination between vaccine and circulating field viruses [21, 27, 47]. This phenomenon has profound implications for disease control, as recombinant viruses may exhibit altered virulence, transmissibility, and antigenic profiles that could compromise vaccine efficacy.

From 2019 onward, the pace of LSDV’s geographic conquest accelerated dramatically. Vietnam experienced a massive epizootic beginning in October 2020, officially reporting 207,687 infected cattle and buffaloes and necessitating the culling of 29,182 animals [39]. Subsequent spread throughout mainland Southeast Asia was remarkably rapid; Malaysia, which had never reported LSDV prior to May 2021, saw the virus spread to 65 of 92 districts in Peninsular Malaysia by December of the same year [16]. Thailand, Indonesia, Cambodia, Laos, Myanmar, and South Korea all documented incursions between 2020 and 2023 [7, 10, 22, 50]. The introduction into Indonesia represents a particularly concerning epidemiological milestone, as it marks the establishment of LSDV in an archipelagic nation with extensive livestock trade connections to Australia, a country currently free of LSDV but considered at high risk of introduction [24, 40]. Geospatial modeling exercises assessing entry pathways for LSDV into Australia have identified two primary risk corridors: windborne dispersal of infected arthropod vectors from Indonesia into northern Australia, particularly during summer monsoon periods, and the transport of infected vectors via shipping channels, with ports in Western Australia (Port Hedland and Dampier) and Far North Queensland identified as the most vulnerable points of entry [40].

The mechanisms driving LSDV’s remarkable transboundary spread are multifaceted. Mechanical transmission by hematophagous arthropod vectors is unequivocally the dominant mode of local and regional dissemination [1, 24, 29]. Seminal experimental studies have demonstrated that multiple mosquito species, including Aedes aegypti, Culex tritaeniorhynchus, and Culex quinquefasciatus, are highly susceptible to LSDV infection and can transmit the virus following ingestion of infectious blood meals [29]. Critically, Sri-In and colleagues provided the first experimental evidence of transovarial transmission of LSDV in Ae. aegypti, demonstrating that the virus can be vertically transmitted from infected female mosquitoes to their progeny, a finding that elevates the epidemiological significance of mosquito vectors beyond simple mechanical carriage and suggests the potential for LSDV to overwinter in mosquito populations [15]. Stable flies (Stomoxys calcitrans) have also been definitively implicated as vectors, with Haegeman and colleagues showing that S. calcitrans feeding on subclinically infected cattle can transmit LSDV to naïve acceptor animals, even in the absence of visible skin nodules on the donor [36]. This finding has profound epidemiological implications, as it indicates that subclinically infected animals, which may escape detection during routine clinical surveillance, can serve as cryptic reservoirs for vector-mediated transmission [30, 36]. Furthermore, Shumilova and colleagues demonstrated that recombinant vaccine-like strains can cause subclinical infections characterized by viremia and viral shedding in nasal and ocular discharges without the development of pathognomonic skin nodules, making these animals exceptionally difficult to identify through passive surveillance [30]. The contribution of subclinical infections to LSDV transmission dynamics likely represents a major, and historically underestimated, driver of the virus’s rapid geographic expansion.

Complementing vector-borne transmission, anthropogenic factors, particularly the long-distance movement of infected cattle through trade networks, have been instrumental in translocating LSDV across national and continental boundaries. Spatiotemporal analyses of the Malaysian epizootic conclusively linked the initial introduction with cross-border cattle movements from Thailand, demonstrating that the spatial distribution of outbreaks closely mirrored major livestock transport routes [16]. Similarly, phylogenetic evidence from Bangladesh reveals co-circulation of genetically distinct LSDV strains, including isolates clustering with South Asian lineages and others grouping with strains from Africa, the Middle East, and Europe, strongly suggesting multiple independent introductions via livestock trade [13]. The Qinghai-Tibet Plateau, historically considered a natural barrier to viral incursion due to its extreme altitude and cold climate, has now experienced LSDV outbreaks in yaks (Bos grunniens), cattle-yaks, and cattle, with the causative strain (LSDV/China/GS/Yak) belonging to the recombinant cluster 1.2 subclade and exhibiting high homology to strains circulating in East and Southeast Asia [12, 28]. This incursion into high-altitude ecosystems previously considered inhospitable to LSDV transmission underscores the virus’s expanding ecological plasticity and raises concerns about its potential to establish endemicity in novel environments.

The host range of LSDV has also broadened during its global expansion, with potentially significant epidemiological consequences. While cattle (Bos taurus and Bos indicus) remain the primary reservoir and clinically affected species, natural LSDV infection has now been documented in water buffalo (Bubalus bubalis), although experimental infections suggest that buffalo are less susceptible and may act as accidental, non-adapted hosts rather than efficient reservoirs [35]. More alarmingly, LSDV has been detected in free-ranging Indian gazelles (Gazella bennettii) in Rajasthan, India, indicating the virus’s ability to cross species barriers and infect wild ruminants [37]. The potential establishment of sylvatic cycles in wild ungulate populations would represent a paradigm shift in LSDV epidemiology, as wildlife reservoirs could sustain the virus independently of domestic cattle populations and complicate eradication efforts. Even more concerning are reports of LSDV infection in camels (Camelus dromedarius), with Kumar and colleagues confirming LSDV isolation from skin nodules of clinically affected camels in India and demonstrating seroconversion [42]. The emergence of LSDV in camelids, coupled with the detection of viral reads in human upper respiratory tract samples from India (although this finding requires further investigation to confirm active infection versus environmental contamination), suggests that the host range of LSDV may be broader than previously appreciated [33, 42]. The World Organization for Animal Health (WOAH) continues to classify LSD as a notifiable disease, and its expanding host range necessitates ongoing vigilance from veterinary authorities globally.

Genomic epidemiology has provided unprecedented resolution into the evolutionary dynamics driving LSDV’s global spread. Whole-genome sequencing of historical and contemporary isolates has revealed an ‘open’ pan-genome structure, with ongoing gene gain and loss events contributing to lineage-specific genetic diversity [5]. Haplotype network analyses and mutation pattern assessments across global genomes have identified a total of 2,212 variants, with Indian isolates forming a distinct and conservative haplogroup reflecting recent common ancestry and regional adaptation [6]. Critically, phylogeographic analyses have identified three genomic regions, located in the 5′ and 3′ flanking regions of the LSDV genome, that exhibit disproportionately high genetic variability; these regions encode proteins involved in immune evasion, including the 001/156 protein that suppresses interferon production by impairing IRF3 dimerization, the ORF142 protein that inhibits type I interferon responses through NBR1-mediated autophagic degradation of STING, and the ORF127 protein that suppresses IFN-β production by inhibiting K63-linked ubiquitination of TBK1 [3, 9, 11, 25, 27]. The rapid evolution of these immunomodulatory genes likely reflects strong selective pressure exerted by host immune responses and may contribute to the enhanced virulence phenotypes observed in recently emerging strains.

Vaccine-induced immune pressure has emerged as a potent selective force shaping LSDV evolution, as evidenced by the emergence of recombinant strains in Asia. The widespread deployment of heterologous goatpox virus (GTPV) vaccines in China has been associated with the isolation of recombinant LSDV strains containing genomic segments derived from GTPV, suggesting that co-infection of cattle with vaccine and field strains can generate novel chimeric viruses with unpredictable phenotypic properties [21]. Chang and colleagues isolated a recombinant LSDV strain (LSDV/China/SX/2023) from a cattle herd immunized with GTPV attenuated vaccine and identified 15 recombination events, including one associated with GTPV, indicating that heterologous vaccine pressure can drive recombination and potentially facilitate the emergence of vaccine-escape variants [21]. This finding underscores the critical importance of implementing DIVA (Differentiating Infected from Vaccinated Animals) strategies and deploying homologous vaccines, such as Neethling-based live attenuated vaccines, that have been demonstrated to provide robust protection against currently circulating recombinant strains [38]. The development of molecular tools capable of discriminating between wild-type, vaccine, and recombinant strains, such as multi-locus high-resolution melting (HRM) assays targeting ORF095, ORF126, and ORF145, is essential for accurate epidemiological surveillance and informed vaccine policy [47].

In conclusion, the epidemiology and global spread of LSDV represent a case study in emerging infectious disease dynamics driven by viral evolution, vector ecology, anthropogenic factors, and vaccine-induced selection. The virus has transformed from a geographically restricted African pathogen into a transcontinental threat affecting livestock industries across Eurasia. Continued genomic surveillance, coupled with rigorous vector ecology studies and the rational deployment of homologous vaccines, will be essential for tracking and mitigating the ongoing expansion of this economically devastating pathogen.

Transmission Dynamics and Vector Ecology

The dissemination of Lumpy Skin Disease Virus (LSDV) represents a complex interplay between viral pathogenesis, vector biology, and host population dynamics, culminating in a pattern of transboundary spread that has fundamentally altered the global distribution of this pathogen over the past decade. Unlike many directly transmitted viral infections of livestock, LSDV transmission is predominantly mechanical, reliant on hematophagous arthropod vectors that bridge the gap between infected and naïve bovine hosts. This vector-borne paradigm imposes a unique set of ecological and epidemiological constraints on the virus, shaping its seasonal incidence, geographical expansion, and the efficacy of control interventions. The rapid emergence of LSDV from its historical confines in sub-Saharan Africa into the Middle East, Europe, and across the vast expanse of Asia, reaching as far east as China, Southeast Asia, and Indonesia by the early 2020s, underscores the urgent need for a sophisticated, evidence-based understanding of its transmission ecology [1, 7, 16, 24]. The World Organisation for Animal Health (WOAH) classifies LSD as a notifiable disease, reflecting its significant socio-economic impact on the global cattle industry, and the expanding vector ranges driven by climate change and globalized trade networks only amplify this threat [1, 4, 24].

The Primacy of Arthropod Vectors: Culicidae and Stomoxys

The preponderance of experimental and field evidence points to blood-feeding dipterans, particularly mosquitoes (Culicidae) and stable flies (Stomoxys calcitrans), as the principal agents of LSDV transmission. The mechanical mode of transmission, wherein the virus is carried on the mouthparts of the vector from a viremic host to a susceptible one, is a well-established paradigm for poxviruses. Critically, however, recent research has challenged the notion that LSDV transmission is purely mechanical, revealing a more intimate and biologically complex relationship between the virus and its vectors.

A landmark series of studies utilizing Aedes aegypti, Culex tritaeniorhynchus, and Culex quinquefasciatus has demonstrated that these mosquitoes are not merely passive flying needles. Following ingestion of a blood meal containing LSDV, the virus can infect the mosquito midgut epithelium, disseminate through the hemolymph to secondary tissues, and ultimately reach the salivary glands, from which it can be expectorated during subsequent feeding events [29]. This process, which mirrors the biological transmission of arboviruses like dengue or West Nile virus, was observed with variable extrinsic incubation periods (EIPs), the time between ingestion of the virus and the ability to transmit it. Ae. aegypti exhibited rapid dissemination, with virus detectable in the head and salivary glands as early as 2 days post-infection (dpi), while Cx. tritaeniorhynchus and Cx. quinquefasciatus demonstrated EIPs of 8 and 14 dpi, respectively [29]. These findings indicate that mosquito species differ significantly in their vector competence, a factor that must be integrated into regional risk assessments.

Perhaps the most startling revelation for LSDV epidemiology is the demonstration of transovarial transmission in Ae. aegypti. In a controlled experimental setting, female mosquitoes infected with a sufficiently high viral titer were able to pass the virus to their progeny, resulting in infected F1 generation larvae and adults [15]. This finding has profound implications for the maintenance and overwintering of LSDV in temperate and subtropical regions. If the virus can survive within mosquito eggs through unfavourable seasons, it could re-emerge the following spring without the need for a persistently infected vertebrate reservoir, which is currently thought to be absent for LSDV [15]. This mechanism of vertical transmission would fundamentally alter our understanding of LSDV persistence and necessitate a shift in control strategies from simply targeting adult vectors to considering larval habitat management.

Beyond mosquitoes, the stable fly Stomoxys calcitrans has been repeatedly implicated as a highly efficient mechanical vector. The painful, interrupted feeding behaviour of Stomoxys makes it an ideal candidate for mechanically transferring virus-laden blood between animals in close proximity. A critical in vivo transmission study using S. calcitrans provided robust experimental evidence for this role. The study demonstrated that flies fed on subclinically infected donor cattle, those with active viral replication but without the characteristic skin nodules, could successfully transmit LSDV to naïve acceptor animals [36]. This is a pivotal finding, as it shows that transmission can occur from animals that would escape clinical detection, forming a cryptic transmission chain that undermines standard "stamping out" policies based solely on visible lesions. Furthermore, the study found that one of the acceptor animals developed a subclinical infection itself, further perpetuating the cycle of undetected spread [36]. The role of subclinical infection in LSDV transmission is a high-risk condition that must be accounted for in epidemiological models and control programs [30].

Ixodid Ticks as Putative Biological Vectors

While the focus has historically been on insects, a growing body of evidence implicates hard ticks (Ixodidae) in the transmission cycle. The ixodid tick Rhipicephalus annulatus has been shown to harbour LSDV in field conditions. In a comprehensive study from Egypt, LSDV was isolated and molecularly confirmed from both cattle and the ticks feeding upon them [41]. Virus was detected in skin biopsies, blood, and, critically, in the ticks themselves using multiplex PCR and quantitative real-time PCR. The cyclic threshold (Ct) values from the tick samples indicated a significant viral load, suggesting active replication within the tick rather than mere mechanical contamination [41]. Given that ticks are long-lived, have multi-year life cycles, and can feed on multiple hosts across different stages (larva, nymph, adult), they represent a potential overwintering reservoir and a mechanism for long-term virus persistence in an ecosystem, a role that warrants extensive further investigation.

Host Range and Viral Shedding: The Amplifying Host

The transmission dynamics of LSDV are inexorably linked to the range of susceptible hosts and the routes by which the virus exits the infected animal. While Bos taurus and Bos indicus cattle are the primary hosts, the virus has demonstrated a concerning capacity to expand its host range. Natural infections have been confirmed in water buffalo (Bubalus bubalis), with experimental infections suggesting that while susceptibility may be lower than in cattle, seroconversion and limited clinical signs can occur, raising questions about their role as spillover or maintenance hosts [35]. More alarmingly, LSDV has been detected in free-ranging Indian gazelles (Gazella bennettii), yaks (Bos grunniens) on the Qinghai-Tibet Plateau, and even in camels (Camelus dromedarius) [12, 37, 42]. The infection in yaks was particularly severe, with a systemic pathogenesis, high mortality (46.67%), and lesions in the respiratory and digestive tracts, indicating that recombinant LSDV strains can be highly pathogenic for novel hosts [12]. This broadening host range expands the ecological niche of the virus and introduces new challenges for wildlife-livestock interface management.

Viral shedding occurs through multiple routes, which dictates the potential for both vector-borne and direct contact transmission. The skin nodule, the pathognomonic feature of LSD, is a concentrated reservoir of infectious virus. Skin scabs and nodule biopsies contain the highest viral loads, serving as a rich source for mechanical vector acquisition and direct contact exposure through fomites [14, 31]. Viral DNA is also consistently detected in nasal, oral, and conjunctival swabs, as well as in semen [4, 52]. The presence of virus in semen is of particular concern for artificial insemination and international trade of bovine germplasm. Viremia, detected in blood samples, is a prerequisite for vector acquisition, although the timing and duration of viremia can be variable. In experimental infections, peak viral loads in blood often coincide with the peak of clinical signs, typically between 11 and 14 days post-infection [2]. Interestingly, blood samples are considered suboptimal for routine diagnosis compared to skin biopsies due to lower and more transient viral titers, but they are critical for vector-borne transmission [2, 14]. Subclinical animals, which exhibit viremia and shed virus from mucosal surfaces without nodules, represent a particularly insidious source of infection for vectors [36].

Cellular Dissemination and Immune Evasion

At the cellular level, LSDV has evolved sophisticated strategies to ensure its dissemination and persistence within the host, which in turn facilitates its acquisition by arthropods. Following entry into the host via a vector bite, the virus infects permissive cells. Bovine peripheral blood mononuclear cells (PBMCs) have been identified as critical vehicles for viral dissemination. While LSDV does not undergo productive replication, characterized by the release of substantial infectious progeny, within PBMCs, it does execute active viral transcription and gene expression [43]. Crucially, infecting naïve PBMCs, LSDV effectively suppresses their innate antiviral response, preventing the upregulation of interferon-stimulated genes (ISGs) that would normally restrict viral spread [43]. These infected PBMCs then act as "Trojan horses," migrating through the bloodstream and lymphatic system. In co-culture experiments, infected PBMCs were shown to transmit LSDV to permissive cells (e.g., fibroblasts) through direct cell-to-cell contact, forming new infection foci [43]. This mechanism allows the virus to navigate the host's circulatory system and establish infection in distant anatomical sites, including the skin, where it creates the nodular lesions that are so attractive to biting flies.

The suppression of the host’s type I interferon (IFN-I) response is a hallmark of LSDV pathogenesis and is orchestrated by a suite of viral proteins. LSDV encodes multiple dedicated immune antagonists. For instance, the LSDV 001/156 protein, encoded within the inverted terminal repeat (ITR) region, acts as a virulence factor by interacting with interferon regulatory factor 3 (IRF3) and preventing its dimerization and nuclear translocation, thereby shutting down IFN-β production [3]. Similarly, the ORF142 protein employs a dual strategy: it directly interacts with IRF3 to block its recruitment to TANK-binding kinase 1 (TBK1) [9], and it also facilitates the autophagic degradation of STING (MITA) by recruiting the cargo receptor NBR1, thus crippling the cGAS-STING signaling pathway [11]. By neutralizing the IFN-I response at multiple points, LSDV ensures efficient replication and the generation of high viral loads in the skin and blood, maximizing the probability of vector-mediated transmission.

Genomic Plasticity and Emerging Transmission Patterns

The recent global expansion of LSDV has been accompanied by significant genomic evolution, which may have direct consequences for transmission dynamics. Phylogenetic analyses of whole-genome sequences have revealed the emergence of distinct clades and recombinants. The pandemic spread into Asia has been dominated by recombinant clade 2.5 viruses, which are hybrids of wild-type field strains and vaccine strains [7, 21, 27]. This recombination, which likely occurred in the field under immune pressure from capripoxvirus vaccines, has resulted in strains with novel genetic signatures [21]. In Southeast Asia, including Malaysia, Thailand, Vietnam, and Indonesia, these recombinant clade 2.5 strains have become dominant, ousting earlier field strains and manifesting a distinct pattern of monophyletic evolution [7, 16, 22, 39]. The emergence of these novel genotypes, including distinct variants in West and Central Africa (clade 1.2.3) and in the Indian subcontinent (clade 1.2), suggests that the virus is actively adapting to different ecological and immunological landscapes [23, 27]. The rapidity of this spread, moving thousands of kilometers from Africa through Europe and into the heart of Asia, is consistent with a highly efficient vector-borne pathogen exploiting a competent and abundant vector population [7]. Genomic surveillance, supported by tools such as high-resolution melting (HRM) assays to differentiate field, vaccine, and recombinant strains, is essential to track these evolving transmission patterns and ensure that control strategies remain relevant [47]. The detection of LSDV reads in the human upper respiratory tract microbiome, while not indicating zoonotic potential, raises intriguing questions about environmental contamination and the potential for indirect fomite transmission, warranting further investigation into non-vectorial pathways [33].

Clinical Manifestations and Pathological Findings

Lumpy skin disease (LSD) represents one of the most clinically distinctive viral infections of bovine species, characterized by a constellation of systemic and cutaneous manifestations that reflect the profound pathophysiological impact of LSD virus (LSDV) infection. The disease, classified as a notifiable transboundary animal infection by the World Organization for Animal Health (WOAH), presents a clinical spectrum ranging from subclinical infection to severe, fatal systemic disease. The clinical expression and pathological outcomes are influenced by viral strain virulence, host immune status, breed susceptibility, age, and environmental factors, with recent evidence demonstrating that recombinant and emerging strains are capable of inducing more aggressive disease phenotypes than classical field isolates [2, 12, 31].

Prodromal Phase and Systemic Signs

The incubation period following natural or experimental infection typically ranges from 4 to 14 days, with clinical signs generally emerging between days 5 and 7 post-infection [2, 52]. The prodromal phase is marked by an abrupt onset of pyrexia, with rectal temperatures frequently exceeding 39.5°C and reaching 41°C in severe cases [50, 52]. This febrile response is often biphasic in nature, with an initial temperature spike coinciding with primary viremia, followed by a second elevation concomitant with the eruption of cutaneous lesions and systemic dissemination [35]. Concurrent with fever, affected animals exhibit pronounced depression, anorexia, reduced ruminal motility, and a sharp decline in milk production that can persist for weeks beyond the acute phase [4, 14, 53]. Oculonasal discharges, initially serous and subsequently becoming mucopurulent, are common, alongside excessive salivation and lacrimation that reflect viral tropism for mucosal epithelial surfaces [12, 49].

Lymphadenopathy is a consistent and early clinical feature, with the superficial lymph nodes, particularly the prescapular, prefemoral, and submandibular nodes, becoming markedly enlarged, firm, and palpable on physical examination [2, 4, 12]. This lymphadenopathy reflects the intense lymphoid hyperplasia and inflammatory cell infiltration that occur as the virus establishes infection within lymphoid tissues, and it often precedes the appearance of cutaneous nodules by several days [30]. Importantly, generalized lymphadenopathy in the absence of skin nodules has been recognized as a hallmark of subclinical infection, a phenomenon that poses significant challenges for field-based diagnosis and disease surveillance [30, 36].

Cutaneous Manifestations: The Hallmark Lesion

The pathognomonic feature of LSD is the development of circumscribed, firm, cutaneous nodules that typically appear 7 to 14 days after the onset of fever [50, 52]. These nodules, which measure 1 to 5 cm in diameter, are round, raised, and well-demarcated from the surrounding skin. They initially present as erythematous macules that rapidly progress to firm papules and then to deep-seated nodules involving the full thickness of the dermis and often extending into the subcutaneous tissue [2, 31]. The distribution of lesions is generalized but exhibits a predilection for the head, neck, perineum, udder, scrotum, and limbs, although nodules can appear on virtually any body surface [31, 50]. The number of nodules per animal is highly variable, with mild cases presenting fewer than 10 lesions and severe cases exhibiting hundreds of confluent nodules covering large areas of the body surface [31]. In a comprehensive analysis of 513 clinically suspected cases during the Indian epidemic, disease stages were categorized as early (20.81%), mid (42.02%), and late (37.17%), with lesion distribution classified as mild (34.14%), moderate (39.39%), and severe (26.47%) [31].

The histopathological evolution of the skin nodule is a dynamic process characterized by progressive tissue destruction and inflammatory infiltration. In the early stages, the epidermis exhibits acanthosis, hyperkeratosis, and ballooning degeneration of keratinocytes, with prominent intracytoplasmic eosinophilic inclusion bodies (known as Bollinger bodies) visible within infected epithelial cells [31, 55]. As the lesion matures, the dermis becomes edematous and infiltrated by a mixed population of mononuclear cells, including lymphocytes, macrophages, and plasma cells, accompanied by neutrophils in areas of necrosis [12, 31]. A defining pathological feature is necrotizing vasculitis affecting small and medium-sized blood vessels, with fibrinoid necrosis of the vessel wall, thrombosis, and perivascular cuffing by inflammatory cells [2, 31]. This vasculitic process is the underlying cause of the ischemic necrosis that characterizes the center of mature nodules, leading to the formation of a characteristic “sit-fast” or sequestrum, a firm, necrotic core that ultimately sloughs, leaving a deep ulcer that heals slowly by granulation [31].

Vascular thrombosis and edema are particularly prominent in the deep dermis and subcutis, contributing to the extensive tissue damage observed in severe cases [2, 12]. Immunohistochemical analysis has localized viral antigens predominantly to hair follicle epithelia, macrophages, and endothelial cells, confirming the critical role of vascular endothelium as a target cell type in LSDV pathogenesis [12]. The prolonged healing process, often requiring several weeks to months, frequently results in fibrous scar formation, which can be disfiguring and permanently damage the hide, significantly reducing its commercial value [31].

Secondary bacterial infection of the necrotic nodules is a frequent complication, profoundly altering the clinical course and pathological picture. Metagenomic profiling of pox lesions has revealed a complex polymicrobial community dominated by Fusobacterium necrophorum, Streptococcus dysgalactiae, Helcococcus ovis, and Trueperella pyogenes, organisms that are typically opportunistic pathogens of the skin and mucous membranes [54]. These secondary invaders exacerbate tissue necrosis, promote suppuration, and can lead to the formation of deep abscesses and cellulitis, further delaying healing and increasing the risk of septicemia [49, 54]. In tropical and subtropical environments, myiasis (fly strike) is an additional complication that can cause extensive tissue loss and contribute to mortality in severely affected animals [54].

Lameness is a frequent and debilitating clinical sign, observed in up to 81.8% of naturally infected Bos indicus cattle in one study [50]. It arises from the development of painful nodules on the coronary bands of the hooves, interdigital spaces, and limb joints, leading to reluctance to bear weight, recumbency, and in chronic cases, hoof deformities [50]. Similarly, nodules on the teat and udder interfere with milking and predispose affected animals to secondary mastitis, while lesions on the scrotum can cause temporary or permanent infertility in bulls [4, 31].

Systemic Pathological Findings

LSDV is not a pathogen confined to the skin; rather, it produces a truly systemic infection with viral dissemination to multiple internal organs [2, 12, 31]. Post-mortem examination of fatally affected animals reveals that necrotic and ulcerative nodules analogous to those in the skin are present on the mucous membranes of the oral cavity, nares, pharynx, larynx, trachea, esophagus, and throughout the gastrointestinal tract [12, 31]. In the respiratory tract, tracheal congestion, pulmonary hemorrhagic plaques, and consolidation have been described, particularly in infections with recombinant strains [12]. The lungs may exhibit interstitial pneumonia, alveolar edema, and the presence of nodular lesions within the pulmonary parenchyma [2]. These respiratory lesions, combined with the obstructive effects of pharyngeal and laryngeal nodules, account for the dyspnea and respiratory distress observed in severe cases [12].

Within the gastrointestinal system, the rumen and reticulum frequently exhibit serosal hemorrhagic masses and mucosal ulcerations [12]. The abomasum and intestines may show multifocal erosions and ulcerations, reflecting viral replication in the epithelial lining. Hepatic changes, including congestion, fatty degeneration, and focal necrosis, have been described, though these are generally considered secondary to systemic toxemia rather than direct viral cytopathology [2, 31].

The lymphoid system bears a disproportionate burden of the pathological changes. The spleen is consistently enlarged, congested, and may contain necrotic foci, reflecting the intense lymphoid hyperplasia and viral replication that occur within this organ [2]. Lymph nodes, particularly those draining the cutaneous lesions, are markedly enlarged, edematous, hemorrhagic, and may contain necrotic abscesses [2, 31]. Histologically, lymphoid follicles exhibit hyperplasia with prominent germinal centers, accompanied by lymphocyte depletion and necrosis in the paracortical zones, indicating a profound disruption of lymphoid architecture and immune cell homeostasis [31]. The highest viral loads are consistently detected in skin nodules and scabs, trachea, tongue, and lymph nodes, with significantly higher viral genome copy numbers observed in the mid- and late-stages of nodule development [31]. Importantly, blood samples from the early stages of infection also show high viral loads, underscoring the hematogenous route of dissemination [14, 31].

Subclinical and Atypical Infections

A critical aspect of LSD epidemiology is the occurrence of subclinical infections, which have been increasingly recognized as a significant challenge for disease control [30, 36]. Subclinically infected animals exhibit no visible skin nodules but may present with mild fever, transient viremia, and viral shedding in nasal and ocular secretions in the absence of overt clinical signs [30]. Enlarged lymph nodes are often the only detectable abnormality, a finding that is easily overlooked by inexperienced observers [30]. Experimental studies have demonstrated that these subclinically infected animals are fully capable of transmitting LSDV to naïve contact animals, either through direct contact or via mechanical arthropod vectors such as Stomoxys calcitrans [30, 36]. Indeed, transmission from subclinical donors showing evidence of productive virus replication but without nodule formation was achieved in two out of five acceptor animals in a controlled study, while no transmission was observed from preclinical donors that developed nodules after vector feeding [36]. These findings have profound implications for stamping-out policies, as the culling of only clinically diseased animals may be insufficient to halt the spread of the virus [30, 36].

Recombinant vaccine-like strains (RVLS) have been shown to induce subclinical infections in experimentally inoculated animals, with prolonged viremia and viral shedding that persisted for up to 14 days post-infection [30]. The ability of these strains to establish subclinical infections in vaccinated or previously exposed populations represents an emerging concern, as it complicates the differentiation of infected from vaccinated animals (DIVA) and undermines the effectiveness of vaccination-based control strategies [30, 32, 47].

Hematological and Biochemical Alterations

LSDV infection induces profound perturbations in the hematological and biochemical profiles of affected animals, reflecting the systemic inflammatory and viremic nature of the disease. Significant leukopenia, characterized by reduced total white blood cell counts (reduced from approximately 6.90 × 10⁹/L in healthy animals to 4.44 × 10⁹/L in infected animals), is a hallmark of the acute phase, likely reflecting the sequestration of leukocytes at sites of tissue damage and the direct cytopathic effects of the virus on immune cells [56]. Concurrently, relative lymphocytosis (from 0.70 to 1.86 × 10⁹/L) is observed, indicative of the robust adaptive immune response that is mounted against the virus [56]. Granulocytopenia is frequently documented, potentially reflecting the suppression of innate immune mechanisms or the consumption of neutrophils at sites of necrotizing inflammation [56]. Hematocrit levels are significantly elevated in infected animals (30.13% versus 26.68% in controls), indicative of dehydration secondary to fever, anorexia, and reduced water intake [56]. Biochemical analyses have revealed elevated serum levels of hepatic transaminases (SGPT and SGOT), blood urea nitrogen (BUN), creatinine, albumin, globulin, and the albumin-to-globulin ratio in LSDV-infected cattle, although these changes may not reach statistical significance in all studies [14]. These alterations collectively reflect multi-organ involvement, with hepatic and renal dysfunction contributing to the metabolic derangements observed in severe disease.

Host Range and Species-Specific Pathology

While Bos taurus and Bos indicus cattle are the primary hosts, LSDV has demonstrated the capacity to infect a broadening range of susceptible species, with distinct pathological manifestations in each. Water buffalo (Bubalus bubalis) are generally less susceptible than cattle, with experimental infection studies demonstrating that only a subset of inoculated buffaloes develop fever, skin nodules, and lymphadenitis, and even then, viral detection by PCR remains negative despite seroconversion detected by ELISA [35]. This suggests that buffaloes may serve as accidental, non-adapted hosts with limited viral replication capacity, though their role in the epidemiological chain remains to be fully elucidated [35, 41].

In yaks (Bos grunniens), recombinant LSDV strains have proven to be particularly devastating, with field outbreaks on the Qinghai-Tibet Plateau characterized by a 46.67% mortality rate, substantially higher than the typical case fatality rate of 1–5% reported in cattle [12]. Affected yaks exhibited fever, dyspnea, cutaneous pox lesions, lymphadenopathy, and mucosal lesions, with 100% positivity for viral DNA in skin samples and systemic involvement of the respiratory and digestive tracts [12]. Histopathological examination revealed severe dermal vasculitis, lymphocytic infiltration, and viral inclusion bodies, indicative of a particularly aggressive disease course in this species [12, 28].

Camels (Camelus dromedarius) have also been confirmed as susceptible hosts, with naturally infected animals presenting nodular skin lesions on the body surface from which LSDV was successfully isolated and confirmed by PCR and nucleotide sequencing [42]. The virus identified in camels (LSDV/Camel/India/2022/Bikaner) was related to historical field strains circulating in the Indian subcontinent, indicating cross-species transmission from infected cattle [42]. Additionally, free-ranging Indian gazelles (Gazella bennettii) have been found with nodular skin lesions and molecularly confirmed LSDV infection, further expanding the known host range and raising concerns for wildlife conservation and the potential establishment of sylvatic cycles [37]. Notably, LSDV reads have also been detected in the human upper respiratory tract microbiome through metagenomic sequencing, though the clinical significance of this finding remains unclear and requires further investigation [33].

Molecular Pathogenesis Correlates of Clinical Disease

The clinical and pathological manifestations of LSD are the direct result of a sophisticated interplay between viral virulence factors and host immune responses. Recent molecular characterization has identified specific viral proteins that orchestrate the pathological cascade. The LSDV001 protein functions as a key virulence factor by positively regulating the inflammatory response through interaction with TAK1 and TAB2/3, promoting assembly of the TAK1-TAB2/3 complex, and driving IKK-dependent activation of NF-κB [8]. This leads to excessive transcription of pro-inflammatory cytokines, including IL-1β and TNFα, which are central to the development of the pronounced inflammatory infiltrate, vasculitis, and tissue necrosis that characterize the skin nodules [8]. Infection with LSDV001-deficient virus results in significantly smaller skin nodules and reduced inflammation compared to wild-type virus, directly demonstrating the role of this protein in shaping the clinical phenotype [8].

Conversely, LSDV has evolved multiple strategies to subvert the host interferon (IFN) response, which would otherwise limit viral replication and dissemination. The LSDV 001/156 protein, encoded within the inverted terminal repeat (ITR) region, acts as a potent negative regulator of the IFN signaling pathway by interacting with interferon regulatory factor 3 (IRF3), disrupting its dimerization and nuclear translocation, thereby attenuating IFN-β production [3]. Similarly, the ORF127 protein suppresses type I interferon responses by inhibiting K63-linked ubiquitination of TANK-binding kinase 1 (TBK1), a critical step in the activation of IRF3 and subsequent IFN induction [25]. The ORF137 and ORF142 proteins further antagonize the IFN response by interacting with IRF3 and preventing its recruitment to TBK1, or by promoting autophagic degradation of STING through NBR1 recruitment [9, 11, 18]. The net effect of these multifaceted immune evasion strategies is to create a permissive environment for unchecked viral replication, leading to high viral loads, systemic dissemination, and the extensive tissue damage observed in severe clinical cases [3, 9, 11, 18].

The LSDV087 protein presents a fascinating example of functional duality, acting as a decapping enzyme that preferentially targets host transcripts to downregulate host gene expression while simultaneously promoting the cGAS-MITA/STING-mediated innate immune response through inhibition of K48-linked polyubiquitination and promotion of MITA/STING oligomerization [46]. This complex regulatory network highlights the delicate balance the virus must strike between evading host defenses and preventing the excessive inflammation that could otherwise limit viral fitness [46]. The dysregulation of host lysophosphatidic acid (LPA) metabolism, induced by LSDV infection, further contributes to pathogenesis by activating the MEK/ERK signaling pathway and suppressing innate immune responses, creating a metabolic environment that favors viral replication [45].

Understanding the molecular underpinnings of the clinical manifestations and pathological findings is not merely an academic exercise; it provides the essential framework for rational vaccine design, the identification of new therapeutic targets, and the development of diagnostic strategies that can distinguish between infected and vaccinated animals. The emergence of highly virulent recombinant strains in Asia and the expanding host range underscore the urgency of continued research into the mechanisms by which LSDV causes disease [2, 12, 17, 26, 27].

Diagnostic Approaches and Laboratory Detection

The accurate and timely laboratory diagnosis of Lumpy Skin Disease Virus (LSDV) is paramount for implementing effective control measures, facilitating epidemiological surveillance, and differentiating infected from vaccinated animals (DIVA). Given the transboundary nature of LSD and its classification as a notifiable disease by the World Organization for Animal Health (WOAH), diagnostic approaches must be robust, sensitive, and capable of distinguishing LSDV from other capripoxviruses, including sheeppox virus (SPPV) and goatpox virus (GTPV), which share up to 97% nucleotide sequence homology [4, 32]. The diagnostic landscape for LSDV has evolved considerably, encompassing classical virological methods, molecular nucleic acid detection, serological assays, and emerging biosensor technologies. Each modality offers distinct advantages and limitations, and their strategic deployment is dictated by the clinical phase of infection, the purpose of testing (e.g., outbreak confirmation, surveillance, vaccine potency testing), and the available laboratory infrastructure.

Molecular Detection: The Cornerstone of Modern Diagnostics

Nucleic acid amplification techniques (NAATs) represent the gold standard for the rapid and sensitive detection of LSDV, particularly during the acute phase of infection when viral DNA is abundant in skin nodules, scabs, and blood [4, 14]. The high genetic stability of the LSDV genome, while subject to ongoing evolutionary pressures and recombination events [7, 21], provides conserved target regions that are exploited for diagnostic PCR assays.

Conventional and Real-Time PCR: Polymerase chain reaction (PCR) targeting specific viral genes, such as the P32 (ORF011), RPO30, and GPCR genes, is widely employed for initial detection and genotyping [14, 28, 39]. Real-time quantitative PCR (qPCR) has emerged as a superior method, offering quantitative data on viral load, which is critical for understanding pathogenesis and monitoring disease progression. Studies have demonstrated that qPCR-derived viral titers correlate strongly with traditional tissue culture titration (Pearson's r = -0.994) and, more importantly, with in vivo protection outcomes in vaccine efficacy trials [58]. This correlation positions qPCR as a reliable, biosafe surrogate for conventional potency testing, reducing reliance on animal challenge models. The highest viral loads are consistently detected in skin nodules and scabs, followed by lymph nodes and tracheal tissues, while blood samples from early-stage infections also harbor significant viral DNA [31]. Notably, tissue samples exhibit a higher positivity rate (69.23% for the P32 gene) compared to nasal swabs (6.38%), underscoring the diagnostic superiority of lesional material [14]. However, the detection of viral DNA in nasal and oral swabs, as well as semen, confirms that the virus is shed through multiple excretions, necessitating comprehensive sampling strategies for outbreak investigations [4, 52].

High-Resolution Melting (HRM) Assays and Differentiation of Strains: The emergence of recombinant LSDV strains, particularly those belonging to clade 2.5 in Southeast Asia, and the widespread use of live-attenuated vaccines have created an urgent need for assays capable of differentiating wild-type, vaccine, and recombinant strains [7, 47]. Multi-locus real-time PCR coupled with HRM analysis has proven to be a rapid and cost-effective solution. By targeting specific genomic loci, such as ORF095, ORF126, and ORF145, HRM assays can discriminate between field and vaccine-type sequences based on distinct melting temperature profiles. This approach can identify recombinant viruses that possess a mosaic of wild-type and vaccine-type genes, for instance, a virus with wild-type ORF095 and ORF145 but a vaccine-type ORF126, within two hours of the PCR run [47]. This capability is indispensable for molecular epidemiological surveillance and for assessing the ongoing evolution of LSDV under immune pressure from heterologous vaccines [21].

Isothermal Amplification and Point-of-Care Testing: While PCR remains the laboratory standard, its requirement for sophisticated thermal cycling equipment limits its utility in resource-limited field settings. Loop-mediated isothermal amplification (LAMP) assays offer a compelling alternative, enabling rapid DNA amplification at a constant temperature (typically 60-65°C) within 60 minutes. A portable visual LAMP (vLAMP) assay has demonstrated 100% sensitivity and specificity compared to conventional PCR when testing tissue, whole blood, serum, and swab samples, achieving a kappa value of 1.0 [60]. The elimination of complex instrumentation and the ability to visualize results with the naked eye make vLAMP an ideal tool for on-site, real-time detection during outbreak responses, particularly in remote pastoralist communities where LSD is endemic.

CRISPR-Based and Biosensor Technologies: The integration of CRISPR/Cas systems with isothermal amplification has yielded diagnostic platforms with attomolar sensitivity. A label-free fluorescence method combining recombinase polymerase amplification (RPA) with CRISPR/Cas12a and a 10-23 DNAzyme achieved an actual detection limit of 1.29 copies/μL, which is 100-fold more sensitive than RPA with Cas12a alone [61]. This method also demonstrated 100% accuracy on 50 clinical samples and no cross-reactivity with SPPV or GTPV, despite their high genetic homology. Parallel innovations in biosensor technology include paper-based fluorescent sensors utilizing molecularly imprinted polymers (MIPs) and carbon quantum dots (CQDs). These sensors exhibit a turn-on fluorescence response upon binding LSDV, with a detection limit of 10¹ log₁₀ TCID₅₀/mL and high specificity against SPPV [62, 63]. Such platforms represent a paradigm shift towards low-cost, disposable, and equipment-free diagnostics that could be deployed at the pen-side.

Serological Approaches and the DIVA Challenge

Serological detection of anti-LSDV antibodies is critical for retrospective surveillance, assessing vaccine immunogenicity, and understanding the epidemiology of subclinical infections. However, the high antigenic cross-reactivity among capripoxviruses poses a significant challenge for serological DIVA strategies, particularly in regions where GTPV or SPPV vaccines are used to protect cattle against LSD [4, 32].

Enzyme-Linked Immunosorbent Assays (ELISAs): A variety of ELISA formats have been developed, including indirect ELISAs (iELISAs) and competitive ELISAs (cELISAs). The identification of immunodominant and specific antigens has been a major research focus. The recombinant LSDV034 protein has been successfully used as a coating antigen in a cELISA, demonstrating a diagnostic sensitivity of 98.46% and specificity of 100% when validated against the virus neutralization test (VNT), with an overall agreement of 98.97% [64]. Similarly, a truncated recombinant LSDV103 protein (TrLSDV103) was identified through a genome-wide phage display library screening. An iELISA based on TrLSDV103 achieved 100% diagnostic specificity and 86.67% sensitivity, with a lowest detection limit of 1:6400 for positive serum and no cross-reactivity with five other bovine pathogens [57]. Crucially, this assay showed significantly higher reactivity with sera from LSDV-infected cattle compared to GTPV-vaccinated cattle, highlighting its potential for DIVA.

Synthetic Antigens for DIVA: To overcome the inherent cross-reactivity of whole-virus antigens, a synthesized gene unique to LSDV has been developed as a differential antigen. An iELISA employing this synthetic protein exhibited 100% diagnostic specificity and 93.3% sensitivity. In a field evaluation of 200 clinical serum samples, no positive reactions were detected in 141 samples from a GTPV-vaccinated herd, while 33.90% of samples from infected herds were positive [32]. This approach provides a robust mechanism for distinguishing naturally infected animals from those vaccinated with heterologous capripoxvirus vaccines, a critical capability for trade and eradication programs.

Virus Neutralization Test (VNT) and Interferon-Gamma Release Assay (IGRA): The VNT remains the gold standard serological reference method due to its high specificity, but it is labor-intensive, slow, and requires live virus, necessitating BSL-3 facilities [64, 65]. The detection of neutralizing antibodies is also delayed, often appearing after the peak of clinical disease. In contrast, the cell-mediated immune (CMI) response, measured via an interferon-gamma release assay (IGRA), offers a significantly earlier diagnostic window. Following experimental LSDV infection, a uniform CMI response was detectable at plateau levels as early as 7 days post-infection (dpi), preceding seroconversion by several days [59]. Similarly, vaccination with a Neethling-based live attenuated vaccine induced a detectable CMI response by 10 days post-vaccination. The use of heat-inactivated antigen in the IGRA allows the assay to be performed under BSL-2 conditions, enhancing its practicality. This assay can achieve up to 100% specificity, making it a powerful tool for early detection and post-vaccination monitoring, particularly in scenarios where serological responses are weak or delayed [59].

Virus Isolation and Classical Virology

Despite the dominance of molecular methods, virus isolation remains essential for obtaining live virus for vaccine development, pathogenesis studies, and genomic characterization. LSDV can be isolated from skin nodules, scabs, and blood using primary cell cultures (e.g., lamb testis, kidney cells) or continuous cell lines such as MDBK, Vero, and BHK-21 [5, 52, 66]. The virus induces characteristic cytopathic effects (CPE), including cell rounding, detachment, and syncytia formation, typically observable after 3-7 days [66]. The adaptation of LSDV to embryonated chicken eggs via the chorioallantoic membrane (CAM) route is another classical isolation method, producing visible pock lesions [55]. The identification of murine osteoblastic MC3T3-E1 cells as a novel permissive cell line expands the repertoire of in vitro models for studying LSDV replication and pathogenesis [34]. However, virus isolation is time-consuming, requires specialized expertise, and must be conducted in high-containment facilities, limiting its use to reference laboratories.

Advanced Genomic and Metagenomic Approaches

Whole-genome sequencing (WGS) has revolutionized the molecular epidemiology of LSDV, providing unparalleled resolution for tracing transboundary spread, identifying recombination events, and monitoring the emergence of novel variants [6, 7, 23]. Next-generation sequencing (NGS) of skin nodule biopsies has revealed the dominance of clade 2.5 recombinant strains in Southeast Asia and has demonstrated that, despite ongoing circulation, no new mosaic variants have been observed in recent years, suggesting a stabilization of the circulating viral pool [7]. WGS has also been instrumental in identifying the co-circulation of genetically diverse strains in countries like Bangladesh, where isolates from the same outbreak clustered with both South Asian and African/Middle Eastern lineages, indicating multiple independent introductions [13]. Furthermore, metagenomic sequencing of pox lesions has unveiled the complex bacterial communities (e.g., Fusobacterium necrophorum, Trueperella pyogenes) that co-infect these sites, providing insights into the polymicrobial nature of severe LSD lesions [54]. The unexpected detection of LSDV reads in the human upper respiratory tract microbiome via metagenomics, while not indicating zoonotic potential, raises intriguing questions about environmental contamination and potential fomite transmission [33].

Diagnostic Considerations for Subclinical and Atypical Infections

A major challenge in LSDV control is the existence of subclinical infections, where animals exhibit viremia and viral shedding without overt skin nodules [30]. Such animals can transmit the virus to naïve cohorts via direct contact or through mechanical vectors like Stomoxys calcitrans [36]. Diagnostic approaches must therefore be sensitive enough to detect low-level viremia in the absence of clinical signs. Real-time PCR on blood samples is the method of choice for identifying subclinically infected carriers, as serological tests may be negative during the early viremic phase. The detection of LSDV in blood samples from subclinical animals, coupled with the demonstration of transmission to sentinel animals, underscores the critical need for molecular surveillance beyond clinically apparent cases [30, 36]. Additionally, the expanding host range of LSDV, including infections in yaks, Indian gazelles, and camels, necessitates the validation of diagnostic assays across different bovine and non-bovine species [12, 37, 42].

Vaccination Strategies and Disease Control

Effective vaccination remains the cornerstone of comprehensive lumpy skin disease (LSD) control programs, serving as the most powerful tool for mitigating the devastating economic and animal health impacts of LSD virus (LSDV) across endemic and emerging regions. The World Organization for Animal Health (WOAH) classifies LSD as a notifiable disease, and its rapid transboundary spread from Africa through the Middle East, Europe, and across Asia underscores the urgent need for robust, scientifically informed vaccination strategies. The biological complexity of LSDV, including its large double-stranded DNA genome encoding 156 open reading frames, its capacity for immune evasion, and the emergence of recombinant strains, presents profound challenges for vaccine development and deployment [1, 17]. However, recent advances in molecular virology, immunology, and vaccinology are illuminating new pathways toward safer, more efficacious, and more discriminatory vaccination approaches.

The Landscape of Live-Attenuated Vaccines: Homologous versus Heterologous Platforms

Historically, the control of LSDV has relied almost exclusively on live-attenuated vaccines (LAVs), which have demonstrated variable but often substantial efficacy in reducing clinical disease and curbing viral spread. Two principal categories of LAVs exist: homologous vaccines derived directly from attenuated LSDV strains, and heterologous vaccines based on closely related capripoxviruses, specifically sheeppox virus (SPPV) and goatpox virus (GTPV) [1, 4]. The biological rationale for heterologous vaccination stems from the high degree of antigenic similarity, up to 97% nucleotide sequence identity, among members of the Capripoxvirus genus, which can induce cross-protective immune responses [32]. This approach has been pragmatically adopted in numerous Asian countries, including China, where the attenuated GTPV AV41 vaccine has been widely administered to cattle to control LSD outbreaks [21, 57].

Despite their widespread use, the relative efficacy of heterologous versus homologous vaccines has been a subject of intense scrutiny. A comprehensive review of the available literature indicates that homologous live-attenuated vaccines prepared using LSDV, particularly those derived from the Neethling strain, are significantly more efficacious in protecting cattle against LSD compared to heterologous SPPV or GTPV-based vaccines [1]. This differential efficacy likely reflects subtle but important antigenic mismatches between capripoxvirus species, which may impact the breadth and durability of protective immunity. However, the superiority of homologous vaccines must be balanced against practical considerations, including production capacity, cost, and the risk of residual virulence or reversion to pathogenicity.

The Neethling strain vaccine, belonging to clade 1.1, has been the most extensively studied and deployed homologous LAV globally. Its historical success is exemplified by the eradication of LSDV from Europe through high-coverage vaccination campaigns employing attenuated Neethling vaccines [4]. This strain has established a robust safety and efficacy record, providing strong protection against severe clinical disease, viremia, and viral shedding. Crucially, recent evidence demonstrates that the attenuated LSDV Neethling strain vaccine retains a promising protective phenotype against currently circulating recombinant strains belonging to the emergent cluster 2.5 lineage, which is now dominant across Southeast Asia [38]. In a controlled challenge study, vaccinated animals exhibited only transient, mild febrile responses without clinical signs, local reactions, or vaccine-induced viremia, while control animals developed moderate to severe clinical disease with high levels of virus shedding [38]. These findings are of paramount importance for contemporary field vaccination programs, as they confirm that established vaccine platforms remain relevant despite ongoing viral evolution.

Nevertheless, the deployment of LAVs is not without significant risks and limitations. One of the most pressing concerns is the phenomenon of recombination between vaccine strains and circulating wild-type or recombinant field strains. The emergence of mosaic recombinant LSDV strains, particularly cluster 2.5, has been linked to the co-circulation of vaccine and field viruses in regions with incomplete vaccination coverage [7, 21]. In China, vaccination of cattle with the heterologous GTPV vaccine has been associated with the isolation of recombinant LSDV strains carrying genomic segments derived from both vaccine and field viruses [21]. Phylogenetic analyses have confirmed that all strains within the dominant Asian recombinant clade share a common set of 15 recombination events, with at least one event specifically involving GTPV sequences [21]. This underscores a critical biological reality: the widespread use of live vaccines, particularly in situations where herd immunity is insufficient to block viral transmission, can inadvertently drive the emergence of novel viral variants with unpredictable phenotypes, including altered virulence, host range, and transmissibility. The potential for subclinically infected, vaccinated animals to serve as reservoirs for recombination and onward transmission represents a fundamental challenge to current vaccination paradigms [30, 36].

Safety, Potency, and the Challenge of Vaccine-Induced Disease

The safety profile of LAVs, particularly in relation to adverse reactions and residual pathogenicity, remains a critical consideration for vaccination strategies. Some Neethling-based vaccine batches have been associated with transient local reactions, mild fever, and, in rare instances, the development of skin nodules at the injection site [38]. While these reactions are generally self-limiting and significantly less severe than natural disease, they can cause concern among livestock owners and may complicate efforts to achieve high vaccination coverage. More worryingly, the use of certain vaccine strains, such as those derived from the KSGP 0240 lineage, has been linked to more severe post-vaccinal reactions and even disease outbreaks in some settings [19]. The genetic instability of some LAV strains, particularly those that have undergone extensive passage in cell culture without rigorous clonal purification, can lead to the accumulation of mutations that may restore virulence or alter immunogenicity.

Accurate potency assessment is essential to ensure that each vaccine batch delivers a reliably protective dose. Historically, vaccine potency has been determined through tissue culture titration, which is labor-intensive, time-consuming, and requires significant technical expertise. A landmark study has validated the use of quantitative real-time PCR (qPCR) as a rapid, sensitive, and reliable surrogate for conventional potency testing of live attenuated LSDV vaccines [58]. By analyzing ten commercial vaccine batches, researchers demonstrated that qPCR-derived viral genome copy numbers showed a remarkably strong correlation with conventional tissue culture titers (Pearson’s r = -0.994; p < 0.0001) and, critically, with protection outcomes following virulent challenge in a calf model (r = 0.777; p = 0.008) [58]. Batches with molecular titers below a certain threshold consistently failed to protect animals, confirming that qPCR can serve as a biosafe, time-efficient, and objective tool for lot-release testing. The integration of molecular quantification into routine vaccine quality control can harmonize vaccine deployment across different manufacturing sites and countries, reduce reliance on live animal challenge testing, and ultimately enhance field protection.

The Imperative for DIVA: Differentiating Infected from Vaccinated Animals

A major limitation of current LAV-based control programs is the inability to serologically distinguish vaccinated animals from those that have been naturally infected with LSDV. This is because conventional serological assays, including ELISA and virus neutralization tests, detect antibodies against viral antigens that are shared between vaccine and field strains [32]. The inability to differentiate infected from vaccinated animals (DIVA) severely compromises surveillance efforts, outbreak investigations, and international trade, as it is impossible to determine whether seropositive animals have been exposed to virulent field virus or simply mounted a vaccine response. This challenge is particularly acute in areas where heterologous GTPV vaccines are used, as available serological tests cannot discriminate between antibodies induced by GTPV vaccination and those elicited by LSDV infection [32].

To address this critical gap, researchers have developed innovative DIVA strategies targeting viral antigens that are unique to LSDV and absent from vaccine strains. One promising approach involves the identification and recombinant expression of the LSDV103 protein, which was discovered through a phage display library screening of the entire LSDV proteome [57]. The truncated recombinant TrLSDV103 protein demonstrated significantly stronger reactivity with sera from LSDV-infected cattle compared to sera from GTPV-vaccinated cattle. An indirect ELISA based on TrLSDV103 achieved 100% diagnostic specificity (95% CI: 90.11–100) and 86.67% diagnostic sensitivity (95% CI: 70.32–94.69), with a detection limit of 1:6400 for positive serum and no cross-reactivity with five other common bovine pathogens [57]. Furthermore, TrLSDV103 induced robust humoral and cellular immune responses in mice, including significant increases in IgG titers (up to 1:204,800), IFN-γ, IL-1β, and the expansion of CD3+CD4+ T cells, highlighting its dual potential as both a DIVA antigen and a vaccine candidate [57].

Parallel work has advanced the development of a synthesized gene unique to LSDV as a differential antigen, yielding an in-house iELISA with similarly impressive performance characteristics [32]. This assay demonstrated 100% diagnostic specificity (95% CI: 88.43–100) and 93.3% diagnostic sensitivity (95% CI: 77.93–99.18) with no cross-reactivity against other bovine pathogens. In field testing of 200 clinical serum samples, the DIVA test correctly identified zero positive samples among 141 GTPV-vaccinated cattle, while detecting 33.9% positivity in naturally infected herds [32]. When applied to 59 infected and 171 vaccinated samples, the LSDV034-based competitive ELISA developed by the same group achieved 98.97% overall concordance with the gold-standard virus neutralization test [64]. These DIVA tools represent a transformative advance for LSD control programs, enabling for the first time a clear epidemiological picture of viral circulation in vaccinated populations and facilitating evidence-based decisions regarding vaccination cessation, outbreak investigations, and international trade certification.

Cellular Immunity and the Interferon-Gamma Release Assay as a Diagnostic Adjunct

The immune response to LSDV infection and vaccination is not solely humoral; cell-mediated immunity (CMI) plays a demonstrably critical role in protective immunity against poxviruses. Despite this, most diagnostic and monitoring efforts have historically focused on antibody detection, which can be suboptimal in sensitivity, slow to develop, and may be absent in some infected animals. The interferon-gamma release assay (IGRA) has emerged as a powerful tool for assessing CMI responses to LSDV [59]. In experimental studies, the CMI response to Neethling-based LAV was detectable in all animals by 10 days post-vaccination, significantly preceding the appearance of antibodies [59]. Following virulent infection, a uniform CMI response was already at plateau levels by day 7 post-infection, again preceding seroconversion [59].

The operational advantages of the IGRA are substantial. When optimized using whole blood and heat-inactivated antigen, the assay can be performed under BSL2 conditions, eliminating the need for high-containment facilities and expanding its accessibility for field and regional laboratories. Under optimized conditions, the LSDV IGRA demonstrated up to 100% specificity [59]. The ability to detect exposure within days rather than weeks makes the IGRA an invaluable tool for early outbreak detection, rapid implementation of containment measures, and monitoring of vaccine-induced CMI responses in real time. The incorporation of CMI assays into routine surveillance programs could significantly shorten the window between viral introduction and detection, a critical advantage given the explosive potential of LSD outbreaks amplified by vector-borne transmission.

New Horizons: Subunit, mRNA, and Reverse Vaccinology Approaches

The intrinsic limitations of LAVs, including safety concerns, the potential for recombination, cold-chain dependence, and the lack of DIVA capability, have spurred intensive research into next-generation vaccine platforms. Subunit vaccines, which deliver defined immunogenic proteins rather than whole live virus, offer the promise of enhanced safety, DIVA compatibility, and greater thermostability. The F13L protein, a component of the extracellular enveloped virion (EEV), has been identified as a particularly promising candidate for both serological detection and vaccine development. Recombinant F13L showed 85.7% and 75% positivity for naturally infected and vaccinated groups, respectively, with 100% sequence conservation between the Neethling-type strain and multiple Thai LSDV field isolates [10]. This conservation across geographically diverse strains suggests that F13L-based vaccines could provide broad protection.

Immunoinformatics and reverse vaccinology have accelerated the rational design of multi-epitope peptide vaccines. By computationally screening the entire LSDV proteome for B-cell and T-cell epitopes with optimal antigenicity, allergenicity, and toxicity profiles, researchers have developed candidate vaccine constructs incorporating membrane glycoprotein-derived epitopes linked with appropriate adjuvants [69]. The LSDVvac database, a comprehensive web-based platform, now integrates 3,913 unique B-cell epitopes and 6,473 unique T-cell epitopes derived from 73 LSDV strains, alongside three-dimensional structural predictions for all 156 LSDV proteins and tools for genome analysis [67]. These resources empower the scientific community to design and test peptide-based vaccines tailored to specific geographic variants.

The mRNA vaccine platform, which proved transformative during the COVID-19 pandemic, is now being explored for LSDV. A multi-epitope mRNA vaccine construct (LSDV-V2) designed through genome-scan vaccinomics and agent-based modeling demonstrated significant predicted binding affinity and stability with bovine immune receptors in molecular dynamics simulations [70]. The incorporation of Kozak sequences, MITD, tPA, and 3′ and 5′ UTRs alongside a poly(A) tail was designed to optimize translational efficiency and immunogenicity [70]. If validated experimentally, an mRNA-based LSDV vaccine could offer unparalleled advantages in production speed, scalability, and the ability to rapidly update antigenic composition in response to viral evolution.

Furthermore, the Cre-loxP recombination system has been established for the rapid genetic manipulation of LSDV, enabling the construction of targeted gene-deleted recombinant viruses that can serve as rationally attenuated vaccine candidates [68]. The successful deletion of the thymidine kinase (TK) gene, a classic attenuation strategy, has been achieved, and the platform can be extended to delete multiple immune evasion genes in a sequential manner [68]. For instance, deletion of the LSDV 001/156 gene, a virulence factor that suppresses interferon production by impairing IRF3 dimerization, resulted in a mutant virus with reduced replication and virulence in cattle while likely retaining immunogenicity [3]. Similarly, deletion of the ORF137 gene, which inhibits IFN-β signaling by reducing IRF3 phosphorylation, yielded a recombinant strain with attenuated replication in MDBK cells and a diminished capacity to suppress interferon-stimulated gene transcription [18]. These genetically defined deletion mutants represent a new generation of LAVs with enhanced safety profiles and potentially improved immunogenicity.

Integrated Disease Control: Vaccination within a Comprehensive Framework

Vaccination alone, regardless of the platform employed, cannot achieve sustainable LSD control in isolation. The biology of LSDV transmission, primarily mechanical via arthropod vectors, demands a multi-faceted control strategy that integrates vaccination with rigorous biosecurity, vector management, surveillance, and movement controls. Studies using geospatial modeling for risk-based surveillance have identified key risk factors for LSDV introduction and spread, including long-distance airborne movement of infected vectors, cattle movements across land borders, and environmental factors such as temperature, humidity, and vector population distributions [24]. In Australia, a country free of LSD but facing incursion risk, integrated geospatial models have identified ports in Western Australia and areas in Far North Queensland as having the highest suitability for LSDV-carrying vector entry via wind currents and shipping channels [40]. Such models can guide targeted surveillance and pre-emptive vaccination strategies in high-risk zones.

In endemic settings, the timing and coverage of vaccination campaigns are critical determinants of success. The Neethling-based LAV provided complete protection against a virulent recombinant challenge in a controlled study [38], but field effectiveness is contingent on achieving high and uniform herd immunity. The waning of passive maternal immunity in calves born to vaccinated dams represents a particularly vulnerable window. Longitudinal studies have demonstrated that maternal antibodies against LSDV persist for less than four months in calves, leaving them susceptible at an age when they are often not yet vaccinated [65]. Current recommendations for vaccination at six months of age may need to be reassessed to close this immunity gap and protect youngstock during their most vulnerable period [65].

Vector control remains an essential pillar of integrated LSD management. Experimental evidence has confirmed the susceptibility of multiple mosquito species, including Aedes aegypti, Culex tritaeniorhynchus, and Culex quinquefasciatus, to LSDV infection, dissemination, and transmission [15, 29]. Critically, the first demonstration of transovarial transmission of LSDV by Ae. aegypti suggests that mosquitoes can serve not only as mechanical vectors but also as potential biological reservoirs, with implications for viral persistence during inter-epidemic periods [15]. The stable fly *

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