Caprine Arthritis-Encephalitis Virus

Overview and Taxonomy of Caprine Arthritis-Encephalitis Virus

Caprine arthritis-encephalitis virus (CAEV) is a monocyte/macrophage-tropic lentivirus belonging to the family Retroviridae, genus Lentivirus, and is a member of the group collectively termed small ruminant lentiviruses (SRLVs) [1, 2, 3]. The virus is the etiological agent of caprine arthritis-encephalitis (CAE), a multisystemic, slowly progressive inflammatory disease of goats that causes substantial economic losses globally through reduced milk production, increased culling rates, diminished animal welfare, and heightened veterinary costs [1, 4, 5, 6]. The disease is notifiable in many countries and is included in the World Organisation for Animal Health (WOAH) list of infections of importance for international trade, underscoring its significance for the global small ruminant industry. CAEV is endemic in most regions where goat farming is practiced, with documented seroprevalences ranging from as low as 0.77% in sheep in eastern China to over 60% in some dairy goat populations in Taiwan [7, 8]. The virus imposes a particularly heavy burden on intensive dairy operations, where management practices such as pooled colostrum feeding and high stocking densities facilitate transmission [9, 10].

Taxonomic Position and Genetic Classification

CAEV is classified within the Lentivirus genus, which includes other non-oncogenic retroviruses such as human immunodeficiency virus (HIV), simian immunodeficiency virus (SIV), feline immunodeficiency virus (FIV), and equine infectious anemia virus (EIAV). The lentiviruses are characterized by their long incubation periods, persistent infection despite a host immune response, and progressive pathology affecting multiple organ systems [2, 3]. Within the SRLV group, CAEV and the ovine maedi-visna virus (MVV) are antigenically and genetically related, and the International Committee on Taxonomy of Viruses (ICTV) has recently reclassified the species into Lentivirus capartenc (formerly CAEV) and Lentivirus ovivismae (formerly MVV) [11]. Despite this taxonomic distinction, natural cross-species infections occur, and both viruses can infect sheep and goats, complicating control efforts [11, 12, 13, 14].

Phylogenetic analyses based on sequences of the gag, pol, and env genes have delineated at least five distinct SRLV genetic groups, designated A through E, with subtypes within several groups. Genotype A (MVV-like) is predominantly found in sheep, while genotype B (CAEV-like) is most common in goats [7, 15, 16, 12, 17, 6]. The prototypic CAEV strain is CAEV-Co (or CAEV-Cork), originally isolated from a goat with arthritis in the United States; this strain serves as the reference for genotype B, subtype B1 [17, 13]. Molecular characterization of field isolates from diverse geographic regions, including Iran, Argentina, Russia, China, Switzerland, and Italy, consistently identifies B1 as the predominant circulating subtype, although genotype A viruses are occasionally detected in goats, particularly in multispecies farms where sheep serve as reservoirs [1, 7, 16, 12, 17, 6, 14]. A notable finding from the Swiss cross-border outbreak in 2017 revealed high genetic heterogeneity within a single imported flock, with multiple B1 variants co-circulating, indicating that multiple introductions or within-host recombination can occur [6]. The U3 region of the viral long terminal repeat (LTR) contains transcription factor binding sites (TFBS) that modulate tissue-specific expression; a rare duplication of the gamma-activated site (GAS) has been reported in one isolate associated with severe renal and cardiac lesions, suggesting that subtle regulatory changes may influence pathogenicity [3].

Genomic Organization and Viral Structure

The CAEV genome is a single-stranded, positive-sense RNA of approximately 9.2 kb, flanked by LTRs. It encodes the canonical retroviral structural and enzymatic proteins: Gag (matrix, capsid, nucleocapsid), Pol (protease, reverse transcriptase, integrase), and Env (surface and transmembrane glycoproteins) [2, 3]. Additionally, like other lentiviruses, CAEV possesses regulatory and accessory genes, rev and vif, which are essential for replication and nuclear export of viral mRNAs. The Rev protein of CAEV contains a monopartite nuclear localization signal (NLS) composed exclusively of arginine residues between amino acids 59–75, a partially overlapping nucleolar localization signal (NoLS), and an unconventional nuclear export signal (NES) spanning residues 89–101 with a spacing pattern of hydrophobic residues distinct from other lentiviral Rev proteins [18]. This unique architecture may influence the kinetics of viral RNA trafficking and contribute to the virus’s ability to establish persistent infection.

Transmission Dynamics and Pathogenesis

CAEV is transmitted primarily through the ingestion of colostrum or milk from infected dams, a route known as lactogenic transmission [15, 19, 20]. Indeed, early studies demonstrated that heating colostrum at 56°C for 1 hour inactivates the virus, forming the basis for pasteurization protocols used in many eradication programs [20]. Horizontal transmission, via prolonged direct contact, contaminated equipment, or shared feeding troughs, is considered less efficient but has been documented, especially under intensive management where infected and uninfected animals are densely housed [20, 10]. Sexual transmission via infected semen has been modeled and reported, though its epidemiological significance remains debated [5, 21]. Intrapartum vertical transmission (transplacental) was traditionally considered negligible; however, recent Western blot evidence has demonstrated that approximately 1.4% of kids born to seropositive dams by sterile cesarean section are seropositive at birth, indicating that intrauterine infection can occur, albeit at very low frequency [15, 19]. This observation has been corroborated by negative PCR results in newborn kids from infected does in other studies, highlighting that such events are rare and unlikely to sustain endemicity [15].

Once inside the host, CAEV targets cells of the monocyte/macrophage lineage. Infected macrophages harbor proviral DNA and serve as vehicles for dissemination to target organs, including the synovium of joints, the central nervous system (CNS), the pulmonary interstitium, the mammary gland, and, as recently documented, the kidney [3]. The virus induces chronic, non-suppurative inflammation characterized by lymphocytic infiltration, fibrosis, and, in the CNS, demyelinating leukoencephalomyelitis. Apoptosis of glial cells, particularly astrocytes, at the edges of demyelinated plaques has been demonstrated through TUNEL labeling and detection of pro-apoptotic Bax and caspases-3, -8, and -9, while neurons express higher levels of anti-apoptotic Bcl-2, suggesting that glial cell death is a key mechanism in the pathogenesis of CAEV encephalitis [22]. Clinically, infected goats may present with arthritis (most common in adults >9 months), progressive paresis in kids (2–6 months old), interstitial pneumonia, indurative mastitis, and, more rarely, nephritis [23, 3, 24, 25]. Joint ultrasonography has revealed cartilage erosion, subchondral bone exposure, and loss of echogenicity in CAEV-associated arthritis [24]. Importantly, seroconversion can be delayed for many months after infection, with proviral DNA detectable by PCR before antibodies appear, complicating serology-based eradication campaigns [26]. The immune response itself contributes to pathology, as demonstrated by vaccine-enhanced arthritis in goats given inactivated virus and then challenged [27].

Global Distribution and Epidemiological Significance

CAEV has a worldwide distribution, with prevalence varying widely by region, management system, and diagnostic method (serology vs. molecular). Large-scale surveys report herd-level seroprevalences exceeding 70% in the United States and Japan, while animal-level prevalences range from 2% to 62% [8, 28, 10, 29]. In Europe, compulsory eradication programs in regions such as South Tyrol (Italy) have dramatically reduced seroprevalence, but a “tailing phenomenon”, persistent low-level seropositivity, has been attributed to spillover from co-housed sheep carrying genotype A viruses [12, 30]. In Africa, Sudan reported a 2.99% prevalence limited to imported breeds, while Bangladesh found 4.26% [4, 31]. The first molecular survey in western Iran using nested PCR revealed 17.1% positivity across three provinces, with no significant association with age, sex, or season, indicating year-round transmission [1]. Similarly, recent reports from Iraq, China, Russia, and Brazil underscore the ongoing global expansion of CAEV and the need for region-specific control strategies [7, 16, 32, 33, 34]. The economic impact is substantial: infected goats produce less milk with altered composition, experience shorter lactations, and are more likely to be culled early [35]. The disease also predisposes to secondary infections such as Mycoplasma agalactiae and subclinical mastitis, further compounding losses [35, 25].

From a taxonomic and evolutionary perspective, the genetic diversity of SRLVs necessitates careful molecular surveillance to track introduction events and inform vaccine or diagnostic design. The WOAH and FAO emphasize the importance of harmonized genotyping to support international trade and eradication programs. As CAEV continues to spread via animal movement, including cross-border trade, the integration of serological screening with PCR-based confirmation is essential for accurate diagnosis and eventual global control [9, 6, 36].

Molecular Pathogenesis and Host-Virus Interactions

The molecular pathogenesis of Caprine Arthritis-Encephalitis Virus (CAEV) is a paradigm of lentiviral chronicity, immunopathology, and tissue-specific tropism that unfolds over years rather than days. As a member of the Retroviridae family and a constituent of the small ruminant lentivirus (SRLV) group, CAEV establishes lifelong infections that are characterized by a protracted asymptomatic phase followed by the insidious development of inflammatory lesions in the synovium, mammary gland, pulmonary interstitium, central nervous system (CNS), and, as increasingly recognized, the renal parenchyma [15, 22, 3]. Understanding the molecular underpinnings of these interactions, from the cellular entry and transcriptional regulation to the mechanisms of immune evasion and tissue destruction, is essential for designing effective intervention strategies. The World Organisation for Animal Health (WOAH) recognizes CAEV as a significant constraint to small ruminant production globally, underscoring the need for a deeper comprehension of its pathobiology.

Viral Life Cycle, Genetic Regulation, and Cellular Tropism

CAEV, like all lentiviruses, relies on a complex interplay of viral and host factors to execute its replication cycle. The proviral genome, once integrated into the host cell DNA, encodes the structural gag, pol, and env genes, as well as regulatory proteins essential for viral gene expression. Among these, the Rev protein orchestrates the critical transition from early to late viral gene expression. Detailed mutational analyses have precisely mapped the functional domains of CAEV Rev: a monopartite nuclear localization signal (NLS) composed exclusively of arginine residues between amino acids 59–75, a partially overlapping nucleolar localization signal (NoLS), and a nuclear export signal (NES) positioned between amino acids 89–101 [18]. The NES of CAEV Rev exhibits an unconventional spacing of hydrophobic residues, distinguishing it from other retroviral Rev/Rev-like proteins yet fulfilling the same essential function of exporting unspliced and partially spliced viral mRNAs from the nucleus to the cytoplasm [18]. This nucleocytoplasmic shuttling is the linchpin of late gene expression, enabling the production of structural proteins and, ultimately, the assembly of infectious progeny.

The cellular tropism of CAEV is primarily restricted to cells of the monocyte/macrophage lineage, a hallmark shared with other lentiviruses [3]. The virus exploits the differentiation state of these cells; monocytes harbor the provirus in a transcriptionally quiescent state and only support active viral replication upon their maturation into tissue macrophages. This Trojan horse strategy facilitates dissemination throughout the body while evading immune detection. Proviral DNA can be detected in peripheral blood mononuclear cells, and the virus has been isolated from a wide array of tissues, including the synovial membrane, mammary gland, lung, brain, and, notably, the kidney [23, 3]. Immunohistochemical studies have confirmed the presence of viral antigen within glial cells and macrophages in the CNS, as well as within tubular epithelium of the kidney [23, 3]. The U3 region of the viral long terminal repeat (LTR) contains transcription factor binding sites (TFBSs) that govern cell-type-specific expression. While these TFBSs are generally conserved among CAEV isolates, reports of duplications, such as an additional gamma-activated site (GAS) in one isolate from a goat with renal and cardiac lesions, suggest that minor genetic variations in the LTR may influence tissue-specific pathogenesis and viral fitness [3].

Host-Virus Interactions: Immune Evasion, Antigenic Variation, and Delayed Seroconversion

A defining feature of CAEV infection is the profound delay between infection and detectable humoral immune response, a phenomenon that has direct and significant implications for eradication programs. Seroconversion can be delayed for many months following natural infection, during which time proviral DNA is detectable by polymerase chain reaction (PCR) in the absence of circulating antibodies [26]. This phenomenon was elegantly demonstrated in a longitudinal study where 20 of 81 seronegative goats were found to harbor proviral CAEV DNA; ten of these animals subsequently seroconverted within eight months, confirming that seronegativity does not equate to freedom from infection [26]. This delayed seroconversion is a critical obstacle to eradication campaigns that rely solely on serological testing, such as agar gel immunodiffusion (AGID) or enzyme-linked immunosorbent assay (ELISA), as infected animals may escape detection for extended periods [9, 26].

The virus employs multiple strategies to subvert host immunity. The error-prone nature of the lentiviral reverse transcriptase, coupled with the high recombination rate typical of retroviruses, generates a swarm of closely related but antigenically distinct variants, termed quasispecies. Sequence analysis of CAEV isolates from a single outbreak in Switzerland revealed a surprisingly high genetic heterogeneity, with evidence of multiple distinct strains infecting individual animals simultaneously [6]. This diversity, particularly within the env gene encoding the surface (SU) glycoprotein, allows the virus to escape neutralizing antibody responses. Phylogenetic analyses have consistently classified circulating CAEV strains into genotype B, predominantly subtype B1, based on gag and env sequences [7, 16, 17]. However, cross-species transmission and recombination events complicate this picture; goats on multispecies farms are frequently infected with genotype A strains, which are typically associated with ovine maedi-visna virus (MVV) [12]. This interspecies transmission blurs the traditional host-virus boundaries and poses a significant challenge for eradication, as sheep can serve as a reservoir for the CAEV-like virus capable of infecting goats [12, 13].

Molecular Mechanisms of Tissue Injury: Immunopathology and Apoptosis

The lesions characteristic of CAEV, chronic interstitial pneumonia, non-suppurative leukoencephalomyelitis, arthritis, and interstitial nephritis, are not the result of direct viral cytolysis but rather of immune-mediated damage driven by a persistent, dysregulated host response. The arthritic form of the disease provides the most compelling evidence for this immunopathological mechanism. Experimental studies have demonstrated that goats vaccinated with inactivated CAEV developed more severe arthritis upon challenge than unvaccinated controls, and that persistently infected goats developed acute arthritis following repeated injections of infectious virus [27]. These findings strongly suggest that the immune response itself, particularly the humoral and cell-mediated arms, is the primary driver of synovial inflammation and joint destruction [27]. Complement activation, deposition of immune complexes, and recruitment of inflammatory macrophages and lymphocytes contribute to the proliferative synovitis, pannus formation, and erosion of articular cartilage observed clinically.

Within the central nervous system, the pathogenesis of the acute demyelinating leukoencephalomyelitis seen in kids involves a highly regulated program of apoptotic cell death. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) positivity, coupled with immunolabeling for pro-apoptotic proteins Bax and caspases-3, -8, and -9, is prominent in glial cells, particularly at the edges of demyelinated plaques and in perivascular cuffs [22]. Double-labeling experiments have identified the majority of these apoptotic cells as astrocytes, with a smaller population of oligodendrocytes [22]. The activation of both caspase-8 (extrinsic pathway) and caspase-9 (intrinsic/mitochondrial pathway) suggests a multifaceted induction of apoptosis, likely triggered by inflammatory cytokines (e.g., tumor necrosis factor-alpha) and direct viral interactions [22]. In contrast, neurons within the brainstem, cerebellum, and spinal cord motor neurons predominantly express the anti-apoptotic protein Bcl-2, which may explain the relative preservation of neuronal cell bodies even in the face of extensive white matter destruction [22]. This selective vulnerability of glial cells, particularly the myelin-producing oligodendrocytes and the supportive astrocytes, underlies the characteristic demyelination that is the pathologic hallmark of the neurological form of CAE.

Beyond the joints and CNS, CAEV induces a similar chronic inflammatory process in other organ systems. Interstitial nephritis has been identified as a consistent finding in goats with multisystemic CAEV infection, with viral antigen detectable in tubular epithelium and proviral sequences amplified from renal tissue [3]. The presence of CAEV-associated thrombotic arteritis, leading to infarction of both kidney and heart in one reported case, indicates that vascular endothelium may also be a target or bystander in the inflammatory cascade [3]. In the mammary gland, chronic indurative mastitis results from mononuclear cell infiltration and fibrosis, which impairs milk secretion and contributes to the reduced lactational performance and elevated somatic cell counts observed in infected does [37]. Concurrently, CAEV-seropositive dams have been shown to produce colostrum with significantly lower immunoglobulin G (IgG) concentrations compared to seronegative controls, although levels of insulin-like growth factor 1 (IGF-1) and vitamin A remain unaffected [37]. This reduction in colostral passive immunity may render newborn kids more susceptible to other infections, compounding the economic impact of the disease.

The interplay between CAEV infection, the host immune response, and the development of clinical disease is further complicated by co-infections and environmental stressors. The presence of CAEV has been associated with an increased risk of clinical arthritis and subclinical mastitis, and the economic ramifications, including reduced milk yield, altered milk composition, and premature culling, are well-documented [35, 38]. Understanding these molecular and cellular interactions is not merely an academic exercise; it is the foundation upon which rational control programs, including targeted diagnostic strategies, vaccine development, and management interventions, must be built.

Global and Regional Epidemiology of CAEV Infection

The global distribution of caprine arthritis-encephalitis virus (CAEV) is a testament to its remarkable adaptability and the consequences of international animal movement. As a member of the small ruminant lentivirus (SRLV) group, CAEV has been identified on every continent where caprine husbandry is practiced, though its prevalence and strain composition exhibit profound regional heterogeneity. This epidemiological mosaic is shaped by a complex interplay of management practices, host genetics, diagnostic capacities, and, crucially, the presence of other SRLV reservoirs, particularly sheep. The economic impact of CAEV, recognized by the World Organisation for Animal Health (WOAH), is significant, as it directly undermines dairy production, reduces animal longevity, and necessitates costly control measures [2, 9]. This section provides a detailed analysis of the global and regional epidemiology of CAEV, drawing upon recent molecular and serological surveys to elucidate the current state of infection and its underlying dynamics.

Global Perspectives on Prevalence and Strain Diversity

At a global level, CAEV seroprevalence varies dramatically, ranging from near-zero in successfully eradicated populations to over 80% in heavily infected, intensively managed herds. A landmark survey from the early 1990s in the United States estimated that 31% of goats were seropositive, with 73% of herds having at least one positive animal [29]. While this provided a baseline for that era, contemporary data from diverse regions reveal an even more complex picture. For instance, a large-scale serological survey in Taiwan between 2011 and 2012, utilizing a commercial ELISA, reported an astonishing overall seroprevalence of 61.7% (2,120/3,437 goats) and a herd-level prevalence of 98.5% (64/65 farms), marking one of the highest ever recorded globally [8]. This starkly contrasts with a 2015 study in Japan, which found a far lower animal-level seroprevalence of 10.0% (86/857) and a herd-level prevalence of 15.0% (17/113), suggesting that husbandry practices and import policies can profoundly influence viral circulation [28]. The phylogenetic basis for these differences has been established through molecular characterization. CAEV isolates predominantly cluster within SRLV genotype B, with subtype B1 being the most commonly described worldwide, including in China, Argentina, the Russian Federation, and Switzerland [7, 16, 17, 6]. However, genetic heterogeneity within this subtype is substantial, as demonstrated by the high intra-flock diversity observed in imported goats into Switzerland, where multiple B1 variants were found even within single animals [6]. This indicates that even a single introduction of infected stock can seed a genetically diverse viral population, complicating eradication efforts.

The African Continent: A Mosaic of Low and Focal High Prevalence

Data from Africa and the Middle East, while limited, reveal a pattern of generally low to moderate prevalence, often with distinct foci of high infection linked to imported breeds and intensive management. In Sudan, a comprehensive 2016 survey of 368 sera from various states (Khartoum, River Nile, Gedarif, Sinnar, North Kordofan, Gazira, and Northern) using ELISA revealed an overall seroprevalence of only 2.99% [31]. Critically, all 11 positive samples were from foreign breed goats (Cyprus Shami) in the Gazira state (39.29% prevalence), while indigenous goat breeds and sheep across all tested states were negative [31]. This strongly implicates the importation of seropositive exotic animals as the primary route of introduction into naive populations. Similarly, in Bangladesh, a study conducted between 2021 and 2022 reported an overall seroprevalence of 4.26% (19/446) using a commercial indirect ELISA [4]. While low, this study was the first to confirm CAEV circulation in the country and identified key risk factors including female sex (OR: 3.98) and poor biosecurity status (OR: 1.66), highlighting the vulnerability of the sector to future spread [4]. In contrast, a molecular prevalence study in Western Iran using nested PCR on 591 goats across three provinces (Lorestan, Ilam, Kurdistan) found a significantly higher overall prevalence of 17.08% (101/591), with notable regional variation: Kurdistan (27.38%), Lorestan (14.87%), and Ilam (12.13%) [1]. This study also reported that 13 of 15 herds were positive, with within-herd prevalence ranging from 7.1% to 40%, indicating that CAEV is well-established in western Iran and circulates year-round, with no significant seasonal effect [1]. Further south in Iraq, the first molecular survey in Babylon province, using PCR targeting the pol gene, detected CAEV in 5.9% (5/85) of goats, confirming a low but persistent presence [32].

The Asian Landscape: High Endemicity in Dairy Sectors

Asia presents a stark dichotomy between regions with intensive dairy operations and those with more traditional, extensive systems. The aforementioned Taiwan study, with its 61.7% seroprevalence, exemplifies the risk associated with high-density, commercial dairy farming without rigorous control programs [8]. In Japan, while the national seroprevalence was 10.0%, the risk was significantly higher on larger farms with more than 10 goats and on those dedicated to dairy and breeding purposes [28]. This aligns with the classic understanding that the primary transmission route, lactogenic transmission through colostrum and milk, is amplified in systems where many does are milked together and kids are pooled. In mainland China, a 2024 study using a highly sensitive TaqMan-based qPCR on 780 blood and brain samples from sheep (note: sheep, not goats) in Eastern China reported a dramatically lower prevalence of just 0.77% [7]. This low molecular detection rate in sheep is consistent with the idea that CAEV is primarily a goat pathogen, and that cross-species transmission into sheep, while possible, may result in lower viral loads and reduced transmission efficiency [7, 13]. However, the study’s phylogenetic analysis confirmed that the detected strains belonged to subtype B1 and were closely related to Chinese goat isolates from Shanxi and Gansu provinces, demonstrating viral spillover between species [7]. The situation in Thailand adds another layer of complexity; a study evaluating diagnostic test performance found that ELISA (with 83.3% specificity) was more sensitive than conventional PCR (69.6% sensitivity) for detecting infected animals, and the authors recommended a combined testing approach to overcome the limitations of any single assay, especially given the potential for antigenic variation in circulating strains [39].

The European Challenge: Tailing Phenomena and Cross-Species Reservoirs

Europe offers the most instructive examples of both successful eradication and the persistent challenges that remain. In Italy, the Autonomous Province of Bolzano–South Tyrol implemented a compulsory eradication program starting in 2007, based on annual serological testing of all goats and culling of positives. This campaign dramatically reduced seroprevalence and eliminated clinical disease; however, it has been hampered by a “tailing phenomenon,” where sporadic, unexplained positive tests continue to appear, preventing complete eradication [12, 30]. A pivotal investigation revealed that this phenomenon was strongly linked to multispecies farms where goats and sheep cohabitated. Goats on such farms had a significantly higher seroprevalence and seroconversion rate, with more frequent detection of SRLV genotype A (the “sheep-related” maedi-visna virus) infections. The study concluded that sheep act as a reservoir for SRLVs, introducing new infections into goats and contributing to the observed tail [12]. This finding is biologically sound, as cross-species transmission has been experimentally confirmed; lambs inoculated with CAEV developed arthritis and antibodies, and young goats inoculated with progressive pneumonia virus (PPV) replicated the virus and developed arthritis, indicating that these viruses are not strictly host-specific [14]. The Dutch accreditation program, which has been in place for over 40 years, has also faced challenges, particularly in large dairy herds (up to 4,600 goats). An analysis of 38 herds that lost accreditation between 2012 and 2022 found that larger herds were more prone to losing status and had greater difficulty regaining it, with management factors and introduction routes being critical variables [9]. The Swiss eradication program faced a direct challenge in 2017 with the importation of a large flock of seropositive goats. Phylogenetic analysis of the imported viruses revealed a surprisingly high genetic heterogeneity, including multiple B1 subtypes within single animals, and confirmed that the spleen was a prime organ for virus isolation [6]. This case underscored the need for robust border checks and diagnostic capabilities to defend a negative status. In contrast to these intensive programs, other parts of Europe show high endemicity. In Croatia, seroprevalence in French Alpine goats on production farms has been reported at 50.8% to 53.7%, with a positive association between clinical arthritis and seropositivity [35, 38]. A preliminary study in North-Eastern Romania found a 31.86% seroprevalence using AGID, all in clinically normal animals, indicating a large asymptomatic reservoir [40]. In Turkey, a serological survey in the Yozgat region reported a 17.5% seroprevalence in goats for Lentivirus capartenc (CAEV), while a separate study focusing on the indigenous Honamlı breed found only a 1.6% seroprevalence, suggesting that local breeds may have lower susceptibility or are kept in less intensive systems [11, 41]. Epidemiological modelling has been identified as a critical tool to understand these dynamics, though its application to CAE has been limited, highlighting a significant gap in the literature for predicting the trajectory of infection under different management scenarios [5].

The Americas: From Low to High Prevalence in a Vast Landscape

In North America, CAEV has been endemic for decades. The early 1986 study in California found that 53% of goats in 13 dairies were seropositive, with age and feeding of unpasteurized milk being key risk factors [10]. A later study from 1993 on a dairy using a modified eradication program found that 25 of 108 goats were seropositive by PCR, but 20 of the 81 seronegative goats also harbored proviral DNA. Ten of these 20 later seroconverted, a phenomenon of delayed seroconversion that poses a major challenge for serology-based eradication programs, as infected but seronegative animals can remain undetected for months [26]. In South America, Brazil provides extensive data. A large-scale study from 2019 across seven Northeastern states (Maranhão, Ceará, Piauí, Alagoas, Sergipe, Rio Grande do Norte, and Paraíba) using Western blot on 531 breeder-goat sera found an overall prevalence of 6.2%, with significant state-level variation from 2.0% in Maranhão to 17.6% in Alagoas [21]. The study identified several management risk factors, including the breeding season and the origin of breeders, emphasizing the role of trade in spreading infection [21]. In the Paraíba state, a separate 2021 study using AGID found a 6.4% seroprevalence in dairy and beef goats, with no significant association with herd size, age, or gender [33]. However, a deeper analysis in Ceará state using Western blot found that 56.25% of farms were positive for CAEV (with a 16.6% animal-level seroprevalence by WB), with intensive management systems being more affected [25]. In Argentina, a 2017 study isolated and characterized the first Argentine CAEV strain, confirming it clustered within genotype B, subtype B1, and showed high homology with the CAEV-Co prototype [17]. This finding is crucial for developing local diagnostic tools that can accurately detect circulating strains, as reliance on kits designed for foreign isolates may reduce sensitivity [39]. The renal pathology associated with CAEV has also been documented in the USA, where six goats with multisystemic CAEV infection had interstitial nephritis and detectable CAEV antigen within the kidney, expanding our understanding of the virus’s tissue tropism and potential clinical impact [3].

Transmission Dynamics Shaping Epidemiology

The fundamental epidemiological patterns of CAEV are underpinned by its transmission biology. The primary route, established as early as 1983, is lactogenic, the ingestion of virus-laden colostrum and milk from infected does [20]. This mechanism drives the high prevalence seen in dairy operations, where kids are born and fed in communal environments. However, the virus is not exclusively transmitted via milk. Experimental studies show that while intrauterine transmission is rare, it is possible. A 2018 study in Brazil found that 1.4% (4/283) of kids born via cesarean section from seropositive does were positive by Western blot immediately after birth, providing evidence for transplacental transmission, albeit at a very low frequency [19]. In contrast, a 2024 Russian study using PCR on kids obtained by sterile kidding from confirmed infected does found no evidence of intrauterine infection, supporting the view that this route is negligible under most circumstances and that careful management (pasteurizing colostrum, feeding from negative sources) can effectively break the cycle [15, 20]. Horizontal transmission, particularly via the aerosol route, has been experimentally difficult to demonstrate. Early work showed that short-term contact during breeding did not result in transmission, but prolonged contact for over 12 months between infected and uninfected adult goats was necessary for spread in non-dairy conditions [20]. However, when uninfected does were milked in the same parlor as infected ones, a high percentage became infected rapidly, implicating contaminated milking equipment as a major iatrogenic horizontal route [20]. This explains why milking orders and hygiene are paramount in control programs [9]. Furthermore, the possibility of sexual transmission has been raised; although not definitively proven, epidemiological modelling suggests that incorporating sexual transmission into models better predicts infection dynamics in some settings [5].

The global epidemiology of CAEV is thus a dynamic equilibrium, where the forces of viral transmission (lactogenic, iatrogenic, low-frequency vertical) are pitted against the effectiveness of control measures (pasteurization, segregated kidding, serological surveillance). The presence of sheep as a SRLV reservoir adds a layer of complexity, particularly in mixed-species farming systems. As strict eradication programs in countries like the Netherlands, Switzerland, and South Tyrol continue to battle the tailing phenomenon, the focus is shifting to the importance of bulk milk testing, strategic pooled sampling, and molecular genotyping to trace infection sources and detect the introduction of new viral subtypes [9, 12, 6]. The evidence overwhelmingly indicates that without rigorous, region-specific interventions that account for local husbandry and the presence of co-mingled sheep populations, CAEV will remain a persistent and economically damaging threat to global goat production.

Clinical Manifestations and Disease Progression in Goats

Caprine Arthritis-Encephalitis Virus (CAEV) infection in goats is a paradigm of a slow, progressive, multisystemic disease, characterized by a protracted incubation period and a spectrum of clinical syndromes that reflect the virus’s tropism for monocyte/macrophage lineage cells. The clinical course is insidious, and the majority of infected animals remain asymptomatic for months or even years, serving as reservoirs for transmission within the herd [1, 4, 26]. The disease manifests primarily in four distinct, though often overlapping, forms: arthritic, neurological (encephalitic), mammary (indurative mastitis), and respiratory (interstitial pneumonia). The expression of these syndromes is influenced by viral strain, host genetics, age at infection, and environmental stressors, with the arthritic form being the most prevalent in adult goats and the neurological form predominantly affecting young kids [22, 23, 24].

The Arthritic Syndrome: A Chronic, Debilitating Polyarthritis

The arthritic form is the most frequently recognized clinical manifestation in adult goats, typically emerging in animals over eight months of age and becoming progressively more severe with age [24, 38]. The pathogenesis is rooted in a chronic, immune-mediated inflammatory process within the synovial membranes. CAEV infects synovial macrophages, leading to the release of pro-inflammatory cytokines and the recruitment of mononuclear cells. This results in synovial hyperplasia, villous hypertrophy, and effusion, which ultimately progresses to fibrosis, cartilage erosion, and periarticular osteophyte formation [24, 27]. Clinically, this presents as a progressive, non-suppurative polyarthritis, most commonly affecting the carpal (knee) joints, followed by the tarsal (hock) and stifle joints [23, 24]. Affected animals exhibit varying degrees of lameness, joint swelling, and stiffness. The joints are typically firm and distended on palpation, and in advanced cases, the animal may become recumbent due to pain and loss of joint function [23, 24, 38]. Ultrasonographic examination of affected joints reveals a loss of normal echogenicity, surface erosions, exposure of subchondral bone, and the presence of hyperechoic areas indicative of osteophyte formation and cartilage fragmentation [24]. The severity of arthritis is directly correlated with the host’s immune response; intriguingly, goats vaccinated with inactivated CAEV developed more severe arthritis upon challenge than naive controls, suggesting that the immune response itself is a primary driver of joint pathology [27]. This immune-mediated pathogenesis explains why clinical arthritis is often a late-stage manifestation, as it requires a sustained, dysregulated immune response against persistent viral antigens. The economic impact of this form is substantial, as it leads to reduced mobility, decreased feed intake, and ultimately, premature culling [35, 38]. Studies from Croatia and Brazil have demonstrated a strong positive association between CAEV seropositivity and the presence of clinical arthritis, with seropositive goats being significantly more likely to exhibit joint disease [21, 38].

The Neurological Syndrome: Acute Leukoencephalomyelitis in Kids

The neurological form is a dramatic and often fatal manifestation of CAEV, primarily observed in kids aged two to six months [22, 23]. This syndrome presents as an acute or subacute, progressive, demyelinating leukoencephalomyelitis. The virus targets the central nervous system (CNS), infecting microglial cells and perivascular macrophages. The resulting pathology is characterized by perivascular cuffing with mononuclear cells, multifocal areas of demyelination, and gliosis, predominantly in the brainstem, cerebellum, and cervical spinal cord [22, 23]. Apoptosis plays a critical role in the pathogenesis of these brain lesions. A detailed immunohistochemical study demonstrated that TUNEL-positive apoptotic cells, predominantly astrocytes and, to a lesser extent, oligodendroglia, are concentrated at the edges of demyelinated plaques and in perivascular areas. This apoptotic process is driven by the activation of both the intrinsic (caspase-9) and extrinsic (caspase-8) apoptotic pathways, with a marked upregulation of the pro-apoptotic protein Bax in glial cells. Conversely, the anti-apoptotic protein Bcl-2 is primarily expressed in neurons, which may explain the relative preservation of neuronal cell bodies despite extensive white matter damage [22]. Clinically, affected kids present with a progressive, ascending paresis or paralysis, beginning with hindlimb ataxia and weakness, which may advance to tetraplegia. Other neurological signs include head pressing, depression, intention tremors, and loss of proprioception [23]. The disease is rapidly progressive, often leading to recumbency and death within weeks. While the neurological form is less common than the arthritic form, it represents a significant source of mortality in young stock and is a hallmark of highly pathogenic CAEV strains.

The Mammary and Respiratory Syndromes: Subclinical Production Losses

The mammary form, or "hard udder," is a chronic, indurative mastitis that is often subclinical but has profound effects on milk production and quality. CAEV infects mammary gland macrophages, leading to a progressive, lymphocytic interstitial inflammation and fibrosis. This results in a firm, non-painful udder with reduced milk secretion [23, 34, 35]. The impact on milk production is significant; seropositive goats have been shown to have reduced lactation length, lower total milk yield, and altered milk composition, including decreased fat and protein percentages [35]. Furthermore, the quality of colostrum is compromised. A study on Saanen goats found that colostrum from CAEV-seropositive dams had significantly lower concentrations of immunoglobulin G (IgG) and total protein compared to seronegative dams, potentially compromising passive transfer of immunity to newborn kids [37]. The virus is shed into milk and colostrum, making these secretions the primary vehicle for lactogenic transmission to kids [20, 42]. The pattern of viral shedding in milk is intermittent; real-time PCR analysis has detected proviral DNA in milk samples from infected does, with the highest concentration observed around parturition and the early postpartum period, decreasing to undetectable levels by 40 days postpartum in some cases [42]. This intermittent shedding complicates diagnostic efforts and underscores the importance of pasteurizing colostrum and milk for feeding kids [15, 20].

The respiratory form, while less frequently diagnosed than arthritis or mastitis, is a consistent pathological finding. It manifests as a chronic, progressive interstitial pneumonia, characterized by a diffuse infiltration of the alveolar septa by mononuclear cells (lymphocytes and macrophages), leading to a thickening of the interstitium and a reduction in gas exchange capacity [23, 3]. Clinically, this may present as a chronic cough, exercise intolerance, and progressive dyspnea. In many cases, the respiratory form is subclinical and only detected at necropsy, but it contributes to the overall debilitation and reduced productivity of infected animals [23].

Beyond the Classical Syndromes: Emerging Clinical Associations

Recent research has expanded the recognized clinical spectrum of CAEV infection. A landmark study by Murphy et al. (2021) provided the first peer-reviewed evidence of CAEV-associated renal lesions [3]. In a series of six goats with chronic, multisystemic CAEV infection, microscopic examination revealed a lymphocytic interstitial nephritis. CAEV antigen was detected via immunohistochemistry in the renal tubular epithelium of three animals, and proviral DNA was amplified from renal tissue in all six. Notably, one animal exhibited a CAEV antigen-associated thrombotic arteritis, leading to infarction of both the kidney and the heart [3]. This finding suggests that CAEV can directly contribute to renal injury, potentially leading to chronic kidney disease, and that the vasculature may be an underappreciated target of viral pathology. Cardiac lesions, including myocardial and endocardial inflammation, were also identified in four of the six animals, indicating a potential for CAEV to contribute to myocarditis and cardiac dysfunction [3]. These findings challenge the traditional view of CAEV as a disease limited to the joints, CNS, lungs, and mammary gland, and suggest that a broader, systemic inflammatory process may be at play in chronically infected animals.

Disease Progression and the Role of Co-infections

The progression from infection to clinical disease is highly variable and influenced by a complex interplay of factors. Following initial infection, typically via ingestion of infected colostrum or milk, the virus establishes a lifelong, persistent infection in cells of the monocyte/macrophage lineage [1, 16, 26]. A critical feature of CAEV pathogenesis is the phenomenon of delayed seroconversion. Studies have demonstrated that goats can harbor proviral DNA, detectable by PCR, for many months before seroconverting to antibody-positive status [26]. This "serologically silent" period can last up to eight months or longer, during which the animal is infectious but escapes detection by standard serological screening tests [26]. This has profound implications for eradication programs, as reliance solely on antibody detection can lead to a false sense of security and allow infected animals to remain in the herd.

Once clinical signs appear, the disease is relentlessly progressive. The arthritic form, in particular, worsens with age, as cumulative joint damage leads to increasing pain and disability [35, 38]. The presence of co-infections can accelerate disease progression. For instance, infection with Mycoplasma agalactiae, the causative agent of contagious agalactia, can produce clinical signs (arthritis, mastitis) that are clinically indistinguishable from CAEV, complicating diagnosis and potentially exacerbating the overall disease burden [25]. While a direct correlation between CAEV and M. agalactiae seropositivity was not found in one study, the presence of a lentivirus-induced immunosuppression could theoretically predispose animals to secondary bacterial infections [25]. The World Organisation for Animal Health (WOAH) recognizes CAEV as a significant pathogen of small ruminants, and its control is a priority for many national veterinary authorities due to its impact on trade and animal welfare. The insidious nature of the disease, characterized by a long subclinical phase and progressive, debilitating clinical syndromes, makes it a formidable challenge for goat producers worldwide.

Diagnostic Methods: Molecular Detection and Phylogenetic Analysis

The accurate diagnosis of Caprine Arthritis-Encephalitis Virus (CAEV) infection presents a formidable challenge, intrinsically linked to the virus's nature as a lentivirus within the group of small ruminant lentiviruses (SRLVs). The hallmark of CAEV infection is the establishment of a lifelong, persistent infection characterized by a prolonged subclinical phase, a restricted viral replication rate, and the proviral integration into the host genome, primarily within cells of the monocyte/macrophage lineage [22, 3]. This unique biology renders traditional diagnostic approaches, particularly serology, susceptible to limitations such as delayed seroconversion, which can extend for many months post-infection, and the inability to differentiate between current infection and past exposure [26]. Consequently, molecular detection methods, specifically those targeting proviral DNA, have become indispensable tools for direct pathogen detection, enabling early diagnosis, surveillance, and the detailed characterization of circulating viral strains through phylogenetic analysis. These molecular techniques are not merely confirmatory; they are foundational to understanding the molecular epidemiology, transmission dynamics, and evolutionary biology of CAEV on a global scale, aligning with the WOAH (World Organisation for Animal Health) guidelines for the diagnosis of SRLV infections.

Molecular Detection: PCR-Based Platforms and Target Genes

The cornerstone of molecular detection for CAEV is the Polymerase Chain Reaction (PCR), which has evolved from conventional formats to highly sensitive real-time quantitative platforms. The choice of target gene, primer design, and assay chemistry are critical determinants of diagnostic performance. The most frequently targeted genomic region for molecular detection is the highly conserved gag gene, which encodes the viral core proteins. This region is favored for its sequence conservation across SRLV genotypes, which is essential for developing broadly reactive assays capable of detecting diverse circulating strains [1, 7, 17, 42]. For instance, Hemmati et al. (2025) successfully employed a nested PCR (nPCR) targeting the gag gene to achieve a molecular prevalence of 17.08% in goats from Western Iran, demonstrating the utility of this approach across different geographical regions [1]. Similarly, Panneum and Rukkwamsuk (2017) utilized a gag-targeted PCR in a comparative study in Thailand, although they reported a sensitivity of only 69.6% compared to viral culture, potentially due to primer-template mismatches with locally circulating strains, a critical issue for assay design [39].

While the gag gene is a primary target, other genomic regions are also exploited, particularly for enhancing phylogenetic resolution. The pol gene, encoding the viral reverse transcriptase and integrase, offers a balance of conservation and variability. Mosa et al. (2022) designed primers to amplify a 573-base pair (bp) fragment of the proviral pol gene in their first molecular survey of CAEV in Iraq [32]. The env gene, which codes for the surface envelope (SU) and transmembrane (TM) glycoproteins, is the most variable region of the SRLV genome. Although less commonly used as a primary diagnostic target due to its high diversity, amplification and sequencing of env fragments are invaluable for subtyping and phylogenetic clustering, as demonstrated by Kolbasova et al. (2023), who used an env-gene fragment to characterize the “Mordovia-2018” isolate as belonging to subtype B1 [16]. The U3 region of the long terminal repeat (LTR), which contains critical transcription factor binding sites (TFBSs) like the gamma-activated site (GAS), also provides a target for analysis, offering insights into viral promoter activity and potential pathogenicity, as Murphy et al. (2021) explored in their study of CAEV-associated renal lesions [3].

The comparative performance of different PCR platforms is a subject of intense investigation. Conventional PCR, including single-step and nested formats, offers high sensitivity but is limited to qualitative (positive/negative) results and is more prone to contamination. Nested PCR, in particular, can detect low copy numbers of proviral DNA, as shown in studies of suspected intrauterine transmission where it was used to confirm the absence of infection in kids from seropositive dams [15, 13]. In contrast, real-time quantitative PCR (qPCR), particularly using TaqMan probes, provides several decisive advantages: quantification of viral load, a broader dynamic range, reduced risk of cross-contamination due to a closed-tube system, and significantly higher sensitivity. Tian et al. (2024) developed a TaqMan-based qPCR targeting the gag gene that achieved a sensitivity of approximately 10 copies/µL, a 1000-fold improvement over conventional PCR [7]. This assay demonstrated an R² > 0.999 and absolute specificity when tested against other common sheep viruses. The quantitative nature of qPCR is particularly powerful for tracking viral shedding in biological fluids. Gaeta et al. (2018) applied real-time PCR to monitor the pattern of CAEV elimination in milk, revealing that viral cDNA was detectable only during the first 40 days postpartum, with a peak concentration around day 40, a finding with profound implications for managing lactogenic transmission [42].

Diagnostic Performance and Multi-Target Strategies

No single diagnostic test is infallible for CAEV, and the optimal approach often involves a combination of molecular and serological methods in a multi-target strategy. The performance characteristics of PCR relative to serological tests like ELISA have been rigorously evaluated. Panneum and Rukkwamsuk (2017) provided a benchmark analysis, reporting that PCR had a specificity of 100% but a sensitivity of only 69.6% when compared to viral culture as the gold standard, whereas ELISA showed a sensitivity of 95.7% but a specificity of 83.3% [39]. The lower sensitivity of PCR in this study was attributed to possible genetic variation of CAEV strains in Thailand, where the primers were designed based on a heterogeneous sequence database. This underscores the critical need for continuous monitoring of circulating strains and periodic re-evaluation of primer sets. The study demonstrated that the parallel use of both tests (testing positive if either test is positive) yielded a combined sensitivity of 98.7%, while series testing (positive only if both are positive) yielded 100% specificity [39]. This logic underpins the multi-target approach advocated by Shuralev et al. (2021), who applied both ELISA and an in-house real-time PCR test for monitoring CAEV on hobbyist farms in Russia. Their results showed that while seroprevalence ranged from 87.50-92.31%, proviral DNA was detected in only 53.85-62.50% of the same animals, highlighting the discordance that can occur between the two methods, particularly in animals with low proviral loads or during the early, serologically silent phase of infection [34].

The phenomenon of delayed seroconversion, elegantly documented by Rimstad et al. (1993), further solidifies the necessity of PCR for accurate diagnosis. In their study of a dairy herd undergoing an eradication program, 20 out of 81 seronegative goats were found to harbor proviral CAEV DNA by PCR. Ten of these animals subsequently seroconverted within eight months, confirming that PCR can detect infection weeks to months before antibodies become detectable by ELISA or agar gel immunodiffusion (AGID) [26]. This finding has direct and critical implications for eradication programs: relying solely on serology can leave a reservoir of latently infected animals that can perpetuate the infection within a herd. Peterson et al. (2022) emphasized this point in their analysis of Dutch dairy goat herds that had lost CAEV accreditation, noting that strategic deployment of PCR on pooled sera, bulk milk, or tissue samples was essential for identifying newly infected animals and tracing the route of viral introduction back to the source [9].

Phylogenetic Analysis: Unraveling Global Diversity and Transmission

Phylogenetic analysis of molecular sequences is the most powerful tool for classifying CAEV isolates, understanding their evolutionary relationships, and tracking transmission patterns on local, regional, and international scales. The SRLV group is broadly divided into five genetic groups (A-E), with genotypes A and B being the most prevalent and widely studied. Genotype A is primarily associated with maedi-visna virus of sheep but can infect goats, while genotype B is the classical caprine arthritis-encephalitis virus group, which is further subdivided into several subtypes, with B1 being the most globally dominant and virulent [12, 6]. The molecular characterization of Argentine CAEV strains by Panei et al. (2017) via sequencing of the gag gene revealed that all local isolates clustered with genotype B, subtype B1, showing high homology (97-98%) with the CAEV-Co prototype strain [17]. Similarly, all three strains isolated from sheep in eastern China by Tian et al. (2024) belonged to subtype B1, with 97.7-98.9% nucleotide homology to strains from Shanxi and Gansu provinces, indicating a widespread distribution of this subtype within China [7]. Kolbasova et al. (2023) further confirmed the B1 subtype for a Russian isolate from Mordovia, which clustered with an older isolate from the Tver region, suggesting a stable, endemic circulation of this subtype in the Russian Federation [16].

The choice of genomic region for phylogenetic analysis dictates the level of resolution. The highly conserved gag gene is excellent for confirming the genotype (A vs. B) and for broad-scale epidemiological studies [17]. However, for distinguishing between closely related strains and investigating transmission chains within a herd, more variable regions are required. The env gene, particularly the SU5 region (encoding the fifth hypervariable domain of the surface envelope), provides this high resolution. A landmark study by Martin et al. (2019) on a cross-border CAEV challenge in Switzerland perfectly illustrates the power of multi-region phylogenetic analysis. When a large flock of seropositive goats was imported into the CAEV-negative Swiss population, a thorough diagnostic and phylogenetic investigation was launched. Using designed primers for gag-pol and env regions, the analysis revealed a surprisingly high degree of sequence heterogeneity. Phylogenetic trees constructed from env sequences showed that the infecting virus was a B1 subtype, but the sequences were not a single, homogenous cluster. Instead, they formed a diverse group, suggesting that individual animals had been infected with multiple, distinct viral variants, and that the imported flock represented a melting pot of different viral quasispecies [6]. This level of phylogenetic detail is only achievable through sequencing of multiple, hypervariable genomic loci and is essential for effectively managing biosecurity breaches and tracing the origin of incursions.

Furthermore, phylogenetic analysis has been instrumental in addressing a critical epidemiological question: the role of sheep as a reservoir for CAEV in goats, a phenomenon known as the “tailing phenomenon” in eradication programs. Nardelli et al. (2020) investigated the persistence of seropositive goats in the South Tyrol eradication campaign in Italy, which had been ongoing for years. By comparing serological and genotyping data from mono-species goat farms versus multispecies farms (where goats and sheep cohabited), they found a clear link. Goats on multispecies farms had a higher prevalence and were more likely to have genotype A (sheep-related) infections, demonstrating conclusively that sheep can serve as a source of SRLV infection for goats, frustrating eradication efforts [12]. These findings align with early experimental work by Banks et al. (1983), who showed that CAEV could productively infect sheep, causing arthritis and seroconversion, and that progressive pneumonia virus of sheep could infect goats, highlighting the bidirectional potential for cross-species transmission and the profound implications for control programs [14]. The integration of molecular detection with robust phylogenetic analysis, as championed by these studies, is not an academic exercise but a practical necessity for designing science-based, effective eradication and control strategies for CAEV.

Transmission Dynamics and Risk Factors

The transmission dynamics of Caprine Arthritis-Encephalitis Virus (CAEV) are characterized by a complex interplay of vertical and horizontal pathways, host factors, and management practices that collectively determine the virus’s persistence and spread within and between goat populations. As a member of the small ruminant lentivirus (SRLV) group, CAEV establishes lifelong infections, making an understanding of its transmission routes and associated risk factors paramount for designing effective control and eradication programs. The World Organisation for Animal Health (WOAH) recognizes CAEV as a significant pathogen impacting small ruminant production globally, and its control is a priority for many national veterinary services.

Vertical Transmission: The Dominant Route and the Colostral/Milk Pathway

The preponderance of evidence firmly establishes the ingestion of infected colostrum and milk as the primary and most efficient route of CAEV transmission from dam to offspring. This lactogenic pathway is the cornerstone of CAEV epidemiology. Early seminal work demonstrated that CAEV could be isolated from goat milk and that feeding kids infected colostrum or milk resulted in efficient transmission [20]. This finding has been replicated across numerous studies and geographic regions, solidifying the understanding that the postpartum period is the most critical window for new infections. The virus is shed into the mammary gland, and the consumption of this contaminated secretion by the naive neonatal gut provides a direct portal of entry. The efficiency of this route is underscored by the success of eradication programs that rely on the immediate separation of kids from their dams at birth and the feeding of heat-treated colostrum and pasteurized milk or milk replacer. For instance, in a study in Russia, a 100% CAEV-infected herd was successfully sanitized within two years by implementing sterile kidding and feeding pasteurized colostrum and milk, without the need to purchase new animals [15]. This practical outcome provides powerful, real-world validation of the lactogenic route’s primacy.

The biological basis for this high efficiency lies in the susceptibility of the neonatal intestinal tract. The gut of a newborn kid is permeable to macromolecules, including immunoglobulins and, unfortunately, viral particles. The virus, present in cell-free or cell-associated forms within the colostrum and milk, can traverse the intestinal epithelium and establish infection in target cells, primarily monocytes and macrophages. The quality of colostrum itself may be compromised in infected dams. Research has shown that CAEV-seropositive Saanen dams produce colostrum with significantly lower concentrations of immunoglobulin G (IgG) compared to seronegative dams, potentially impacting passive immunity transfer and making kids more vulnerable to other pathogens, even if CAEV transmission is prevented [37]. This dual impact, direct transmission of CAEV and reduced passive immunity, compounds the negative effect of the virus on herd health.

The Enigma of Intrauterine Transmission: A Low-Frequency Event

While the lactogenic route is unequivocally the main driver of vertical transmission, the possibility of in utero infection has been a subject of considerable debate and investigation. The evidence points to this being a rare, but not impossible, event. Early studies using cesarean-derived kids found that intrauterine infection may have occurred in a small fraction of cases, though postpartum horizontal transmission could not be entirely ruled out [20]. More recent work has provided both supporting and contradictory evidence.

A study in Brazil using the highly sensitive Western blot technique detected CAEV antibodies in 4 out of 283 (1.4%) newborn kids immediately after birth, before any ingestion of colostrum, suggesting transplacental transmission [19]. This low frequency is consistent with the idea that the placenta provides a significant, albeit imperfect, barrier to the virus. Conversely, a rigorous study in Russia, which collected fetuses via sterile cesarean section from PCR- and ELISA-confirmed positive does and tested them for the CAEV genome, found no evidence of infection in any of the samples [15]. This study concluded that intrauterine transmission did not occur in their cohort. The discrepancy between these findings may be attributable to differences in viral strain pathogenicity, maternal viral load, the timing of infection during gestation, or the sensitivity of the diagnostic methods used. The consensus, therefore, is that while transplacental transmission is biologically possible, it is a sporadic event with a low probability. For practical purposes, control programs that focus on preventing lactogenic transmission are highly effective, but the existence of this rare vertical pathway may contribute to the "tailing phenomenon" observed in some eradication campaigns, where a low level of infection persists despite rigorous control measures [12, 30].

Horizontal Transmission: The Role of Prolonged Contact and Iatrogenic Spread

Horizontal transmission, or transmission between animals of any age that are not in a maternal-offspring relationship, is a significant but more insidious route of CAEV spread. Unlike many acute respiratory viruses, CAEV is not efficiently transmitted by casual contact or the aerosol route. Seminal studies demonstrated that even short-term direct contact between infected bucks and virus-free does during breeding did not result in transmission [20]. Instead, horizontal transmission requires prolonged, close contact. The same study found that it took over 12 months of continuous cohabitation for transmission to occur under non-dairy conditions [20]. This slow rate of spread is a hallmark of lentivirus infections.

However, the dynamics change dramatically under intensive management conditions. When uninfected does were milked alongside infected does, a high percentage became infected in less than 10 months [20]. This implicates the milking process as a potent iatrogenic route of transmission. The virus can be spread via contaminated milking equipment, milkers' hands, or udder cloths, effectively bypassing the need for prolonged contact. This finding has profound implications for dairy operations, where the milking parlor can become a central hub for viral dissemination. The presence of CAEV proviral DNA in milk [42] confirms that the mammary gland is a site of active viral replication and shedding, making milk a vehicle not only for vertical transmission to kids but also for horizontal spread among adult does during the milking process.

Other forms of horizontal transmission include the sharing of contaminated feeding and watering equipment, as well as the use of common needles or surgical instruments. The virus can be present in blood and other bodily fluids, and any practice that breaks the skin or transfers infected secretions can facilitate transmission. Crowded housing conditions, poor biosecurity, and the mixing of animals of different ages and infection statuses are all significant risk factors that amplify the potential for horizontal spread [4, 41].

Risk Factors: A Multifactorial Web

The probability of a goat or a herd becoming infected with CAEV is influenced by a constellation of host, management, and environmental risk factors. Identifying these factors is crucial for targeted interventions.

Age is a consistently identified risk factor, with seroprevalence typically increasing with age [10, 29]. This is a function of cumulative exposure over time; older animals have had more opportunities for both vertical and horizontal transmission. In a study of Honamlı goats in Turkey, seropositivity was significantly higher in the 4-year-old and older age group (8.70%) compared to younger age groups [41]. Similarly, a large-scale US survey found that prevalence increased with age up to 3 years [29]. However, some studies, such as the one in Western Iran, found no significant association with age [1], which may reflect a high force of infection in younger animals or a different epidemiological context.

Sex has shown variable associations. While many studies report no significant difference between males and females [1, 10, 29], a study in Bangladesh identified female sex as a significant risk factor (Odds Ratio: 3.98) [4]. This is likely a management-related artifact rather than a true biological difference in susceptibility. In many production systems, females are kept longer and are subjected to the repeated stress of lactation and milking, increasing their exposure risk. Males, particularly breeding bucks, may be kept in smaller groups or for shorter periods.

Breed and Production System are also important. Dairy breeds, such as Saanen, French Alpine, and Damascus goats, often show higher seroprevalence than meat or fiber breeds like Angora [23, 35, 29, 40]. This is largely due to the intensive management systems associated with dairy production, which involve higher stocking densities, frequent milking, and the commingling of animals, all factors that facilitate both horizontal and vertical transmission. In contrast, extensive or semi-extensive systems, where animals are kept in smaller groups with less contact, tend to have lower prevalence [33]. The importation of foreign breeds, as seen in Sudan where only Cyprus shami goats were seropositive, can introduce the virus into naive local populations [31].

Herd Size and Management Practices are powerful determinants. Larger herds are more prone to CAEV infection and face greater challenges in regaining accreditation after a breakdown [9, 28]. Poor biosecurity, including the lack of quarantine for new animals, the absence of separate pens for different age groups, and the failure to implement "all-in-all-out" management, are all significant risk factors [4, 41]. The practice of communal feeding and, most critically, the failure to pasteurize colostrum and milk for kids are among the highest-risk management failures [20, 10].

Interspecies Transmission and the Role of Sheep as a Reservoir

A critical and often overlooked dimension of CAEV transmission dynamics is the potential for interspecies transmission between goats and sheep. SRLVs are a closely related group of viruses, and the traditional host species barrier is not absolute. Experimental infections have demonstrated that CAEV can infect sheep, causing arthritis and seroconversion, and that the ovine progressive pneumonia virus (OPPV) can infect goats [14]. Furthermore, a study demonstrated that CAEV transmission among sheep is possible, albeit with low frequency, and that the virus can be detected by PCR even in the absence of seroconversion [13].

This experimental evidence is supported by field observations. In the South Tyrol eradication program in Italy, a "tailing phenomenon" was observed, where a low level of seropositivity persisted despite years of rigorous testing and culling. A detailed investigation revealed that goats on multispecies farms (where goats and sheep cohabited) had a higher prevalence, higher antibody titers, and were more likely to be infected with SRLV genotype A, which is typically associated with sheep [12]. This strongly suggests that sheep can act as a reservoir for SRLVs that can spill over into goat populations, undermining eradication efforts. This finding has significant implications for control programs, which must consider the entire small ruminant population on a farm, not just the goats. The diagnostic challenge is compounded by the fact that serological tests may not always distinguish between infections caused by different SRLV genotypes, and the presence of sheep can confound the interpretation of surveillance data [6].

Prevention, Control Strategies, and Economic Impact

The management of Caprine Arthritis-Encephalitis Virus (CAEV) infection represents one of the most formidable challenges in contemporary small ruminant medicine, demanding a multifaceted approach that integrates rigorous biosecurity protocols, strategic diagnostic deployment, and an unyielding commitment to herd-level interventions. The economic ramifications of this insidious lentivirus are profound and pervasive, extending far beyond the immediate clinical manifestations to undermine the very foundations of productive goat husbandry. Consequently, any discussion of prevention and control must be grounded in a sober appreciation of the fiscal devastation that CAEV exacts upon the global caprine industry, a reality that compels the implementation of aggressive, evidence-based eradication strategies.

Economic Impact: The Hidden Tax on Caprine Productivity

The economic burden of CAEV is both direct and indirect, manifesting as reduced production efficiency, increased veterinary expenditures, premature culling, and diminished genetic progress. The virus exerts a particularly insidious effect on dairy operations, where the most quantifiable losses are observed. Infection with CAEV has been definitively linked to significant reductions in milk yield, alterations in milk composition, and a shortened productive lifespan. In a comprehensive study of French Alpine goats in Croatia, seropositive animals demonstrated demonstrably lower milk production and altered lactation parameters compared to their uninfected counterparts, with clinical arthritis further compounding these losses [35]. This finding is corroborated by research from the same region, which documented a 50.8% seroprevalence in intensively managed herds, with a statistically significant positive association between CAEV seropositivity and the presence of clinical arthritis, a condition that inevitably leads to reduced mobility, impaired feeding behavior, and early culling [38]. The economic impact is not limited to milk; the quality of colostrum is also compromised. Studies have shown that CAEV-seropositive dams produce colostrum with significantly lower concentrations of immunoglobulin G (IgG), thereby reducing the passive transfer of immunity to newborn kids and increasing their vulnerability to neonatal diseases, which in turn drives up mortality rates and veterinary costs [37].

Beyond the individual animal, the economic consequences ripple through the entire production system. Infected herds experience higher culling rates due to chronic, progressive conditions such as arthritis, mastitis, and pneumonia, forcing producers to incur the substantial costs of replacement stock acquisition and rearing [1, 2]. The chronic, subclinical nature of the infection, where animals may remain asymptomatic carriers for years, creates a hidden reservoir of disease that silently erodes profitability. This is compounded by the fact that clinical disease often manifests only in adult animals, after significant investment in their rearing has already been made. The World Organisation for Animal Health (WOAH) recognizes CAEV as a disease of significant economic importance, and its presence in a herd can restrict trade in breeding stock and animal products, further amplifying financial losses [2]. The cumulative effect is a substantial, often underappreciated, tax on productivity that can render goat farming economically unsustainable in the absence of effective control measures.

Foundational Prevention: Breaking the Chain of Lactogenic Transmission

The cornerstone of all CAEV prevention and control programs is the interruption of the primary route of transmission: the ingestion of infected colostrum and milk by newborn kids. This lactogenic route is overwhelmingly the most efficient mechanism for establishing infection in a naïve herd, and its disruption is the single most impactful intervention available to producers [20]. The seminal work by Adams et al. (1983) established that CAEV is shed in high titers in both colostrum and milk, and that feeding these materials to kids results in a very high rate of transmission [20]. This fundamental understanding has shaped the standard protocol for CAEV eradication: the immediate separation of kids from their dams at birth, followed by feeding with a heat-treated, safe source of colostrum and milk.

The efficacy of heat treatment is well-documented. Early studies demonstrated that heating colostrum at 56°C for one hour effectively inactivates CAEV to below titratable levels, preventing transmission to kids fed this treated material [20]. More recent field applications have refined this protocol. In a landmark study from Russia, where 100% of a goat population was infected, a comprehensive control program was implemented that included sterile kidding (kids taken by cesarean section or immediately at birth before any contact with the dam) and feeding with colostrum and milk pasteurized at 60°C for 30 minutes, followed by the use of a whole milk substitute. This rigorous approach successfully produced a CAEV-free herd within two years, without requiring the culling of adult animals and without incurring significant economic losses from purchasing replacement stock [15]. This demonstrates that a well-executed prevention program can be both effective and economically viable, even in the face of extreme prevalence.

However, the success of this approach is contingent upon strict adherence to protocol. The use of pooled colostrum from multiple dams is particularly risky, as a single infected doe can contaminate the entire batch. Furthermore, the phenomenon of delayed seroconversion, where infected animals may not test positive for antibodies for many months after infection, complicates the identification of safe colostrum donors [26]. Therefore, the gold standard remains the use of heat-treated colostrum from known negative dams, or the use of commercial bovine colostrum replacers and milk powders, which are inherently free of CAEV. The implementation of these practices, while requiring labor and infrastructure investment, is the most cost-effective long-term strategy for maintaining a negative herd status.

Strategic Control and Eradication: Test-and-Cull and Accreditation Programs

For herds that are already infected, or for regions aiming for regional eradication, a more aggressive approach is required, typically centered on a test-and-cull strategy combined with rigorous biosecurity. This approach has been successfully implemented in several European nations, most notably in the Autonomous Province of Bolzano – South Tyrol, Italy, where a compulsory eradication campaign was launched in 2007 [12, 30]. The program was based on a strict census of all small ruminants, annual serological testing of every animal using validated enzyme-linked immunosorbent assays (ELISA), and the immediate culling of all seropositive individuals. The results were dramatic: the campaign succeeded in completely eliminating clinical cases of CAE and drastically reducing seroprevalence at both the herd and individual animal levels [30]. This success underscores the power of a centralized, well-funded, and legally enforced eradication program.

However, the South Tyrol experience also revealed a critical challenge: the "tailing phenomenon." After the initial rapid reduction in prevalence, the program encountered a plateau where complete eradication proved elusive, with sporadic new infections continuing to appear in sanitized flocks [12, 30]. This phenomenon was investigated in detail, and a key contributing factor was identified: the role of sheep as a viral reservoir. In multispecies farms where goats and sheep cohabitated, goats had a significantly higher prevalence and seroconversion rate compared to those on monospecies farms. Furthermore, genotyping revealed that goats on mixed farms were more frequently infected with SRLV genotype A, which is typically associated with sheep (Maedi-Visna virus), rather than the goat-adapted genotype B [12]. This demonstrates that sheep can serve as a source of infection for goats, complicating eradication efforts in mixed-species operations and highlighting the need for integrated control programs that address both species.

The Dutch SRLV accreditation program offers another model for control, one that is more tailored to the realities of large commercial dairy goat farms. In the Netherlands, herds that lose their CAEV accreditation are provided with a "tailor-made" approach from the Royal GD, which includes strategic deployment of various diagnostic tests. This involves retrospective testing of bulk milk samples and strategic pooled milk sample testing to pinpoint the time and route of viral introduction. By intensifying surveillance through bulk milk testing, the program enables the rapid identification of new infections and allows for swift action to prevent further transmission within and between herds [9]. This approach is particularly valuable for large herds (400 to 4600 goats), where individual animal testing is logistically and financially prohibitive. The Dutch experience also highlights that larger herds are more prone to losing accreditation and face greater difficulty in regaining it, emphasizing the need for robust internal biosecurity and continuous monitoring [9].

Diagnostic Strategies: The Indispensable Tool for Control

The success of any control or eradication program is inextricably linked to the performance of the diagnostic tests employed. The choice of test, serological (ELISA, AGID, Western Blot) versus molecular (PCR, qPCR), and the sampling strategy (individual sera, pooled sera, bulk milk, tissue) must be carefully considered based on the goals of the program and the epidemiological context [9, 36]. Serological tests, particularly ELISA, are the mainstay of large-scale screening programs due to their high throughput, relative low cost, and good sensitivity and specificity. A competitive-inhibition ELISA (cELISA) has been shown to have 100% sensitivity and 96.4% specificity, making it a powerful tool for establishing and maintaining CAEV-free herds [43]. However, serology has a critical limitation: the window period of delayed seroconversion. Infected animals may not develop detectable antibodies for months, leading to false-negative results and allowing the virus to circulate undetected [26]. This is a major reason why eradication programs can fail; a single seronegative but infected animal can reintroduce the virus into a sanitized herd.

Molecular tests, such as conventional PCR and real-time quantitative PCR (qPCR), offer a solution to this problem by detecting proviral DNA directly, regardless of serological status. TaqMan-based qPCR assays have demonstrated exceptional sensitivity, capable of detecting as few as 10 copies of the viral genome per microliter, which is 1000 times more sensitive than conventional PCR [7]. This allows for the identification of infected animals during the serological window period. The strategic combination of serology and PCR is therefore recommended. Parallel testing (running both tests simultaneously) maximizes sensitivity, while serial testing (confirming a positive ELISA with PCR) maximizes specificity [39]. The choice of target gene is also important; the gag gene is highly conserved and widely used for both PCR and phylogenetic analysis, while the env gene is more variable and useful for subtyping [7, 16, 17]. The development of diagnostic tests using local circulating strains is also crucial, as antigenic variation can lead to false negatives if the test is based on a heterologous strain [17, 39].

Biosecurity and Management: Preventing Introduction and Spread

Beyond the specific interventions of kid management and test-and-cull, a comprehensive biosecurity plan is essential for preventing the introduction of CAEV into a negative herd and for limiting its spread within an infected herd. The virus can be introduced through the purchase of infected animals, which is the most common route [9, 6]. A strict quarantine and testing protocol for all incoming stock is non-negotiable. New animals should be isolated for a minimum of 30-60 days and tested serologically and, ideally, by PCR before being introduced to the main herd. The importation of animals from regions with unknown or poor CAEV status is a significant risk, as demonstrated by the introduction of a B1 subtype into Switzerland via imported goats, which challenged the country's eradication program [6].

Horizontal transmission, though less efficient than lactogenic transmission, is a significant concern, particularly in intensively managed herds. Prolonged direct contact, sharing of feed and water troughs, and the use of contaminated equipment (e.g., dehorning tools, tattoo equipment, needles) can facilitate the spread of the virus [15, 20]. Management practices that reduce contact density, such as avoiding overcrowding and maintaining clean, well-ventilated housing, can help mitigate this risk. The role of breeding bucks in transmission is also a consideration. While sexual transmission is not considered a major route, infected bucks can shed virus in semen, and the close contact during mating can facilitate transmission [5, 21]. Therefore, using only CAEV-negative bucks for breeding is a prudent control measure. Furthermore, the practice of mixed-species grazing or housing of goats and sheep should be avoided, given the demonstrated risk of cross-species transmission of SRLVs [12, 14]. The implementation of these management practices, while requiring vigilance and discipline, is a critical component of a sustainable control program, reducing reliance on reactive testing and culling and moving towards a proactive prevention paradigm.

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Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.