Pathogenicity and virulence of lumpy skin disease virus: A comprehensive update

ABSTRACT

Lumpy skin disease (LSD), which was confined to the Africa for many decades, has expanded its geographical distribution to numerous countries across Asia and Europe in recent years. The LSD virus (LSDV) is a relatively poorly studied virus. Its 151 Kb genome encodes 156 open reading frames (ORF); however, the exact number of the proteins encoded by the viral genome and their specific functions remain largely unknown. Arthropod vectors primarily transmit the LSDV mechanically, but the precise nature of these vectors in different regions and their role in transmission is not fully understood. Homologous live-attenuated vaccines prepared using LSDV have proven to be highly efficacious compared to heterologous vaccines based on sheep pox virus or goatpox virus, in protecting cattle against LSD. This review offers the latest insights into the molecular biology and transmission of LSDV and discusses the safety and efficacy of available vaccines, along with the challenges faced in controlling and eradicating the disease in endemic regions.

Introduction

Lumpy skin disease (LSD) is a transboundary animal disease, characterized by the development of skin nodules over the body surfaces, fever, swollen lymph nodes, decreased milk yield and a temporary or permanent sterility in bulls. Mortality rates are generally low but can be exceptionally high in certain naive populations [1,2]. LSD has a severe impact on productivity and livestock economies, posing a threat to food security [3,4].

The disease was first reported in Zambia, Africa, in 1929. For many years, LSD was confined to Africa. Its first transcontinental spread occurred in 1988 in Egypt, followed by an outbreak in Israel in 1989 [5,6]. Since then the disease has spread throughout the Middle East [7], parts of Europe [8] and more recently, numerous Asian countries [9,10] (Figure 1).

Figure 1.

Global distribution of LSD.

Along with sheepox virus (SPV) and goatpox virus (GTPV), LSD virus (LSDV) is classified under the genus Capripoxvirus of the family Poxviridae. The LSDV genome is 151-kbp long and consists of a central coding region and identical 2.4 kbp-inverted terminal repeat on each side of the genome [11]. Capripoxviruses cross-react with each other and cannot be distinguished serologically [7]. Similar to SPV and GTPV strains [12], some LSDV strains are known to produce more severe disease than others. However, there is limited experimental evidence on strain-dependent LSDV virulence [2].

LSDV strains circulating worldwide are classified into three genetic lineages [13]. The classical LSDV strains, are divided into two clades: Clade 1.1, which is restricted to Africa, and Clade 1.2, which is more widely distributed across Africa, Europe, and Asia [13]. Additionally, vaccine-like recombinant LSDV strains form a third clade comprises of five subclades (2.1, 2.2, 2.3, 2.4 and 2.5) [13–15].

Cattle are the primary hosts for LSDV, while buffaloes can sometimes develop a milder form of the disease. The reasons remain unknown, but sheep and goats generally exhibit resistance to LSDV. Occasionally LSDV has been shown to infect camels [16], Gazelles [17], yak [18]. The virus is primarily transmitted by insect vectors, though direct close contact with infected animals and indirect transmission via contaminated equipment can also contribute to its spread [19–21].

Given the devastating impact of LSD on the livestock economy, its prevention and control in endemic areas are top priorities. Vaccination has proven to be one of the most effective measures for controlling outbreaks and preventing viral spread. Homologous live-attenuated vaccines have been particularly successful in protecting cattle from LSD [19,22–26]. Due to the cross reactivity of LSDV with SPV and GTPV [27,28], some countries have utilized heterologous vaccines (SPV and GTPV-based) in cattle upon initial outbreaks [29–33]. However, the quantity, quality and persistence of the immune responses induced by these heterologous vaccines is often weaker than the homologous vaccines [34–36], potentially resulting in incomplete protection against LSD in cattle [22,37]. Inactivated LSD vaccines have also been developed, offering greater safety than live-attenuated vaccines, but they provide only short-term immunity and are less effective [38].

The effectiveness of LSD vaccines can depend on factors such as the specific vaccine strain used and the immune status of the cattle population being vaccinated [39,40]. The antibody response to LSD vaccination is complex; not all vaccinated animals necessarily seroconvert, yet they may still be fully protected from the disease [19,41,42]. Achieving disease-free status after a successful mass vaccination campaign requires the ability to differentiate between infected and vaccinated animals (DIVA) [43,44], which remains challenging for most vaccines, except for a recently licenced LSD vaccine based on the Ranchi LSDV strain [22,45]. This review explores the latest insights into the molecular biology and pathogenesis of LSDV and highlights challenges in its prevention and control of the disease using vaccination.

Genome structure

LSDV belongs to the genus Capripoxvirus of the family Poxviridae. The capripoxviruses are highly conserved, sharing about 97% nucleotide similarity. LSDV is thought to have emerged roughly 500 years ago through recombination events [46,47]. However, its first official documentation occurred in Zambia in 1929. The LSDV genome is a linear, double-stranded DNA molecule approximately 151 kbp in size (Figure 2). It has a 73% AT content and encodes 156 open reading frames (ORFs) annotated as putative genes.

Figure 2.

Genome organization. Capripoxvirus genome is organized into a central coding region which is relatively conserved and comprises of ORF024 to ORF123. It codes for viral genes related with DNA replication and nucleotide metabolism, RNA transcription and modification and virus structure and assembly. The central conserved region is flanked by variable inverted terminal repeats (ITRs) regions of about 2.4 kbp which are repeated in an inverted orientation at both ends of the genome. The terminal genomic region, comprises ORF001-ORF023 and OR124 to ORF156, respectively at 5’ ad 3’ end of the genome is highly variable and mostly code for virulence and host range genes.

The genome is organized into a central conserved region flanked by variable inverted terminal repeat (ITR) regions, each measuring 2.4 kbp. These ITRs consist of DNA sequences repeated in an inverted orientation at both genome ends. The central conserved region of LSDV genes shows a high degree of collinearity and amino acid sequence similarity (averaging 65%) with genes of other known mammalian poxviruses. This conservation indicates that these genes play critical roles in the virus life cycle, including mRNA biogenesis, DNA replication, virion structure, and assembly, which are under strong evolutionary pressure to maintain their function [11]. LSDV contains at least 26 conserved poxviral genes involved in DNA replication, viral mRNA transcription, and post-transcriptional RNA processing [41]. Additionally, it includes over 30 poxviral homologue proteins involved in the virion core, intracellular mature virus (IMV) formation, and potential enzymes related to protein modification and DNA packaging. In contrast, the ITR regions exhibit lower sequence similarity (averaging 43%), disrupted collinearity, and limited or absent poxviral homologues. These ITR regions often contribute to virulence and host adaptation, displaying a greater tendency towards genetic variation and possibly giving rise to new viral strains with altered pathogenicity.

LSDV is closely related to leporipoxviruses, yatapoxviruses, GTPV, and SPV [43]. The capripoxviruses share a common ancestor and have evolved to adapt to their respective hosts [43]. Genetic analysis of LSDV isolates from 1954 to 2018 suggests a substitution rate of 7.4  ×  10^{− 6} substitutions/site/year [47]. Capripoxvirus evolution is influenced by various factors, including host immune pressure, genetic drift, and recombination events. Major mutations in attenuated Capripoxvirus strains are predominantly located in the variable terminal regions, which encode virulence and host range genes such as ankyrin repeat and kelch-like proteins [48]. One limitation of most live-attenuated vaccines is the induction of the Neethling response, characterized by local site reactions, generalized micronodules, fever, and a temporary reduction in milk yield [49,50]. Notably, the Indian LSDV vaccine strain (Ranchi), which has a unique 801-nucleotide deletion in its variable terminal regions [22,23], does not induce the Neethling response, suggesting a potential link with virulence attenuation.

Host-range genes

Like other poxviruses, LSDV possesses a variety of potential host-range genes that likely influence tissue tropism and play a role in modulating or evading host immune responses [51]. The host range of capripoxviruses is determined by specific viral genes that interact with host cell receptors [52–54]. These genes are often located in the variable regions of the genome and may differ significantly between different Capripoxvirus species [48]. Viral ankyrin-repeat proteins, for instance, may determine host range by interacting with host proteins involved in immune signalling and apoptosis [55].

Capripoxviruses have developed several mechanisms to adapt to their specific hosts [52–54]. These adaptations are often mediated by changes in the viral genome. For example, SPV063 from SPV exhibits a specific defect in murine cells due to particular amino acid residues. It was demonstrated that substituting residues 134 and 135 can restore its functionality in both human and murine cells, highlighting the role of these residues in host range determination [56,57].

Poxviruses have been known to enhance their immune evasion capabilities by acquiring host-derived genes through recombination. A notable example is the vaccinia virus, which encodes a viral growth factor similar to human growth factors. This similarity is believed to result from the virus capturing a host gene, thereby aiding in evading the host’s immune response [58]. However, studies documenting the acquisition of host genes by capripoxviruses are currently lacking [59].

Interestingly, LSDV encodes a unique complement of genes related to its specific host range properties. Based on homology analysis with other poxviral proteins, six LSDV proteins, including IL-10, IL-1 R, IFN-α/β binding protein, IFN-γ receptor, and IL-18 binding protein, are predicted to be involved in disrupting or modulating host immune responses [11,60].

LSDV also encodes two IL-1 R-like proteins (LSDV013 and LSDV006), alongside four potentially membrane-localized immunomodulatory proteins. Like gammaherpesvirus proteins, LSDV010 may play a role in immune evasion [11]. LSDV014 and LSDV034, homologous to vaccinia virus PKR inhibitors E3L and K3L, likely modulate immune responses by targeting the host PKR pathway. LSDV014 May function like E3L, binding dsRNA to prevent PKR activation, thereby blocking eIF2α phosphorylation and maintaining viral protein synthesis. LSDV034 likely mimics K3L, acting as a pseudosubstrate to inhibit PKR activity [11]. These mechanisms help LSDV evade interferon-induced antiviral responses, similar to vaccinia virus, ensuring efficient viral replication and immune evasion [11,61,62]. Poxviruses, including LSDV, can encode one or more serine proteinase inhibitors (serpins). LSDV has a single serpin (LSDV149) with potential anti-inflammatory functions [11].

The virus also encodes various poxviral homologue proteins such as EGF, N1L, VV C7L, A14.5 L, MYX M004/M011L, and N1R (p28), which are implicated in virulence and cell-specific virus replication [11]. LSDV encodes at least five proteins with ankyrin repeat motifs, two of which (LSDV145 and LSDV147) appear to be orthologues of proteins encoded by leporipoxviruses and SPV. These poxviral ankyrin repeat genes may inhibit virus-induced apoptosis and regulate host range functions [63,64].

Certain virulence and/or host range proteins found in other poxviruses are absent in LSDV. These include tumour necrosis factor receptor homologue (leporipoxviruses and orthopoxviruses), MDA-7 cytokine-like protein (Yaba-like DV), the 35-kDa secreted chemokine binding protein (leporipoxviruses and orthopoxviruses), MHC-I-like proteins (Yaba-like DV, SPV, and MCV), glutathione peroxidase (MCV and FPV), hydroxysteroid dehydrogenase (Yaba-like DV), semaphorin-like protein (orthopoxviruses, FPV), lysophospholipase (Yaba-like DV and orthopoxviruses), CPD photolyase (leporipoxviruses and FPV), and sialyltransferase (leporipoxviruses) [11].

A 56 bp element of an early/late bi-directional promoter element in LSDV exhibits structural similarities with promoters in other poxviruses, further indicating the conservation of transcriptional elements within the Poxviridae family. This suggests that identified LSDV promoters could be used to express foreign genes in a recombinant LSDV system [65].

Genetic lineages of LSDV

Current nucleotide sequences categorize LSDV into three major phylogenetic lineages (clades). Clade 1.1 includes classical Neethling-type field strains, predominantly found in Africa. Clade 1.2 comprises Kenyan/KSGP-type field strains widely distributed across Asia, Europe, and Africa. Most recombinant vaccine-like LSDV strains fall into Clade 2.5, primarily circulating in Southeast Asia, including Russia, China, Vietnam, Thailand, and Taiwan. The minor population of clade 2.1, 2.2, 2.3, and 2.4 strains are found in Russia and Kazakhstan. Initially, it was thought that these recombinant strains emerged naturally in the field [14]. However, later it was confirmed that this was the result of the vaccine spillover and generated due to mishandling of the multiple LSDV strains in the cell culture in the laboratory in Kazakhstan [15].

Homologous recombination between the Neethling vaccine strain and the Kenyan KSGP strain occurred during cell culture for Lumpivax vaccine development. Vaccination in Kazakhstan subsequently facilitated natural transmission to regions such as Russia, China, Thailand, and Mongolia [66–68]. Nevertheless, natural recombination events in capripoxviruses have never been demonstrated. Irrespective of the nature of the LSDV strain involved, homologous live-attenuated LSD vaccines prepared using a specific LSDV strain, provide complete protection against other LSDV strains, including recombinant strains, in cattle [19,22–24].

Understanding the evolutionary dynamics of Capripoxviruses is critical for predicting and managing future outbreaks. Continued comparative genomics and phylogenetic studies can help identify emerging strains with enhanced virulence or altered host specificity, informing targeted control measures.

Replication and gene expression

Unlike other DNA viruses, which replicate in the nucleus, poxviruses replicate in the cytoplasm of the infected host cells [69]. The replication cycle of poxvirus involves early gene expression, DNA replication, intermediate gene expression, late gene expression, and virion assembly and release [70]. Early genes are transcribed immediately after the virus enters the host cell. These genes encode proteins that are involved in viral DNA replication, transcriptional regulation, and host immune evasion [71]. Many early gene products modulate the host cell environment to favour viral replication, often by inhibiting host antiviral responses [72]. For example, superoxide dismutase (SOD) homologue in LSDV influences host transcriptional activity, apoptosis, and histopathology [73]. Recombinant LSDV vaccines lacking the SOD gene showed altered induction and inhibition of apoptosis, suggesting the SOD homologue’s role in modulating immune responses [73]. Similarly, the early expressed LSDV genes LSDV005, LSDV008, and LSDV142, encoding a putative RNA polymerase, Kelch-like protein, and Ankyrin-repeat domain protein, respectively, play vital roles in transcription, immune evasion, and host interactions. These genes are crucial for establishing infection before viral DNA replication begins [72]. The LSDV with these knocked-out genes revealed attenuation in vivo and provided protection to cattle from infection [74].

Following early gene expression, the viral DNA is replicated [75]. Capripoxviruses use a rolling-circle mechanism of replication, similar to other poxviruses [72]. The replication process involves the formation of concatemeric DNA, which is then resolved into individual genomes that are packaged into new virions. Intermediate genes are expressed after the initiation of DNA replication. These genes typically encode factors required for late gene expression and virion assembly [76]. Late genes are expressed just before the assembly of new virions and encode structural proteins and enzymes necessary for the formation of infectious virus particles [77].

Transmission

Despite a relatively narrow host range, LSDV continues to spread into new territories. This unprecedented spread of LSDV in the recent decades has necessitated further investigation into the transmission mechanisms of LSDV [1]. Most poxviruses are commonly transmitted by direct contact, although other transmission routes such as ingestion, parenteral inoculation, mechanical vector transmission, fomites, droplets or aerosols are also possible. The route of poxvirus transmission may even vary within the genus. For example, the SPV and GTPV are predominantly transmitted via aerosols, whereas the most commonly known mechanism of LSDV transmission is mechanical, via the arthropod vector, by biting to their hosts. Likewise, SPV and GTPV but not LSDV were shown to be transmitted to animals through flea infestation [78].

Some researchers have suggested that direct contact transmission or indirect (vector-free) contact are either ineffective or significantly less effective routes for LSDV transmission [1,79,80]. However, association of the occurrence of LSD with communal cattle grazing and watering points suggests that a vector free transmission is also possible [1,7,51,80]. The most LSD outbreaks usually occur during hot and humid climates (summer rainy season), when potential vector species numbers are high [1,81,82]. This pattern strongly suggests that vector-borne transmission plays a major role in the spread of the disease. The majority of the transmission studies have been conducted by involving arthropod species (Rhipicephalus appendiculatus ticks) [83] that are restricted to the southern hemisphere. However, LSD outbreaks have also been reported outside the warmer summer period under certain conditions [84,85]. This raises the possibility of a non-vector-borne transmission route [1,80]. Climate change or shifts in vector distribution may contribute to this pattern, but further research is needed to confirm these factors.

Infected animals can excrete the virus through various body excretion and secretions such as nasal and ocular discharge, faeces, semen, and milk, contributing to the spread of the disease [1,86,87]. Transmission can also occur indirectly through contaminated fomites, such as equipment, bedding, and feed, though this is less common than vector-borne transmission [88].

Multiple blood-sucking arthropods, such as biting flies, stable flies, midges, horse flies, and ticks, have been suggested to play a potential role in the transmission of LSDV between cattle [89]. Mechanical transmission occurs when these vectors feed on the blood of an infected animal and then transfer the virus to a susceptible host through subsequent bites [84,85]. The virus does not seem to replicate within the insect vector, but it can survive on the mouthparts of the vector long enough to be transmitted to another host. This notion is supported by experimental evidence obtained with mosquitoes (Aedes aegypti and Anopheles stephensi) [79,83], biting midge (Culex quinquefasciatus) [79,83,90], Ixodid ticks [Amblyomma hebraeum, Rhipicephalus appendiculatus and Rhipicephalus (Boophilus) decoloratus] [21,82,89,91,92], biting flies (Stomoxys calcitrans) [20,93,94] and horse flies (Haematopota spp.) [20]. In addition, mechanical transmission [21] or intrastadial transmission by R. appendiculatus and Amblyomma hebraeum ticks has also been reported [89]. Transstadial transmission by A. hebraeum adults, moulted from nymphs fed on experimentally infected cattle is also possible [95].

LSDV transmission by Stomoxys calcitrans is relatively well studied by experimental infections wherein naïve cattle were shown to develop typical LSDV skin nodules, viraemia, and seroconversion following exposure to S. calcitrans, which had fed upon donor bulls that had developed skin nodules after experimental inoculation [20,25,93,94]. Likewise, LSDV remained detectable until 14 dpi in Aedes aegypti, Culex tritaeniorhynchus, and Culex quinquefasciatus following artificial membrane feeding of LSDV on sheep blood. This suggests that these may act as vectors during the LSDV outbreak; their involvement may extend beyond being solely mechanical [90].

Experimental infection

The outcome of experimental LSDV infection greatly varies, all the LSDV-infected animals may not necessarily develop clinical disease and may remain uninfected or else develop subclinical infection [20,80,85,96]. In a study by Carn and Kitching, cattle were infected with the Neethling strain of LSDV by different routes. The cattle inoculated via the conjunctival sac did not show any clinical signs and did not seroconvert [80]. The intradermal inoculation produced local lesions in most of the animals but generalized infection in only 16% of the infected animals [80]. However, over 72% of the intravenously infected animals developed generalized skin nodules, which suggests that naturally occurring cases of LSD with generalized skin nodules may be the result of intravenously feeding arthropods. The intraepidermal inoculation resulted in the development of clinical disease in only 40% to 50% of animals. The animals that were housed with infected animals for 1 month did not develop the disease and did not seroconvert [80]. Moreover, these animals were fully susceptible to challenge infection [80]. A localized swelling at the site of inoculation after 4–7 days and enlargement of the regional lymph nodes develop after subcutaneous or intradermal inoculation of cattle with LSDV [97]. Therefore, it was concluded that LSDV transmission between animals by contagion is a rare event and that parenteral LSDV inoculation may be essential for a productive infection [80].

Susceptibility of laboratory animals to the LSDV

Mice, hamsters, guinea pigs, and rabbits have been tested for their sensitivity to LSDV. The intradermal inoculation of LSDV produced local skin nodules in rabbits and hamsters [98]. The virus could be isolated from the local skin nodules, besides detecting anti-capripoxvirus antibodies, starting from 10 days after infection in mice, 27 days in rabbits, and 14 days in hamsters. However, these laboratory animals did not develop generalized skin nodules, like those seen in the natural host (cattle) [98].

Transmission of LSDV from subclinically infected animals

Typically, LSD is characterized by characteristic skin nodules and lesions on mucosal surfaces. The LSDV infection may range from the development of generalized skin nodules to a subclinical or asymptomatic infection. The subclinical infection of LSDV is characterized by viraemia, shedding of the virus through nasal and ocular discharges, and enlarged lymph nodes but without any skin nodules. Subclinically LSDV-affected cattle shed the virus via nasal, ocular and oral excretion, whereas clinically affected animals shed more virus via the sequestrated skin nodules [99]. The role of animals, with subclinical infection, in epidemiology and transmission of LSDV is just beginning to be appreciated. In a modelling study [85], cattle with subclinical infection was not shown to play a significant role in epidemiology and transmission of LSD, as compared to the cattle with skin lesions, potentially due to a low amount of virus acquired by the feeding vectors [85]. However, later Haegman et al., showed successful LSDV transmission by Stomoxys calcitrans flies from subclinical donors to the naïve cattle, demonstrated proof of productive virus replication but without the formation of skin nodules [84].

In a study by Aerts et al., over 67% of the skin biopsies from subclinically LSDV-infected animals were found to be positive for viral genome by PCR [100], suggesting their role in transmission of the disease. Although the precise mechanisms remain unknown, the susceptibility of LSD infection, in part, may depend on the genetic makeup of the host (breed effect), nature of the vector, and pre-existing immunity [43].

Factors influencing virus spread

Several factors influence the spread of LSDV in cattle populations. Environmental conditions, such as temperature, humidity, and rainfall, significantly affect vector abundance and activity, thereby impacting virus transmission rates [101]. Outbreaks of LSD are often associated with periods of high vector activity, such as during the rainy season, when the populations of biting flies and mosquitoes increase [3,102].

Human activities, including livestock movement for trade, grazing, or cultural practices, are also major contributors to LSDV spread. In regions where cattle are frequently transported across borders or between areas, the risk of introducing the virus into new locations is high [1,103]. This has been observed in multiple outbreaks where the introduction of LSDV into previously unaffected areas was linked to the movement of infected cattle [104].

Management practices, such as the use of communal grazing areas, inadequate vector control, and poor biosecurity measures, can further exacerbate the spread of LSDV [105]. Infected animals introduced into a herd or grazing area can rapidly transmit the virus to other susceptible animals, leading to outbreaks [103]. Conversely, implementing strict biosecurity measures such as isolating infected animals, controlling vectors, and minimizing cattle movement can significantly reduce LSDV transmission risks [1].

Multiple transmission modes may contribute to LSD outbreaks, with the dominant mode depending on specific environmental and biological factors present before the outbreak [106]. This underscores the need for further research into LSDV transmission routes, particularly in non-endemic countries, to develop more effective control measures. Currently, combating LSD relies heavily on vector control, vaccination, and restricting cattle movement, while sources like contaminated feed, water, and litter are often overlooked but may require consideration. Effective control strategies must therefore be multifaceted, addressing both vector management and biosecurity practices while considering the ecological and social contexts in which cattle are raised.

Persistence of LSDV

In endemic regions, LSDV can persist in the environment for extended periods, especially under cooler and wetter conditions, contributing to ongoing transmission cycles even in the absence of vectors. LSDV infection does not result in a carrier state [84]. Additionally, wildlife may also appear to play a limited role in the epidemiology of disease [107]. Recent studies have identified transstadial persistence of LSDV in Rhipicephalus appendiculatus and Amblyomma hebraeum ticks, as well as transovarial viral persistence in Rhipicephalus decoloratus, R. appendiculatus, and A. hebraeum ticks [95]. A. hebraeum and R. appendiculatus overwinter on the ground as engorged nymphs/unfed adults, while R. decoloratus overwinters as engorged females. One study demonstrated transstadial and transovarial persistence of LSDV in A. hebraeum nymphs and R. decoloratus females exposed to cold temperatures of 5°C at night and 20°C during the day for two months, suggesting possible overwintering of the virus in these tick species [108].

Tuppurainen et al. placed laboratory-bred, uninfected R. decoloratus larvae to feed on experimentally infected “donor” cattle, demonstrating transovarial transmission of LSDV by R. decoloratus ticks in cattle and highlighting a vertical mode of transmission [21,109].

LSDV has also been detected in the eggs and larvae hatched from A. hebraeum and R. (Boophilus) decoloratus eggs that fed on infected cattle [89,109]. Similarly, Stomoxys calcitrans was shown to excrete LSDV through regurgitation and defaecation, with viral persistence for at least three days [110]. The transovarial passage of LSDV in female ticks suggests the potential role of A. hebraeum, R. appendiculatus, and R. decoloratus as reservoir hosts. Mosquitoes (Culicidae) and biting midges (Culicoides spp.) can retain LSDV for 7–10 days after feeding on virus-spiked blood [111], highlighting the role of arthropod vectors in virus transmission and maintenance.

Transmission of recombinant LSDV

Emerging evidence suggests that recombinant LSDV strains (Clade 2.5), such as Saratov/2017, can be efficiently transmitted through direct and indirect contact without vector involvement, unlike Clade 1.1 (Neethling strains) and Clade 1.2 (Kenyan-type strains) viruses [1,106,109,112–114]. However, the genetic elements responsible for insect-free transmission are not well understood [106]. This potential transmission risk requires further epidemiological investigation.

Subclinically infected cattle with recombinant vaccine-like strain Udmurtiya/2019 have shown the ability to transmit the disease to naïve cattle in insect-proof shared spaces, with 40% of exposed animals developing clinical symptoms [115]. This highlights the need for more research into the role of subclinical carriers in LSDV transmission to improve disease control measures.

Venereal transmission

LSDV is excreted in milk and semen in the naturally infected cattle. Following experimental infection, LSDV can be isolated from semen for up to six weeks post-infection, while viral DNA can be detected for up to five months [116]. High infection doses may lead to persistent infection in the testis and epididymis, with prolonged viral shedding [117]. The World Organization of Animal Health (WOAH) recommends that semen used for breeding should test negative for the LSDV genome by PCR, show no clinical signs of LSD, and originate from LSDV-free regions for at least 28 days prior to collection [118].

Transmission of vaccine virus

Experiments with various field strains of LSDV have failed to show contact transmission [99,116,119]. While live-attenuated vaccine strains were thought to be incapable of transmission, recent evidence suggests otherwise [14]. Although some animals vaccinated with live-attenuated vaccines may excrete the virus in milk [51,120], there is no evidence of live-attenuated LSDV being excreted in semen [22,121].

Immune response

The immune response to LSDV in cattle involves a coordinated action of the innate and adaptive immune systems to control infection. This response varies based on host factors, the immune status of the animal, and the virulence of the LSDV strain. Most animals that recover from clinical disease develop long-lasting cell-mediated immunity. Calves born to infected cows acquire maternal antibodies, offering protection against clinical disease for 3–4 months.

Innate immune response

Similar to responses against other pathogens, the immune response to Capripoxvirus infection involves both innate and adaptive immunity, which play crucial roles in virus elimination [122]. The innate immune response, which occurs soon after infection, includes anatomical, endocytic, physiological, and inflammatory barriers, forming the first line of defence against the pathogen [123]. This response is mediated by the interaction between pattern recognition receptors (PRRs) and pathogen-associated molecular patterns (PAMPs). PRRs are expressed by macrophages, lymphoid cells, neutrophils, natural killer (NK) cells, dendritic cells, basophils, mast cells, and eosinophils [122]. In a study by Chibssa, among thirteen PRRs evaluated, only RIG-I expression levels were found to increase following SPV infection, indicating that host sensing of SPV RNA intermediates is predominantly through RLRs rather than TLRs [124]. However, the specific role of these cells in Capripoxvirus infection remains unclear.

Humoral immune response

Humoral immunity plays a significant role in conferring protection against reinfection by poxviruses, while virus clearance involves both humoral and cell-mediated immune responses [12,29,123]. B cells, the primary component of the humoral immunity, recognize antigens and produce virus-specific antibodies. Additionally, B cells can serve as antigen-presenting cells (APCs) [125–127]. While virus-specific antibodies offer protection, only 30%–80% of vaccinated cattle show detectable antibody levels after LSD vaccination [42,128,129]. Cattle can still be protected against LSD even without detectable antibody levels [130]. Although circulating antibodies may limit virus spread, they might not restrict virus replication at the infection site [130].

Berguido et al. observed that LSDV and SPV sera effectively neutralized their homologous viruses, while GTPV sera neutralized both GTPV and SPV. However, while LSDV sera could neutralize SPV, SPV and GTPV sera did not effectively neutralize LSDV [131]. The specific viral antigens or epitopes involved in the humoral response to LSDV in cattle remain unknown, as do the mechanisms behind inability of some animals to generate antibodies.

The LSD vaccination of buffaloes is also considered safe [132,133], with both homologous and heterologous [132,133] LSD vaccines inducing an antibody response. However, compared to cattle, the detectable antibody levels in buffaloes appear later in the serum [132].

Cell-mediated immune response

Certain animals resist infection despite the absence of detectable anti-LSDV antibodies, indicating the importance of cell-mediated immunity (CMI) in protection against LSDV [129,134]. T cells, produced in the bone marrow and maturing in the thymus, are essential for the CMI response to poxviruses. Unlike B cells, T cells require APCs to recognize specific antigens [135]. T cells differentiate into poxvirus-specific cytotoxic T cells (CD8 + ) and helper T cells (CD4 + ). However, their exact roles in Capripoxvirus protection are not fully understood.

The cell-mediated response to LSDV is poorly characterized. Initial attempts measured delayed-type hypersensitivity (DTH) reactions to LSDV antigen [127,136]. DTH primarily involves T lymphocytes but may also include neutrophils, monocytes, and eosinophils [127]. The DTH reaction to LSDV involves a local inflammatory response that recruits T cells to the infection site, leading to cytokine release, inflammation, cell death, and tissue damage [127]. Positive DTH reactions, indicated by increased skin thickness at the injection site, can detect CMI responses to LSDV, similar to testing for Mycobacterium bovis infection in cattle. Carn and Kitching measured DTH reactions by intradermal LSDV inoculation in cattle recovered from primary infection, noting reactions within 24 hours that persisted for 1–2 weeks [136]. However, this study lacked quantitative data. DTH responses were also examined in relation to LSDV vaccines, showing that subcutaneous vaccination produced stronger reactions than intradermal vaccination [137]. Later, CMI responses were assessed using IFN-γ release assays (IGRAs), measuring IFN-γ production in PBMCs stimulated with LSDV antigen [138]. Kara et al. generated knockout viruses (LSDV_WB008KO) with disrupted IL-10 and IFN-γ receptor-like genes. Following vaccination and challenge with virulent LSDV at 21 days, no clinical signs of LSD appeared. CMI responses in PBMCs, analysed through ELISpot and intracellular cytokine staining, showed increased re-stimulation responses [139]. Intracellular cytokine staining further revealed elevated IFN-γ production by CD4 + and CD8 + T cells in response to antigen stimulation [139,140]. Hageman et al. (2021) investigated CMI responses in cattle inoculated with different LSDV vaccines and then challenged with virulent LSDV, observing varying IFN-γ responses across vaccinated groups [25]. However, the specific PBMC populations responsible for IFN-γ production were not identified.

In a recent study, Kumar et al. observed increased CD8 + T cell counts, but no significant changes in CD4 + T cell counts, upon antigenic stimulation of sensitized PBMCs from cattle recovered from natural LSDV infection. Stimulated PBMCs from LSDV-sensitized cattle effectively cleared LSDV infection compared to unsensitized cells [22], suggesting a critical role for CD8 + T cells in LSDV clearance [141–146].

In conclusion, these studies highlight a detectable CMI response in cattle to LSDV following vaccination and challenge infection [147–150]. Both CD4 + and CD8 + T cells appear involved in IFN-γ production. However, the kinetics and magnitude of CMI responses remain unclear. Differences, if any, in CMI responses between clinical and non-clinical LSD cases, as well as the duration of the response and the specific PBMC subpopulations involved in IFN-γ production, are still unknown. The impact of different LSDV inoculation routes on CMI also requires further investigation.

Quality, quantity and persistence of immune response following LSD vaccination

Poxviruses induce both antibody and cell-mediated immune response [151]. Cattle develop anti-LSDV neutralizing antibodies, both following vaccination and after acquiring the natural infection. A full protective immunity usually develops at 2–3 weeks post-vaccination [19]. Antibodies can appear as early as 15 days post-vaccination, with peak titres reached between 30 and 40 days [22]. Interestingly, susceptibility to virulent LSDV infection does not always correlate with neutralizing antibody levels, as animals that do not seroconvert after vaccination can still be fully protected against virulent LSDV [19,41,42]. Thus, anti-LSDV antibodies are not reliable as correlates of protection. In cases where anti-LSDV antibodies are undetectable, cell-mediated immunity plays a crucial protective role. During serosurveillance, antibody-positive cattle are considered protected; however, antibody-negative animals may or may not be protected. In such cases, CMI responses is essential for deciding whether to vaccinate seronegative animals. The persistence of antibodies following vaccination or natural infection varies among individuals. More data on seroconversion and antibody persistence are needed to provide guidelines and tools for countries conducting serosurveillance during or after an outbreak or for those monitoring vaccination efficacy through serological studies.

LSDV exposure is believed to induce long-lasting immunity in cattle. However, the precise duration of protective immune responses following vaccination or natural infection remains unclear. Determining the persistence of these responses requires maintaining large numbers of vaccinated cattle under controlled conditions, followed by challenge infections with virulent virus at regular intervals, which is a complex and costly endeavour. In a recent controlled study, a homologous live-attenuated LSD vaccine was shown to offer protection for at least 12–18 months, leading to the recommendation of annual revaccination [152], In contrast, inactivated LSD vaccines provided protection for up to six months [152]. For heterologous vaccines, it is recommended that animals receive a revaccination at 4–6 months after the primary dose, followed by regular booster vaccinations [153].

Maternal immunity

Following vaccination with a homologous live-attenuated vaccine, antibody persistence in calves after colostrum intake varies from 3 to 6 months [12,42,154–156]. In a study by Agianniotaki, the neutralizing antibody titres of vaccinated cattle against LSDV were measured in colostrum and serum samples, with colostrum antibody titres above 1:160 correlating with detectable antibodies in calf serum 3 days after birth. Only calves with high neutralizing antibody titres on day 3 retained detectable serum antibodies up to 90 days after birth [154]. However, many calves were not protected by maternal antibodies beyond two months of age. These findings can help optimize recommendations for the timing of initial vaccination in calves to enhance disease control measures [154].

With heterologous LSD vaccines, it was observed that passively transferred antibodies persist at protective levels for two, three, or four months in calves born to cows vaccinated during ≤ 4^th, 4.5–6^th, or > 6–8^th months of pregnancy, respectively. While neutralizing antibody titres were comparable in pregnant cows vaccinated at different stages of gestation, the persistence of maternally derived antibodies depended on the timing of the vaccination of dam during pregnancy [157].

Host susceptibility

LSD primarily affects cattle, with domestic buffaloes generally considered relatively resistant to the LSDV than cattle, often exhibiting milder clinical symptoms [3,102,103]. For reasons that remain unclear, sheep and goats generally do not exhibit susceptibility to LSDV infection [1,158]. The susceptibility of cattle to LSDV is influenced by various factors, including breed, age, and immune status [159]. Younger animals tend to be more vulnerable to infection and often develop more severe clinical signs compared to older cattle [103]. A significant number of cattle exposed to natural or experimental infection do not exhibit lesions or only show reactions at the site of inoculation [147]. Antibodies to LSDV have been detected in wild ruminant species such as impala, waterbuck, reedbuck, giraffe, kudu, blue and black wildebeest, eland, springbok, camel, buffalo, and others in Africa [160]. However, aside from African buffalo living near endemic regions [161], seroprevalence in wildlife is relatively low, ranging from 10% to 27%. Clinical disease has been observed only in captive Arabian oryx (Oryx leucoryx) [162], yak [18], camel [16], Mithun [163] and in experimentally infected buffalo, giraffes and impalas [164]. The potential for wildlife to affect vector abundance and survival may be relevant for LSDV transmission. Despite numerous studies on LSDV spread and maintenance in wildlife [43,165–168], their precise role in the epidemiology and transmission of LSDV remains unclear and requires further investigation.

Breed susceptibility

Studies of LSD outbreaks in Africa have shown that some indigenous cattle breeds are at lower risk for LSD compared to exotic or crossbred cattle [159,169,170]. For example, Khalafalla et al. observed in Khartoum State, Sudan, that purebred Friesian cattle experienced more severe disease, with morbidity and mortality rates of 37.9% and 4.2%, respectively, compared to indigenous cattle, which exhibited milder symptoms. Bos indicus (zebu) cattle generally demonstrate greater resistance to LSDV than Bos taurus (European) breeds, which are more susceptible to severe disease [18]. In Ethiopia, seroprevalence of LSD exposure was found to be significantly higher in crossbred than local zebu cattle [169]. A study conducted by our group during the 2022 LSD outbreak in India found the case fatality rate in the indigenous Rathi breed to be significantly lower (27.27%) compared to crossbred cattle (69.51%) [2]. Genetic factors, combined with environmental and management conditions, comorbidities, and vector susceptibility, play a crucial role in determining the susceptibility of cattle to LSDV. Indigenous breeds may have an additional advantage from prolonged natural selection for resistance to local diseases, though this may not apply to native African (Kenyan) breeds, as LSD was first reported in Kenya in the 1950s, not long after the introduction of exotic breeds [171,172]. In India, where LSD was first reported in 2019 [173] the crossbreeding programme began in 1910. Indigenous breeds may also undergo subclinical infection, necessitating further field or experimental studies. Our group analysed potential mechanisms for differing susceptibility to LSDV infection by studying Rathi and crossbred cattle that recovered from natural LSDV infection ( ~ 6 months prior) with a virus-neutralizing antibody titre of ≥ 32 [2]. Compared to crossbred cattle, upon antigenic stimulation, PBMCs from sensitized Rathi animals exhibited higher levels of IFN-γ, increased CD8 + T cell counts, and significant lymphoproliferative responses, suggesting Rathi cattle mount a stronger cell-mediated immune response, which may confer increased resistance to LSD [2].

Pathogenesis

Understanding the biological process from viral entry into the host to infection outcomes, including mechanisms by which the virus interacts with the host immune system, disseminates, and causes disease, is critical for developing effective treatments. Despite some understanding, many aspects of LSDV pathogenesis remain poorly defined. Understanding how LSDV evades the immune system, establishes systemic infection, and causes tissue damage could help develop better prevention and control strategies. This section provides a comprehensive overview of LSDV pathogenesis.

Viral entry

The LSDV enters the host primarily through the skin, either via bites from infected vectors or through abrasions and cuts. The virus gains access to the dermal and epidermal layers of the skin, where it targets the susceptible cells. The entry of LSDV into host cells is mediated by interactions between viral surface proteins and host cell receptors, however, the specific receptors involved in LSDV entry are not known. Once the virus binds to the host cell receptor, it induces endocytosis, allowing the virion to enter the cell within an endocytic vesicle [12]. The acidic environment within the endosome triggers the uncoating of the virus, releasing the viral core into the cytoplasm [174–176]. The viral core contains the viral DNA and associated proteins necessary for initiating replication [70,71]. Unlike many other DNA viruses, LSDV replicates entirely in the cytoplasm of the host cell.

Formation of skin nodules, viremia and systemic spread

The characteristic nodular skin lesions of LSD represent a granulomatous reaction in the dermis and hypodermis, extending to surrounding tissues [29]. Poxvirus replication often starts at the skin entry site, infecting keratinocytes, fibroblasts, and endothelial cells (Figure 3). Keratinocytes experience swelling, degeneration, and cytoplasmic vacuolation [177], accompanied by inflammatory cell infiltration, vasculitis, panniculitis, thrombosis, infarction, and formation of microvesicles, which coalesce into larger vesicles [178]. The nodules may eventually ulcerate, leading to secondary bacterial infections and further complications [177]. While nodules may appear anywhere, they are often more concentrated on the neck, head, and limbs.

Figure 3.

Pathogenesis of LSDV. LSDV infects the animal by vector bite or through direct contact (abrasions/cuts). The initial replication takes place in keratinocytes in the dermal/epidermal layer. Thereafter, the virus spread to regional lymph nodes via the lymphatics, where it infects various immune cell types. Via the lymphatics, the virus spreads from the regional lymph nodes to various organs and tissues. Viremia occurs when LSDV enters the bloodstream, either through direct infection of endothelial cells lining blood vessels or via the infected immune cells that migrate into the circulation. From the blood, the virus then spread into various organs such as skin, lungs, liver, intestine, spleen, kidneys which eventually results in the development of more severe clinical signs, generalized lymphadenopathy, fever, and widespread skin lesions.

After initial skin replication, LSDV travels through the lymphatic system to regional lymph nodes [178], where it infects macrophages, dendritic cells, and other immune cells, facilitating further viral amplification and dissemination to distant tissues [178]. Thereafter, virus spread to the secondary sites of replication, primarily via the monocytes [178,179]. The involvement of the lymphatic system in LSDV dissemination is a key factor in the development of viraemia [180].

Viraemia occurs when LSDV enters the bloodstream, either through direct infection of endothelial cells lining blood vessels or via infected immune cells that migrate into the circulation (Figure 3). [181]. Once in the bloodstream, LSDV can disseminate to various organs, including the skin, lungs, liver, spleen, and kidneys [182,183]. The presence of virus in the blood also increases the likelihood of transmission to other animals, particularly through vector bites. The systemic spread is associated with the development of more severe clinical signs, such as generalized lymphadenopathy, fever, and widespread skin lesions [180].

The severity of viraemia can vary depending on the host’s immune response, the virulence of the virus strain, and other factors such as co-infections. In some cases, viraemia may be transient, with the virus being rapidly cleared from the bloodstream by the host immune system. However, in other cases, viraemia can persist for an extended period, leading to widespread dissemination and severe disease. Understanding the factors that influence tissue tropism and the mechanisms by which LSDV causes tissue damage is essential for developing effective treatments and preventive measures.

LSDV exhibits a broad tissue tropism, meaning that it can infect a wide range of cell types and tissues within the host [181,183]. The primary target organs for LSDV infection include the skin, lymph nodes, lungs, liver, and spleen [181,183]. The virus shows a particular affinity for epithelial cells, endothelial cells, and immune cells such as macrophages and dendritic cells. The virus multiplies in the macrophages, fibroblasts, pericytes, and endothelial cells of blood and lymphatic vessels causes vasculitis and lymphagitis [183]. The vascular changes consist of multifocal areas of necrosis and infiltration of inflammatory cells. In more severe cases, thrombosis and infarction may result [181]. These necrotizing lesions are followed by proliferation of various cell types. Apart from the cytopathogenic changes in poxvirus-infected cells, the virus can also have a proliferative effect on non-infected cells [184].

In the lungs, LSDV can cause interstitial pneumonia, characterized by the infiltration of inflammatory cells and the presence of viral inclusions within alveolar cells [185]. This can lead to respiratory distress and increased susceptibility to secondary bacterial infections. The liver and spleen may also show signs of infection, including necrosis and lymphoid depletion.

Intradermal inoculation of the wild-type LSDV in cattle causes localized inflammation at the injection site (4–7 days post-infection), followed by lymph node enlargement and generalized skin nodules (7–10 days post-infection) [22]. The skin nodules attain peak size at 15–19 days post-infection. Typical viraemia is usually observed from day 3–14 post infection. Viral shedding through oral, faecal, and nasal secretions may persist from day 3 to day 28 post-infection [22]. The virus can also be excreted in semen and milk for 3–6 weeks [10,116].

Malnourished animals, lactating cows, and young calves often experience more severe disease [186]. Antibodies are detectable by 15 days post-vaccination [22]. Most animals clear the infection without becoming carriers.

Histopathological examination of skin nodules from naturally infected cattle at various disease stages reveals orthokeratotic hyperkeratosis, vasculitis, epidermal microvesicles, and Borrel’s cellules in early infection stages. Severity of histological changes correlates with the severity of clinical disease. Late-stage lesions show epidermal hyperkeratosis, dermal lymphocytic, and histiocytic infiltration. Early-stage lesions predominantly feature IFN-γ + cells and CD4 + T lymphocytes, with macrophages appearing in the later stages. These findings suggest that IFN-γ + cells, CD4 + T lymphocytes, and macrophages are critical for immunity against the LSDV and that cutaneous vasculopathy is immune-mediated [187].

Vaccines

In recent years, LSD has rapidly expanded its geographical reach. Mass immunization and restrictions on cattle movement have proven to be the most effective control measures. However, choosing the optimal LSD vaccine remains a significant challenge for policymakers and farmers. The commercially available vaccines for LSD differ in their efficacy, safety profiles, side effects, and pricing. This section discusses the advantages and disadvantages of various vaccine types, with emphasis on their immunogenicity and the correlates of protection used to evaluate the success of LSD vaccination campaigns in the field.

Homologous live-attenuated vaccines

Irrespective of the nature of the LSDV strain being used, homologous live-attenuated vaccines, prepared by using a specific LSDV strain, provide complete protection against all other LSDV field strains, including recombinant LSDV strains [19,22–26,188]. Homologous vaccines had always been proven to be better than heterologous (SPV/GTPV-based vaccines) vaccines in terms of immunogenicity, efficacy, and persistence of the immune response. Three live-attenuated LSDV strain-based vaccines have been licenced, which include the well-known South African Neethling strain, Kenyan sheep and goat pox (KSGP) or KS-1 strains, and Indian Ranchi strain (Table 1).

Table 1.

Attributes of commercial live-attenuated lumpy skin disease vaccines.

Manufacturer	Product Name	Virus Strain	Virus titre per dose	DIVA (Serology)	DIVA (PCR)	Neethling response	Reference
Biovet Pvt Ltd (India)	BIOLUMPIVAXIN	Ranchi strain	103.5 TCID50	Yes	Yes	No	[22,23]
Intervet (Pty) South Africa/MSD Animal Health	Lumpyvax™	Neethling strain	104.0 TCID50	No	Yes	Yes	[19,189]
MCI Santé Animale (Morocco)	Bovivax-LSD™	Neethling strain	103.5 TCID50	No	Yes	Yes	https://mci-santeanimale.com/
Jordan Bio-Industries Center (Jordan)	LumpyShield-N™	Neethling strain	104.0 TCID50	No	Yes	Yes	http://www.jovaccenter.com
MEVAC (Egypt)	MEVAC LSD	Neethling strain	103.5 TCID50	No	Yes	Yes	[36]
National Veterinary Institute (Ethiopia)	Lumpy Skin Disease Vaccine	Neethling strain	103.0 TCID50	No	Yes	Yes	https://www.nvi.com.et/
Jordan Bio-Industries Center (Jordan)	Kenyavac®	KSGP 0240 strain	102.5 TCID50	No	Yes	Yes	http://www.jovaccenter.com
(National Veterinary Institute Ethiopia)	Lumpy Skin Disease (LSD) Vaccine	KSGP (KS1 O-180) strain	103.0 TCID50	No	Yes	Yes	https://www.nvi.com.et/
Kenya Veterinary Vaccines Production Institute (Kenya)	Lumpivax™	Capripoxvirus strains*	Not Known	No	Yes	Yes	[190,191]

*Claimed as Neethling strain but it was found to be a complex mixture of multiple strains of the LSDV, including Neethling-like, KSGP-like, and Sudan-like strains; TCID50: Tissue Culture Infective Dose 50; KSGP: Kenyan Sheep and Goat Pox; DIVA: Differentiation of infected and vaccinated animals, LK: Lamb Kidney; CAM: Chorioallantoic membrane

South African neethling strain

The Neethling strain underwent attenuation through 61 sequential passages in lamb kidney cells, followed by 20 passages in embryonated chicken egg chorioallantoic membranes, and three additional passages in lamb kidney cells [192]. It is evident that a higher number of passages were required to attenuate the Neethling virulent field strain. Since the Neethling strain-based vaccine was first developed, it is most commonly used vaccines in Africa and the Middle East (Table 1). It was also employed in Balkan countries (Europe) during the 2016 and 2017 LSD outbreak to successfully eliminate the disease by coordinated mass vaccination. Importantly, the efficacy of the Neethling strain vaccines was experimentally evaluated using a challenge trial by the experts of the European Union LSD reference laboratory at Sciensano in Belgium [25]. However, like other live-attenuated poxviruses, this vaccine can produce side effects, including localized skin inflammation, small generalized nodules, fever, and temporary reductions in milk yield, collectively termed the “Neethling response” (Table 1) [49,193].

Kenyan sheep and goat pox (KSGP) strains

A Kenyan vaccine known as Kenyan sheep-and-goatpox (KSGP or KS-1), was developed using limited passage in cell culture from field strains isolated from sheep and goats. The level of attenuation of KSGP strain vaccines varies generally between 13 to 27 passages [12,190], which is quite low as compared to the Neethling strain based vaccines. Although the name of the strains refers to SPV and GTPV, the real identity of the Kenyan sheep and goat pox (KSGP) and KS-1 strains (Table 1) has been revealed to be an LSDV strain [33,48,194,195]. A KSGP strain-based vaccine has been used in cattle against LSD in Ethiopia [196], Israel [197], and Egypt [191]. However, adverse post-vaccinal reactions, including fever and skin lesions have been reported, which was probably due to insufficient level of attenuation of the KSGP strain for cattle [33].

Indian Ranchi strain

India reported LSD in 2019 [198]. Initially confined to the eastern regions [22,198]. a major outbreak occurred in 2022, affecting the north-western states, including Rajasthan, Gujarat, Punjab, Jammu and Kashmir, and western Uttar Pradesh, leading to an 80% morbidity rate, 10–50% mortality, a 26% reduction in milk production, and economic losses to the tune of INR 18,337.76 crore (USD 2,217.26 million) [2,199,200]. Herd immunity, acquired through natural infection, appeared to curb disease incidence temporarily. The use of goatpox vaccine yielded minimal impact, evidenced by subsequent outbreaks in vaccinated herds [201].

To address this, the National Centre for Veterinary Type Cultures at Hisar (India) developed a live-attenuated vaccine from a local LSDV strain (LSDV/Ranchi) [22]. The virus, initially isolated in goat kidney cells, subsequently underwent 50 passages (P) in Vero cells [22]. The whole genome sequencing of the LSDV/Ranchi/P50 indicated a unique molecular signature (deletion of 801 bps in its ITR region), besides many other mutations that are commonly observed in other Capripoxvirus vaccine strains [23]. Experimental trials in calves demonstrated 100% safety, eliciting robust antibody and cell-mediated immune responses [22]. Large-scale field trials involving 26,900 cattle demonstrated over 85% seroconversion, confirmed complete safety, showed no Neethling response and ensured suitability for use in pregnant animals. (Table 1) [22]. The Central Drugs Standard Control Organization (CDSCO), Government of India, licenced the vaccine to Biovet Pvt. Ltd., Bengaluru (BIOLUMPIVAXIN), in 2024 for its commercial production (Table 1). Most importantly, its unique 801 deletions in the ITR region, allows easy differentiation between field and vaccine strains through PCR, besides facilitating serological DIVA testing.

Gene deleted LSD vaccine

Live-attenuated vaccines, achieved through extended passage in unnatural hosts, accumulate random mutations. A more targeted approach involves inactivating virulence genes. Chervyakova et al. sequentially knocked out four virulence genes (LSDV005, LSDV008, LSDV066, LSDV142) in the LSDV/Atyrau genome, producing a recombinant virus genetically stable over ten passages [74]. The recombinant virus efficiently replicated in cell culture, did not produce any disease in cattle, induced generation of virus-neutralizing antibodies, and resisted challenge infection with virulent virus in the immunized animals, suggesting the suitability of the recombinant virus as a vaccine candidate [74]. However, the recombinant virus replicated at a maximum titre of 10^5.5-10⁶ TCID₅₀/ml [74], which is quite low as compared to the conventional live-attenuated vaccines, where the vaccine virus grows at a titre of 10⁷-10⁸ TCID₅₀/ml [22]. Such studies are under preclinical development and have not yet been licenced.

Heterologous LSD vaccines, friends or foe?

The Capripoxvirus genus, which includes LSDV, SPV and GTPV, are genetically quite similar and cross reacts with each other [32]. Therefore, SPV- or GTPV-based vaccines have also been used to protect cattle against LSD, particularly in circumstances where an LSDV-based vaccine is not available [29–33].

SPV-based vaccine

The Yugoslavian RM65 strain-based attenuated SPV vaccine was used against LSD in cattle in Israel [189] and Jordan [24] but it could provide incomplete protection [35]. Seroconversion following field vaccination with SPV-based vaccines has generally been low, typically ranging between 30% and 60% in most cases [19,137,202]. The Romanian SPV strain-based live-attenuated vaccine was used in Egypt and Saudi Arabia [203] with a ten times higher dose than that used in sheep [204]. However, it could also provide a partial cross-protection against LSD in cattle [196,197,205]. Upon first introduction of LSD, Georgia and Azerbaijan used a Turkish SPV strain (Bakirköy) in 3–10- times higher dosages (as compared to the sheep dose) against LSD in cattle [19]. However, the disease control and elimination were not successful, like that observed by the use of a homologous vaccine in Balkan countries [19]. One of the advantage of SPV-based vaccines is that they rarely induce any side effect, as compared to those caused by homologous attenuated LSD vaccines. However, a high dosage of SPV RM65 vaccine has been shown to induce generalized skin lesions in cattle [32].

Goatpox virus based vaccine

The Gorgan strain was the first GPPV-based vaccine strain evaluated against LSD in cattle in Ethiopia where it induced a good level of seroconversion and prevented the development of clinical disease [137]. In a comparative study of Gorgan GTPV vaccine and Romanian SPV vaccine, seroconversion, IFN-γ, and IL-4 levels were found to be higher in cattle vaccinated with the GTPV vaccine as compared to the SPV vaccine strain [206]. In Kazakhstan, Niskhi SPV and GTPV (G2-LKW) strains based vaccines were shown to induce comparable levels of seroconversion but the latter one was found to be comparatively more efficacious against LSD in cattle [207]. In a study by Abitaev et al., a heterologous vaccine, prepared by using the “G20-LKV” strain of the GTPV, was found to be protective for cattle against virulent LSDV but at doses of more than 1.5 × 10⁴ TCID₅₀ [208]. Under field conditions, like SPV-based vaccines, the seroconversion following field vaccination with GTPV-based vaccines has also been observed to be low, ~ 50% [19,137,202].

In 2021, India authorized the use of Uttarkashi GTPV strain-based goatpox vaccine against LSD in cattle [199,209]. However, subsequent experimental trials and extensive field evidence indicated that the Uttarkashi strain of the goatpox vaccine offered minimal protection against LSD in cattle [22,37]. For instance, in Maharashtra, approximately 14 million cattle were vaccinated with this vaccine, yet more than 418,000 of these goatpox vaccinated animals contracted the disease, resulting in over 32,000 deaths [210]. Although the goatpox vaccine generated some cross-neutralizing antibodies against LSDV, it elicited either minimal or no cell-mediated immune response in cattle [210].

While certain SPV/GTPV-based heterologous vaccines provide substantial protection, others offer only partial or no protection against LSD in cattle [27,39]. Moreover, heterologous vaccines are often administered at ten times the concentration of homologous LSD vaccines [203,211]. Additionally, heterologous vaccines either do not produce a robust antibody response in cattle or, if they do, the response is significantly weaker, lower in quality, and less persistent [36]. Consequently, the WOAH recommends independent evaluation of any heterologous LSD vaccine candidate before its application in cattle [39,40]. This recommendation is especially pertinent given the experience in India, where the Uttarkashi strain-based goatpox vaccine was authorized without adequate assessment of its safety and efficacy against LSD in cattle, ultimately proving to provide suboptimal protection.

Inactivated vaccines

Inactivated vaccines provide a higher degree of safety due to the non-replicative nature of the vaccine virus, which prevents transmission to susceptible in-contact animals. Additionally, there is no risk of reversion to virulence or recombination with field viruses to generate recombinant LSDV strains [212,213]. These vaccines may serve as suitable alternatives in countries that are currently disease-free but at high risk of disease introduction. However, a major limitation of inactivated vaccines is that they require two initial doses, followed by revaccinations every six months, which significantly raises the overall cost of vaccination [213]. Furthermore, inactivated LSD vaccines often induce insufficient immune responses, and their protective immunity tends to be short-lived [38].

A study by Hamdi et al. showed that an inactivated, oil-adjuvanted vaccine based on the Neethling strain induced a high level of specific antibodies and protection against virulent virus challenges, comparable to live-attenuated vaccines, without any adverse effects [213]. Conversely, Wolff et al. achieved only partial protection in cattle vaccinated with the inactivated LSDV-Neethling vaccine strain when challenged with virulent LSDV [214]. Increasing the vaccine dose was found to enhance antibody responses in laboratory animals, but experimental evidence supporting this in cattle is currently lacking [215].

Correlates of immune protection

Given the variability in antibody and IFN-γ responses in LSD-vaccinated animals, evaluating vaccine immunogenicity and efficacy ideally involves challenge infection of vaccinated animals with a virulent virus [22,33,207,216]. However, this approach is cumbersome, costly, and requires a containment facility. MicroRNAs (miRNAs) serve as potential alternative biomarkers due to their stability, specificity, and regulatory roles in gene expression [217]. In search of an alternative biomarker, we conducted miRNA profiling of cells infected with LSDV and identified several dysregulated miRNAs [218]. Some of these dysregulated miRNAs identified were further explored for their role in LSDV infection [218–220] wherein circulating miR-29a levels in LSDV-infected cattle was found to progressively decrease from 0 to 9 dpi in LSDV-infected cattle. In contrast, IFN-γ levels progressively increased from 0 to 9 dpi in infected cattle [2]. IFN-γ and its inducing transcriptional factors, such as T-bet and EOMES, share a conserved miR-29a target site within their 3’ untranslated regions (UTRs) [221–223]. Sensitized PBMCs treated with a miR-29a inhibitor produced higher levels of IFN-γ, indicating that miR-29a expression may antagonize the IFN-γ response in LSDV-infected cattle [2]. Therefore, reduced miR-29a expression could serve as a novel biomarker during the acute phase of LSDV infection [2].

Additionally, we observed that upon antigenic stimulation, a reduction of over 60% in miR-29a expression levels in the PBMCs of LSDV-vaccinated cattle correlated with a strong cell-mediated immune (CMI) response, characterized by significant lymphoproliferation, enhanced CD8 + T cell counts, and increased IFN-γ levels [2]. This could potentially predict protective immunity against LSDV, especially in animals that do not develop detectable levels of antibodies and IFN-γ. Further validation from large-scale controlled trials is needed to confirm these findings. Among the numerous dysregulated miRNAs identified in LSDV-infected cells [218], other candidates may also regulate immune responses to the virus, warranting further validation.

Vaccine potency and batch testing

The WOAH has outlined a challenge model for evaluating LSD vaccine efficacy [224]. This involves using 15 healthy, anti-LSDV antibody-free calves aged six to twelve months. Animals should be acclimatized to a controlled environment for one to two weeks prior to experimentation. Eight animals receive a single field dose (10^3.5 TCID₅₀), two receive ten times the field dose (10^4.5 TCID₅₀), and five remain unvaccinated controls. All vaccinated and control animals are challenged with a virulent LSDV field strain 3–4 weeks post-vaccination. While the outcome of LSDV experimental infection varies, 30–80% of animals typically develop clinical disease [22,224]. Achieving 100% disease development following challenge is rare [224]. A challenge dose of 10^4.0 to 10^6.5 TCID₅₀ of virulent virus may be required for disease manifestation in more than half of unexposed animals [224]. Infection via both the intradermal and the intravenous route increases the likelihood of developing the clinical disease. The challenge virus may be either a low-passaged virus or a homogenate from LSDV-infected skin scabs, with prior titration in calves [22].

For a vaccine to be considered effective, vaccinated animals should not develop local or generalized skin nodules following a challenge infection. While experimental outcomes are variable, virulent viruses must produce disease in at least 50% of unvaccinated controls for the vaccine to be deemed efficacious [22]. These two are the minimum criteria for considering the vaccine to be efficacious. Additional parameters such as fever, viraemia, lymph node enlargement, and virus excretion through nasal, ocular, and faecal routes may also be taken into the consideration in a clinical scoring system [25] for evaluating the vaccine efficacy [22]. Occasionally, vaccinated animals may experience a local inflammatory response following challenge infection, which typically resolves within three to four days without progressing to skin nodule formation [22].

Vaccine potency is often determined by flank titration, wherein unvaccinated control and vaccinated calves (three weeks post-vaccination) are intradermally inoculated with serial ten-fold dilutions and observed for the development of skin nodules at the injection site. A difference of more than 2.5 log10 in virus titre between vaccinated and unvaccinated animals indicates the vaccine to be efficacious against LSD in cattle [36,224].

According to recent WOAH guidelines [225], once the minimum dose required for immunity has been established for a specific vaccine strain, it is not necessary to determine the potency of each batch via animal inoculation (flank titration), provided the vaccine virus titre is known [225].

Differentiation of vaccine and field strains of LSDV

One limitation of live-attenuated vaccines is their association with adverse reactions that may mimic disease symptoms, complicating clinical diagnosis and differentiation of vaccine and field strains. Most live-attenuated LSDV vaccine strains possess a 12 bp insertion in the GPCR gene, which has been utilized for qRT-PCR differentiation between vaccine and field strains [226]. Similarly, the Neethling vaccine strain lacks a 27 bp fragment in the LSDV126 extracellular enveloped virion (EEV) gene, enabling identification and quantification vaccine and field strains using qRT-PCR [227,228]. The Indian vaccine strain (LSDV/Ranchi/P50) has a unique 801 bp deletion in its ITR region, distinguishing it from other LSDV strains. This feature has been exploited to develop a novel PCR method for identifying and quantifying LSDV vaccine and field strains [23].

Serological differentiation of infected and vaccinated animals

At the conclusion of any successful disease control programme, it is crucial to demonstrate freedom from infection through serological testing, as outlined by WOAH guidelines, particularly if vaccination has been part of the control strategy. However, differentiating infected animals from vaccinated ones (DIVA) is not possible when using Neethling- or KSGP strain-based LSD vaccines. In contrast, the Ranchi strain-based LSD vaccine differs from field LSDV strain(s) by a unique deletion of 801 nucleotides in its ITR region (ORF154). We cloned and purified ORF154 and utilized the recombinant protein as a capture antigen in an ELISA assay. This recombinant protein specifically reacted with sera from infected animals but not with sera from vaccinated ones, allowing serological differentiation between LSDV-infected and vaccinated animals. This establishes the Ranchi strain-based vaccine as the first DIVA-compatible LSD vaccine [45,210].

Should vaccination with live-attenuated vaccines be practiced during outbreak?

A full protective immunity following LSDV vaccination is believed to develop at  ~  2–3 weeks post-vaccination [3,19]. During this period, susceptible in-contact animals may encounter natural infection. Typically, vaccination of infected animals and those in close contact, potentially incubating the disease, is not recommended during outbreaks, as it is believed that the stress induced by vaccination could exacerbate disease severity. In practice, naturally infected animals may inadvertently be vaccinated during an outbreak, potentially leading to recombination between the vaccine and field strains, creating a novel recombinant virus. However, experimental evidences substantiating this hypothesis are not available.

During the 2022 LSD outbreak in India, morbidity rates reached up to 80% with case fatality rates as high as 67% [2]. In the absence of a specific LSD vaccine, the goatpox vaccine was authorized for emergency use but proved only moderately effective and faced supply shortages due to dependence on a single manufacturer and surging demand. During pan India outbreak, a new homologous live-attenuated LSD vaccine (Lumpi-Provac^Ind), developed by our group, was undergoing field trials. As infection was widespread, it was nearly impossible to isolate clinically affected animals from apparently healthy ones. Given the high case fatality rate, some farmers consented to vaccinate both infected and susceptible animals using Lumpi-Provac^Ind. This intervention significantly reduced morbidity and case fatality rates among in-contact susceptible animals compared to those left unvaccinated [2]. Furthermore, vaccination of infected animals with LSD vaccine notably reduced mortality as compared to those that left unvaccinated [22]. Given these advantages, we recommend the use of Ranchi strain-based homologous live-attenuated LSD vaccines, such as BIOLUMPIVAXIN (commercially form of Lumpi-Provac^Ind), to reduce the disease severity (mortality) in both infected and in-contact animals during outbreaks [22].

In an in vitro study, we demonstrated that co-infection with vaccine and field strains of LSDV allows the vaccine virus to outcompete the field strain within four sequential passages in primary lamb testicle cells [229]. This interference with the virulent virus could potentially slow disease progression, although further experimental validation is required. Reports on disease outcomes for animals naturally infected shortly after vaccination are inconsistent. For instance, a study by Ayelet observed high morbidity in local breeds but no significant impact on crossbreeds [196]. Another study found disease severity followed the order: non-vaccinated farms > farms vaccinated after infection > vaccinated farms [24].

Side effects

Adverse reactions to the vaccine typically manifest 1–2 weeks post-vaccination and include local reactions at the injection site, fever, decreased milk yield in lactating animals, and sometimes generalized skin micronodules, collectively known as the “Neethling response” [25,49]. The skin nodules induced by the vaccine strain are much smaller than those caused by virulent field strains [193]. However in certain cases, pronounced swelling can be seen at injection sites in up to 12% of the immunized animals [49,189]. According to a study from Israel, a homologous LSDV-based vaccine caused only mild adverse effects at very low incidence (0.4%, n = 9/2356) [189]. In the Balkan region, despite the annual vaccination of 1.8 million cattle with a live-attenuated LSD vaccine, no vaccine-associated disease/outbreak could be reported [120]. In another study, the whole genome sequencing of a skin sample collected from vaccinated animals revealed that vaccine virus remains unchanged after a passage in cattle as the sequences 100% matched with the original vaccine virus [230].

Bamouh et al., compared the adverse reaction induced by relatively high doses (10⁴ TCID₅₀ and 10⁵ TCID₅₀) of KSGP O-240 and Neethling strain based vaccines under controlled conditions and observed the development of LSD-like skin nodules in 12.5% and 6.7% animals respectively [211].

In field, the homologous vaccines are more prone to induce Neethling effect when used for the first time in a previously disease-free country [231]. The high-yielding dairy cows are usually most susceptible to the natural disease [205,211,232]. The booster vaccination usually does not cause adverse reactions [193].

Homologous live-attenuated vaccines, particularly the Neethling strain based vaccine, are usually associated with decrease in milk yield [49,50]. However, Morgenstern et al., could not observed any significant difference in milk yield up to 30 days post-vaccination while using Neethling strain based vaccine [233]. Likewise, Ranchi strain based vaccine was also not shown to induce reduction in the milk yield [22]. However, it may depend upon the breed, immune status of the herd and vaccine strain being used [2,22].

Heterologous vaccines usually do not induce Neethling effect [189,234]. However, KSGP (strain 0240) was shown to induce generalized skin reactions in up to 22.9% vaccinated dairy cattle in two herds, whereas beef cattle did not develop reactions [234] with 3.5% decrease in milk production over a period of 12 days [234]. The vaccine virus could be isolated from the animals with severe lesions, and was also demonstrated by electron microscopy. The histopathological lesions were similar to those of LSD. Likewise, high dose of RM65 SPV strain based vaccine has been shown to produce adverse reactions [32,234].

The vaccine virus can occasionally be detected in milk, skin nodules, blood, and nasal swabs post-vaccination with Neethling strain-based vaccines [49,120]. It can be isolated in the cell culture from vaccinated animals 10 to 21 days after inoculation [120]. In limited field studies, the Ranchi vaccine strain was not detected in milk, semen, or respiratory and ocular secretions [22], although further studies involving more animals are necessary.

Quality control testing

The quality of veterinary vaccines plays a significant role in determining farmers’ acceptance or hesitation towards their use. This is especially true for live-attenuated vaccines, such as those for LSDV, which can be associated with notable adverse reactions. The level of protection offered by LSDV vaccines depends on several factors, including the nature of the seed virus used, the degree of attenuation, and the final titre of the attenuated virus in the product. Vaccine quality is crucial for disease control and for safeguarding the welfare of farmers and the cattle industry. Therefore, vaccine production facilities must adhere to Good Manufacturing Practices (GMP), as outlined by the WOAH guidelines for veterinary biological products. High-quality vaccines must be free of extraneous agents, including other viruses, bacteria, fungi, mycoplasmas, protozoa, and rickettsia [235].

The origin and passage history of the vaccine seed virus must be thoroughly documented and characterized both genetically and antigenically. Poorly characterized vaccine viruses, especially those containing extraneous agents, may lead to the infection of animals with unwanted pathogens, posing significant safety concerns. Over-attenuated live virus vaccines may fail to provide optimal protection against LSDV, while under-attenuated vaccines can cause adverse side effects. It is, therefore, essential to periodically verify the correct LSDV genotype and any subpopulations, particularly regarding the contamination by field strains [33].

Haegeman et al. evaluated the Lumpivax vaccine (produced by Kevevapi) and detected nucleotide sequences of wild-type LSDV and GTPV, in addition to the vaccine-type LSDV, using PCR and nucleotide sequencing. The detection of both GTPV and wild-type LSDV sequences in the vaccine raises concerns about the original content of the Lumpivax vaccine vials. Are three different viruses present in the vaccine vial due to contamination, or is this an outcome of cell culture recombination, or perhaps a combination of both? [236].

Some manufacturers produce both KS1 and Neethling vaccines, which increases the risk of cross-contamination between these vaccine strains, potentially resulting in recombinant strains during cell culture processes. Mishandling multiple vaccine and field strains under laboratory conditions has also led to recombinant strains [237–240]. The recent reports of recombinants LSDV in Russia [112,241] and in China [242], together with the identification of LSDV-GTPV recombinant virus-like sequences in the Lumpivax, suggested that such recombinants were indeed present in the vaccine vials. Despite these findings, the vaccinated animals were protected against a virulent LSDV challenge, demonstrating a comparable level of protection, seroconversion, and IFNγ responsiveness to other attenuated LSDV vaccines [25].

Unlike LSDV, at certain degrees, some of the SPV and GTPV strains can cross infect their respective hosts [243,244]. However, no such data are available to show that GTPVs can multiply in cattle, which further increases the possibility that recombinants in the Lumpivax vaccine may have been generated during vaccine manufacturing processes.

One of the potential problems that arise with the presence of the recombinants is the altered virus transmission [245]. The points of consideration in this regard is relatively easy transmission of recombinant LSDV by Stomoxys [20] and evidence of indirect contact transmission, oral transmission and vector free transmission, of the recombinant viruses in cattle [246,247]. The recent detection of vaccine-like sequences in Musca Domestica flies in Russia [246] and in China [248] further supports the hypothesis.

Vaccines must be safe for use across all sexes, including pregnant cattle, all age groups, and breeds. Any potential side effects should be clearly indicated on the vaccine vial or associated literature to distinguish between disease induced by field strains and vaccine-related side effects [235].

Freeze-dried vaccines should be stored at temperatures between 4°C and 8°C and shipped under a strict cold chain. Capripoxvirus vaccines are sensitive to direct sunlight, emphasizing the need for proper storage away from light exposure [235].

Vaccine affordability

The affordability of LSDV vaccines, particularly for poor, small-scale farmers in developing countries, is a significant barrier to mass immunization programmes. Live-attenuated vaccines require a relatively low quantity of virus particles per dose, as compared to the inactivated vaccine, making bulk production comparatively cheaper. However, processes such as freeze-drying, packaging, and maintaining the cold chain during delivery significantly increase costs. To reduce costs, vaccines are often supplied in vials containing 50 or 100 doses, with diluent provided separately. For small-scale cattle owners with few animals, it is crucial to offer smaller, reasonably priced quantities. The strict requirement to use vaccines within a few hours after reconstitution also prevents sharing among farmers.

Antivirals

Currently, there is no approved antiviral medication specifically designed to treat animals suffering from viral infections, including LSDV infections. Preventing the spread of the virus is crucial to minimize the impact of viral diseases. Since vaccines cannot provide immediate protection, antiviral compounds could potentially serve as an effective means of intervention. While limited in vitro and in vivo studies have been conducted, encouraging results from foot-and-mouth disease (FMD) studies suggest potential success in treating animals with antivirals [249,250]. Administration of Acyclovir and Ivermectin in naturally LSD infected cattle resulted into speedy recovery, besides diminishing clinical severity [251].

One major limitation of antiviral drugs is their tendency to rapidly induce drug-resistant viral mutants [252–256]. Viral infections activate various intracellular signalling pathways, creating an antiviral state. However, viruses have evolved mechanisms to exploit these pathways, thereby facilitating efficient replication [255,257–261]. This reliance on host signalling may be leveraged to develop new antiviral drugs. For instance, hesperetin, a naturally occurring flavonoid found in citrus fruits like oranges, grapes, and lemons, has demonstrated antiviral efficacy against poxviruses, including LSDV, BPXV, and VACV [262]. Researchers found that hesperetin blocks viral protein synthesis by inhibiting the interaction between the eukaryotic translation initiation factor eIF4E and the 5′ cap of viral mRNA [262]. Moreover, hesperetin reduced pock lesions on the chorioallantoic membrane and mortality in an embryonated egg infection model. Importantly, prolonged viral culture in the presence of hesperetin did not lead to the emergence of hesperetin-resistant viral mutants [262].

Unlike drugs that directly target viral factors, those that target essential cellular factors generally have a lower tendency to induce antiviral drug resistance [255,257–261,263–270]. Though still in the preclinical phase, these studies hold promise for reducing drug resistance. However, there is a significant gap in our understanding of how LSDV interacts with host cell signalling pathways; characterizing these interactions may help develop novel antiviral therapies. Although antivirals may not be cost-effective for widespread animal use, they could be employed to support emergency vaccinations and protect valuable zoological collections and breeding stocks from deadly pathogens like LSDV.

Challenges and key recommendations for LSD control

Despite the availability of an effective live-attenuated LSD vaccine, the virus continues to cause outbreaks. Possible reasons include (i) Lack of vaccines in newly endemic countries (ii) Insufficient vaccination coverage to achieve herd immunity (iii) Limited awareness among farmers and veterinarians in endemic regions about LSDV prevention and control (iv) Lack of sustained political commitment to support mass vaccination programmes (v) Social and cultural customs or taboos in some endemic regions that hinder movement restrictions, particularly during festivals (vi) Difficulty in vaccinating free-grazing and wild animals, as well as livestock owned by nomadic tribes (vii) The diverse host and insect vectors of LSDV pose challenges to its prevention and control (ix) Limited international collaboration (x) Insufficient veterinary infrastructure to ensure proper vaccination coverage.

Therefore, the key recommendations for LSD control may include mass vaccination campaigns using effective homologous vaccines, active surveillance, movement restrictions, vector control measures, and farmer awareness programmes. Strengthening diagnostic capacity, ensuring vaccine cold-chain logistics, and establishing regional coordination for outbreak response will enhance disease containment and long-term eradication efforts.

Conclusion and future perspectives

Significant progress has been made in the genetic characterization of multiple LSDV genomes. However, the precise functions of most of the 156 open reading frames (ORFs) encoded by the LSDV genome and many aspects of its pathogenesis remain unclear. Functional genomics approaches, such as siRNA and CRISPR/Cas9, could be employed to elucidate the roles of individual LSDV genes, potentially uncovering novel therapeutic targets. Additionally, proteomic analyses of both vaccine and field strains could provide insights into virulence mechanisms and help identify targets for therapeutic intervention. While LSDV is believed to be primarily transmitted through vector bites, the exact role and distribution of vectors across different geographical regions, as well as their contribution to LSDV transmission dynamics, require further investigation. Similarly, the role of wildlife in the epidemiology and transmission of LSDV remains poorly understood. Homologous vaccines have demonstrated high efficacy, successfully controlling and eradicating the disease in regions such as the Balkans, though they may occasionally cause side effects. In contrast, heterologous vaccines are less effective and provide only short-term immunity. The deployment of a high-quality vaccine with minimal side effects, integrated with DIVA capability, along with the implementation of sustained annual mass vaccination programmes for cattle, is crucial for effective disease control and eventual eradication. Among the available options, the Ranchi strain-based LSD vaccine stands out as a promising choice for endemic regions due to its high safety, efficacy, and DIVA compatibility.

Funding Statement

This work was supported by the Indian Council of Agricultural Research, New Delhi [grant number IXX16675].

Disclosure statement

The authors declare no conflict of interest in the submission of this manuscript. The manuscript has been approved by all the authors for publication. This is original work that has not been published previously and is not under consideration for publication elsewhere, in whole or in part.

We have used Generative AI tools (Chat GPT) for grammatical language correction in this manuscript.

Author contributions

N.K., and B.N.T. conceived and designed the study. N.K. wrote the primary draft. S.S. and B.N.T. reviewed the manuscript. All authors have read and approved the final work.

Data availability statement

Data sharing not applicable to this article as no new data were created or analysed in this study.