Reticuloendotheliosis Virus

Overview and Taxonomy of Reticuloendotheliosis Virus

Reticuloendotheliosis virus (REV) represents a formidable and economically devastating oncogenic retrovirus within the family Retroviridae, genus Gammaretrovirus, that afflicts a broad spectrum of avian species globally [5, 22, 38]. As a pathogen of critical importance to poultry health and food security, REV is recognized by the World Organisation for Animal Health (WOAH) as a significant agent of neoplastic and immunosuppressive disease, contributing to substantial economic losses across commercial, backyard, and wild bird populations [3, 11, 22]. The virus is the etiological agent of reticuloendotheliosis (RE), a disease complex characterized by immunosuppression, runting-stunting syndrome, acute reticulum cell neoplasia, and chronic T- or B-cell lymphomas [22, 29, 36]. Unlike the related avian leukosis virus (ALV) subgroups, REV possesses a distinct genetic architecture and a uniquely broad host range, infecting chickens, turkeys, ducks, geese, quail, pheasants, peafowl, and numerous free-ranging avian species, which complicates eradication efforts and underscores the necessity for comprehensive global surveillance [1, 31, 44]. The Food and Agriculture Organization (FAO) has also highlighted the threat of transboundary animal diseases like RE to sustainable poultry production, particularly in developing regions with intensive farming systems [41, 43].

Taxonomic Classification and Virion Structure

Taxonomically, REV is classified within the genus Gammaretrovirus of the subfamily Orthoretrovirinae within the family Retroviridae [5, 22]. This genus includes other horizontally transmitted, non-acutely transforming retroviruses, though REV is distinguished by its high prevalence of recombination with other avian pathogens and its propensity for integration into the genomes of heterologous viruses, most notably fowlpox virus (FWPV) and Marek’s disease virus (MDV) [1, 4, 6]. The mature REV virion is an enveloped, spherical particle approximately 80–100 nm in diameter, containing a diploid, single-stranded, positive-sense RNA genome [22, 29]. The viral envelope is derived from the host cell plasma membrane and is studded with surface projections comprised of the viral glycoproteins gp90 (surface, SU) and gp20 (transmembrane, TM), which mediate receptor recognition and membrane fusion during entry [20, 33]. Beneath the envelope resides the icosahedral capsid composed of the p30 capsid (CA) protein, which encases the viral core containing the genomic RNA dimer complexed with the nucleocapsid (NC) protein p12 and the essential viral enzymes: reverse transcriptase (RT), integrase (IN), and protease (PR) [8, 13]. The viral protease is particularly critical, cleaving the gag and pol polyproteins into their functional subunits, and recent evidence has demonstrated that the REV PR possesses the capacity to cleave host cell vimentin, a key intermediate filament protein, at specific sites (leucine-239/glutamine-240, alanine-261/alanine-262, and histidine-431/serine-432), which may facilitate cytoskeletal remodeling essential for viral replication and egress [8].

Genomic Organization and Subtypes

The proviral DNA genome of REV is approximately 8.2 to 8.3 kilobase pairs in length, flanked by identical long terminal repeat (LTR) sequences of approximately 570 bp that contain the U3, R, and U5 regions crucial for viral transcription, integration, and polyadenylation [13, 29, 45]. The genome encodes the canonical retroviral structural and enzymatic genes: gag (encoding matrix p18, capsid p30, and nucleocapsid p12), pol (encoding protease, reverse transcriptase/ribonuclease H, and integrase), and env (encoding surface gp90 and transmembrane gp20) [1, 13]. Notably, REV exhibits substantial genetic diversity, with three major subtypes recognized based on phylogenetic analysis of the env gene, particularly the gp90 sequence, and the LTR region: REV subtype 1 (prototype strain REV-A), REV subtype 2 (prototype strain SNV, spleen necrosis virus), and REV subtype 3 (a globally predominant lineage including strains such as APC-566, MD-2, and numerous Chinese field isolates) [1, 13, 36]. This classification has been corroborated by comprehensive phylogenetic analyses of complete genome sequences, which consistently resolve into three robust clades (bootstrap value 100) corresponding to these subtypes [1, 32]. Recent deep sequencing and comparative genomics have further refined this taxonomy, revealing that REV subtype 3 can be subdivided into distinct ‘East’ and ‘West’ subclusters with notable geographical segregation, comprising 38 strains in the former and 24 in the latter, suggesting divergent evolutionary trajectories and potentially distinct pathogenic signatures [1]. The LTR region, particularly the U3 domain, is the most variable portion of the genome and serves as a key determinant of viral replication efficiency and cell tropism [13, 45]. Indeed, the integration of the REV LTR into the MDV genome has been shown to confer a selective advantage by enhancing horizontal transmission, as demonstrated by the field strain GX0101, which outcompetes its LTR-deleted counterpart in contact transmission studies [42].

Genetic Diversity and Chimeric Associations

A hallmark of REV biology is its remarkable propensity for genetic recombination and integration into the genomes of unrelated avian DNA viruses. The integration of near-full-length REV proviral sequences or remnant LTRs into the FWPV genome, located between ORF201 and ORF203, is an exceptionally common phenomenon observed worldwide [6, 15, 26]. This chimeric state has been documented in FWPV isolates from chickens, turkeys, and pigeons across North America, South America, Europe, Asia, and Africa, with some FWPV strains carrying populations of heterogeneous genome molecules that contain either only the REV LTR or the entire functional REV provirus [6, 25, 26]. The biological significance of this integration is profound: REV sequences integrated into FWPV can be reactivated and expressed, leading to the induction of reticuloendotheliosis in co-infected or even singly infected birds [15, 24]. The 2022 Austrian epidemic of cutaneous fowlpox in naïve layer and broiler flocks, which involved 65 cases across multiple farms, was directly linked to FWPV strains carrying full-length REV proviruses, with the chimeric virus detected not only in lesions but also in environmental dust, highlighting the epidemiological importance of these recombinants [15]. Similarly, REV-LTR insertions have been documented in field isolates of MDV, particularly in China and Egypt, with 22.2% of Egyptian MDV field isolates harboring REV-LTR sequences [4, 21]. These insertions are associated with enhanced MDV pathogenesis, increased tumor incidence, and reduced vaccine efficacy against Marek’s disease, representing a significant threat to poultry health management [21, 39]. The high prevalence of REV integration in FWPV and MDV genomes underscores the critical role of these large DNA viruses as vectors for REV dissemination and evolution [1, 28].

Host Range and Global Epidemiology

REV is unique among the avian retroviruses in its exceptionally broad host range, which encompasses Galliformes (chickens, turkeys, pheasants, quail, prairie chickens), Anseriformes (ducks, geese), and even Columbiformes (pigeons) and Pelecaniformes (pelagic seabirds) [1, 18, 31, 44]. This host promiscuity has been demonstrated by the detection of REV in a Laysan albatross (Phoebastria immutabilis) in Hawaii, marking only the second virus documented in native Hawaiian birds and the first report in a pelagic seabird [18]. The virus has been identified on every continent where poultry farming occurs, with seroprevalence rates in commercial flocks varying dramatically by region: 13.91% in China (2005–2015), 21.13% in Bangladesh, 35% in Egyptian breeder flocks, 39.23% in Thai chickens, and a staggering 74.6% in Sudanese flocks [3, 36, 37, 41, 43]. In wild bird populations, prevalence is generally lower but ecologically significant, with 43.6% of sampled wild turkeys in the United States testing positive for REV DNA, and 4% of Rio Grande wild turkeys in Texas harboring the virus [9, 14]. The detection of REV in critically endangered species, such as the Attwater’s prairie chicken (Tympanuchus cupido attwateri), where it caused catastrophic mortality (nearly half of captive adult mortality in one facility), underscores the conservation implications of this pathogen [23, 24, 34]. Molecular characterization of REV from Brazilian mallard ducks, chickens, and Muscovy ducks has confirmed that subtype 3 circulates extensively in South America, with phylogenetic analyses revealing close relationships to North American and Chinese strains, indicative of global viral traffic likely mediated through contaminated biological products and international trade [1, 35, 40].

Molecular Pathogenesis and Immunosuppression

The pathogenicity of REV is multifaceted, driven by its capacity to establish lifelong persistent infections, induce severe immunosuppression, and trigger oncogenic transformation. At the molecular level, REV infection initiates a cascade of host cellular responses that the virus exploits for its replication. The virus activates the endoplasmic reticulum stress (ERS) response, specifically the PERK-eIF2α signaling axis, which simultaneously suppresses apoptosis and aggravates immunosuppression, thereby creating a permissive environment for viral replication [2]. Concurrently, REV modulates the expression of host microRNAs, particularly miR-155, which is synergistically upregulated during co-infection with ALV-J or MDV and serves to enhance viral replication by targeting dual pathways (PRKCI-MAPK8 and TIMP3-MMP2) that interact with the viral U3 region [10, 27]. The viral gp90 glycoprotein is the primary target of neutralizing antibodies and contains a conserved linear B-cell epitope (195REESVRERL203) that is critical for vaccine development [20]. Exosomes derived from REV-infected cells and from meconium of infected chicks carry the complete viral genome and three major viral proteins, enabling horizontal transmission that is completely resistant to neutralization by REV-specific antibodies, representing a significant immune evasion mechanism [7, 17, 19]. This exosome-mediated pathway has been demonstrated in semen, meconium, and cell culture supernatants, providing a robust vehicle for vertical, horizontal, and venereal transmission even in the presence of maternal immunity [7, 17, 19]. The complex interplay between REV, its avian hosts, and co-infecting pathogens, including ALV-J, MDV, and FWPV, drives a synergistic replication dynamic that amplifies pathogenicity, extends tumor spectra, and compromises vaccine efficacy, posing a persistent challenge to the global poultry industry [12, 16, 30, 39].

Molecular Pathogenesis and Immunosuppression by REV

Reticuloendotheliosis virus (REV), a gammaretrovirus within the family Retroviridae, represents one of the most insidious pathogens affecting global poultry production, not merely for its direct oncogenic potential but fundamentally for its profound and multifaceted capacity to subvert the host immune system. The molecular pathogenesis of REV is a complex, orchestrated process involving direct cytopathic effects on immune organs, intricate manipulation of host cell signaling pathways, exploitation of non-classical transmission routes via exosomes, and synergistic interactions with other avian pathogens. Understanding these mechanisms at a molecular level is critical for developing effective control strategies, particularly given the absence of a commercial vaccine and the virus's ability to persist in flocks through both vertical and horizontal transmission [22, 47]. The World Organisation for Animal Health (WOAH) recognizes REV as a significant pathogen due to its economic impact on the poultry industry, and its immunosuppressive nature complicates the control of other endemic diseases.

Induction of Endoplasmic Reticulum Stress and the Unfolded Protein Response

A cornerstone of REV’s pathogenic strategy is its ability to hijack the host cell’s stress response machinery. Upon infection, REV induces significant endoplasmic reticulum (ER) swelling and upregulates the ER stress marker HSPA5 in infected cells, such as DF-1 chicken fibroblast cells [2]. This triggers the unfolded protein response (UPR), a cellular adaptive mechanism designed to restore ER homeostasis. Critically, REV selectively activates the PERK-eIF2α signaling branch of the UPR, while the other branches (IRE1 and ATF6) remain largely unaffected [2]. This is not a passive consequence of viral replication but a deliberate viral strategy. Activation of PERK-eIF2α serves a dual purpose that is exquisitely beneficial for the virus. First, it suppresses apoptotic cell death, thereby prolonging the survival of the host cell and allowing for sustained viral replication. Second, and perhaps more importantly for the host, this pathway directly exacerbates immunosuppression [2]. The phosphorylation of eIF2α leads to a global attenuation of cap-dependent translation, which preferentially inhibits the synthesis of short-lived proteins, including many cytokines and immune mediators. This creates a cellular environment that is permissive for viral replication while simultaneously dampening the host's ability to mount an effective antiviral response. The REV-induced PERK-eIF2α activation thus represents a critical molecular switch that shifts the cellular balance from a defensive, apoptotic state to a pro-viral, immunosuppressed state.

Subversion of Innate Immune Signaling: The TLR and Interferon Pathways

REV’s immunosuppressive capabilities are further manifested through its direct interference with the innate immune system, particularly the Toll-like receptor (TLR) and interferon (IFN) signaling cascades. The TLR-3/IFN-β pathway is a primary antiviral defense mechanism, where recognition of viral double-stranded RNA by TLR-3 leads to the activation of transcription factors like IRF-7 and NF-κB, culminating in the production of type I interferons. In specific-pathogen-free (SPF) chickens infected with REV, a dynamic and biphasic disruption of this pathway is observed in central immune organs such as the thymus and bursa of Fabricius [48]. In the early stages of infection, there is an upregulation of TLR-3, IRF-7, and NF-κB p65 at both the mRNA and protein levels, suggesting an initial attempt by the host to mount an antiviral response. However, as the infection progresses, a dramatic collapse occurs. The levels of NF-κB p65 decrease, and this is contemporaneous with a significant fall in IFN-β production at both the transcriptional and translational levels [48]. This late-stage suppression is a key molecular event underlying REV-induced immunosuppression, as it cripples the host's ability to control not only REV itself but also secondary opportunistic infections. The data strongly suggest that the changes in IFN-β content are intimately linked to the availability of NF-κB p65, indicating that REV may target this transcription factor for degradation or inhibition, effectively silencing the interferon response in the central lymphoid organs [48]. This is corroborated by transcriptomic analyses showing that REV infection modulates the expression of key pattern recognition receptors (PRRs) and downstream signaling molecules, including MyD88, STAT1, and various interferon-stimulated genes (ISGs), with a pronounced effect on the JAK-STAT and NF-κB signaling pathways [54, 58].

The Role of MicroRNAs in Synergistic Replication and Immunosuppression

REV pathogenesis is also profoundly influenced by its manipulation of the host microRNA (miRNA) network, particularly miR-155. This miRNA acts as a central hub in the regulation of immune responses and oncogenesis. REV infection, especially by the highly oncogenic Rev-T strain, directly activates the transcription of gga-miR-155 through the binding of the v-rel oncoprotein to NF-κB binding sites within the miR-155 promoter [57]. This upregulation is not merely a byproduct of infection but is mechanistically linked to the transformation process and the establishment of an immunosuppressive state. The role of miR-155 becomes even more critical in the context of co-infection, which is the rule rather than the exception in the field. Co-infection with avian leukosis virus subgroup J (ALV-J) and REV synergistically increases miR-155 levels to a much greater extent than either virus alone [10, 56]. This synergistic activation of miR-155 is a key driver of the enhanced viral replication and exacerbated pathology seen in co-infected birds. Mechanistically, miR-155 promotes synergistic replication by targeting a dual pathway: it suppresses PRKCI, which in turn prevents the inhibition of MAPK8, and it suppresses TIMP3, which prevents the inhibition of MMP2 [10]. Both MAPK8 and MMP2 then interact with the U3 region of the ALV-J and REV genomes, directly enhancing their transcription and replication [10]. This creates a powerful positive feedback loop where co-infection drives miR-155 expression, which in turn drives the replication of both viruses. Furthermore, miR-155 contributes to immunosuppression by targeting pro-apoptotic factors like caspase-6 and FOXO3a, thereby inhibiting apoptosis and promoting cell survival, which paradoxically allows for greater viral persistence and dissemination [27]. The role of miR-155 extends beyond co-infection; it is a central node in REV pathogenesis, linking viral replication, immune evasion, and oncogenic transformation.

Exosome-Mediated Immune Evasion and Transmission

A paradigm-shifting aspect of REV molecular pathogenesis is its exploitation of host exosomes for both immune evasion and transmission. Exosomes are small extracellular vesicles that mediate intercellular communication. REV-infected cells, including DF-1 cells and cells in the reproductive tract, release exosomes that carry the complete REV genome and viral proteins [7, 19]. These REV-loaded exosomes are functionally infectious and, critically, are resistant to neutralization by REV-specific antibodies [7, 19]. This provides the virus with a cloaked mode of transmission that bypasses the humoral immune response. The implications are profound. Semen-derived exosomes can mediate the entry of REV into semen, providing a mechanism for vertical transmission via artificial insemination that is impervious to maternal antibodies [19]. Similarly, exosomes present in the meconium of newly hatched chicks can mediate early horizontal transmission, even in the presence of maternally derived antibodies that would normally neutralize free virus particles [17]. This exosome-mediated pathway explains the paradoxical observation that REV can spread efficiently within flocks despite the presence of specific antibodies. The exosomes effectively shield the virus from the adaptive immune system, allowing it to establish infection in new hosts and contributing to the virus's persistence and the failure of antibody-based control strategies. This mechanism represents a sophisticated form of immune escape that is distinct from classical antigenic variation.

Cleavage of Host Cytoskeletal Proteins and Disruption of Cellular Integrity

At a more fundamental level, REV directly targets the host cell cytoskeleton through the activity of its viral protease (PR). Beyond its essential role in cleaving viral polyproteins (Gag and Pol) into their mature, functional components, the REV PR has been shown to cleave chicken vimentin, a key intermediate filament protein that maintains cellular structure and integrity [8]. This is the first demonstration of such an activity for an avian retrovirus protease. The REV PR cleaves vimentin at specific sites between leucine-239 and glutamine-240, alanine-261 and alanine-262, and histidine-431 and serine-432 [8]. The cleavage of vimentin is a significant pathogenic event. Vimentin is not merely a structural protein; it is also involved in cell signaling, migration, and the immune response. Its degradation by the REV PR likely contributes to the cytopathic effects observed in infected cells, including cell rounding and detachment [3]. Furthermore, disruption of the vimentin network can impair the formation of the immunological synapse, potentially hindering the ability of T cells to interact with antigen-presenting cells and further contributing to the overall state of immunosuppression. This direct proteolytic attack on a major host structural protein underscores the aggressive and multifaceted nature of REV's pathogenic strategy, where the virus actively dismantles host cell architecture to facilitate its own replication and spread.

Metabolic Reprogramming and Oxidative Stress

REV infection induces a profound metabolic reprogramming in host cells, particularly in lymphocytes, which is intimately linked to the observed immunosuppression. Transcriptomic and proteomic analyses of REV-infected lymphocytes reveal a significant shift in lipid metabolism. Infected cells preferentially utilize exogenous fatty acids for energy via β-oxidation, as evidenced by increased free fatty acid content and carnitine palmitoyltransferase-1 activity, while simultaneously downregulating genes involved in de novo lipid and fatty acid biosynthesis [49]. This metabolic switch is orchestrated through the peroxisome proliferator-activated receptor (PPAR) signaling pathway [49]. This shift likely provides the energy and biosynthetic precursors necessary for viral replication but comes at a cost to the host cell's normal function. Concomitantly, REV infection induces a state of severe oxidative stress in immune organs like the thymus. Levels of reactive oxygen species (H₂O₂) and lipid peroxidation markers (MDA) increase, while the activity and expression of key antioxidant enzymes, such as superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx1), are significantly reduced [52]. This imbalance leads to oxidative damage, mitochondrial swelling, and nuclear damage within thymic cells, contributing directly to thymic atrophy and the loss of T lymphocytes [52]. The resulting oxidative environment is highly immunosuppressive, impairing lymphocyte proliferation and function. Proteomic studies of the bursa of Fabricius have confirmed that REV infection alters the expression of multiple proteins involved in oxidative stress response, including peroxiredoxins and glutathione peroxidases, further solidifying the link between REV-induced oxidative damage and immunosuppression [50].

Synergistic Pathogenesis with Other Oncogenic Viruses

In the field, REV rarely acts alone. Its immunosuppressive nature makes it a frequent accomplice in complex polymicrobial infections, most notably with Marek's disease virus (MDV) and ALV-J. The molecular basis of this synergy is becoming increasingly clear. Co-infection with MDV and REV enhances the replication of both viruses in a limited time frame, leading to increased viral loads and more severe disease [12]. This synergistic replication is driven by the modulation of host proteins. Quantitative proteomics has identified that co-infection alters the abundance of key immune regulators, including IRF7, MX1, TIMP3, and AKT1, which are likely involved in the enhanced replication [12]. The presence of an REV long terminal repeat (LTR) integrated into the genome of some field MDV strains (e.g., GX0101) provides a direct molecular mechanism for synergy. This REV LTR acts as a powerful enhancer, driving the expression of adjacent MDV genes and conferring a significant horizontal transmission advantage to the recombinant MDV, making it a more prevalent and virulent strain [42]. Similarly, co-infection with ALV-J and REV leads to a dramatic increase in viral replication and pathogenicity. This is mediated, as discussed, through the synergistic activation of miR-155 [10] and the dysregulation of integrin signaling pathways, which are critical for cell adhesion, migration, and immune cell trafficking [16]. The co-infection also leads to the accumulation of exosomal miRNAs that further modulate the cellular environment to favor viral replication [56]. These synergistic interactions result in a clinical picture of heightened mortality, an expanded tumor spectrum, and a profound breakdown of vaccine efficacy, as seen with MDV vaccines [39]. The molecular interplay between REV and these other viruses creates a pathogenic synergy that is far more devastating than the sum of its parts, highlighting the critical need for diagnostic tools that can detect these complex co-infections, such as the multiplex qPCR assays now being developed [46, 51].

Disruption of Lymphocyte Homeostasis and Adaptive Immunity

The ultimate consequence of these molecular events is a severe disruption of adaptive immunity. REV directly targets the central and peripheral immune organs, causing atrophy of the thymus, bursa of Fabricius, and spleen [47, 55]. Within the thymus, REV infection inhibits the proliferation of thymic lymphocytes, induces apoptosis, and decreases the ratio of CD4⁺ to CD8⁺ T cells [53]. This imbalance in T cell subtypes is a hallmark of immunosuppression, compromising both cell-mediated and humoral immune responses. The virus achieves this by modulating key cell cycle regulators, such as CyclinD1, and anti-apoptotic proteins like Bcl-2 [53]. In peripheral blood lymphocytes, REV infection reduces cell numbers by inhibiting the S/G1 phase transition through the FOXO and p53 pathways [55]. Simultaneously, it cripples the immune defense functions of these lymphocytes by suppressing the secretion of critical cytokines like IL-8 and IL-18 through the disruption of Toll-like receptor, NOD-like receptor, and MAPK-AP1 signaling pathways [55]. The cumulative effect is a bird that is profoundly immunocompromised, with a diminished capacity to respond to vaccines and a heightened susceptibility to a wide range of secondary bacterial, viral, and parasitic infections. This is the clinical reality of REV infection: a primary pathogen that acts as a gateway for a host of other diseases, making it a major driver of morbidity and mortality in poultry flocks worldwide.

Epidemiology, Transmission, and Host Range of REV

Reticuloendotheliosis virus (REV) represents a paradigmatic example of a gammaretrovirus that has successfully breached species barriers, established global distribution, and exploited multiple transmission modalities to achieve persistent circulation in both domestic and wild avian populations. The epidemiological landscape of REV is characterized by its remarkable host breadth, its capacity for both vertical and horizontal dissemination, and its frequent association with co-infections that amplify its pathogenic impact. Understanding the intricate dynamics of REV transmission and its host range is not merely an academic exercise; it is a critical prerequisite for designing effective control strategies, particularly given the virus's documented ability to contaminate commercial vaccines and its emergence in endangered species.

Global Distribution and Seroprevalence: A Pervasive Threat

The distribution of REV is truly global, with serological and molecular evidence confirming its presence on every continent where poultry production exists. The World Organisation for Animal Health (WOAH) recognizes REV as a significant pathogen of galliform birds, and the Food and Agriculture Organization (FAO) has highlighted its economic impact on smallholder and commercial flocks alike. The seroprevalence data paint a picture of a virus that is endemic in many regions, often circulating at high levels within seemingly healthy flocks.

In China, a landmark serological survey spanning 2005 to 2015 examined 25,224 sera from 573 flocks and found an overall individual seroprevalence of 13.91%, with 56.20% of flocks harboring at least one seropositive bird [43]. This study revealed striking regional variation, with Guangxi province exhibiting a staggering 57.84% seroprevalence, underscoring the focal nature of REV transmission. More recent surveillance using a multiplex qPCR assay in Guizhou province detected REV positivity in 9.3% (11/118) of local chickens, with a notable 20.4% of positive samples representing mixed infections with avian leukosis virus subgroup J (ALV-J) and chicken infectious anemia virus [46]. In Egypt, a targeted investigation of breeder flocks from 2022–2023 identified a 35% seroprevalence rate using ELISA, with molecular confirmation of REV subtype III circulating in the Ismailia, El-Sharqia, and El-Dakahliya governorates [3]. The Egyptian isolates showed 99–100% nucleotide identity with American, Chinese, and Taiwanese strains, highlighting the transnational movement of REV lineages.

In Thailand, the first comprehensive molecular survey of REV in chickens from 2013–2016 detected REV in 39.23% (51/130) of clinical samples and 72.41% (21/29) of farms across nine provinces [36]. All Thai isolates clustered within REV subtype III, reinforcing the dominance of this subtype in Asian poultry. In Bangladesh, a massive serological study of 3,555 samples from 144 flocks yielded an overall seroprevalence of 21.13%, with 73.61% of flocks being seropositive; notably, the highest rates were observed during the onset of lay (19–24 weeks of age), suggesting that reproductive stress may reactivate latent infections [41]. Sudan presented an even more alarming picture, with a seroprevalence of 74.6% in local and commercial breeds, including 79.5% in commercial flocks; PCR detection of proviral DNA in liver (10%) and spleen (15%) samples confirmed active viral replication [37]. In South America, Brazil has emerged as a region of particular interest. Retrospective analysis of backyard chickens revealed the first molecular characterization of REV in South America, with the virus classified as REV subtype 3 [32]. Subsequent surveys documented REV in 65% of sampled chickens from farms with multiple viral coinfections, and a broader ecological survey detected REV in 16.8% of birds from the Amazon biome, including Muscovy ducks, wild turkeys, and chickens [13, 35]. The discovery of REV in a mallard duck in Brazil, the first complete genome from South America, demonstrates that the virus is actively circulating in free-ranging waterfowl [1].

Host Range: Beyond the Domestic Chicken

While Gallus gallus domesticus remains the primary host for REV, the virus exhibits an extraordinarily broad host range encompassing at least 20 avian species across multiple orders. This host promiscuity is a defining feature of REV epidemiology and presents unique challenges for disease control.

Domestic poultry are the most extensively studied hosts. Beyond chickens, REV causes significant disease in turkeys (Meleagris gallopavo), Muscovy ducks (Cairina moschata), and Pekin ducks [31, 35]. In China, an outbreak of neoplastic disease in breeding Muscovy ducks led to the isolation of strain CH-GD2019, which was phylogenetically closely related to chicken-origin REV strains, suggesting interspecies transmission [31]. Turkeys appear particularly susceptible; an Austrian epidemic of cutaneous fowlpox in naïve layer chickens and turkeys revealed that all FWPV field strains harbored integrated REV provirus, and the clinical presentation was markedly more severe in these co-infected birds [15].

Wild birds serve as both reservoirs and sentinels for REV circulation. In the United States, extensive surveillance of wild turkeys has revealed a prevalence of 43.6% (75/172) across 15 states, with co-infections with lymphoproliferative disease virus (LPDV) detected in 34.9% of birds [9]. REV was significantly more common in winter, suggesting seasonally influenced transmission dynamics. In Texas, Rio Grande wild turkeys (Meleagris gallopavo intermedia) showed a 5% prevalence in REV-affected counties, and an Eastern wild turkey imported from West Virginia was also REV-positive, indicating that translocation of birds may spread the virus [14, 34]. Perhaps most concerning is the detection of REV in endangered species. During a 2016–2017 outbreak at a captive breeding facility, nearly half of all adult Attwater's prairie chickens (Tympanuchus cupido attwateri) died from REV infection, and the proviral genome sequence was nearly identical to fowlpox virus-integrated REV strains [23, 24]. This finding implicates FWPV as a vector for REV transmission in this critically endangered population. In Hawaii, REV was identified as only the second virus ever documented in native Hawaiian birds associated with pathology, infecting two threatened Hawaiian geese (Branta sandvicensis) and a Laysan albatross (Phoebastria immutabilis); the detection of REV in a pelagic seabird is unprecedented and raises questions about marine transmission pathways [18]. Green peafowls (Pavo muticus) in Iran have also been found co-infected with REV and ALV, further expanding the host range [44].

Transmission Dynamics: A Multimodal Strategy

REV has evolved a sophisticated arsenal of transmission mechanisms that collectively ensure its persistence across diverse ecological niches. The virus can be transmitted vertically, horizontally, iatrogenically, and via vector-borne mechanisms.

Vertical (Egg-borne) Transmission: REV is efficiently transmitted from infected hens to their progeny through the egg. This vertical route is of paramount importance because it allows the virus to persist across generations and evade detection in breeder flocks. Infected chicks shed virus in their meconium immediately after hatching, providing an early source of infection for hatchmates [17]. The ability of REV to establish congenital infection was elegantly demonstrated using a high-dose spleen necrosis virus (SNV) model, where one-day-old SPF chickens inoculated with REV developed persistent infections and showed profound transcriptomic changes in the spleen [54, 61]. Vertical transmission is also implicated in the contamination of chicken embryos used for vaccine production, a documented source of REV outbreaks in vaccinated flocks [45].

Horizontal Transmission and the Role of Exosomes: Horizontal transmission through direct contact, contaminated feces, and environmental fomites is a major driver of REV spread within flocks. Infected birds shed virus in feces, and the virus can remain infectious in litter and dust for extended periods [15]. A critical breakthrough in our understanding of REV transmission came with the discovery of exosomes as vehicles for viral spread. Mechanistically, exosomes purified from REV-positive semen contain viral genomic RNA and all three major viral proteins, and these exosomes can establish productive infections both in vitro and in vivo, crucially evading neutralization by REV-specific antibodies [19]. Semen-derived exosomes are more efficient than free virions at establishing infection, implicating a novel mechanism for REV entry into semen and subsequent vertical transmission. Similarly, exosomes purified from REV-infected DF-1 cells were shown to infect 7-day-old embryonated eggs, 1-day-old chicks, and 23-week-old hens, even in the presence of neutralizing antibodies [7]. Most alarmingly, exosomes extracted from the meconium of REV-positive chicks can mediate horizontal transmission and immune evasion; when these exosomes were inoculated into maternal antibody-positive chicks, the maternal antibodies failed to inhibit the pathogenic effects, including growth retardation and hepatosplenomegaly [17]. This exosome-mediated transmission provides a mechanistic explanation for the observation that REV can spread even in flocks with high levels of maternally derived antibodies.

Iatrogenic Transmission via Vaccine Contamination: One of the most insidious routes of REV transmission is through contaminated biologics. Historical incidents of REV contamination in commercial Marek's disease vaccines, fowlpox vaccines, and even infectious bursal disease vaccines have been documented globally. In China, a REV strain named MD-2 was isolated from a commercial Marek's disease vaccine, and its complete genome showed >99% identity with the prairie chicken isolate APC-566 from the US [45]. Similarly, a REV strain isolated from a contaminated IBD vaccine was used to develop a gp90-based vaccine candidate, highlighting the irony of using a contaminant as a vaccine antigen [62]. The detection of REV contamination in vaccines has driven the development of highly sensitive assays, such as droplet digital PCR (ddPCR), which can detect as little as 0.1 TCID₅₀/1,000 feathers, an improvement of 1,000-fold over conventional PCR [59]. The presence of REV in attenuated vaccines is suspected to be a primary cause of massive REV outbreaks in China [59].

Vector-Borne Transmission via Fowlpox Virus: Perhaps the most unique and epidemiologically significant aspect of REV transmission is its integration into the genome of fowlpox virus (FWPV). This chimeric relationship has been observed globally. The REV provirus, either as a full-length genome or as remnant long terminal repeats (LTRs), is stably integrated between ORF201 and ORF203 of the FWPV genome [6, 26]. This integration has profound implications: FWPV, an epitheliotropic virus transmitted mechanically by biting insects (e.g., mosquitoes, Culicoides midges), can deliver REV directly to new hosts. In Austria, an epidemic of cutaneous fowlpox in naïve layer chickens and turkeys revealed that all FWPV field strains carried integrated REV, and the outbreaks were unusually severe [15]. In Brazil, atypical fowlpox outbreaks in vaccinated commercial laying hens were linked to FWPV strains containing both partial (LTR-only) and complete REV insertions, and these chimeric viruses caused feathering abnormalities and diphtheritic lesions that mimicked infectious laryngotracheitis [25]. Phylogenetic analysis of 27 chimeric FWPV-REV genomes placed them across all three REV subtypes, demonstrating that integration events have occurred multiple times independently [1]. The REV LTR insertion in FWPV can also occur in the MDV genome; in Egypt, 22.2% of MDV field isolates contained REV-LTR insertions, and these insertions were closely related to those in FWPV, suggesting horizontal exchange of genetic material between these DNA viruses [4].

Enhancement of Horizontal Transmission by REV LTR: The REV LTR, when integrated into the genome of Marek's disease virus (MDV), provides a selective advantage that explains its prevalence in field strains. The recombinant MDV strain GX0101, which harbors an REV LTR insert, was shown to possess significantly higher horizontal transmission capacity compared to its LTR-deleted counterpart (GX0101ΔLTR) [42]. In co-infection experiments, GX0101 became the predominant strain even when initially inoculated at a 100-fold lower dose. Transcriptomic analysis revealed that 16 genes related to virus replication and transmission were significantly up-regulated in GX0101, including genes involved in tegument proteins, glycoproteins, and immune evasion [42]. This LTR-driven enhancement of transmission likely explains why recombinant MDVs have become dominant in Chinese poultry flocks over the past two decades.

Co-infections and Synergistic Transmission

REV rarely circulates alone; field surveys consistently demonstrate that REV-positive flocks are frequently co-infected with other immunosuppressive and oncogenic viruses. In Brazil, 82.5% of REV-positive samples harbored up to seven additional viruses, including ALV-J, MDV, chicken infectious anemia virus, and fowl adenoviruses [13]. In China, the reemergence of REV and MDV co-infection in Yellow-Chickens from 2018–2020 was associated with 5–20% morbidity and 2–10% mortality, and the MDV strains were characterized as very virulent plus (vv+) type [21, 60]. The co-infection of MDV and REV enhances viral replication: in CEF cells, dual infection increased MDV and REV loads compared to single infections, and proteomic analysis identified IRF7, MX1, TIMP3, and AKT1 as potential mediators of this synergy [12]. Furthermore, MDV-REV co-infection significantly reduces the efficacy of Marek's disease vaccination; the protective index of CVI988 vaccine dropped from 80.0 to 47.7 in co-challenged birds [39].

The synergy between REV and ALV-J is equally well-documented. Co-infection promotes synergistic replication through the upregulation of miR-155, which targets a dual pathway involving PRKCI-MAPK8 and TIMP3-MMP2, ultimately interacting with the U3 region of both viruses [10]. Exosomes from co-infected cells contain higher levels of miR-155 and other miRNAs compared to singly infected cells, and these exosomes can modulate the actin cytoskeleton pathway through TRIM62 regulation [30, 56]. The epidemiological consequence is that mixed infections are common; in Guizhou, 20.4% of all positive samples were co-infections of ALV-J, REV, and CIAV [46].

Temporal and Seasonal Patterns

Epidemiological studies have identified seasonal patterns in REV prevalence. In wild turkeys in the US, REV prevalence was significantly higher in winter compared to other seasons, a finding that may reflect increased crowding at feeding sites or stress-induced reactivation of latent infections [9]. In Bangladesh, seroprevalence did not vary significantly by season, ranging only from 20.43% (winter) to 22.96% (summer), suggesting year-round transmission in tropical climates [41]. Age is another critical risk factor: REV seroprevalence increases with age, with the highest rates observed in birds older than 24 weeks [41, 43]. This age-related increase likely reflects cumulative exposure and the chronic nature of REV infection.

Molecular Epidemiology and Phylogenetic Subtypes

Phylogenetic analyses consistently classify REV into three major subtypes (REV-1, REV-2, and REV-3), with REV-3 being globally dominant in chickens. The complete genome sequencing of REV from diverse hosts has revealed a complex evolutionary history shaped by recombination, positive selection, and geographical clustering. The recent discovery of two distinct REV-3 subclusters, designated 'East' (38 strains) and 'West' (24 strains), with notable geographical associations suggests that regional viral evolution is occurring independently [1]. Selective pressure analysis has identified positively selected codons in the matrix (MA), p18, reverse transcriptase/ribonuclease H, and surface (SU) domains of the envelope protein, indicating that adaptive evolution is driving diversification of key functional domains [1, 13]. Notably, the pol gene of REV shows remarkable plasticity, with variable lengths and amino acid deletions after position 675 in different strains [6]. This genetic flexibility likely contributes to the virus's ability to infect such a wide range of hosts and to integrate into the genomes of unrelated DNA viruses.

Geographical Hotspots and Emerging Threats

While REV is globally distributed, certain regions have emerged as hotspots of high prevalence and genetic diversity. Southern China, particularly Guangxi and Guangdong provinces, consistently reports the highest seroprevalence rates (57.84% in Guangxi) and is the epicenter of recombinant MDV-REV and FWPV-REV strains [21, 43, 60]. Brazil is now recognized as a major reservoir for REV in South America, with the virus detected in chickens, turkeys, ducks, and wild birds across multiple biomes, including the Amazon [1, 13, 35, 40]. The detection of REV in a pelagic seabird in Hawaii raises the alarming possibility that the virus may be spreading via marine bird migration routes, potentially introducing REV to naive island ecosystems [18].

The emergence of REV in captive breeding programs for endangered species represents a conservation crisis. The Attwater's prairie chicken outbreak, which killed nearly half of the captive adult population at a Texas facility, underscores the vulnerability of genetically homogeneous captive populations to REV [23, 24]. With no commercial vaccine available against REV, the only mitigation strategy is rigorous biosecurity and surveillance. The discovery of REV in wild turkeys in Texas, which roam near captive APC facilities, suggests that wild birds may serve as a reservoir from which the virus can be transmitted to these vulnerable populations [34].

The epidemiological picture of REV is one of a highly adaptable, globally distributed retrovirus that exploits every conceivable transmission route, vertical, horizontal, exosome-mediated, iatrogenic, and vector-borne, to maintain its presence in avian populations. Its broad host range, encompassing domestic poultry, game birds, waterfowl, and endangered species, combined with its propensity for co-infection and genetic recombination, makes REV a formidable challenge for the poultry industry and wildlife conservationists alike. The continued emergence of chimeric FWPV-REV and MDV-REV strains, the detection of REV in previously naïve species and ecosystems, and the demonstration of exosome-mediated immune evasion all point to a virus that is far from contained and that will require innovative, multi-pronged control strategies.

Clinical Manifestations and Pathological Lesions of Reticuloendotheliosis

Reticuloendotheliosis virus (REV) induces a spectrum of clinical disease that ranges from subclinical immunosuppression to acute fatal neoplasia, with the specific manifestations governed by viral strain, host species, age at infection, immune status, and the presence of concurrent infections. The clinical and pathological features of REV infection are profoundly influenced by the virus's dual capacity to both transform lymphoid cells and induce severe immunosuppression, creating a pathogenic cascade that often culminates in multiple organ dysfunction and heightened susceptibility to secondary pathogens [3, 13, 29]. Understanding the full breadth of these manifestations is critical for differential diagnosis, particularly given the overlapping presentations with Marek's disease virus (MDV) and avian leukosis virus (ALV), and is essential for implementing effective surveillance and control programs [4, 21, 39].

Runting-Stunting Syndrome and Growth Retardation

One of the most economically significant clinical presentations of REV infection is the runting-stunting syndrome, characterized by profound growth retardation, poor feathering, and general unthriftiness. This syndrome is particularly pronounced in chicks infected vertically or horizontally at a very young age [29, 38, 47]. Experimentally, specific-pathogen-free (SPF) chickens inoculated with REV at one day of age exhibit significantly depressed weight gain compared to uninfected controls, with the severity of growth impairment correlating directly with viral load and the degree of lymphoid organ damage [29, 47]. Wang and colleagues demonstrated that REV-infected chicks showed marked developmental delay, hepatomegaly, and thymic atrophy, with mortality rates significantly elevated [47]. The runting phenomenon is not merely a consequence of reduced feed intake but reflects a fundamental metabolic reprogramming induced by the virus. Transcriptomic analyses of peripheral blood lymphocytes from REV-infected chickens reveal a profound shift in lipid metabolism, characterized by reduced triglyceride content and increased free fatty acid utilization via the peroxisome proliferator-activated receptor (PPAR) signaling pathway, suggesting that the virus redirects host energy metabolism away from growth and toward viral replication [49]. Clinically, affected birds present with small body size, pale combs and wattles, and a characteristic "dwarf" appearance that distinguishes them from flock mates, often becoming economically culled or succumbing to secondary infections [29, 66].

Immunosuppression: The Foundation of Disease Progression

Immunosuppression represents the most insidious and consequential clinical manifestation of REV infection, as it not only directly impairs host defense but also exacerbates the pathogenicity of co-infecting agents and compromises vaccine efficacy. The virus establishes a state of profound immune dysfunction through multiple, synergistic mechanisms targeting the central and peripheral lymphoid organs.

Thymic and Bursal Atrophy

The hallmark pathological lesion underlying REV-induced immunosuppression is the progressive atrophy of the thymus and bursa of Fabricius. In experimentally infected SPF chickens, the thymus index is significantly reduced compared to controls, with histopathological examination revealing a marked depletion of cortical lymphocytes, increased reticular endothelial cells, inflammatory cell infiltration, and evidence of mitochondrial swelling and nuclear damage [50, 52]. The thymic atrophy is driven by a combination of increased apoptosis and inhibited proliferation of thymic lymphocytes. Fu and colleagues demonstrated that REV infection leads to a significant reduction in the proliferation potential of thymic lymphocytes, with decreased transition through the S/G1 phase of the cell cycle, accompanied by increased apoptosis and a reduced CD4+/CD8+ T-cell ratio [53]. This T-cell imbalance is a critical component of the immunosuppressive phenotype, as it impairs both cell-mediated and humoral immune responses. Concurrently, the bursa of Fabricius undergoes similar degenerative changes, with follicular atrophy, loss of cortical lymphocytes, and disruption of normal bursal architecture, as confirmed by TMT-based quantitative proteomic analysis showing massive alterations in proteins involved in immune responses, energy metabolism, and the cell cycle [50].

Oxidative Stress and Antioxidant Depletion

A key mechanistic driver of lymphoid organ atrophy is the induction of oxidative stress. REV infection results in a significant increase in hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) levels in the thymus, coupled with a marked decrease in total antioxidant capacity (TAC), superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase 1 (GPx1) [52]. This oxidative imbalance leads to enhanced lipid peroxidation and direct cellular damage, accelerating the apoptotic loss of lymphocytes. The expression of antioxidant enzymes is transcriptionally downregulated, as evidenced by reduced CAT and GPx1 mRNA levels in the thymus of infected birds [52]. The resulting oxidative stress not only damages lymphoid tissue directly but also impairs the functional capacity of surviving lymphocytes, further compounding the immunosuppressed state.

Disruption of Intracellular Signaling and Apoptosis Regulation

The virus orchestrates a sophisticated manipulation of host cell signaling to evade immune clearance and promote its own replication while simultaneously suppressing protective immune responses. REV infection induces endoplasmic reticulum (ER) stress, leading to the specific activation of the PERK-eIF2α signaling axis [2]. This activation serves a dual purpose: it suppresses apoptotic cell death, allowing infected cells to survive and produce viral progeny, while simultaneously exacerbating immunosuppression [2]. The PERK pathway-mediated attenuation of apoptosis is a critical survival strategy for the virus, as it prevents the premature death of infected cells that would otherwise limit viral output. Furthermore, the virus modulates the expression of microRNAs, particularly miR-155, which is upregulated in a time- and dose-dependent manner in REV-infected cells [27, 57]. miR-155 targets caspase-6 and FOXO3a to inhibit apoptosis and accelerate cell cycle progression, enhancing the viability of infected cells and facilitating viral replication [27]. This miRNA-mediated regulation extends to the broader immune response, as miR-155 has been shown to facilitate synergistic replication between REV and ALV-J by targeting the PRKCI-MAPK8 and TIMP3-MMP2 dual pathway, which then interacts with the U3 region of both viruses to enhance replication [10].

Inhibition of Innate and Adaptive Immunity

At the molecular level, REV infection profoundly disrupts both innate and adaptive immune pathways. The virus impairs the TLR-3/IFN-β pathway, a critical component of the antiviral innate immune response. In REV-infected chickens, the mRNA and protein levels of TLR-3, IRF-7, and NF-κB p65 show significant alterations over the course of infection, with an initial upregulation followed by a later decline in NF-κB p65 that correlates with a fall in IFN-β levels in the thymus and bursa [48]. This suppression of interferon production allows the virus to replicate more efficiently and predisposes the host to secondary infections. Additionally, microarray and RNA-sequencing studies have revealed that REV downregulates genes involved in antigen processing and presentation, T-cell receptor signaling, and cytokine receptor interactions, while upregulating inflammatory mediators such as IL-1β, IL-6, and TNF-α [38, 54, 58]. This dysregulated inflammatory state contributes to the pathological changes observed in lymphoid organs without providing protective immunity. The virus also targets peripheral blood lymphocytes specifically, inhibiting their proliferation through FOXO and p53 pathways and suppressing the secretion of IL-8 and IL-18 via the MAPK-AP1 pathway downstream of Toll-like receptor and NOD-like receptor signaling [55].

Neoplastic Disease: Lymphoreticular Tumors

The oncogenic potential of REV is fully realized in the development of neoplastic lesions, primarily lymphoreticular tumors, which can affect multiple visceral organs. These tumors are a common presenting sign in commercial flocks and are frequently the reason for diagnostic investigation.

Gross Pathology and Organ Distribution

On necropsy, the most consistent gross lesions are hepatosplenomegaly with multifocal to coalescing white to grayish tumor nodules. The liver is often massively enlarged, with the parenchyma replaced by nodular growths that can vary from miliary to several centimeters in diameter [3, 29, 31, 60, 64]. The spleen is similarly affected, showing marked enlargement and the presence of neoplastic foci [31, 64]. Tumors are also frequently observed in the kidney, proventriculus, heart, lung, ovary, and cecal tonsils, and occasionally in the skeletal muscle and peripheral nerves, though nerve involvement is less prominent than in Marek's disease [21, 60, 64]. In breeder ducks, the most significant gross lesions were reported as tumors bearing livers and enlarged spleens, with proliferation of malignant lymphoreticular cells [31]. The proventriculus may be thickened and exhibit a characteristic "pork chop" appearance due to neoplastic infiltration, a lesion also associated with ALV-J infection but observed in REV cases as well [64].

Histopathological Characteristics

Histologically, REV-induced tumors are composed of a uniform population of lymphoreticular cells, which are large, pleomorphic, and exhibit a high mitotic index [3, 31, 67]. These neoplastic cells infiltrate and efface the normal architecture of affected organs. In the liver, there is diffuse or nodular infiltration by neoplastic lymphoid cells and primitive reticular cells, often accompanied by areas of necrosis and hemorrhage [31]. The spleen shows proliferation of reticuloendothelial cells and lymphoid elements, with disruption of the normal follicular structure [3]. Immunohistochemical staining confirms the presence of REV antigens in neoplastic cells and in the surrounding non-neoplastic lymphoid tissue, with positive signals consistently detected in the liver, kidney, spleen, bursa, and ovaries, but notably absent in the heart in some studies [3]. The histopathological picture can be further complicated by the presence of multiple viral coinfections, as REV is frequently found in concert with ALV-J and MDV. In such cases, tissues may be infiltrated by both neoplastic lymphocytes (associated with MDV and REV) and myeloblastic cells or primitive reticular cells (associated with ALV-J), with all three viral antigens detectable within the same tissue and even within the same cell [64].

Coinfection and Synergistic Pathogenicity

In commercial poultry operations, REV rarely acts in isolation. The immunosuppression it induces creates a permissive environment for a host of other pathogens, and the resulting coinfections dramatically alter the clinical and pathological picture. The synergistic interactions between REV and other oncogenic viruses, particularly MDV and ALV-J, are of paramount importance.

REV and Marek's Disease Virus Coinfection

The coinfection of REV and MDV is a well-documented and increasingly recognized cause of severe tumor outbreaks. Compared to infection with MDV alone, coinfection with REV significantly increases mortality and tumor rates. Sun and colleagues reported that in co-challenged groups, mortality increased by 20% (from 76.7% to 96.7%) and tumor rates by 26.7% (from 53.3% to 80.0%) compared to MDV-only challenged birds [39]. This synergistic pathogenicity is accompanied by a marked reduction in the efficacy of MD vaccines, with the protective index of CVI988 decreasing by 33.3% in the presence of REV coinfection [39]. Field investigations in China have confirmed this phenomenon, with reports of breeder flocks experiencing 5-20% morbidity and 2-10% mortality due to coinfection with REV and very virulent plus (vv+) MDV-like strains [21]. The genetic basis for this synergy is partially explained by the integration of the REV long terminal repeat (LTR) into the MDV genome. REV-LTR insertions have been detected in 22.2% of MDV field isolates in Egypt [4], and the presence of this LTR confers a significant horizontal transmission advantage to the recombinant MDV, allowing it to outcompete non-recombinant strains and become the predominant circulating virus [42]. The LTR insertion upregulates 16 MDV genes related to replication and transmission, further enhancing the pathogenicity of the recombinant virus [42].

REV and Avian Leukosis Virus Subgroup J Coinfection

Coinfection with ALV-J and REV is another common and highly pathogenic combination. The two viruses synergistically enhance each other's replication through mechanisms involving miR-155 and exosomal miRNAs [10, 56]. This increased viral load leads to a broader tumor spectrum and more severe immunosuppression than either virus alone [16]. Proteomic analyses of coinfected cells reveal the involvement of integrins as key regulators of this synergy, with 8 integrins (ITGα1, ITGα3, ITGα5, ITGα6, ITGα8, ITGα9, ITGα11, and ITGβ3) being differentially expressed [16]. Furthermore, the coinfection induces TRIM62 regulation of the actin cytoskeleton, which appears to be an important host response to limit viral replication [30]. Clinically, birds coinfected with ALV-J and REV present with severe growth inhibition, profound immunosuppression, and a higher incidence of tumors in multiple organs, including the proventriculus, which is a characteristic lesion of this coinfection [44, 64].

Exosome-Mediated Pathology and Immune Evasion

A paradigm-shifting understanding of REV pathogenesis has emerged from the study of exosomes. REV-infected cells, including those in semen and meconium, release exosomes that contain the complete viral genome and all three major viral proteins (Gag, Pol, and Env) [7, 17, 19]. These REV-positive exosomes are not neutralized by REV-specific antibodies and can establish productive infections both in vitro and in vivo [7, 17, 19]. This mechanism is particularly important for vertical transmission, as semen-derived exosomes can mediate viral entry into the egg, and for early horizontal transmission, as meconium-derived exosomes shed by infected chicks can infect naïve hatchlings even in the presence of maternal antibodies [17, 19]. The pathogenicity of exosome-mediated infection is distinct from that of free virions; in embryonated eggs, REV exosomes caused reduced hatching rates and increased mortality after hatching, along with severe growth inhibition and immune organ damage [7]. This exosome pathway represents a critical immune evasion strategy that complicates control efforts based solely on neutralizing antibodies.

Lesion Distribution Across Host Species and Age

While chickens are the primary host of concern for commercial poultry, REV has a remarkably broad host range that includes turkeys, ducks, geese, pheasants, peafowl, and wild birds [1, 9, 18, 22, 31, 44]. The clinical manifestations and lesion distribution can vary across species. In Muscovy ducks, natural outbreaks present with neoplastic disease primarily affecting the liver and spleen, with histopathological findings of malignant lymphoreticular cell proliferation [31]. In wild turkeys, REV infection is often subclinical but can be associated with lymphoproliferative lesions, with bone marrow showing the highest detection rate (41.4%), suggesting a tissue tropism that differs from chickens [9]. Laysan albatross and Hawaiian geese, species in which REV was recently documented for the first time, presented with multicentric histiocytoma and concurrent poxvirus infection, indicating that REV can contribute to mortality in endangered wild bird populations [18]. The age at infection is a critical determinant of clinical outcome. Young birds, particularly those infected in ovo or within the first few days of life, develop the most severe disease, including runting, profound immunosuppression, and early mortality [7, 47]. In contrast, infection of adult hens often results in transient viremia with less severe clinical signs, though vertical transmission to progeny remains a significant concern [7, 65].

Molecular Pathology: Vimentin Cleavage and Cellular Disruption

At the subcellular level, REV exerts direct cytopathic effects through the activity of its encoded protease (PR). Zhu and colleagues demonstrated that the REV PR cleaves chicken vimentin, a key component of the intermediate filament cytoskeleton, at three specific sites: between leucine-239 and glutamine-240, alanine-261 and alanine-262, and histidine-431 and serine-432 [8]. This cleavage disrupts the structural integrity of infected cells, potentially facilitating viral release and contributing to the cytopathology observed in lymphoid organs. The cleavage sites are distinct from those targeted by other retroviral proteases, suggesting a unique adaptation of REV to its avian host [8]. This vimentin degradation likely contributes to the observed loss of lymphocyte viability and the architectural disruption of lymphoid follicles seen histologically.

Diagnostic and Economic Implications

The clinical manifestations of REV infection have profound implications for poultry health management and economic productivity. The immunosuppression induced by REV leads to increased susceptibility to secondary bacterial and viral infections, poor response to vaccinations (including those for Newcastle disease and Marek's disease), and reduced overall flock performance [39, 66]. The economic losses stem from increased mortality, culling of runted birds, reduced egg production (with an average decrease of 6% observed in some outbreaks [15]), and the cost of diagnostic testing and biosecurity measures. The World Organisation for Animal Health (WOAH) recognizes the significance of REV as an immunosuppressive and oncogenic agent, and its presence can complicate international trade of poultry and poultry products. The frequent integration of REV sequences into the genomes of fowlpox virus (FWPV) and MDV further complicates the epidemiological picture, as these chimeric viruses can serve as vehicles for REV transmission and may exhibit altered pathogenicity [4, 6, 15, 25, 26, 28, 42, 63].

In conclusion, the clinical manifestations and pathological lesions of reticuloendotheliosis are diverse, ranging from subclinical immunosuppression to severe runting and fatal neoplasia. The pathological hallmark is the atrophy of central lymphoid organs due to oxidative stress, apoptosis, and disrupted cell signaling, which underlies a state of profound immune dysfunction. Neoplastic lesions, primarily lymphoreticular tumors affecting the liver, spleen, and other viscera, are a common end-stage presentation, particularly in birds coinfected with MDV or ALV-J. The discovery of exosome-mediated immune evasion and the integration of REV into other viral genomes have expanded our understanding of its pathogenic mechanisms and highlight the challenges facing eradication efforts.

Advanced Diagnostics: Multiplex qPCR and Molecular Detection of REV

The accurate and timely diagnosis of reticuloendotheliosis virus (REV) infections has become a cornerstone of modern poultry health management, particularly given the virus's capacity for immunosuppression, vertical and horizontal transmission, and its frequent involvement in complex coinfections with other oncogenic and immunosuppressive agents. Conventional diagnostic approaches, including virus isolation in cell culture, serological assays such as enzyme-linked immunosorbent assay (ELISA), and histopathological examination, while foundational, suffer from inherent limitations in sensitivity, specificity, and throughput, especially when confronted with the subtlety of subclinical infections or the genetic diversity of field strains. The evolution of advanced molecular diagnostic platforms has therefore been imperative, and among these, multiplex quantitative real-time PCR (multiplex qPCR) has emerged as a preeminent tool for the simultaneous, high-throughput, and exquisitely sensitive detection of REV, often in conjunction with other critical poultry pathogens.

The Multiplex qPCR Paradigm for Simultaneous Pathogen Surveillance

The development and application of a TaqMan-based multiplex qPCR (M-qPCR) assay for the simultaneous detection of REV, avian leukosis virus subgroup J (ALV-J), and chicken infectious anemia virus (CIAV) represents a significant leap forward in diagnostic capability for breeder-derived pathogens (BDPs) [46]. This assay, validated with high linearity (R² > 0.99) and exceptional intra- and inter-assay repeatability (coefficient of variation < 5%), demonstrated a detection limit of 10 copies/μL for each target, a sensitivity benchmark that aligns with the most rigorous international standards for pathogen surveillance (WOAH/FAO guidelines for emerging infectious disease detection) [46]. Critically, the assay exhibited no cross-reactivity against a panel of common non-target avian pathogens, confirming its analytical specificity [46]. This specificity is paramount in field settings where clinical signs of REV, ranging from runting-stunting syndrome to lymphomas, can be clinically indistinguishable from those caused by Marek's disease virus (MDV) or ALV-J [46, 64]. The ability to reliably identify mixed infections, even under conditions of unbalanced target concentrations (e.g., a high load of one virus masking a low load of another), is a particular strength of this multiplex approach, mirroring the complex ecological reality of viral coinfections in commercial poultry [46].

The practical utility of this M-qPCR assay was underscored in an epidemiological survey of three local chicken breeds in Guizhou, China. The assay revealed positivity rates of 33.9% for ALV-J, 9.3% for REV, and 7.6% for CIAV, with a striking 20.4% of all positive samples representing mixed (double or triple) infections [46]. This data not only quantifies the prevalence of REV but also exposes the high frequency of its co-occurrence with other BDPs, a finding that has profound implications for control and eradication programs. The study also established that serum samples were significantly more suitable than cloacal swabs for virus detection using this M-qPCR, a practical insight that can guide optimal sampling protocols for maximum diagnostic yield [46].

Addressing the Chimeric Complexity: FWPV-REV Detection

A unique and formidable challenge in REV molecular diagnostics arises from the virus's propensity for genomic integration into the genome of fowlpox virus (FWPV). Field strains of FWPV frequently carry integrated REV sequences, ranging from remnants of the long terminal repeat (REV-LTR) to nearly the entire REV proviral genome [4, 6, 15, 25, 26]. This chimeric state can confound standard diagnostic PCRs, as a positive REV signal might originate from the integrated provirus within an FWPV genome rather than from a replicating, autonomous REV. Conversely, a negative signal for FWPV does not rule out REV infection. To resolve this diagnostic ambiguity, advanced molecular strategies have been specifically developed.

A multiplex PCR assay designed for the simultaneous detection of infectious laryngotracheitis virus (ILTV), FWPV, REV-integrated FWPV, and autonomous REV represents a sophisticated solution to this problem [51]. By incorporating primer sets that target sequences flanking the REV integration site within the FWPV genome (between ORF 201 and ORF 203), this assay can differentiate between FWPV genomes containing only an REV-LTR remnant, those carrying a full-length REV provirus, and the presence of REV itself [51]. This is not merely an academic distinction; it has direct practical relevance. The integration of a full-length REV provirus into FWPV can confer enhanced virulence and atypical clinicopathological manifestations in vaccinated flocks, as documented in outbreaks in Brazil and Austria [15, 25]. The molecular differentiation of these forms is therefore critical for epidemiological tracing, virulence assessment, and implementing appropriate biosecurity measures. The detection of heterogeneous FWPV populations, some carrying only an REV-LTR and others carrying a near full-length REV provirus, within a single host further highlights the dynamic and unstable nature of these chimeric genomes [26]. The widespread detection of REV-LTR insertions in MDV field isolates in Egypt and FWPV strains across multiple avian species underscores that this is a global phenomenon requiring vigilant molecular surveillance [4, 28].

Differential Diagnosis in Complex Coinfections and Tissue Tropism

The clinical and pathological presentation of REV overlaps extensively with other avian oncogenic viruses, particularly MDV and ALV-J, and coinfections with two or even three of these viruses are common [32, 60, 64]. The detection of REV in such complex scenarios demands molecular assays with unambiguous discriminatory power. The use of multiplex qPCR or PCR panels that include specific targets for REV (e.g., the env, pol, or LTR regions), MDV (e.g., meq, gB, or ICP4 genes), and ALV-J (e.g., gp85) is essential for accurate etiological diagnosis [21, 32, 60]. For instance, an investigation of a high-mortality outbreak in three-yellow chickens in China using PCR and sequencing successfully identified a coinfection of REV with a vv+ MDV-like strain, a diagnosis that would have been impossible to confirm through histopathology alone [21, 60]. Similarly, comprehensive PCR screening of Brazilian chickens revealed that 82.5% of samples positive for REV were involved in multiple viral coinfections, including up to seven different viruses, emphasizing that REV is rarely a sole etiological agent in field conditions [13].

Tissue tropism is another critical factor influencing diagnostic sensitivity. A large-scale survey of wild turkeys (Meleagris gallopavo) in the United States, using PCR targeting the REV 3' LTR and pol gene, demonstrated that bone marrow had the highest detection rate for REV (41.4%), significantly higher than spleen (51.7%) or liver (34.5%) [9]. This finding challenges the traditional reliance on spleen or tumor tissue and suggests that bone marrow sampling may offer superior diagnostic yield, particularly for detecting latent or low-level infections [9]. This observation resonates with the pathophysiology of REV, a retrovirus that establishes persistent infection in hematopoietic and lymphoid tissues. The pronounced detection in bone marrow, which is also observed for lymphoproliferative disease virus (LPDV) in the same study, points to a shared tissue reservoir for these avian retroviruses [9, 14].

The Imperative for Ultra-Sensitive Detection: Vaccine Safety and Hidden Reservoirs

The contamination of commercial live attenuated vaccines with REV has been a historically significant and economically damaging problem, leading to widespread iatrogenic spread of the virus in poultry flocks [45, 60]. Detecting low-level REV contamination in vaccines or in birds with very low proviral loads requires diagnostics that far exceed the sensitivity of conventional PCR. Droplet digital PCR (ddPCR) has emerged as a transformative technology in this context. A ddPCR assay designed for REV detection in vaccines exhibited a limit of detection of 0.1 TCID₅₀ per 1,000 feathers, which was 1,000-fold more sensitive than conventional PCR (10² TCID₅₀/1000 feathers) and 10-fold more sensitive than standard quantitative PCR [59].

This level of sensitivity is not merely a technical achievement; it has profound practical implications. It allows for the confident certification of vaccine batches as REV-free, a critical requirement in many regulatory frameworks (e.g., China's Ministry of Agriculture requirements for exogenous virus screening). Furthermore, ultrasensitive detection is vital for identifying "carrier" birds that may harbor extremely low levels of provirus and yet remain seronegative, acting as silent reservoirs within a flock. The detection of REV in formalin-fixed, paraffin-embedded (FFPE) tissues by PCR, yielding 99% sequence homology to other REV isolates, demonstrates the robustness of molecular techniques even when sample integrity is compromised, offering retrospective diagnostic and epidemiological capabilities [67].

The development of antigen-capture ELISA (AC-ELISA) using monoclonal and polyclonal antibodies against the immunogenic gp90 envelope protein provides an orthogonal, protein-level detection method that complements nucleic acid-based tests [68]. This assay, with a detection limit of 195 TCID₅₀, is less sensitive than ddPCR but offers a rapid, high-throughput, and cost-effective screening tool for detecting active REV infection or contamination in cell culture material. The reported 90.63% positive coincidence rate between this AC-ELISA and RT-qPCR for detecting REV in contaminated vaccines indicates that a dual-testing approach, combining sensitive molecular screening with antigen detection, can provide a robust, multi-layered surveillance strategy [68].

Genomic Surveillance and the Molecular Epidemiology of REV

Molecular detection is not an endpoint but the gateway to deeper genomic characterization. Phylogenetic analysis of full-length REV genomes or specific genes (e.g., env, pol, LTR) provides critical insights into viral evolution, geographic distribution, and transmission dynamics [1, 13, 36]. Complete genome sequencing of REV isolates from Brazil, Thailand, China, and the Middle East has revealed a global circulation of REV subtype 3, with distinct geographical subclusters emerging (e.g., "East" and "West" subclusters of REV-3) [1, 36]. The detection of REV in novel hosts, such as mallard ducks in Brazil, wild turkeys in Texas, and Hawaiian geese, highlights the virus's expanding host range and the necessity for continuous genomic surveillance across wild bird populations [1, 18, 34].

Selective pressure analyses on REV genomes have identified positive selection acting on codons within the gag, pol, and env genes, particularly in functional domains like the matrix protein (p18), reverse transcriptase/ribonuclease H, and the surface (SU) glycoprotein [1, 13]. These findings have immediate diagnostic relevance: highly variable regions within these genes can serve as targets for molecular assays but also present a risk for primer-template mismatches that could lead to false-negative results. Therefore, the design of any REV-specific PCR or qPCR assay must be informed by a comprehensive, globally representative sequence database to ensure conserved primer binding sites. The identification of novel antigenic epitopes, such as the conserved linear B-cell epitope ¹⁹⁵REESVRERL²⁰³ in the gp90 protein, provides new targets for developing more specific diagnostic antibodies and immunodiagnostic assays [20].

Exosomes and the Challenge of Immune Evasion in Detection

Recent discoveries have unveiled an additional layer of complexity in REV detection: the existence of infectious REV components within exosomes. Studies have demonstrated that exosomes purified from REV-positive semen, REV-infected DF-1 cell cultures, and even meconium from infected chicks contain viral genomic RNA and structural proteins [7, 17, 19]. Critically, these REV-loaded exosomes can establish productive infections both in vitro and in vivo and, most importantly, can evade neutralization by REV-specific antibodies [7, 19]. This exosome-mediated transmission provides the virus with a stealth mechanism for immune evasion and horizontal spread, even in the presence of maternal antibodies [17].

For diagnostics, this finding presents both a challenge and an opportunity. A standard PCR performed on plasma or tissue homogenates would detect REV nucleic acid regardless of whether it originates from free virions or within exosomes. However, the presence of exosome-encapsulated viral genomes could explain discordant results between PCR (DNA/RNA detection) and serology (antibody detection), where a bird might be PCR-positive but seronegative due to the immune-evasive nature of exosomal transmission. The development of diagnostic protocols that can differentiate between free virus and exosome-associated REV, perhaps through exosome isolation or specific capsid-targeting steps prior to PCR, represents a frontier in molecular diagnostics that could yield more nuanced interpretations of infection status. This is particularly relevant for understanding the dynamics of vertical transmission, where exosomes in the meconium and semen of infected birds provide a direct conduit for early-life infection, bypassing the immune system's first line of defense [17, 19]. The integration of direct detection methods (e.g., PCR on untreated serum) with functional assays (e.g., virus neutralization tests) is essential to fully understand the interplay between free virions and exosome-encapsulated viral particles in the pathogenesis and transmission of REV.

Coinfections with Avian Leukosis Virus and Chicken Infectious Anemia Virus

The intricate interplay between Reticuloendotheliosis virus (REV), Avian Leukosis virus (ALV), and Chicken Infectious Anemia virus (CIAV) represents one of the most clinically and economically significant challenges in modern poultry medicine. These three pathogens, collectively classified as breeder-derived pathogens (BDPs), share the capacity for vertical transmission, induce profound immunosuppression, and frequently circulate concurrently within commercial and indigenous chicken flocks [46]. The co-circulation of these agents creates a complex pathogenic synergy that exacerbates disease severity, expands the tumor spectrum, and significantly complicates eradication programs worldwide. Understanding the specific dynamics of coinfections involving ALV and CIAV in the context of REV is essential for developing effective surveillance strategies and intervention protocols.

Epidemiological Prevalence and Global Distribution of ALV-CIAV Coinfections

The recent development and application of advanced multiplex molecular diagnostic tools have revolutionized our capacity to detect and quantify mixed infections involving REV, ALV, and CIAV simultaneously. Wang et al. (2025) [46] established a highly sensitive TaqMan-based multiplex quantitative PCR (M-qPCR) assay capable of detecting all three pathogens with a limit of 10 copies/μL and excellent specificity, demonstrating no cross-reactivity with common non-target avian pathogens. When applied to three indigenous chicken breeds in Guizhou Province, China, this assay revealed positivity rates of 33.9% (40/118) for ALV-J, 9.3% (11/118) for REV, and 7.6% (9/118) for CIAV, with mixed infections (double and triple) constituting 20.4% of all positive samples [46]. These findings underscore the substantial burden of co-circulation among these viruses in field populations and highlight the critical importance of simultaneous detection methodologies for accurate epidemiological assessment.

The prevalence of ALV and CIAV coinfections with REV extends well beyond China, with global surveillance efforts revealing a widespread distribution of these pathogens across diverse avian populations. Serological surveys have demonstrated that REV infections are extensively distributed across major poultry-producing regions, with seroprevalence rates reaching 56.20% at the flock level and 13.91% in individual chickens across 23 provinces in China over a decade-long surveillance period from 2005 to 2015 [43]. In Bangladesh, a comprehensive seroprevalence study involving 3,555 serum samples from 144 flocks across 10 districts documented an overall REV seroprevalence of 21.13%, with 73.61% of flocks testing seropositive [41]. The highest prevalence rates were observed during the onset of laying (38.21% in birds aged 19–24 weeks), and broiler breeders exhibited substantially higher seropositivity (42.15%) compared to broilers (6.86%) or domestic chickens (6.49%) [41]. These epidemiological patterns indicate that REV, ALV, and CIAV are not merely sporadic pathogens but represent persistent, endemic threats that frequently overlap in their distribution.

In Brazil, molecular detection of REV in commercial and backyard poultry revealed a prevalence of 65% (26/40) among tested samples, with concomitant viral infections detected in 82.5% of samples and 90% of farms [13]. Alarmingly, multiple infections included up to seven different viruses simultaneously, highlighting the complexity of field coinfections [13]. The presence of REV has also been documented in Muscovy ducks, wild turkeys, and chickens in the Amazon biome, with a relatively high overall prevalence of 16.8% [35, 40]. Phylogenetic analyses of Brazilian REV strains demonstrated close relationships with variants from the United States and fowlpox virus (FWPV)-related strains, suggesting potential intercontinental transmission patterns and the importance of recombinant viral forms in REV dissemination [13, 25]. In Thailand, REV was detected in 39.23% of clinical samples from 72.41% of farms across nine provinces, with subtype III being the predominant circulating lineage [36]. The detection of REV in Sudan revealed an overall seroprevalence of 74.6%, with PCR positivity in 10% of liver samples and 15% of spleen samples, further confirming the extensive global distribution of this pathogen in both local and commercial chicken breeds [37].

Molecular Mechanisms of Synergistic Pathogenesis

The synergistic interactions between REV and ALV, particularly ALV subgroup J (ALV-J), represent a paradigm of viral cooperation that amplifies pathogenic outcomes beyond what either virus could achieve independently. Co-infection of ALV-J and REV in poultry flocks induces synergistic replication, which dramatically worsens immunosuppression, growth inhibition, and mortality [10]. The mechanistic basis for this synergy has been progressively elucidated through sophisticated molecular analyses, revealing a complex network of host-virus interactions centered on microRNA regulation, exosomal communication, and cytoskeletal remodeling.

MicroRNA-155 (miR-155) has emerged as a central orchestrator of ALV-J and REV synergistic replication. Xue et al. (2023) [10] demonstrated that ALV-J and REV co-infection synergistically upregulates miR-155 levels in both infected cells and tissues. Remarkably, all structural proteins of both viruses activated miR-155 expression, with the gag proteins being particularly potent, and this activation occurred through transcriptional induction rather than insertional mutagenesis [10]. Once upregulated, miR-155 facilitates synergistic viral replication through a dual-pathway mechanism: it directly suppresses PRKCI (protein kinase C iota), which in turn prevents MAPK8 (mitogen-activated protein kinase 8) expression, while simultaneously suppressing TIMP3 (tissue inhibitor of metalloproteinases 3), which allows MMP2 (matrix metalloproteinase 2) expression [10]. Both MAPK8 and MMP2 then co-interact with the U3 regions of both ALV-J and REV, creating a feed-forward loop that enhances the replication of both viruses [10]. This elegant molecular circuitry explains how two distinct retroviruses can cooperate to maximize their replicative success within the same host cell.

The role of exosomes in mediating intercellular communication and immune evasion during ALV-J and REV coinfections adds another layer of complexity to their synergistic pathogenesis. Exosomes purified from REV-infected DF-1 cells contain viral genomic RNA and structural proteins, and importantly, these exosomes can establish productive infections even in the presence of REV-specific neutralizing antibodies [7]. Comparative studies revealed that REV-exosomes caused more severe pathology than free virions, including reduced hatching rates, increased post-hatch mortality, severe growth inhibition, and more pronounced immune organ damage in 1-day-old chicks [7]. The ability of exosomes to escape antibody neutralization has profound implications for coinfection dynamics, as it allows REV to persist and replicate in hosts that have mounted humoral immune responses, thereby maintaining a reservoir for ALV coinfection. Furthermore, Zhou et al. (2018) [56] demonstrated that ALV-J and REV co-infection synergistically increases the accumulation of exosomal miRNAs, with five key miRNAs, miR-184-3p, miR-146a-3p, miR-146a-5p, miR-3538, and miR-155, being particularly enriched. These exosomal miRNAs target genes involved in virus-vector interaction, oxidative phosphorylation, energy metabolism, and cell growth, thereby reprogramming the cellular environment to favor co-infection [56].

The discovery that REV-positive meconium exosomes mediate early horizontal transmission and immune evasion in newly hatched chicks provides critical insights into how these viruses establish footholds in naive populations. Fu et al. (2026) [17] isolated exosomes from the meconium of REV-positive chicks and confirmed that these exosomes contain the complete REV genome and all three major viral proteins. Crucially, when these meconium exosomes were inoculated into chicks with maternal antibodies against REV, the antibodies were unable to neutralize the exosome-associated virus, resulting in productive infection and pathogenicity equivalent to that observed in antibody-negative chicks [17]. This exosome-mediated immune evasion likely facilitates the early establishment of REV infection, which subsequently creates an immunocompromised environment permissive for ALV and CIAV coinfection.

The immunosuppressive effects of REV that create vulnerability to ALV and CIAV coinfection are mediated through multiple interconnected pathways. REV infection induces significant endoplasmic reticulum stress (ERS) through the specific activation of the PERK-eIF2α signaling axis, which simultaneously suppresses apoptosis and aggravates immunosuppression, thereby creating a favorable environment for viral replication [2]. The suppression of apoptosis is particularly significant for coinfection dynamics, as the extended survival of infected cells allows for prolonged viral replication and increased probability of dual infection events. Additionally, REV disrupts the TLR-3/IFN-β pathway, with the transcriptional and translational levels of TLR-3, IRF-7, and NF-κB p65 showing significant alterations during the early stages of infection, followed by a decline in NF-κB p65 and IFN-β in the central immune organs during later infection [48]. This disruption of innate antiviral immunity likely contributes to the enhanced susceptibility of REV-infected birds to subsequent ALV and CIAV infection.

Proteomic and Transcriptomic Alterations During Coinfection

The host cellular response to ALV-J and REV co-infection involves profound reprogramming of the proteome and transcriptome, with specific alterations in integrin signaling, actin cytoskeleton regulation, and immune pathway components. Cui et al. (2022) [16] employed tandem mass tag (TMT)-based quantitative proteomics to compare differentially expressed proteins in chicken embryo fibroblast (CEF) cells infected with ALV-J, REV, or both. Co-infection resulted in 719 differentially expressed proteins (292 upregulated, 427 downregulated) compared to ALV-J mono-infection and 64 proteins (35 upregulated, 29 downregulated) compared to REV mono-infection [16]. Gene ontology and KEGG pathway analyses revealed that these proteins were predominantly involved in virus-vector interaction, biological adhesion, and immune response pathways. Most notably, a large number of integrins were dysregulated in co-infected cells, with eight integrins, including ITGα1, ITGα3, ITGα5, ITGα6, ITGα8, ITGα9, ITGα11, and ITGβ3, being validated by quantitative RT-PCR and western blotting [16]. Integrins are critical mediators of cell-matrix adhesion and signal transduction, and their dysregulation during co-infection likely contributes to enhanced viral entry, cell-to-cell spread, and altered cellular signaling that favors viral replication.

The actin cytoskeleton emerges as a key battleground during ALV-J and REV co-infection, with the tripartite motif containing 62 (TRIM62) protein playing a central regulatory role. Li et al. (2020) [30] demonstrated that TRIM62 negatively regulates viral replication in co-infected CEF cells, with its inhibitory effect on REV being more pronounced than on ALV-J. Mechanistically, TRIM62 modulates the actin cytoskeleton by decreasing the expression of NCK-associated protein 1 (NCKAP1, also known as Nap125) and increasing the expression of actin-related 2/3 complex subunit 5 (ARPC5) [30]. These proteins are critical components of the WAVE regulatory complex and the Arp2/3 complex, which together control actin polymerization and cytoskeletal dynamics. The involvement of TRIM62 in restricting viral replication during co-infection is further supported by its interaction with Ras-related protein Rab-5b (RAB5B) and Arp2/3 complex subunit 2 (ARPC2), both of which are involved in actin cytoskeletal pathway regulation [69]. The observation that REV infection first upregulates and then downregulates TRIM62 expression suggests that the virus has evolved mechanisms to overcome this host restriction factor, potentially facilitating co-infection.

The spleen, as a major immune organ, undergoes dramatic molecular changes during REV infection that likely impact susceptibility to ALV and CIAV coinfection. Gao et al. (2019) [61] performed integrated miRNA and mRNA expression profiling in the spleens of REV-infected specific-pathogen-free chickens and identified 63 differentially expressed miRNAs (30 known and 33 novel) and 482 differentially expressed target genes. KEGG enrichment analysis revealed significant involvement of immune system pathways, cell growth and death pathways, and signaling molecule interactions [61]. The identification of immune-relevant miRNA-mRNA interaction pairs provides a molecular framework for understanding how REV-induced alterations in the splenic microenvironment may facilitate secondary infections with ALV and CIAV. Similarly, transcriptomic analysis of chicken spleen tissues infected with the REV strain SNV identified 1,507 differentially expressed genes, with pattern recognition receptors (PRRs), chemokine signaling, T cell receptor signaling, JAK-STAT, TNF, and NF-κB pathways being prominently affected [54]. The dysregulation of inflammatory factors (CCL4, TNFRSF18, CDKN2), apoptosis regulators (IRF1, PDCD1, WNT5A), innate immunity components (TLR, MDA5, TRIM25), and adaptive immunity molecules (LY6E, CD36, LAG3) collectively creates an immunocompromised state permissive for co-infection [54].

Clinical Manifestations and Diagnostic Implications of Triple Coinfections

The simultaneous presence of REV, ALV, and CIAV in affected flocks produces clinical presentations that are often more severe and diagnostically challenging than infections with any single agent. Liu et al. (2019) [64] investigated the histologic findings and viral antigen distribution in cases of natural coinfection of layer hens with ALV-J, Marek's disease virus (MDV), and REV. Affected hens exhibited hepatosplenomegaly, thickened proventriculi, and white tumor nodules distributed across multiple organs including the liver, spleen, lung, kidney, and ovary [64]. Microscopic examination revealed infiltration of most tissues by neoplastic lymphocytes, with the spleen, lung, proventriculus, heart, and liver showing infiltration by both neoplastic lymphocytes and myeloblastic cells and/or primitive reticular cells [64]. Most significantly, fluorescence multiplex immunohistochemistry staining demonstrated that ALV-J, MDV, and REV antigens were co-expressed in the same tissues, and even within the same cells [64]. This finding has profound implications for understanding viral synergy, as it confirms that these three viruses can simultaneously infect individual cells, creating opportunities for direct molecular interactions, recombination events, and synergistic modulation of host cellular pathways.

The diagnostic challenges posed by coinfections involving REV, ALV, and CIAV are substantial, particularly given that immunosuppression induced by one virus can facilitate the replication and detection of another. The development of multiplex qPCR assays has significantly improved our ability to discriminate between these pathogens and quantify their relative loads

Control Strategies and Therapeutic Interventions

The management of reticuloendotheliosis virus (REV) represents one of the most formidable challenges in contemporary avian medicine, owing to the virus’s unique biological properties, its capacity for integration into host genomes, its exploitation of exosomal pathways for immune evasion, and its propensity to synergize with other oncogenic and immunosuppressive pathogens [7, 46, 47]. Unlike many viral diseases of poultry for which commercial vaccines are available, no licensed REV vaccine exists globally, and therapeutic options remain largely experimental [22, 33, 41, 43, 46, 47]. The development of effective control strategies is further complicated by the virus’s ability to contaminate live viral vaccines, its integration into the genomes of fowlpox virus (FWPV) and Marek’s disease virus (MDV), and its capacity to establish persistent infections that evade both humoral and cell-mediated immune responses [4, 6, 15, 28, 45, 51]. Consequently, control of REV necessitates a multipronged approach encompassing immunomodulatory therapeutics, strategic vaccination, enhanced biosecurity and surveillance, and the rigorous screening of biological products.

Immunomodulatory Interventions: Transfer Factor and Beyond

Among the most promising non-vaccine therapeutic strategies is the use of transfer factor (TF), a low-molecular-weight lymphocyte extract capable of transferring cell-mediated immune memory from immune donors to naive recipients. Wang et al. (2025) provided the first comprehensive evaluation of TF as a control agent against REV, examining its efficacy across three distinct application scenarios: pre-exposure prophylaxis, post-exposure intervention in day-old chicks, and treatment of chicks infected via the vertical route during incubation [47]. The results were striking: TF administration significantly mitigated REV-induced growth retardation, hepatomegaly, and thymic atrophy, while concurrently reducing mortality rates and suppressing both viral replication in vivo and cloacal virus shedding [47]. Notably, TF demonstrated particular utility in reducing egg-associated vertical transmission, suggesting a role in breaking the cycle of congenital infection [47]. The effects were dose-dependent and frequency-dependent, with higher doses or repeated administration yielding superior outcomes. Although the precise mechanism by which TF exerts its anti-REV effects remains to be fully elucidated, the observed reduction in viral load and organ pathology suggests that TF enhances antiviral T-cell responses, potentially compensating for REV-mediated immunosuppression [47, 53]. Given that REV is known to induce profound thymic atrophy and disrupt CD4+/CD8+ T-cell ratios [53, 55], TF’s capacity to restore cell-mediated immunity represents a critical therapeutic advantage.

The immunomodulatory potential of other natural compounds has also been investigated. Allicin, the principal bioactive component of garlic, has been shown to alleviate REV-induced immunosuppression via the ERK/mitogen-activated protein kinase (MAPK) pathway [38]. In specific-pathogen-free (SPF) chickens, dietary allicin supplementation ameliorated REV-induced dysplasia, restored the balance of inflammatory cytokines (reducing IL-1β, IL-6, IL-10, and TNF-α while increasing IFN-α, IFN-β, and IL-2), and suppressed the activation of toll-like receptors (TLRs) and melanoma differentiation-associated gene 5 (MDA5) [38]. Crucially, allicin inhibited the phosphorylation of ERK, JNK, and p38, key mediators of the MAPK pathway, and the anti-REV effect was specifically linked to ERK inhibition, as confirmed by pharmacological blockade experiments [38]. Allicin also mitigated the oxidative damage induced by REV in the thymus and spleen, reducing lipid peroxidation and restoring antioxidant enzyme activities [38, 52]. These findings position allicin as a candidate adjunctive therapy that targets both the immunopathological and oxidative stress components of REV infection.

Polysaccharides derived from the marine alga Enteromorpha clathrata (EPS) have similarly demonstrated immune-restorative properties in REV-infected broilers [66]. When administered to chicks that had been infected with REV at one day of age and subsequently vaccinated against Newcastle disease, EPS treatment significantly enhanced immune organ indices, small intestinal secretory IgA levels, peripheral blood lymphocyte transformation rates, and serum antibody titers against the Newcastle disease vaccine [66]. The elevation of both IL-2 and IFN-γ levels following EPS treatment suggests a capacity to restore Th1-type responses that are suppressed by REV [54, 66]. This is particularly relevant given that REV infection is known to disrupt the JAK-STAT, TNF, and NF-κB signaling pathways, leading to impaired adaptive immunity [54, 55].

Vaccination Strategies: The Quest for an Effective Immunogen

The development of an effective REV vaccine has been a persistent goal, with the viral envelope glycoprotein gp90 emerging as the primary target antigen. Gp90 is the major immunogenic protein of REV, responsible for inducing neutralizing antibodies and mediating viral entry [20, 33]. Pan et al. (2020) developed an optimized secretory expression system for glycosylated gp90 using suspension culture technology, achieving high yields of properly folded, immunogenic protein in serum-free medium [33]. The resulting oil-emulsion subunit vaccine induced rapid and robust immune responses in chickens, including high titers of gp90-specific antibodies and neutralizing antibodies, with a preferential T-helper 2 (Th2) response characterized by IL-4 secretion rather than the Th1-associated IFN-γ [33]. Vaccinated chickens challenged with REV showed significantly reduced viremia, indicating that the glycosylated gp90-based vaccine could effectively protect against horizontal infection [33].

Building on this concept, Ren et al. (2018) evaluated a gp90 protein vaccine derived from an REV strain isolated from a contaminated infectious bursal disease (IBD) vaccine, using cytosine-phosphate-guanine oligodeoxynucleotide (CpG-ODN) as an adjuvant [62]. When administered to breeding hens, the vaccine induced high levels of maternal antibodies that were passively transferred to progeny chicks. Offspring from vaccinated hens were significantly protected against REV challenge at one day of age, demonstrating reduced viremia and improved growth rates compared to chicks from unvaccinated controls [62]. This maternal immunization strategy holds considerable promise for protecting chicks during the early, most vulnerable period of life, particularly given that REV can be vertically transmitted through the egg and that infected chicks begin shedding virus in meconium within hours of hatching [17, 19, 65].

However, the development of a broadly effective REV vaccine faces several obstacles. The virus exists as at least three distinct subtypes (REV-1, REV-2, and REV-3) based on phylogenetic analysis of the env gene, with subtype 3 being the most prevalent globally [1, 3, 13, 36]. Furthermore, REV exhibits considerable genetic diversity, particularly in the gp90 gene, with polymorphic regions that could affect antigenic cross-reactivity [1, 13]. The identification of a novel conserved linear B-cell epitope (195REESVRERL203) on gp90 by Han et al. (2026) provides a potential target for designing epitope-based vaccines that could confer broad protection across REV subtypes [20]. Additionally, the observation that REV can be transmitted via exosomes that resist neutralizing antibodies [7, 17, 19] raises the troubling possibility that even robust humoral immunity may be insufficient to block all routes of infection. Exosomes purified from REV-positive semen and meconium have been shown to establish productive infection in vitro and in vivo even in the presence of REV-specific neutralizing antibodies, and maternal antibodies failed to protect chicks against exosome-mediated infection [7, 17, 19]. These findings underscore the need for vaccines that elicit strong cell-mediated immunity in addition to humoral responses, and they highlight the importance of targeting the exosomal pathway as a therapeutic intervention point.

The Challenge of Vaccine Contamination and the Need for Stringent Screening

A critical dimension of REV control that has garnered increasing attention is the problem of iatrogenic transmission through contaminated live virus vaccines. REV is the most frequent exogenous virus contaminating poultry vaccines, and its presence in commercial products has been documented across multiple countries and vaccine types, including vaccines against Marek’s disease (MDV), fowlpox, and infectious bursal disease [29, 45, 59, 60, 62]. Li et al. (2015) isolated the REV strain MD-2 from a commercial freeze-dried MDV vaccine, demonstrating that the vaccine had been contaminated with REV despite regulatory requirements for exogenous virus screening [45]. Subsequent molecular characterization revealed that the contaminating strain was phylogenetically close to prevalent field isolates, suggesting that vaccine contamination may directly contribute to REV dissemination in the field [45]. Indeed, epidemiological evidence from China has linked REV outbreaks in three-yellow chickens to strains that are closely related to REV isolates recovered from contaminated turkey herpesvirus (HVT) vaccines [21, 60].

The detection of REV contamination in vaccines has driven the development of highly sensitive screening methods. Meng et al. (2021) established a droplet digital PCR (ddPCR) assay capable of detecting REV at a dose as low as 0.1 TCID50 per 1,000 feather samples, a 1,000-fold improvement over conventional PCR and a 10-fold improvement over quantitative PCR [59]. The ddPCR method is particularly valuable for its ability to quantify low-level contamination in vaccines with high precision and intuitiveness [59]. Complementary approaches include the antigen-capture ELISA (AC-ELISA) developed by Miao et al. (2022), which uses monoclonal and polyclonal antibodies against gp90 to detect REV in vaccines with a detection limit of 195 TCID50 units [68]. The AC-ELISA showed a 90.63% positive coincidence rate with RT-qPCR and a 96.88% rate with indirect immunofluorescence assay (IFA), confirming its utility as a high-throughput screening tool [68]. Multiplex qPCR assays capable of simultaneous detection of REV, ALV-J, and CIAV further enhance the efficiency of vaccine screening and flock surveillance [46].

Co-infection Dynamics and the Compromised Efficacy of Existing Vaccines

The control of REV cannot be viewed in isolation from the complex co-infection ecology that characterizes modern poultry production. REV commonly co-infects chickens with other oncogenic viruses, including MDV, ALV-J, and fowlpox virus, and these co-infections result in synergistic pathogenicity that dramatically exacerbates disease severity [10, 12, 16, 21, 39, 42, 60, 64]. Perhaps most concerning is the documented ability of REV to compromise the efficacy of vaccines against other pathogens. Sun et al. (2017) demonstrated that co-infection with MDV and REV significantly increased mortality (from 76.7% to 96.7%) and tumor rates (from 53.3% to 80.0%) compared to MDV infection alone, and that the protective indices of the MD vaccines CVI988 and 814 were reduced by 33.3 and 13.3 points, respectively, in the presence of REV co-infection [39]. The synergistic replication observed in MDV-REV co-infections is mediated in part by the REV long terminal repeat (LTR), which, when integrated into the MDV genome, enhances viral horizontal transmission and alters the transcriptional profile of MDV genes involved in replication, immune evasion, and pathogenesis [42].

The role of REV LTR integration in MDV and FWPV genomes represents a unique evolutionary adaptation that complicates control efforts. Field isolates of MDV and FWPV frequently carry REV LTR insertions, and in some cases, near-full-length REV proviral sequences [4, 6, 15, 26, 28, 42]. The presence of REV sequences within FWPV genomes has been associated with atypical clinicopathological manifestations in vaccinated flocks, including more severe and persistent fowlpox lesions, and has been linked to outbreaks in Brazil and Austria [15, 25]. Mosad et al. (2020) detected REV-LTR integration in 30 of 40 avipoxvirus field isolates from Egypt, encompassing fowlpox, turkeypox, and canarypox viruses, but notably absent in pigeonpox virus isolates [28]. The widespread nature of these chimeric viruses underscores the importance of diagnostic assays capable of distinguishing between REV infection and REV sequences integrated into the genomes of other pathogens [5, 51].

Exosome-Mediated Transmission as a Therapeutic Target

A paradigm-shifting insight into REV biology that carries profound implications for control is the discovery that REV can be transmitted via exosomes, small extracellular vesicles that are produced by infected cells and carry viral genomic RNA, viral proteins, and host-derived molecules [7, 17, 19, 30]. Exosomes derived from REV-infected DF-1 cells have been shown to establish productive infection in embryonated eggs, day-old chicks, and adult hens, and, critically, these exosome-associated virions are resistant to neutralization by REV-specific antibodies [7]. When 7-day-old embryonated eggs were inoculated with REV-positive exosomes in the presence of neutralizing antibodies, infection was established, whereas free virions were blocked under the same conditions [7]. Similarly, exosomes purified from the semen of REV-infected roosters mediated horizontal and vertical transmission of the virus, evading antibody neutralization and establishing infection more efficiently than free virions [19].

The most recent studies have extended these findings to the meconium of newly hatched chicks, demonstrating that REV-positive exosomes in meconium can mediate horizontal transmission even in the presence of maternal antibodies [17]. Chicks with maternal REV antibodies that were inoculated with REV-positive meconium exosomes showed no significant differences in growth retardation, organ pathology, or viral replication compared to antibody-negative chicks, indicating that maternal immunity, while effective against free virions, is powerless against exosome-mediated infection [17]. This exosomal immune evasion mechanism likely explains the persistence and spread of REV in flocks that are seropositive for REV antibodies and provides a mechanistic basis for the failure of antibody-based control strategies [7, 17, 19].

The therapeutic implications of these findings are significant. Strategies aimed at disrupting exosome biogenesis, release, or uptake could complement traditional antiviral approaches. The identification of specific exosomal cargo, including miR-155, which is synergistically upregulated during REV-ALV-J co-infection and promotes viral replication through the PRKCI-MAPK8 and TIMP3-MMP2 dual pathway [10], suggests that exosomal miRNAs could serve as therapeutic targets. Additionally, the observation that TRIM62, a member of the tripartite motif family, negatively regulates REV replication and interacts with components of the actin cytoskeleton pathway (RAB5B and ARPC2) provides a potential host-directed therapeutic avenue [30, 69].

Biosecurity, Surveillance, and Integrated Control Programs

Given the limitations of current therapeutic and vaccine options, biosecurity and surveillance remain the cornerstones of REV control. The virus is capable of both horizontal transmission (through direct contact, contaminated fomites, and exosome-containing meconium and feces) and vertical transmission (through the egg), making it exceptionally difficult to eradicate once established in a flock [7, 17, 19, 46, 47, 65]. The detection of REV in wild bird populations, including wild turkeys, mallard ducks, Muscovy ducks, and even pelagic seabirds like the Laysan albatross, highlights the potential for transmission from wild reservoirs to commercial poultry [1, 9, 14, 18, 23, 24, 34, 35, 63]. Surveillance programs must therefore extend beyond commercial flocks to include wild and feral bird populations, particularly in regions where captive breeding programs for endangered species (such as the Attwater’s prairie chicken) are at risk [23, 24, 34].

Advanced molecular diagnostic tools have greatly enhanced the capacity for surveillance. Multiplex qPCR assays, such as those developed by Wang et al. (2025) for the simultaneous detection of REV, ALV-J, and CIAV, offer high sensitivity (10 copies/μL), specificity (no cross-reactivity with non-target pathogens), and reproducibility (coefficients of variation below 5%) [46]. The selection of appropriate sample types is critical: Wang et al. demonstrated that serum samples were more suitable than cloacal swabs for REV detection using the multiplex qPCR assay [46]. For large-scale serosurveillance, the use of yolk antibody detection, where egg yolks are tested at a dilution of 1:300, provides a non-invasive and cost-effective alternative to serum sampling, with 100% agreement between yolk and serum antibody results in at least one validation study [65]. However, the interpretation of serological results requires caution, as commercially available ELISAs may have suboptimal specificity; Mamczur et al. (2024) recommended increasing the cut-off values for the IDEXX and BioChek ELISAs to 2000 and 3050, respectively, to achieve 100% agreement in distinguishing REV infection from cross-reactivity with fowlpox virus [5].

The integration of REV control with broader poultry health management programs is essential. REV-induced immunosuppression predisposes birds to secondary infections with a wide range of pathogens, including Escherichia coli, Salmonella, and respiratory viruses, and synergizes with ALV-J and MDV to produce more severe disease [10, 12, 16, 21, 39, 46, 64]. The presence of REV in a flock can compromise the efficacy of vaccination programs for other diseases, as demonstrated for MD vaccines [39] and Newcastle disease vaccines [66]. Therefore, REV control must be considered a component of comprehensive immunoprophylaxis. The use of immunomodulatory feed additives, such as allicin [38] and Enteromorpha polysaccharides [66], in conjunction with vaccination may help restore immune competence and improve vaccine responses in REV-infected flocks.

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