Small Ruminant Lentiviruses: Maedi-Visna and CAE Context

Overview and Taxonomy of Small Ruminant Lentiviruses: Maedi-Visna and CAE Context

Taxonomic Classification and Phylogenetic Position

Small ruminant lentiviruses (SRLV) are a genetically and antigenically related group of viruses classified within the genus Lentivirus, family Retroviridae, subfamily Orthoretrovirinae [1, 3, 8]. This genus is characterized by slow, progressive, and persistent infections that ultimately lead to chronic inflammatory disease and death of the host [3]. The lentivirus genus includes several well-known pathogens of significant veterinary and medical importance, such as the human immunodeficiency viruses (HIV-1 and HIV-2), simian immunodeficiency viruses (SIV), feline immunodeficiency virus (FIV), and the equine infectious anemia virus (EIAV). Within the context of small ruminants, the SRLV group comprises two historically distinct but now recognized as closely related viral entities: the Visna/Maedi virus (VMV), which primarily infects sheep, and the Caprine Arthritis-Encephalitis virus (CAEV), which primarily infects goats [1, 3]. The World Organisation for Animal Health (WOAH) classifies both MV and CAE as notifiable diseases due to their significant economic impact on the global sheep and goat industries, underscoring the need for robust surveillance and control programs.

Historically, VMV and CAEV were considered separate species based on their host species of origin and distinct clinical presentations. However, extensive molecular and phylogenetic analyses have demonstrated that these viruses are not strictly host-specific and that cross-species transmission events have occurred frequently in both natural and experimental settings [3]. Consequently, the current taxonomic consensus groups them collectively under the umbrella term "small ruminant lentiviruses" (SRLV), acknowledging their shared genetic, structural, and pathogenic features. This unified classification is critical for understanding the epidemiology of these infections, as it implies that control strategies must consider the potential for interspecies transmission between sheep and goats cohabiting the same premises [1, 2]. The genetic diversity within SRLV is substantial, with multiple genotypes and subtypes identified globally, which complicates both serological diagnosis and vaccine development [4, 8].

Genomic Organization and Structural Features

As members of the Retroviridae family, SRLV possess a single-stranded, positive-sense RNA genome approximately 9.2 to 9.4 kilobases in length [3, 8]. The genomic organization follows the canonical retroviral structure: 5′-LTR-gag-pol-env-LTR-3′, flanked by long terminal repeats (LTRs) that contain essential regulatory elements for viral transcription and integration [3]. The gag gene encodes the structural polyprotein precursor, which is cleaved by the viral protease into the matrix (MA), capsid (CA), and nucleocapsid (NC) proteins. The capsid protein, designated p28 in SRLV (analogous to p24 in HIV), is the most abundant viral protein and serves as the primary target for serological diagnostic assays, including the indirect ELISA and Western blotting techniques [1, 4]. The pol gene encodes the enzymatic machinery essential for viral replication: the reverse transcriptase (RT), integrase (IN), and protease (PR). The env gene encodes the surface (SU) and transmembrane (TM) glycoproteins, which mediate viral entry into host cells and are the primary targets of neutralizing antibodies [3].

A distinguishing feature of SRLV, shared with other lentiviruses, is the presence of additional regulatory and accessory genes. These include vif, vpr-like, and rev genes, which are critical for viral replication, immune evasion, and pathogenesis [3]. The rev gene, for instance, is essential for the nuclear export of unspliced and singly-spliced viral mRNAs, a process that is tightly regulated and rate-limiting for viral production. The vif gene encodes a protein that counteracts the host restriction factor APOBEC3, a cytidine deaminase that induces hypermutation in the viral genome. This intricate interplay between viral accessory proteins and host restriction factors is a hallmark of lentiviral biology and contributes to the establishment of lifelong persistent infections.

Cellular Tropism and Pathogenesis

SRLV exhibit a restricted cellular tropism, primarily infecting cells of the monocyte/macrophage lineage [3]. This tropism is a key determinant of their pathogenesis and persistence. Monocytes, which circulate in the blood, are relatively resistant to viral replication and carry the proviral DNA in a latent state. As these monocytes differentiate into tissue macrophages, the cellular environment changes, and viral replication is activated, leading to the production of new virions [3]. This mechanism allows the virus to evade immune surveillance while disseminating throughout the body. The primary target organs for SRLV replication include the lungs, central nervous system, joints, and mammary glands, corresponding to the principal clinical manifestations of the disease [3].

The pathogenesis of SRLV infection is driven by a chronic, dysregulated inflammatory response. Infected macrophages release pro-inflammatory cytokines and chemokines, which recruit additional immune cells to the site of infection. This persistent immune activation leads to the progressive accumulation of lymphocytes, plasma cells, and macrophages in affected tissues, resulting in the characteristic lesions of interstitial pneumonia (maedi), demyelinating encephalitis (visna), chronic arthritis (CAE), and indurative mastitis [3]. The disease progression is slow, often taking months to years before clinical signs become apparent. This long incubation period, coupled with the fact that the majority of infected animals remain asymptomatic carriers, makes SRLV infections particularly insidious and difficult to control [1, 3].

Epidemiology and Global Distribution

SRLV infections are distributed worldwide, with seroprevalence rates varying dramatically between regions, countries, and even individual flocks [1-3, 5]. The global distribution is a consequence of international trade in live animals and germplasm, as infected but clinically normal animals can introduce the virus into naive populations. The World Organisation for Animal Health (WOAH) recognizes the significant economic burden imposed by these diseases, which includes reduced milk production, decreased weight gain, increased culling rates, and trade restrictions [1].

Epidemiological studies have revealed stark contrasts in seroprevalence between sheep and goat populations in different geographic regions. For example, a serological survey conducted in São Paulo state, Brazil, using the agar gel immunodiffusion (AGID) test, found a seroprevalence of 0.3% (4/1235) for MVV in sheep, compared to 15.1% (30/199) for CAEV in goats [2]. This high prevalence of CAEV in goats in São Paulo suggests widespread dissemination and underscores the urgent need for effective control measures [2]. In contrast, a study in the Low Parnaíba microregion of Maranhão, Brazil, reported a very low frequency of SRLV, with only 0.39% (5/1260) of goats and 0.08% (1/1150) of sheep testing positive by AGID [5]. These regional differences highlight the influence of management practices, flock density, and historical trade patterns on the epidemiology of SRLV.

In China, a recent large-scale serological survey using a newly developed recombinant CA protein-based iELISA tested 4,786 clinical serum samples from 13 cities across six provinces. The overall seroprevalence was found to be 10.64%, with positivity rates ranging from 0.85% to 40.00% depending on the region [1]. This study is particularly significant because it provides the first comprehensive data on SRLV prevalence in China, a country with a large and rapidly growing small ruminant population. The wide range of seroprevalence observed across different provinces suggests that localized control efforts could be highly effective if targeted appropriately.

Diagnostic Challenges and the Need for Sensitive Tools

The accurate diagnosis of SRLV infection is fraught with challenges, primarily due to the genetic and antigenic diversity of the viruses, their ability to establish latent infections, and the fluctuating nature of the humoral immune response [4]. The agar gel immunodiffusion (AGID) test has been the traditional mainstay for serological diagnosis, but it is limited by its relatively low sensitivity, particularly in the early stages of infection and in animals with low antibody titers [1, 4, 7]. Comparative studies have consistently demonstrated that indirect enzyme-linked immunosorbent assays (iELISA) are significantly more sensitive than AGID. For instance, one study reported that an iELISA detected 259 (37.2%) positive samples out of 696, whereas AGID detected only 128 (18.4%) [7]. Another study found that the iELISA was 400 to 1600 times more sensitive than the AGID test [1].

Western blotting (WB) is considered the most accurate serological technique for SRLV, offering superior sensitivity and specificity compared to both AGID and ELISA [4]. WB can detect antibodies at dilutions up to 256 times greater than AGID and 32 times greater than ELISA [4]. This enhanced sensitivity is crucial for identifying infected animals during the early seroconversion window, when antibody levels are low, and for confirming equivocal results from screening tests. The ability of WB to simultaneously resolve multiple viral antigens (e.g., p25, p28, gp135) provides a high degree of specificity, reducing the risk of false-positive results [4]. Despite these advantages, WB is more labor-intensive, expensive, and technically demanding, making it less suitable for large-scale screening programs. Therefore, a tiered diagnostic approach is often recommended, where a highly sensitive screening test (e.g., iELISA) is used for initial surveillance, followed by confirmation of positive results using a highly specific test (e.g., WB) [1, 4].

Genetic Determinants of Host Susceptibility

Recent advances in host genetics have opened new avenues for controlling SRLV infections without resorting to mass culling. The identification of genetic markers associated with resistance or susceptibility to SRLV infection offers the potential for selective breeding programs to reduce the prevalence of these diseases in endemic flocks [3, 6]. One of the most promising candidate genes is TMEM154, which encodes a transmembrane protein of unknown function. Several single nucleotide polymorphisms (SNPs) within the TMEM154 gene have been associated with differential susceptibility to SRLV infection in sheep [6].

A study conducted in Poland investigated the presence of SNPs in the TMEM154 gene in 107 sheep from three different breeds and assessed their association with SRLV infection status and proviral load [6]. The study found a positive association between the E35K polymorphism and SRLV status in one of the breeds analyzed. Furthermore, when analyzing the relationship between SNPs and proviral load across the entire study population, five SNPs showed a strong association [6]. These findings suggest that selecting for specific TMEM154 genotypes could reduce the susceptibility of sheep to SRLV infection and lower the viral burden in infected animals. However, the authors caution that these results require validation in larger, more diverse populations before they can be implemented in practical breeding programs [6]. The integration of genetic selection with traditional biosecurity and management practices represents a holistic and sustainable approach to SRLV control.

Molecular Pathogenesis of Small Ruminant Lentiviruses: Viral Entry, Replication, and Host Immune Evasion

The molecular pathogenesis of small ruminant lentiviruses (SRLV), encompassing the Maedi-Visna virus (MVV) in sheep and the Caprine Arthritis-Encephalitis virus (CAEV) in goats, represents a paradigm of host-pathogen co-evolution characterized by lifelong persistence, slow progressive disease, and intricate immune evasion strategies. Understanding the precise molecular mechanisms governing viral entry, replication, and the subversion of host immunity is paramount for the development of effective control measures, diagnostics, and potential therapeutic interventions. As the World Organisation for Animal Health (WOAH) recognizes these infections as significant impediments to global small ruminant production, the need for a mechanistic dissection of pathogenesis has never been more urgent.

Cellular Tropism and Molecular Determinants of Viral Entry

The foundation of SRLV pathogenesis is established at the point of viral entry into the target cell. SRLVs exhibit a highly restricted cellular tropism, primarily infecting cells of the monocyte/macrophage lineage [3]. This tropism is a defining feature of the lentivirus subfamily and directly dictates the nature of the ensuing pathology and immune response. The virus does not infect the circulating, quiescent monocyte efficiently; rather, it establishes a silent, non-productive infection within these precursor cells. The critical permissiveness for active viral replication is acquired upon monocyte differentiation into tissue macrophages, a process driven by inflammatory signals and migration into tissues [3]. This mechanism explains the insidious nature of SRLV disease, as the infected monocytes constantly seed target tissues, the lungs, joints, mammary glands, and central nervous system, where differentiation triggers viral production and localized inflammation.

The initial attachment and entry process is mediated by the interaction of the viral envelope glycoprotein (Env) with specific host cell surface receptors. While the primary receptor for SRLVs has not been definitively characterized with the same clarity as for HIV (CD4), recent genetic and functional studies have illuminated a critical role for the transmembrane protein 154 (TMEM154). Source [6] identifies TMEM154 as a strong candidate gene for SRLV susceptibility in sheep. The presence of specific single nucleotide polymorphisms (SNPs) within the TMEM154 gene, notably the E35K polymorphism, is significantly associated with infection status [6]. Mechanistically, it is hypothesized that TMEM154 acts as a necessary entry co-factor or receptor, and the truncated or altered protein products resulting from certain alleles (particularly those encoding a lysine at position 35) confer resistance by disrupting the virus’s ability to bind and fuse with the host cell membrane [6]. This molecular-level interaction directly explains the genetic basis for differential resistance observed across sheep breeds and provides a clear target for selective breeding programs as an alternative to culling, as proposed by Materniak-Kornas et al. [6].

Following receptor binding, the virus undergoes fusion with the host cell membrane, a pH-independent process typical of lentiviruses. This allows the viral core, consisting of the capsid protein (CA, p28) and the viral RNA genome, to be released into the cytoplasm. The capsid protein p28, a highly immunogenic structural component and the target of most serological diagnostic tests, is not merely a passive container [1, 4]. It shuttles the viral replication complex towards the nucleus and must disassemble (uncoat) in a regulated manner to permit reverse transcription. The efficiency of these early post-entry steps is likely modulated by host restriction factors and cellular environment, which may differ between quiescent monocytes and activated macrophages, thereby contributing to the cell-cycle-dependent nature of SRLV replication.

The Replication Cycle: Reverse Transcription, Integration, and Proviral Latency

Once inside the cytoplasm, the viral RNA genome is transcribed into double-stranded DNA by the viral reverse transcriptase (RT) enzyme. SRLV reverse transcription is a highly error-prone process due to the lack of proofreading activity in RT. This results in a mutation rate approximately one million times greater than that of host cell DNA replication. This inherent genetic diversity, a hallmark of all retroviruses, is a primary driver of SRLV pathogenesis. The resulting quasispecies swarm within a single host allows the virus to rapidly adapt to selective pressures, including the host immune response and the microenvironments of different tissues [4]. The diagnostic implications of this genetic variability are profound; as noted by Peixoto et al. [4], it creates a moving target for antibody detection and can lead to false-negative results in serological tests like ELISA or AGID, which rely on a limited set of conserved antigens.

The newly synthesized linear double-stranded viral DNA is then transported into the nucleus as part of a pre-integration complex. A defining feature of lentiviruses, including SRLV, is their ability to infect non-dividing cells, which distinguishes them from other retroviruses like murine leukemia virus. This capacity is crucial for infecting terminally differentiated macrophages. Once inside the nucleus, the viral integrase enzyme catalyzes the stable integration of the viral genome into the host cell chromosome. This integrated provirus becomes a permanent genetic fixture of the infected cell and its progeny. The site of integration is non-random and can influence the transcriptional activity of the provirus.

For SRLV, the establishment of proviral latency is the cornerstone of lifelong infection. As described in Source [3] and [4], the virus can persist in a transcriptionally silent state for extended periods. This occurs particularly in circulating monocytes, which harbor the provirus but produce minimal viral RNA or protein. This latency directly thwarts the host immune system, as no viral antigens are presented for immune recognition, and it protects the virus from antiviral drugs or immune clearance. Reactivation from latency is intimately linked to the differentiation of monocytes into macrophages. This process is triggered by inflammatory cytokines and the activation of transcription factors, most notably NF-κB, that bind to the viral Long Terminal Repeat (LTR) promoter. The LTRs of SRLV contain binding sites for several host transcription factors, and differences in LTR sequences between strains may account for differences in tissue tropism and the spectrum of disease manifestations (respiratory, arthritic, or neurological) seen in different geographic regions [2, 5]. The sporadic and stochastic nature of this reactivation event drives the fluctuating levels of antigenemia and antibody titers observed in infected animals, contributing to the "viral intermittence" and diagnostic challenges highlighted in Sources [4] and [7].

Mechanisms of Host Immune Evasion: A Multi-Pronged Strategy

The ability of SRLV to cause a persistent infection despite a robust host immune response is a testament to its sophisticated immune evasion arsenal. The virus employs a multi-layered strategy that targets both the innate and adaptive arms of immunity.

Antigenic Variation and the "Moving Target": The error-prone nature of reverse transcriptase yields a constant stream of envelope (Env) variants. These mutations, particularly in immunodominant epitopes, allow the virus to escape from neutralizing antibodies. As the host generates antibodies against an initial viral variant, a new, antigenically distinct variant emerges that is no longer neutralized. This serial selection of escape mutants drives the chronic, relapsing nature of the inflammatory disease, particularly in the central nervous system where a new wave of virus can cause a fresh bout of demyelination. The genetic diversity of SRLV is so extensive that it is a primary reason why no effective vaccine has been developed, and why diagnostic test performance can be variable across different breeds and regions [3, 4].

Compartmentalization and Anatomical Sequestration: SRLV infection is not uniform throughout the body; it is highly compartmentalized. The virus establishes distinct, quasi-species populations in different tissues, lungs, joints, brain, and mammary gland, that evolve independently [4, 8]. This anatomical sequestration allows the virus to evade immune effectors that may be effective in the blood but cannot access the tissues. For example, cytotoxic T lymphocytes (CTLs) found in the blood may be ineffective against viral variants replicating in the synovial fluid of an arthritic joint. This compartmentalization was vividly demonstrated by the first isolation of a CAEV strain (BrRN-CNPC.G1) from a naturally infected goat in Brazil, which required a specific co-culture technique using goat synovial membrane (GSM) cells, highlighting the unique tissue-adapted nature of these viruses [8].

Direct Subversion of the Host Immune System: The core of SRLV immune evasion lies in its specific targeting of macrophages. Macrophages are not only the primary target cells but also central orchestrators of the immune response. By infecting these cells, the virus can directly dysregulate their function.

1. Impaired Antigen Presentation: Infected macrophages may downregulate Major Histocompatibility Complex (MHC) Class II molecules, reducing their ability to present viral antigens to CD4+ helper T cells. This blunts the activation of the adaptive immune response, including both antibody production and the generation of cytotoxic T cells.
2. Dysregulated Cytokine Secretion: SRLV infection alters the delicate balance of macrophage cytokine production. The virus can induce a disproportionate release of pro-inflammatory cytokines (such as TNF-α, IL-1β, and IL-6) and chemokines, which paradoxically serves the virus by recruiting more monocytes/macrophages to the site of infection. These newly recruited cells become fresh targets for propagation. This persistent, low-grade inflammatory state is the direct cause of the pathological lesions seen in maedi (interstitial pneumonia), arthritis (synovitis), and encephalitis (demyelination).
3. Interference with Humoral Immunity: The antibody response, while detectable and diagnostically useful, is often rendered ineffective. As detailed by Peixoto et al. [4], while Western blot and ELISA can detect antibodies, the antibodies are often non-neutralizing or are rapidly overwhelmed by antigenic variation. Furthermore, immune complexes formed between viral antigens and antibodies can deposit in tissues, contributing to the chronic inflammatory pathology. The serological evidence from large-scale studies in Brazil [2, 5] and China [1] shows that seropositivity can fluctuate over time, a phenomenon directly linked to these evasion mechanisms.

The Role of Host Genetics in Susceptibility: The host is not entirely defenseless. The interaction between SRLV and the host genome is a critical determinant of the outcome of infection. Beyond the primary receptor TMEM154 [6], other genetic loci are emerging as key modulators of resistance or susceptibility. These genes may influence the efficiency of the innate immune response (e.g., through pattern recognition receptors like Toll-like receptors) or the adaptive capacity of T cells. The identification of resistance-associated SNPs offers a non-pharmacological route to controlling SRLV. By breeding animals with protective alleles, such as those found in the TMEM154 gene in Polish sheep [6], it may be possible to establish flocks that are genetically resistant to infection, even in the face of ongoing exposure. These genetic markers represent the most promising avenue for sustainable control, circumventing the intractable problem of immune evasion that has foiled vaccine development.

Epidemiology and Risk Factors of SRLV Infections: Global Distribution and Seroprevalence Patterns

The global distribution of small ruminant lentiviruses (SRLVs), encompassing both maedi-visna virus (MVV) in sheep and caprine arthritis-encephalitis virus (CAEV) in goats, presents a complex epidemiological landscape characterized by marked regional variation in seroprevalence, heterogeneous diagnostic approaches, and a multiplicity of interacting risk factors. As members of the Lentivirus genus within the Retroviridae family, these pathogens establish lifelong, persistent infections that, while often subclinical, exact a substantial toll on productivity and animal welfare worldwide [1, 3]. The World Organisation for Animal Health (WOAH) recognizes SRLV infections as notifiable diseases in many member nations, underscoring their economic significance and the imperative for robust surveillance programs. A thorough understanding of the global seroprevalence patterns and the underlying risk factors is essential for designing effective control strategies, particularly given the absence of curative treatments or commercially available vaccines [3, 4].

Global Seroprevalence: A Mosaic of Regional Variation

SRLV infections are truly cosmopolitan, with documented presence on every continent where small ruminant husbandry is practiced. However, the reported seroprevalence rates exhibit extraordinary variability, ranging from less than 1% in certain extensively managed flocks to over 80% in intensively managed, high-production herds [1, 3]. This heterogeneity reflects a complex interplay of factors including diagnostic test sensitivity and specificity, breed susceptibility, management practices, and true geographic differences in viral circulation.

In China, a nation with one of the world’s largest sheep and goat populations, comprehensive serological surveillance has historically been lacking. Recent work by Ma et al. (2025) using a newly developed recombinant capsid protein (p28)-based indirect ELISA applied to 4,786 clinical serum samples from 13 cities across six provinces revealed an overall seroprevalence of 10.64% [1]. Importantly, this study demonstrated substantial regional variation within China, with positivity rates ranging from as low as 0.85% to as high as 40.00%, suggesting that SRLV distribution is far from uniform and may be concentrated in specific production zones or intensive farming systems [1]. The development and validation of this cost-effective ELISA, which proved 400–1,600 times more sensitive than the traditional agar gel immunodiffusion (AGID) test, represent a critical step forward for large-scale surveillance in resource-limited settings [1].

Brazil presents another instructive case study of within-country epidemiological variation. In the state of São Paulo, a serological survey employing AGID found a stark contrast between species: 0.3% (4/1,235) of sheep were seropositive for MVV, whereas 15.1% (30/199) of goats were positive for CAEV [2]. This approximately 50-fold difference in seroprevalence between sheep and goats in the same geographic region highlights species-specific differences in viral transmission dynamics or management practices. Conversely, in the microregion of Low Parnaíba, Maranhão, a separate study using AGID found much lower frequencies overall, 0.39% (5/1,260) in goats and 0.08% (1/1,150) in sheep, with positive flocks clustered in strategically located municipalities critical for animal commercialization [5]. These findings suggest that while SRLV prevalence can be low in certain extensive production systems, the spatial distribution of seropositive flocks may follow trade routes, underscoring the role of animal movement in viral dissemination [5].

Further complicating the Brazilian epidemiological picture, Nascimento et al. (2014) compared AGID and indirect ELISA for diagnosing caprine lentiviruses in 696 serum samples from the state of Piauí. AGID detected 18.4% positivity, whereas ELISA identified 37.2%, revealing that ELISA detected more than twice as many positive animals [7]. The sensitivity of i-ELISA relative to AGID was 94.5%, but specificity was only 75.7%, indicating that AGID substantially underestimates true seroprevalence [7]. This diagnostic discrepancy has profound implications for interpreting global prevalence data: studies relying solely on AGID likely report artificially low seroprevalence rates, potentially masking the true extent of SRLV circulation.

In northeastern Brazil, specifically Rio Grande do Norte, the first isolation of a caprine lentivirus from a naturally infected goat was achieved by Feitosa et al. (2011) using co-cultivation of peripheral blood leukocytes with goat synovial membrane cells [8]. While seroprevalence in that specific flock was low, the successful isolation of the BrRN-CNPC.G1 strain confirms active viral replication and provides a foundation for molecular characterization of circulating strains [8]. Such regional isolates are crucial for understanding the genetic diversity that complicates serological diagnosis and may influence cross-reactivity patterns between diagnostic tests [4].

The Diagnostic Conundrum: Implications for Prevalence Estimation

Any meaningful discussion of SRLV epidemiology must acknowledge the profound influence of diagnostic methodology on reported seroprevalence. As Peixoto et al. (2021) comprehensively review, there is no true gold standard test for SRLV infection [4]. The most commonly employed tests, AGID and various ELISA formats, differ substantially in their performance characteristics. AGID, while highly specific and operationally simple, suffers from poor sensitivity, particularly in early infection or in animals with low antibody titers [4, 9]. Indeed, the recombinant ELISA developed by Ma et al. demonstrated 400- to 1,600-fold greater sensitivity than AGID, and Western blot (WB) can detect antibodies at dilutions 256 times greater than AGID and 32 times greater than ELISA [1, 4].

The implications for epidemiological studies are profound. Comparative studies have shown that AGID may miss a substantial proportion of infected animals. In Piauí, Brazil, ELISA detected 37.2% seropositivity versus 18.4% by AGID, and the micro-AGID format, while showing good agreement with macro-AGID (κ = 0.90), still likely underestimates true prevalence [7, 9]. Furthermore, positive animals exhibit unstable antibody levels over time, oscillating between detectable and undetectable states, leading to intermittent seronegativity even in persistently infected animals [4]. This phenomenon, combined with the genetic diversity of SRLV, viral compartmentalization, and the potential for immune escape, means that a single negative serological test does not rule out infection [4]. Consequently, cross-sectional studies using AGID or even commercial ELISAs may systematically underreport true prevalence, and temporal trends based on such data should be interpreted cautiously.

Biological and Demographic Risk Factors

Among the most consistently identified biological risk factors for SRLV seropositivity is host age. Nascimento et al. (2014) reported a significantly higher prevalence among goats older than six months compared to younger animals (p < 0.05) [7]. This age-dependent increase likely reflects cumulative exposure over time, given the lifelong persistence of infection and the predominantly horizontal transmission routes. As animals age, their opportunities for contact with infected individuals, particularly through prolonged co-housing, shared feeding equipment, and during parturition, increase, resulting in a higher probability of seroconversion.

Sex also appears to modulate seroprevalence, though the biological mechanisms warrant further investigation. In the Piauí study, male goats exhibited a seroprevalence of 56.7%, significantly higher than the 35.4% observed in females (p < 0.01) [7]. This finding may reflect management practices that concentrate males in high-density breeding groups, or it may indicate sex-linked differences in immune response or viral susceptibility. However, such sex-specific differences have not been universally reported, and the biological plausibility remains to be fully elucidated.

Species itself is a critical risk factor. As demonstrated in São Paulo, caprine seroprevalence for CAEV (15.1%) vastly exceeded ovine seroprevalence for MVV (0.3%), despite geographic co-location [2]. This may reflect differences in viral pathogenesis, transmission efficiency, or management intensity between goat and sheep operations in that region. Importantly, SRLV strains are not strictly species-specific; cross-species transmission between sheep and goats is well-documented, adding further complexity to epidemiological interpretations [3].

Management and Environmental Risk Factors

Attempts to identify discrete management risk factors have yielded inconsistent results, likely due to the multifactorial nature of transmission and the difficulty of disentangling correlated variables. Carmo et al. (2013) applied multivariate logistic regression to identify risk factors associated with seropositive properties in São Paulo but found that no single variable emerged as statistically significant for either CAEV or MVV [2]. This absence of identifiable management risk factors in a well-designed study suggests that SRLV transmission is driven by a constellation of factors rather than any single practice. However, some general principles are supported by the literature.

Herd size and stocking density have been implicated in many studies: larger herds with higher animal densities facilitate more frequent animal-to-animal contact, increasing the probability of transmission through respiratory aerosols (particularly for MVV) and through ingestion of colostrum or milk containing cell-associated virus (a major route for CAEV) [3]. The commercialization network is also a critical risk factor. In Maranhão, seropositive flocks were strategically located in municipalities serving as hubs for animal trade, strongly suggesting that animal movement, whether for breeding, sale, or slaughter, is a primary mechanism for introducing SRLV into naïve herds [5]. This spatial clustering along trade routes highlights the vulnerability of extensively managed flocks that may appear isolated but are nonetheless connected through market chains.

Host Genetics as a Critical Risk Factor

In recent years, host genetic factors have emerged as perhaps the most promising avenue for understanding SRLV susceptibility and for developing innovative control strategies. Larruskain and Jugo (2013) emphasized that host genetics play an important role in determining both susceptibility to infection and the rate of disease progression, though they noted that relatively little work had been performed in small ruminants compared to other livestock species [3]. More recent investigations have begun to fill this knowledge gap.

The transmembrane protein 154 gene (TMEM154) has been identified as a key candidate for SRLV resistance. Materniak-Kornas et al. (2024) conducted the first investigation of single nucleotide polymorphisms (SNPs) in the TMEM154 gene in Polish sheep, examining 107 animals from three breeds [6]. They found that the frequency of specific alleles differed markedly among breeds and, critically, that a positive association existed between the E35K polymorphism and SRLV infection status in one breed. Furthermore, when analyzing the relationship between SNPs and SRLV proviral load across the entire study population, five polymorphisms showed strong associations [6]. These findings suggest that certain TMEM154 genotypes confer relative resistance or susceptibility to SRLV infection and that selection for resistant genotypes could be a viable component of integrated control programs. However, as the authors caution, validation in larger, more diverse populations is necessary before such genetic markers can be applied in breeding programs [6].

The Economic and Health Context

The epidemiological patterns of SRLV infection are inseparable from their economic consequences. Although the majority of infected animals remain asymptomatic for extended periods, often years, chronic infection leads to progressive inflammatory lesions in the lungs (maedi), nervous system (visna), joints (arthritis), and mammary glands (mastitis) [1, 3]. These pathological outcomes translate directly into reduced milk production, premature culling, decreased fertility, and increased mortality, all of which impose substantial economic burdens on producers [1, 3, 6]. The insidious nature of the disease, combined with the diagnostic challenges outlined above, means that many producers are unaware of the infection status of their flocks until significant production losses have already accrued [1].

Given the lack of effective treatments or vaccines, current control strategies rely entirely on breaking transmission cycles through serological testing, culling of positive animals, and implementation of strict biosecurity measures, including the separation of offspring from infected dams before colostrum ingestion [3, 4]. The variable seroprevalence patterns observed globally, ranging from less than 1% in some Brazilian regions to 40% in certain Chinese provinces, underscore that these measures are applied with highly inconsistent effectiveness. Regions with low seroprevalence, such as the extensively managed flocks in Maranhão, represent critical opportunities for eradication if incursions can be detected early and contained [5]. Conversely, regions with high seroprevalence, like parts of China and the goat populations of São Paulo, likely require sustained, multi-year control programs incorporating enhanced diagnostics (such as WB or high-sensitivity ELISA), genetic resistance screening, and rigorous management interventions [1, 2, 6].

The global distribution of SRLV, modulated by diagnostic test performance, host genetics, age, sex, and management practices, presents a formidable challenge to small ruminant health. The increasing availability of sensitive, cost-effective diagnostic tools and the growing understanding of host genetic resistance offer pathways toward more effective surveillance and control. However, the heterogeneity of global seroprevalence patterns demands that control strategies be tailored to regional epidemiological contexts, informed by high-quality surveillance data, and adaptive to the evolving genetic landscape of both virus and host.

Clinical Manifestations and Disease Progression in Maedi-Visna and Caprine Arthritis-Encephalitis

The clinical spectrum of small ruminant lentivirus (SRLV) infection, encompassing Maedi-Visna (MV) in sheep and Caprine Arthritis-Encephalitis (CAE) in goats, is characterized by a protracted, insidious onset that belies the relentless pathological progression underlying these diseases. As members of the Lentivirus genus within the Retroviridae family, these viruses establish lifelong, persistent infections that typically culminate in the death of the host [3]. A critical epidemiological feature, one that profoundly complicates surveillance and control efforts, is that the vast majority of infected animals remain asymptomatic for prolonged periods, often years [1, 3]. This subclinical carrier state is the primary driver of within-herd transmission, as apparently healthy animals continuously shed virus through respiratory secretions, colostrum, and milk [1, 6]. The World Organisation for Animal Health (WOAH) recognizes the economic gravity of these diseases, listing them as notifiable due to their impact on international trade and productivity. Furthermore, the Food and Agriculture Organization (FAO) has highlighted the significant welfare concerns and production losses, including reduced milk yield, decreased fertility, and premature culling, associated with SRLV infections, particularly in resource-limited settings where diagnostic infrastructure is often lacking.

The disease progression follows a characteristic lentiviral pattern: infection of cells of the monocyte/macrophage lineage, followed by a prolonged period of viral persistence and slow, cumulative inflammatory damage to target organs [3]. The virus exploits the very cells meant to orchestrate an immune response, using monocytes as Trojan horses to disseminate throughout the body while evading immune clearance. The clinical picture is therefore a mosaic of chronic inflammatory syndromes, most commonly affecting the lungs, nervous system, joints, and mammary glands [3]. The specific constellation of clinical signs and the rate of progression are influenced by a complex interplay of viral strain virulence, host genetic susceptibility, age at infection, and environmental stressors.

Respiratory Syndrome (Maedi)

The pulmonary form, known as Maedi (Icelandic for "dyspnea"), is the most frequently reported manifestation in sheep and is a hallmark of progressive ovine pneumonia [3]. The disease is characterized by a slowly progressive, interstitial pneumonia that evolves over months to years. The underlying pathology is a lymphoproliferative interstitial pneumonitis, wherein the alveolar septa become thickened by an infiltration of lymphocytes, macrophages, and plasma cells, leading to a loss of functional alveolar airspace [3]. Clinically, affected sheep present with a gradual onset of exercise intolerance, tachypnea, and a progressive increase in respiratory effort. Initially, signs may only be evident when the animal is herded or stressed. As the disease advances, the animal develops a characteristic "puffing" respiration at rest, with forceful, abdominal breathing. Auscultation of the lungs may reveal harsh, bronchial sounds, but coughing is notably absent and not a typical feature of uncomplicated Maedi [3]. The disease is relentlessly progressive; the animal loses body condition, becomes emaciated, and eventually succumbs to severe respiratory failure or secondary bacterial pneumonia. The insidious nature of this progression cannot be overstated; a sheep may carry a high proviral load and significant pulmonary pathology for 2–4 years before overt clinical signs become apparent [6]. This prolonged subclinical phase is a major obstacle to eradication, as many infected animals are retained in the flock as breeding stock, perpetually contaminating the environment.

In goats, while respiratory disease is less commonly the primary presenting complaint compared to arthritis, interstitial pneumonia is a frequent finding at necropsy in CAEV-infected animals. The pulmonary lesions are histologically similar to those seen in MVV-infected sheep, suggesting a shared pathogenic mechanism of SRLV-induced interstitial lung disease across the two hosts [3, 8].

Neurological Syndrome (Visna and CAE Encephalitis)

The neurological form, termed Visna (Icelandic for "wasting" or "paralysis"), is a less common but devastating manifestation of MVV infection in sheep. It typically presents as a chronic, progressive meningoencephalomyelitis. The clinical signs reflect the involvement of the central nervous system's white matter, leading to demyelination. Affected sheep initially exhibit subtle proprioceptive deficits, such as knuckling of the fetlocks, a stumbling gait, and ataxia of the hindlimbs [3]. Over weeks to months, this progresses to ascending paralysis, with the animal eventually becoming recumbent and unable to rise. Head tremors, circling, and blindness may also occur. The disease is invariably fatal, often necessitating euthanasia on humane grounds.

In goats, the neurological form is more commonly seen in kids between 2 and 6 months of age, presenting as a rapidly progressive, non-suppurative encephalomyelitis [3, 8]. This acute presentation is a stark contrast to the chronic, insidious course seen in adult sheep. Affected kids typically present with an acute, ascending paralysis that begins in the hindlimbs and progresses to complete tetraplegia within days to weeks. Other common signs include ataxia, hypermetria, head tilt, and opisthotonos [8]. This form is rapidly fatal, with most kids succumbing or being euthanized within a few weeks of onset. The pathogenesis involves a direct, lytic infection of central nervous system cells, particularly microglia and perivascular macrophages, leading to intense inflammation and necrosis. The age-dependent susceptibility is thought to be related to the immaturity of the kid's immune system and the high viral replication rate in the developing brain.

Arthritic Syndrome (CAE)

The arthritic form is the most common chronic clinical manifestation in adult goats infected with CAEV, serving as the namesake for the disease [3]. It presents as a progressive, chronic, and often bilateral polyarthritis, most frequently affecting the carpal (knee) joints, but also the hocks, stifles, and fetlocks [3]. The onset is gradual, and early signs may be subtle, such as a shortened stride, a stiff gait that improves with movement, and a reluctance to rise or bear weight on the affected limbs. Over months to years, the joints become visibly swollen, warm, and painful on palpation. The swelling is typically firm and periarticular, reflecting a chronic proliferative synovitis. Joint effusion may be present, and in advanced cases, there is palpable crepitus as the articular cartilage erodes. Affected goats often adopt a "dog-sitting" posture to relieve pressure on the carpi [3]. The progressive nature of the arthritis leads to severe pain, lameness, and ultimately, debilitation. The animal loses body condition due to reduced mobility and a decreased ability to compete for feed. This chronic pain and immobility represent a significant animal welfare concern. The arthritic syndrome is rarely seen in sheep, where respiratory and neurological signs predominate, although occasional reports of synovitis exist.

Mammary Gland and Other Manifestations

Chronic indurative mastitis is a common but clinically underappreciated manifestation in both sheep (MVV) and goats (CAEV). The infection of mammary gland macrophages and epithelial cells leads to a chronic, interstitial lymphocytic infiltration, resulting in a "hard bag" [3]. The affected udder becomes firm, non-painful, and non-functional. This "hard udder" is often a first clinical sign that alerts a vigilant farmer to the presence of the disease in the herd. The most significant consequence is a profound reduction in milk production, which may be reduced by 10–30% or more in infected, asymptomatic animals [3]. Furthermore, the presence of the virus in colostrum and milk is the primary route of transmission from dam to kid or lamb, perpetuating the infection cycle within the flock. This highlights the critical importance of management interventions such as feeding pasteurized colostrum and milk from seronegative animals.

Other less common clinical associations include chronic progressive pneumonia in goats (less severe than in sheep, but present), weight loss and chronic wasting without overt organ-specific signs, and secondary infections due to compromised overall health. It is crucial to recognize that many infections are subclinical. The high seroprevalence reported in many regions, such as the 15.1% seroprevalence for CAEV in São Paulo state, Brazil [2], or the 10.64% overall SRLV seroprevalence identified in a large-scale Chinese survey using a newly developed iELISA [1], vastly exceeds the number of animals showing clinical signs. This creates a hidden reservoir of infection that is exceptionally difficult to manage, underscoring the need for routine serological testing, such as the highly sensitive Western blotting (WB) techniques, which can detect antibodies at dilutions 256 times greater than AGID and 32 times greater than ELISA [4], but also the use of more cost-effective screening tools like the iELISA for large-scale surveillance [1, 7]. The presence and progression of clinical disease are also modulated by host genetics. Recent research has identified associations between specific single nucleotide polymorphisms (SNPs) in the TMEM154 gene and resistance or susceptibility to SRLV infection and increased proviral load, suggesting a genetic basis for variable disease progression [6]. This opens the door for future selection-based control strategies, though further validation is required [6].

Diagnostic Approaches for Small Ruminant Lentiviruses: Serological Assays (ELISA, AGID) and Molecular Detection

The diagnostic landscape for small ruminant lentiviruses (SRLV), encompassing Maedi-Visna virus (MVV) and caprine arthritis-encephalitis virus (CAEV), is characterized by a complex interplay between serological screening, confirmatory immunoblotting, and direct molecular detection. The accurate identification of infected animals remains the cornerstone of control programs, as these lentiviruses induce a persistent, lifelong infection marked by a protracted asymptomatic phase, during which animals serve as silent reservoirs for horizontal and vertical transmission [1, 3]. The diagnostic challenge is further compounded by the extraordinary genetic diversity of SRLV, their capacity for viral compartmentalization within the host, and the temporal fluctuation of antibody titers, which collectively preclude the establishment of a single, universally reliable gold-standard test [4]. Consequently, a stratified diagnostic approach, integrating high-throughput serological screening with sensitive and specific molecular confirmation, is not merely recommended but essential for effective surveillance, eradication schemes, and the facilitation of international animal trade, in alignment with the standards advocated by the World Organisation for Animal Health (WOAH) and the Food and Agriculture Organization (FAO).

Serological Assays: The Frontline of Surveillance

Agar Gel Immunodiffusion (AGID): The Traditional Mainstay

Historically, the agar gel immunodiffusion (AGID) test has served as the primary serological tool for detecting antibodies against SRLV, and it remains widely employed in many national surveillance programs due to its low cost, technical simplicity, and lack of requirement for specialized laboratory equipment [2, 5, 9]. The assay relies on the diffusion of antibodies from test sera and a standardized viral antigen preparation through an agarose gel matrix; the formation of a visible precipitin line between the antigen and antibody wells indicates a positive reaction, with the test being interpreted subjectively. Early epidemiological surveys, such as those conducted in São Paulo state, Brazil, utilized AGID to establish baseline seroprevalence data, reporting rates of 0.3% for MVV in sheep and 15.1% for CAEV in goats [2]. Similar AGID-based surveys in the Maranhão state of Brazil revealed a much lower frequency, with only 0.08% of sheep and 0.39% of goats testing seropositive, underscoring the geographic variability in SRLV prevalence [5].

Despite its historical utility, the limitations of AGID are profound and well-documented. The most critical drawback is its notoriously low analytical sensitivity compared to modern immunoassays. Multiple independent studies have quantitatively demonstrated this disparity. The recently developed recombinant CA protein-based iELISA described by Ma et al. was found to be 400 to 1600 times more sensitive than the available AGID test [1]. This finding corroborates earlier comparative work by Nascimento et al., which showed that when a standardized i-ELISA for caprine lentiviruses was compared with AGID, the i-ELISA detected 37.2% of samples as positive compared to only 18.4% by AGID, yielding a sensitivity of the i-ELISA relative to AGID of 94.5%, but a specificity of only 75.7% [7]. This discrepancy highlights a crucial point: AGID is prone to false-negative results, particularly in animals with low antibody titers, a common occurrence during the early stages of infection, in latently infected animals experiencing viral intermittence, or in those infected with divergent viral strains that may not react well with the antigen used in the commercial AGID kit [4]. Furthermore, Peixoto et al. noted that Western blotting can detect antibodies at a dilution 256 times greater than that of AGID, underscoring the latter's fundamental insensitivity [4].

Efforts to refine the AGID methodology have led to the development of a micro-AGID variation, which employs smaller volumes of reagents (10 μL of antigen and 30 μL of test serum) compared to the traditional macro-AGID (20 μL each). In a comparative evaluation of 447 caprine serum samples, Arruda et al. demonstrated an excellent adjusted agreement (Kappa = 0.90) between the two formats, with the micro-AGID offering the advantages of clearer precipitin lines and results obtainable up to 24 hours earlier [9]. This modification represents a pragmatic improvement for low-resource settings, yet it does not overcome the fundamental sensitivity deficit inherent to the immunodiffusion principle. Given these limitations, while AGID can serve as a useful herd-level screening tool in regions with high prevalence, its inadequacy for individual animal certification, especially in official eradication campaigns, is unequivocal [7].

Enzyme-Linked Immunosorbent Assay (ELISA): Superior Sensitivity and Scalability

The enzyme-linked immunosorbent assay (ELISA) has progressively superseded AGID as the serological test of choice for large-scale SRLV surveillance, offering unparalleled throughput, objective spectrophotometric readouts, and substantially enhanced sensitivity. The most significant recent advancement in this domain is the development of recombinant protein-based indirect ELISAs (iELISA), which circumvent the batch-to-batch variability and potential biosafety concerns associated with whole-virus antigen production.

The seminal work by Ma et al. in 2025 exemplifies this paradigm shift. They successfully engineered an iELISA utilizing the recombinant SRLV capsid protein (CA), specifically the p28 antigen, a highly conserved structural protein that elicits a robust humoral immune response early in infection [1]. Through rigorous checkerboard titration optimization, they established a stringent cut-off value of 0.09 (based on sample-to-positive ratios, S/P). The assay demonstrated exceptional reproducibility, with intra- and inter-assay coefficients of variation (CV) below 6.60%, satisfying rigorous quality control standards for diagnostic deployment. Critically, the test exhibited no serological cross-reactivity with six common small ruminant pathogens (including Brucella ovis, Chlamydophila abortus, and Coxiella burnetii), confirming its high specificity [1]. When applied to a field trial with 93 clinical serum samples, all iELISA-positive results were subsequently confirmed by Western blotting, indicating a negligible false-positive rate. The true power of this assay was demonstrated in a massive epidemiological application: screening 4,786 clinical serum samples across 13 cities in six provinces of China revealed an overall SRLV seroprevalence of 10.64%, with rates fluctuating from 0.85% to 40.00% depending on the geographic region [1]. This study provides irrefutable evidence that a recombinant CA-based iELISA is not only viable but is the preferred tool for comprehensive, cost-effective national surveillance.

Earlier work by Nascimento et al. on standardizing an i-ELISA for caprine lentiviruses in Brazil further reinforces the advantages of this platform. Their optimization established ideal parameters of 0.25 μg of whole-virus protein per well, a 1:200 serum dilution, and a 1:3,000 dilution of anti-goat IgG conjugate. In their comparative analysis, the i-ELISA detected 259 positive samples (37.2%) versus only 128 (18.4%) by AGID, confirming a markedly higher detection rate [7]. They also identified important epidemiological correlates, documenting a significantly higher prevalence in animals older than six months and a notable disparity between sexes (56.7% in males vs. 35.4% in females, p < 0.01), a statistic that underscores the role of management practices and potential sex-linked transmission dynamics [7].

The superior performance of ELISA over AGID can be attributed to its fundamental mechanism. ELISA amplifies the antibody-antigen binding signal through an enzyme-conjugated secondary antibody and a chromogenic substrate, allowing for the detection of very low concentrations of specific immunoglobulins. This is particularly critical for SRLV diagnosis, as infected animals may exhibit unstable levels of antibodies over weeks, months, and even years due to viral escape mechanisms and compartmentalization [4]. The ability to detect these fluctuating serological responses with high fidelity is what positions the iELISA as the recommended initial screening test for most WOAH-aligned control programs.

Western Blotting (WB): The Confirmatory Arbiter

While ELISA and AGID serve as front-line screening tools, Western blotting (WB) has emerged as the definitive confirmatory method due to its unparalleled ability to resolve antibodies against individual viral proteins (e.g., p25, p28, gp135). The diagnostic power of WB lies in its ability to discriminate between specific immune responses to SRLV structural and envelope antigens and non-specific cross-reactivity that may plague other serological tests. As extensively reviewed by Peixoto et al., WB can detect anti-SRLV antibodies at a dilution 256 times greater than AGID and 32 times greater than ELISA, making it the most sensitive serological technique available [4].

The WB process involves electrophoretic separation of viral lysate or recombinant antigens, their transfer to a membrane, and subsequent incubation with test sera. The appearance of specific bands corresponding to viral capsid (p25/p28) and envelope (gp135) proteins constitutes a positive result. This technology is particularly valuable for resolving ambiguous or borderline ELISA results, as it can discern whether the low-titer reactivity is genuinely SRLV-specific or a false positive. Furthermore, WB is the only serological method capable of detecting the early seroconversion window; production of antibodies against CA proteins (p25 and p28) begins around the third week post-infection, with antibodies to envelope glycoproteins appearing after the fifth week [4]. This early detection capability is crucial for identifying newly infected animals before they become efficient transmitters of the virus.

In the study by Ma et al., the high fidelity of their iELISA was validated by WB; all 93 clinical serum samples that tested positive by iELISA were confirmed by WB, demonstrating that the iELISA had a low false positive rate [1]. However, the practicality of WB as a screening tool is limited by its labor-intensive nature, high cost, requirement for specialized equipment and technical expertise, and longer turnaround time compared to ELISA. Therefore, in the diagnostic algorithm advocated by leading veterinary authorities, WB is reserved as the gold standard for confirmation of positive results from screening tests, particularly in the context of certification for international trade or entry into elite breeding nucleus flocks free of SRLV [4, 8]. The ability to test biological matrices other than serum, such as seminal plasma, further extends the utility of WB for investigating different routes of transmission, including venereal spread [4].

Molecular Detection: Direct Evidence of Infection

Polymerase Chain Reaction (PCR) and Proviral Load Quantification

Given the inherent limitations of serology, particularly the diagnostic window of seroconversion and the phenomenon of seronegative carriers, molecular detection techniques targeting the proviral DNA integrated into host leukocytes have become indispensable. The primary target for PCR-based assays is the integrated SRLV genome within infected monocytes and macrophages, the principal cellular reservoirs of the virus [3]. DNA-based PCR (or nested PCR) directly detects the presence of the pathogen itself, rather than the host's adaptive immune response, thus providing definitive proof of infection even in the absence of detectable antibodies.

The utility of PCR is inextricably linked to the understanding of SRLV pathogenesis. The virus establishes a state of viral latency in circulating monocytes; only upon differentiation into tissue macrophages does the virus become transcriptionally active, leading to productive infection and the release of new virions [3]. This cellular state means that the proviral load in peripheral blood can be low and variable, making the choice of genomic target for PCR critical. Most assays target highly conserved regions of the gag (p25/p28) or pol genes to minimize false negatives due to genetic drift among diverse SRLV strains [6]. The study by Materniak-Kornas et al. exemplifies the modern application of molecular techniques, where they screened 107 sheep from Polish flocks for SRLV infection by both serological testing and PCR, using the results to categorize animals as infected or non-infected for subsequent genetic association studies [6]. Their work demonstrated a positive association between certain single nucleotide polymorphisms (SNPs) in the TMEM154 gene and SRLV infection status, and importantly, they analyzed the relationship between these SNPs and SRLV proviral load. Five SNPs showed a strong association with proviral load across the entire test population, suggesting that genetic markers may influence not only susceptibility to infection but also the capacity for viral replication within the host [6]. This highlights a potential future diagnostic pathway: combining serology with PCR-based genotyping and proviral load quantification to refine risk assessment and guide selective breeding for resistance.

Virus Isolation: The Definitive but Impractical Gold Standard

The most unambiguous evidence of SRLV infection is the isolation of the live virus from a biological sample. This technique, while historically critical for characterizing viral strains, is laborious, time-consuming (often requiring 30–50 days of culture), requires specialized biocontainment facilities, and suffers from a low success rate due to the predominantly cell-associated nature of SRLV [8]. The standard protocol involves co-cultivation of the animal's peripheral blood leukocytes (PBLs) with permissive target cells, such as goat synovial membrane (GSM) cells. The study by Feitosa et al. provides a textbook example: from a naturally infected goat in Rio Grande do Norte, Brazil, they isolated a CAEV strain (designated BrRN-CNPC.G1) by co-culturing PBLs on GSM monolayers. After 50 days of co-culture, characteristic viral cytopathic effects (CPE), including syncytia formation, were observed, and the presence of the viral genome was confirmed by nested PCR on the culture supernatant [8].

This isolate represented the first documented SRLV isolation from a naturally infected goat in that specific Brazilian state, paving the way for subsequent molecular characterization and phylogenetic analysis. However, as a diagnostic method for routine surveillance, virus isolation is entirely impractical. Its primary role is in research settings for generating viral stocks, studying viral evolution, and developing new antigens for serological tests or inactivated vaccines. For field diagnosis, PCR has largely superseded virus isolation as the molecular method of choice, offering a balance between specificity (detecting the viral genome) and practical feasibility (results in 24–48 hours) [6].

Integration of Diagnostic Modalities: A Multi-Tiered Strategy

No single diagnostic test satisfies the criteria of 100% sensitivity and 100% specificity for SRLV. The inherent trade-offs necessitate a multi-tiered diagnostic algorithm. Initial screening of large populations is best performed using a highly sensitive iELISA, such as the recombinant CA-based assay from Ma et al., which can process hundreds of samples daily at a low cost per test [1]. All ELISA-positive samples should ideally be subjected to confirmatory Western blotting to rule out false positives, particularly in low-prevalence populations. Animals yielding negative results on ELISA can be confidently classified as uninfected, provided the assay has been validated for the target population.

For animals that are ELISA-negative but belong to a high-risk epidemiological context (e.g., a confirmed seropositive herd with clinical cases), or for certification of individual animals intended for trade, a parallel PCR test for proviral DNA is recommended. PCR can identify seronegative carriers during the early window period or in the subset of infected animals that exhibit intermittent or very low antibody production [4]. The combined use of serology and PCR maximizes the negative predictive value, ensuring that only truly uninfected animals are certified as SRLV-free. Furthermore, the incorporation of quantitative PCR (qPCR) to measure proviral load, as used in the TMEM154 association study [6], provides a refined metabolic indicator of infection severity and potential transmissibility.

In conclusion, the diagnostic arsenal for SRLV has evolved from the rudimentary but robust AGID towards a sophisticated integration of high-throughput serological screening (iELISA), definitive confirmatory immunoblotting (WB), and direct molecular detection (PCR). The recent development of recombinant protein antigens marks a quantum leap in test standardization and sensitivity. For epidemiologists and veterinary practitioners seeking to control these economically devastating lentiviruses, the path forward is clear: a judicious combination of these technologies, interpreted with an understanding of the host-pathogen dynamics and the limitations of each method, is mandatory for successful eradication and the safeguarding of global small ruminant production systems.

Genetic Diversity and Phylogenetic Classification of SRLV Strains: Implications for Diagnosis and Control

The Molecular Landscape of SRLV Genomic Heterogeneity

The small ruminant lentiviruses (SRLV) represent a profoundly heterogeneous viral group, encompassing the classical prototypes of Visna/Maedi virus (VMV) in sheep and Caprine Arthritis-Encephalitis virus (CAEV) in goats, along with a continuously expanding array of recombinant and intermediate strains [1, 3]. This genetic diversity is not merely a taxonomic curiosity but constitutes the single most formidable obstacle to the development of universal diagnostic platforms and globally effective control programs. The very classification of these viruses as a unified group stems from their shared genomic organization and pathogenic mechanisms, both infect cells of the monocyte/macrophage lineage, establish lifelong persistent infections, and induce slow, progressive inflammatory diseases primarily affecting the lungs, nervous system, joints, and mammary glands [3]. However, beneath this superficial unity lies a genomic plasticity that rivals that of their primate lentivirus counterparts, with profound implications for serological detection, molecular characterization, and the design of intervention strategies.

The phylogenetic architecture of SRLV strains is characterized by a complex branching pattern that has been refined over decades of molecular epidemiology. Initial classifications broadly divided SRLV into two major groups corresponding to the prototype viruses: the MVV-like group (genotype A) and the CAEV-like group (genotype B) [1]. However, subsequent investigations have revealed a far more intricate tapestry, with additional genotypes (C, D, E, and others) identified in specific geographic regions and host species. These genotypes are not monophyletic in the strictest sense, as extensive inter-genotypic recombination events have generated chimeric strains that defy simple categorization. The capsid protein (CA), encoded by the gag gene, has emerged as a critical phylogenetic marker, and it is this very protein, specifically the p28 antigen, that serves as the foundation for many serological diagnostic assays [1]. The genetic variability within the CA region, while constrained by functional requirements for capsid assembly and stability, nevertheless exhibits sufficient polymorphism to influence antibody recognition patterns across different strains and genotypes.

Evolutionary Drivers and Viral Quasispecies Dynamics

The genetic diversity of SRLV is a direct consequence of the fundamental error-prone nature of retroviral replication. The viral RNA-dependent DNA polymerase (reverse transcriptase) lacks proofreading exonuclease activity, resulting in a mutation rate that is orders of magnitude higher than that observed in DNA viruses or host cellular genomes. This intrinsic mutagenic potential is compounded by the viral recombination machinery, which can generate novel genomic configurations through template switching during reverse transcription in cells co-infected with distinct strains [4]. The resulting viral populations exist not as homogeneous entities but as dynamic quasispecies, clouds of closely related but genetically distinct variants that continuously evolve under selective pressures imposed by the host immune system and tissue microenvironments.

The host immune response itself serves as a primary selective force driving SRLV diversification. Infected animals mount both humoral and cellular immune responses, with antibodies directed predominantly toward viral capsid proteins (p25 and p28) appearing around the third week post-infection, followed by immunoglobulins targeting other viral proteins after approximately five weeks [4]. This antibody-mediated selection pressure favors the emergence of escape variants with altered epitope sequences, perpetuating a molecular arms race between viral evolution and host immune surveillance. The process of compartmentalization further exacerbates this diversity, as anatomically distinct tissue sites, such as the lungs, joints, mammary glands, and central nervous system, can harbor genetically differentiated viral subpopulations that evolve semi-independently [4]. This compartmentalization has critical implications for diagnosis, as the viral population circulating in peripheral blood may not accurately reflect the quasispecies present in target organs, potentially leading to discordant results between blood-based molecular tests and the actual tissue viral burden.

Translating Genetic Diversity into Diagnostic Challenges

The profound genetic heterogeneity of SRLV strains directly undermines the sensitivity and specificity of both serological and molecular diagnostic approaches. Serological tests, which form the backbone of SRLV surveillance and control programs worldwide, are particularly vulnerable to antigenic variation. The agar gel immunodiffusion (AGID) test, long considered the standard for herd-level screening, relies on the detection of precipitating antibodies against viral antigens. However, AGID demonstrates markedly lower sensitivity compared to more modern platforms, with one study reporting that a recombinant CA protein-based indirect ELISA (iELISA) was 400–1600 times more sensitive than the available AGID test [1]. This enormous disparity in analytical sensitivity underscores the limitations of AGID in detecting low-level antibody responses, particularly in animals infected with divergent strains that may present altered epitope profiles.

The genetic diversity of the capsid protein itself poses a fundamental challenge for ELISA-based diagnostics. The recombinant CA protein (p28) used in the iELISA developed by Ma et al. (2025) was designed to provide broad reactivity across SRLV strains, and its successful application in screening 4786 clinical serum samples across six provinces in China, revealing an overall seroprevalence of 10.64% with positivity rates ranging from 0.85% to 40.00%, suggests a reasonable degree of cross-reactivity [1]. However, the extent to which this single recombinant antigen captures antibodies elicited by the full spectrum of SRLV genotypes and recombinants remains uncertain. Studies have consistently demonstrated that Western blotting (WB) offers superior diagnostic accuracy compared to both AGID and ELISA, with the capacity to detect antibodies at dilutions 256 times greater than AGID and 32 times greater than ELISA [4]. This enhanced performance derives from WB's ability to resolve multiple immunogenic antigens simultaneously, providing a more comprehensive assessment of the antibody repertoire that compensates for potential epitope mismatches between the diagnostic antigen and circulating viral strains [4].

The "No Gold Standard" Conundrum and the Need for Multi-Assay Platforms

A particularly vexing aspect of SRLV diagnosis is the absence of a universally accepted gold standard test [4]. This situation is not merely a technical inconvenience but a direct reflection of the genetic diversity problem: no single assay can reliably detect all infected animals across the full spectrum of viral genotypes and stages of infection. The interplay between viral genetic diversity and host immune dynamics creates a diagnostic landscape characterized by temporal fluctuations in antibody titers, with infected animals presenting unstable levels of immunoglobulins over weeks, months, and even years [4]. This antibody instability, combined with the phenomenon of viral intermittence, where peripheral blood proviral load may transiently fall below detectable thresholds, means that single-test, single-timepoint diagnostic approaches will inevitably miss a proportion of infected animals.

Comparative studies have consistently highlighted the superiority of more sensitive platforms. Nascimento et al. (2014) demonstrated that an iELISA detected 37.2% positive samples compared to only 18.4% by AGID in a caprine population, yielding an ELISA sensitivity of 94.5% relative to AGID but a specificity of only 75.7% [7]. This lower specificity of ELISA relative to AGID likely reflects both the higher sensitivity of the platform (detecting true positives missed by AGID) and the potential for cross-reactivity with antibodies induced by non-SRLV antigens or divergent viral strains. The micro-AGID format, which uses reduced reagent volumes and provides clearer precipitation lines within a shorter timeframe, has been proposed as a practical alternative to conventional macro-AGID, with a reported kappa agreement of 0.90 between the two formats [9]. However, even this refined AGID variant cannot overcome the fundamental sensitivity limitations inherent to immunodiffusion-based detection.

Genetic Diversity as a Correlate of Control Strategy Efficacy

The existence of multiple SRLV genotypes and recombinant strains has profound implications for the design and implementation of control programs. Classical eradication approaches, which rely on serological testing and removal of positive animals, are predicated on the assumption that available diagnostic tests can reliably identify all infected individuals. The genetic diversity of SRLV strains fundamentally undermines this assumption. If a diagnostic assay is optimized against a particular genotype (e.g., a prototype CAEV strain), it may exhibit reduced sensitivity for animals infected with divergent genotypes (e.g., genotype A or recombinant strains), allowing infected animals to escape detection and perpetuate transmission within the herd.

The geographic distribution of SRLV genetic diversity further complicates control efforts. Serological surveys conducted in Brazil have revealed starkly contrasting prevalence patterns, with a study in São Paulo state reporting 15.1% seropositivity for CAEV in goats but only 0.3% for MVV in sheep [2], while a survey in Maranhão found much lower frequencies of 0.39% in goats and 0.08% in sheep [5]. These regional variations likely reflect differences in viral strains circulating in different geographic areas, as well as management practices and breed-specific susceptibility factors. The first isolation of a SRLV strain from a naturally infected goat in the Rio Grande do Norte state of Brazil, designated BrRN-CNPC.G1, represents an important step toward characterizing the circulating regional strains and understanding their relationship to prototype viruses [8]. Only through comprehensive molecular characterization of such isolates, analyzing structural genes and comparing them with existing sequences, can researchers identify the likely sources of infection and establish the genetic relationships between strains circulating in different regions [8].

Host Genetics and the Prospect of Resistance Breeding

An alternative and increasingly promising approach to SRLV control involves leveraging host genetic resistance to reduce susceptibility to infection and disease progression. The concept of breeding for resistance is particularly attractive in the context of SRLV genetic diversity, as it targets the host rather than the pathogen, potentially conferring protection against a broad spectrum of viral strains. Research has identified several candidate genes associated with SRLV susceptibility, with the most prominent being the gene encoding transmembrane protein 154 (TMEM154) [6]. Multiple single nucleotide polymorphisms (SNPs) within this gene have been identified in sheep of different breeds, and certain alleles have been associated with resistance to SRLV infection.

Materniak-Kornas et al. (2024) conducted the first investigation of TMEM154 SNPs in Polish sheep flocks, genotyping 107 animals representing three breeds for their SRLV infection status and proviral load [6]. Their findings revealed that the frequency of identified alleles differed among breeds, and a significant association between TMEM154 genotype and SRLV status was found for the E35K polymorphism and two polymorphic sites in the 5′ untranslated region (5′UTR) in one breed. Moreover, when the relationship between SNPs and SRLV proviral load was analyzed across the entire study population, five polymorphisms showed a strong association with viral burden [6]. These findings suggest that selecting SRLV-resistant animals based on TMEM154 genotyping might be feasible, although validation in larger sheep populations is required. The potential utility of host genetics in SRLV control is further supported by evidence that host genetics play an important role in determining susceptibility/resistance to SRLV infection and disease progression, though relatively little research has been performed in small ruminants compared to other livestock species [3].

Implications for Vaccine Development and Eradication Campaigns

The genetic diversity of SRLV strains presents a formidable barrier to vaccine development. An effective vaccine must elicit protective immune responses capable of recognizing and neutralizing the full spectrum of circulating viral genotypes and recombinants. The antigenic variation observed in the capsid protein and envelope glycoproteins means that a vaccine based on a single prototype strain is unlikely to provide broad protection. This challenge is compounded by the lentiviral propensity for immune evasion through epitope masking, glycosylation shielding, and the establishment of latent proviral reservoirs that are inaccessible to immune effectors.

From a regulatory and trade perspective, the genetic diversity of SRLV strains has direct economic consequences. The World Organisation for Animal Health (WOAH) recognizes SRLV infections as significant diseases affecting international trade in sheep and goats and their products [1]. Countries seeking to export breeding stock or genetic material must demonstrate freedom from infection, typically through serological testing of donor animals. However, the sensitivity of the testing protocol is critically dependent on the diagnostic platform employed and its ability to detect antibodies induced by the local circulating strains. The use of a recombinant CA protein-based ELISA, such as the one validated by Ma et al. (2025), represents a significant advancement toward standardized, broadly reactive diagnostic tools that can be applied across different geographic regions [1]. The indirect ELISA format offers advantages in terms of cost-effectiveness, scalability, and suitability for large-scale surveillance, key considerations for national control programs and international trade certification.

The World Health Organization (WHO) and the Food and Agriculture Organization (FAO) have emphasized the importance of integrated approaches to livestock disease control that combine diagnostic surveillance, biosecurity measures, and genetic improvement strategies. For SRLV, this integrated framework must explicitly account for the challenge posed by viral genetic diversity. Control programs should incorporate diagnostic algorithms that utilize multiple test platforms, perhaps combining a high-throughput ELISA for initial screening with Western blotting or PCR for confirmation of borderline or discordant results [4, 7]. The superior accuracy of WB in detecting antibodies across a range of viral proteins, including those encoded by diverse genotypes, positions it as a valuable confirmatory tool in the diagnostic cascade [4].

The spatial analysis of SRLV outbreaks, as conducted by Soares et al. (2020) in the microregion of Low Parnaíba, Maranhão, Brazil, reveals that reactive flocks are often distributed in strategic cities for animal commercialization throughout the microregion [5]. This geographic pattern highlights the role of animal movement in disseminating diverse viral strains across wide areas, further complicating control efforts. The seroprevalence data from China, where positivity rates varied dramatically from 0.85% to 40.00% across different cities and provinces [1], suggest that regional strain prevalence and diversity may be influenced by factors such as breed composition, management intensity, importation history, and climate. Understanding these epidemiological patterns at a molecular level is essential for tailoring control strategies to local conditions.

Control Strategies and Biosecurity Measures for SRLV Infections in Sheep and Goat Populations

The control of small ruminant lentivirus (SRLV) infections, encompassing Maedi-Visna virus (MVV) in sheep and caprine arthritis-encephalitis virus (CAEV) in goats, represents one of the most formidable challenges in contemporary veterinary medicine. These viruses, members of the Retroviridae family, establish lifelong, persistent infections characterized by a protracted subclinical phase followed by progressive, debilitating inflammatory disease [3]. The economic ramifications are substantial, manifesting as reduced milk production, premature culling, decreased reproductive efficiency, and impeded international trade in livestock and germplasm [1, 3]. Critically, there is no effective treatment or commercially available vaccine, rendering the entire control paradigm dependent on a multi-pronged strategy of robust biosecurity, accurate diagnostics, and strategic animal management [3, 4]. The World Organisation for Animal Health (WOAH) classifies these infections as notifiable in many contexts due to their economic impact, underscoring the need for rigorous, evidence-based control programs.

The Diagnostic Cornerstone: Serological Surveillance and Test Performance

Effective control is impossible without reliable detection. The cornerstone of any SRLV control program is a rigorous, population-level serological surveillance system. The majority of infected animals remain asymptomatic for years, acting as silent reservoirs that perpetuate viral transmission within and between flocks [1, 3]. Therefore, control strategies must be predicated on identifying these subclinical carriers. The diagnostic landscape has evolved significantly, moving from traditional methods to more sensitive and specific platforms.

The agar gel immunodiffusion (AGID) test has historically been the most widely used serological tool, particularly in resource-limited settings, due to its simplicity and low cost [2, 5, 7, 9]. However, its limitations are profound. Studies consistently demonstrate that AGID suffers from markedly lower sensitivity compared to enzyme-linked immunosorbent assays (ELISA) and Western blotting (WB) [1, 4, 7]. For instance, a recombinant capsid protein (p28)-based indirect ELISA (iELISA) was found to be 400–1600 times more sensitive than the AGID test in detecting anti-SRLV antibodies in Chinese sheep and goat populations [1]. Similarly, comparative studies in Brazil have shown that iELISA detects a significantly higher proportion of positive animals (37.2%) compared to AGID (18.4%) in the same sample set, with the iELISA demonstrating a sensitivity of 94.5% relative to AGID [7]. This discrepancy is critical; reliance on AGID alone will systematically underestimate true prevalence, allowing infected animals to remain in the herd and undermine eradication efforts.

The Western blot (WB) technique represents the current gold standard for confirmatory diagnosis, offering the highest sensitivity and specificity among serological methods [4]. WB can detect antibodies at dilutions 256 times greater than AGID and 32 times greater than ELISA [4]. This is particularly important given the intermittent and often low-level antibody production characteristic of SRLV infections, a phenomenon driven by viral immune evasion mechanisms such as genetic diversity, antigenic variation, and compartmentalization within the host [4]. The ability of WB to simultaneously resolve antibodies against multiple viral proteins (e.g., p25, p28, gp135) provides exceptional diagnostic reliability, making it indispensable for confirming infection status in valuable breeding stock or for resolving ambiguous ELISA results [4]. However, its high cost, technical complexity, and labor-intensive nature preclude its use as a primary screening tool for large populations. Therefore, a tiered diagnostic approach is recommended: initial screening with a high-throughput, sensitive iELISA [1, 7], followed by confirmatory WB testing of all positive or suspect samples [4]. This strategy balances cost-effectiveness with diagnostic accuracy, a prerequisite for any large-scale control program.

Biosecurity: The First Line of Defense Against Horizontal and Vertical Transmission

Biosecurity measures are the operational backbone of SRLV control, designed to interrupt the known routes of viral transmission. SRLVs are transmitted primarily through the ingestion of infected colostrum and milk (lactogenic transmission) and through prolonged, direct contact with infected animals via respiratory secretions and other bodily fluids [3]. Iatrogenic transmission through contaminated needles, dehorning equipment, and tattooing instruments is also a significant, yet preventable, route.

Management of Lactogenic Transmission: The most critical and effective biosecurity intervention is the prevention of lactogenic transmission from dam to offspring. This is achieved through a strict "pasteurization and separation" protocol. Immediately after birth, lambs and kids must be removed from their dams before they have the opportunity to suckle. They should be fed heat-treated colostrum (e.g., 56°C for 60 minutes) from a known negative dam or a commercial bovine colostrum substitute, followed by pasteurized milk or milk replacer. This single intervention can dramatically reduce the incidence of new infections in a replacement stock, as it breaks the primary route of transmission to the next generation. The success of this strategy is contingent upon rigorous adherence; any lapse in protocol can reintroduce infection into a clean cohort.

Herd Structure and Contact Management: Horizontal transmission via direct contact is facilitated by high stocking densities, shared feeding and watering points, and prolonged co-mingling. Control programs must therefore implement strict segregation policies. Flocks should be managed in distinct groups based on serological status: a "negative" or "clean" group, a "positive" or "infected" group, and a "quarantine" group for new introductions. Physical separation, including dedicated equipment, footwear, and personnel, is essential to prevent cross-contamination. The use of separate pastures or barns, with a minimum distance between groups, is strongly advised. Studies from São Paulo state, Brazil, have highlighted the widespread nature of CAEV infection, with a seroprevalence of 15.1% in goats, emphasizing that without such segregation, the virus can become endemic and difficult to manage [2]. In contrast, the low prevalence of MVV (0.3%) in the same region suggests that early intervention and biosecurity can be effective in limiting spread [2].

Quarantine and Testing of New Additions: The introduction of new animals is the single highest-risk activity for introducing SRLV into a naïve flock. A mandatory quarantine period of at least 60–90 days, combined with sequential serological testing (e.g., ELISA at entry and again at the end of quarantine), is non-negotiable. Given the slow seroconversion rate, a single negative test upon arrival is insufficient. Animals should only be introduced into the negative flock after two consecutive negative test results, ideally confirmed by WB if resources permit [4]. This rigorous protocol is the only way to ensure that new stock does not harbor the virus in a pre-seroconversion window.

Genetic Selection: A Paradigm Shift Towards Host Resistance

While biosecurity and culling remain the mainstays of control, a revolutionary approach is emerging: the genetic selection of animals with inherent resistance to SRLV infection. This strategy offers a sustainable, long-term solution that reduces reliance on intensive testing and culling. Recent research has identified the transmembrane protein 154 (TMEM154) gene as a major determinant of susceptibility to SRLV infection in sheep [6]. Specific single nucleotide polymorphisms (SNPs) within this gene are strongly associated with resistance or susceptibility. For example, the E35K polymorphism has been positively associated with SRLV infection status in certain breeds [6]. Furthermore, several SNPs in the 5' untranslated region (UTR) of the TMEM154 gene have shown a strong association with proviral load, a key indicator of viral replication and transmission potential [6].

The implications for control are profound. By genotyping breeding rams and ewes for these resistance-associated alleles, producers can selectively breed a flock that is genetically less permissive to SRLV infection. This approach does not require the immediate culling of infected animals but rather a gradual, generational shift in the genetic makeup of the herd. The identification of these markers in Polish sheep flocks [6] and other breeds globally suggests that this is a broadly applicable strategy. However, further validation in larger, diverse populations is required before it can be universally recommended as a standalone control measure. It is most powerful when integrated with traditional biosecurity and testing, creating a multi-layered defense.

Strategic Culling and Flock Eradication

For flocks with a low to moderate prevalence, a test-and-cull strategy can be effective. This involves periodic serological testing of the entire flock (e.g., semi-annually or annually) and the immediate removal of all seropositive animals. This approach is most successful when combined with the strict biosecurity measures outlined above, particularly the prevention of lactogenic transmission. The goal is to progressively reduce the prevalence to zero. However, this strategy is economically challenging, as it requires the removal of potentially valuable breeding stock. For flocks with very high prevalence (>50%), a complete depopulation and repopulation with certified SRLV-negative stock may be the most cost-effective and rapid route to eradication, despite its high initial cost and emotional toll on producers.

Regulatory Frameworks and International Trade Implications

The implementation of control strategies is often driven by regulatory frameworks and market access requirements. Many countries, particularly in Europe, have established national voluntary or mandatory control programs for SRLV. These programs typically mandate regular testing, movement restrictions on positive animals, and certification of SRLV-free status for flocks. The absence of such a program can severely limit export opportunities, as importing countries demand guarantees of disease freedom. The development of cost-effective, high-throughput diagnostic tools, such as the recombinant CA protein-based iELISA [1], is critical for enabling large-scale surveillance in countries like China, where knowledge of SRLV prevalence is limited but the need for control is urgent [1]. The spatial analysis of outbreaks, as demonstrated in Maranhão, Brazil, where positive flocks were concentrated in strategic commercial hubs [5], underscores the need for targeted, regionally coordinated control efforts that align with animal movement patterns.

In conclusion, the control of SRLV infections demands an integrated, multi-faceted approach. No single intervention is sufficient. The most effective programs combine rigorous diagnostic surveillance using sensitive tools like iELISA and confirmatory WB [1, 4, 7], strict biosecurity protocols that eliminate lactogenic and horizontal transmission [2, 3], strategic culling or segregation of infected animals, and the emerging promise of genetic selection for host resistance [6]. The path to eradication is long and requires unwavering commitment from producers, veterinarians, and regulatory bodies, but the economic and welfare benefits of SRLV-free flocks are substantial and enduring.

References

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