Marek’s disease (MD), an important avian immunosuppressive and neoplastic diseases, has resulted in huge economic losses for the poultry industry worldwide. Over the past 50 years, the long-term prevalence of this disease and the growing immune pressure of MD vaccination have triggered persistently increased virulence. The emerging hypervirulent variant of MDV (HV-MDV) overcomes the protection conferred by commercial vaccines, highlighting the urgent need to develop novel, highly efficient MD vaccines. To address this challenge, using CRISPR/Cas9-based gene editing, we generated a novel candidate vaccine, SQ01ΔmeqΔLAT, which features double deletions of both oncogene meq and LAT-clustered miRNAs. Comprehensive experiments confirmed the gene deletions, genetic stability and retention of the replication kinetics of the mutant, without revealing any adverse impact on the expression of essential viral genes. Further animal experiments showed no histopathological lesions or neoplastic changes in SQ01ΔmeqΔLAT-challenged birds, demonstrating favourable safety to chicken hosts. More importantly, the SQ01ΔmeqΔLAT vaccine achieved a high protection index (PI) of 88.9% against HV-MDV strain HNSQ01; this was an improvement compared to CVI988, which provided a PI value of only 70.6%. Our data provide an important basis for the development of highly efficient MD vaccines and shed new light on the design of herpesvirus vaccines, simultaneously targeting both viral protein-coding genes and non-coding RNAs.

Hepatitis caused by fowl adenovirus serotype 4 (FAdV-4) is a serious avian disease associated with huge losses to the poultry industry worldwide. Herein, we document the first outbreak of FAdV-4 infection in blue-breasted quail (Excalfactoria chinensis). The causative agent, designated as HNSQ2023, was isolated and characterized as an FAdV-4 epidemic strain using PCR amplification, DNA sequencing, and IFA staining. Comparative genomic analysis of the viral genomes of 79 virulent or non-pathogenic FAdV-4 strains showed that the gene order and shared open reading frames of HNSQ2023 were similar to those of previously reported virulent FAdV-4 strains. It also contains 100% conserved amino acid mutations in major structural proteins such as hexon and fiber-2. This is similar to the virulent FAdV-4 strains, except for an irregular mutation at aa46 (A to T) in fiber-1. Notably, the newly identified HNSQ2023 isolated from Excalfactoria chinensis and the other five epidemic FAdV-4 strains previously isolated from peacocks, chickens, ducks, and pigeons during 2018-2023 formed a close evolutionary sub-branch, distinct from the main branch, mostly composed of Chinese FAdV-4 strains originating from chickens. Furthermore, animal experiments demonstrated that HNSQ2023 retained high pathogenicity to SPF chickens and Excalfactoria chinensis, with 100% morbidity and over 90% mortality in both species. Our data shed new light on the high risk of cross-species transmission of FAdV-4 in domestic poultry and ornamental pet birds, stressing the importance of stricter biosecurity measures and necessary vaccinations for future control of this disease.

The avian immunosuppressive and neoplastic diseases are great threats to the poultry industry, causing huge economic losses worldwide. Most recently, the emerging hypervirulent variants of Marek's disease virus (HV-MDV), partially co-infected with avian leukosis virus (ALV) and/or reticuloendotheliosis virus (REV), have been identified as the key driver of tumour outbreaks in vaccinated chicken flocks, but the role of chicken infectious anemia virus (CIAV) remains unclear. Herein, we have investigated the prevalence and co-infection of CIAV in 71 clinical tumour-bearing flocks collected from central China during 2021-2023, which has shown a CIAV positivity rate of 59.2% (42/71). Notably, the incidence of CIAV mono-infection increased significantly from 0% (0/29) in 2021 to 23.7% (9/38) in 2023, whereas CIAV + MDV co-infection decreased from 65.5% (19/29) to 31.6% (12/38). A total of 20 viral genomes of epidemic CIAV isolates from diverse sources were obtained, and the phylogenetic analysis, including 91 reference isolates were clustered into four major lineages (A-D), with clade C further subdivided into subclades C1 and C2. Clade C1 consisted predominantly of Asian isolates, with 88.5% (46/52) of the isolates originating from mainland China. Among the 20 new isolates, 17 were clustered in subclade C1, two in C2, and one in B. The VP1 gene phylogeny showed a topology largely consistent with that of the whole-genome analysis. Moreover, all newly characterized isolates contained glutamine (Q) at VP1 residue 394, a molecular marker associated with high pathogenicity. Collectively, our data suggest that prevalent HV-MDV variants together with CIAV co-infections are the primary drivers of the ongoing tumour outbreaks in Chinese poultry flocks. Notably, the significantly increased CIAV mono-infections, possibly resulting from an independently evolving lineage among circulating Chinese strains, are likely to pose a new challenge for future control of disease.

Marek’s disease (MD), caused by pathogenic Marek’s disease virus serotype 1 (MDV-1), is one of the most important avian immunosuppressive and neoplastic diseases and has led to huge economic losses to the poultry industry worldwide. Rapid and accurate clinical diagnosis is of great significance for efficient control of the disease. Herein, we have established a multiplex PCR (mPCR) method to simply differentiate all of the three types of MDV, using five specific primers targeting to MDV-1 oncogene meq or MDV-2 and MDV-3/HVT gB genes. Simultaneously, it can detect any type of virulent or vaccine MDV strains in one PCR reaction, with amplicons of the short (S) and long (L)-meq of MDV-1 strains, and the gB of MDV-2 and HVT vaccine strains. Non-specific amplifications of avian leukosis virus (ALV), reticuloendotheliosis virus (REV), or fowl adenovirus virus 4 (FAdV-4) were not observed, indicating a good specificity of this method. A total of 522 clinical samples of tumor-bearing or suspected diseased birds collected from 30 poultry farms were detected. The results demonstrated that the newly developed mPCR method accurately detected and differentiated epidemic MDV-1 infections and vaccine strains, and provided nearly 100% consistency for detecting clinical wild-type infections compared with conventional PCR amplification of the meq gene. Collectively, our data has provided a highly efficient method for early differential diagnosis of MD clinical cases, virus identification and future evaluation of vaccination efficacy in healthy chicken flocks, which would be meaningful for efficient control of the disease.

As a core member of the DEAD-box helicase family, DDX3X modulates RNA metabolic networks through its ATPase activity, RNA helicase function, and nucleic acid-binding capacity to participate in bidirectional regulation of innate immune responses and virus-host interactions. Multiple viruses achieve effective genome replication and immune evasion by hijacking DDX3X’s enzymatic activities or interfering with its mediated immune signaling transduction. Nevertheless, hosts have evolved strategies to exploit DDX3X for activating interferon signaling pathways and other antiviral mechanisms, establishing multilayered defense networks. This review systematically elaborates the functional diversity exhibited by DDX3X protein in virus interaction networks. DDX3X orchestrates viral genomic RNA processing during replication. Simultaneously, it interacts with host restriction factors to evade antiviral immunity, establishing a dynamic balance between viral propagation and host defense. The functional plasticity of DDX3X not only elucidates immune regulatory mechanisms in host-pathogen coevolution, but also provides novel molecular perspectives for deciphering zoonotic transmission barriers.

Ferroptosis is an iron-dependent form of programmed cell death that plays a crucial role in regulating intracellular redox homeostasis and lipid metabolism, and in combating viral infections. Viruses have persistently evolved and adapted synergistically with their hosts over a long period and, to some extent, have been able to utilize ferroptosis to promote viral replication. Herein, we summarize the characteristics, mechanisms, and regulatory networks of ferroptosis and provide an overview of the key regulatory steps of ferroptosis involved in viral infection, together with the changes in host indicators and key regulatory signaling pathways. This study intends to deepen our understanding of the critical role of ferroptosis in viral infection, which will be meaningful for further revealing the mechanisms underlying the occurrence and progression of virus diseases, as well as for the future exploration of anti-viral strategies.

In the past decade, research has demonstrated that viral miRNAs encoded by a number of viral genomes, particularly by most of the herpesvirus including Marek's disease virus (MDV), play important regulatory roles in viral infection, replication, and regulation of tumorigenesis. As macrovesicles in cells, exosomes can deliver viral miRNAs and exert gene regulatory functions. Whether the exosomes play a role in the replication, pathogenesis/tumorigenesis of avian herpesviruses such as oncogenic Marek's disease virus (MDV) remains unclear. Herein we extracted and identified the exosomes from MDV-transformed T cell line MSB-1 and demonstrated high abundance of MDV-1 miRNA expression. Using dual luciferase-based reporter assay, we also demonstrated that the exosomes derived from MSB-1 can deliver functional miRNA successfully into primary chicken embryo fibroblasts. These findings provide new insights into the role of exosomes and the mechanisms of how virus-encoded miRNA function in MDV latency/activation switching, viral replication, pathogenesis and/or tumorigenesis.

Marek’s disease virus (MDV) is a highly pathogenic and oncogenic alpha herpesvirus that causes Marek’s disease (MD), which is one of the most important immunosuppressive and rapid-onset neoplastic diseases in poultry. The onset of MD lymphomas and other clinical diseases can be efficiently prevented by vaccination; these vaccines are heralded as the first demonstration of a successful vaccination strategy against a cancer. However, the persistent evolution of epidemic MDV strains towards greater virulence has recently resulted in frequent outbreaks of MD in vaccinated chicken flocks worldwide. Herein, we provide an overall review focusing on the discovery and identification of the strategies by which MDV evades host immunity and attacks the immune system. We have also highlighted the decrease in the immune efficacy of current MD vaccines. The prospects, strategies and new techniques for the development of efficient MD vaccines, together with the possibilities of antiviral therapy in MD, are also discussed.

Marek’s disease virus (MDV) strain GX0101 was the first reported field strain of recombinant gallid herpesvirus type 2 (GaHV-2). However, the splenic proteome of MDV-infected chickens remains unclear. In this study, a total of 28 1-day-old SPF chickens were intraperitoneally injected with chicken embryo fibroblast (CEF) containing 2000 PFU GX0101. Additionally, a control group, consisting of four one-day-old SPF chickens, received intraperitoneal equal doses of CEF. Blood and various tissue samples were collected at different intervals (7, 14, 21, 30, 45, 60, and 90 days post-infection; dpi) for histopathological, real-time PCR, and label-free quantitative analyses. The results showed that the serum expressions of MDV-related genes, meq and gB, peaked at 45 dpi. The heart, liver, and spleen were dissected at 30 and 45 dpi, and their hematoxylin-eosin staining indicated that virus infection compromised the normal organizational structure at 45 dpi. Particularly, the spleen structure was severely damaged, and the lymphocytes in the white medulla were significantly reduced. Furthermore, liquid chromatography-mass spectrometry (LC-MS) and label-free techniques were used to analyze the difference in splenic proteome profiles of the experimental and control groups at 30 and 45 dpi. Proteomic analysis identified 1660 and 1244 differentially expressed proteins (DEPs) at 30 and 40 dpi, respectively, compared with the uninfected spleen tissues. According to GO analysis, these DEPs were involved in processes such as organelle organization, cellular component biogenesis, cellular component assembly, anion binding, small molecule binding, metal ion binding, cation binding, cytosol, nuclear part, etc. Additionally, KEGG analysis indicated that the following pathways were linked to MDV-induced inflammation, apoptosis, and tumor: Wnt, Hippo, AMPK, cAMP, Notch, TGF-β, PI3K-Akt, Rap1, Ras, Calcium, NF-κB, PPAR, cGMP-PKG, Apoptosis, VEGF, mTOR, FoxO, TNF, JAK-STAT, MAPK, Prion disease, T cell receptor, and B cell receptor. We finally screened 674 DEPs that were linked to MDV infection in spleen tissue. This study improves our understanding of the MDV response mechanism in the spleen.

As one of the most important avian immunosuppressive and neoplastic diseases, Marek’s disease (MD), caused by oncogenic Marek’s disease virus (MDV), has caused huge economic losses worldwide over the past five decades. In recent years, MD outbreaks have occurred frequently in MD-vaccinated chicken flocks, but the key pathogenic determinants and influencing factors remain unclear. Herein, we analyzed the pathogenicity of seven newly isolated MDV strains from tumor-bearing chickens in China and found that all of them were pathogenic to chicken hosts, among which four MDV isolates, SDCW01, HNXZ05, HNSQ05 and HNSQ01, were considered to be hypervirulent MDV (HV-MDV) strains. At 73 days of the virus infection experiment, the cumulative incidences of MD were 100%, 93.3%, 90% and 100%, with mortalities of 83.3%, 73.3%, 60% and 86.7%, respectively, for the four viruses. The gross occurrences of tumors were 50%, 33.3%, 30% and 63.3%, respectively, accompanied by significant hepatosplenomegaly and serious atrophy of the immune organs. Furthermore, the immune protection effects of four commercial MD vaccines against SDCW01, CVI988, HVT, CVI988+HVT, and 814 were explored. Unexpectedly, during the 67 days of post-virus challenge, the protection indices (PIs) of these four MD vaccines were only 46.2%, 38.5%, 50%, and 28%, respectively, and the birds that received the monovalent CVI988 or HVT still developed tumors with cumulative incidences of 7.7% and 11.5%, respectively. To our knowledge, this is the first demonstration of the simultaneous comparison of the immune protection efficacy of multiple commercial MD vaccines with different vaccine strains. Our study revealed that the HV-MDV variants circulating in China could significantly break through the immune protection of the classical MD vaccines currently widely used. For future work, there is an urgent need to develop novel, more effective MD vaccines for tackling the new challenge of emerging HV-MDV strains or variants for the sustainable control of MD.

Marek's disease (MD) caused by pathogenic Marek's disease virus type 1 (MDV-1) is one of the most important neoplastic diseases of poultry. MDV-1-encoded unique Meq protein is the major oncoprotein and the availability of Meq-specific monoclonal antibodies (mAbs) is crucial for revealing MDV pathogenesis/oncogenesis. Using synthesized polypeptides from conserved hydrophilic regions of the Meq protein as immunogens, together with hybridoma technology and primary screening by cross immunofluorescence assay (IFA) on Meq-deleted MDV-1 viruses generated by CRISPR/Cas9-gene editing, a total of five positive hybridomas were generated. Four of these hybridomas, namely 2A9, 5A7, 7F9 and 8G11, were further confirmed to secrete specific antibodies against Meq as confirmed by the IFA staining of 293T cells overexpressing Meq. Confocal microscopic analysis of cells stained with these antibodies confirmed the nuclear localization of Meq in MDV-infected CEF cells and MDV-transformed MSB-1 cells. Furthermore, two mAb hybridoma clones, 2A9-B12 and 8G11-B2 derived from 2A9 and 8G11, respectively, displayed high specificity for Meq proteins of MDV-1 strains with diverse virulence. Our data presented here, using synthesized polypeptide immunization combined with cross IFA staining on CRISPR/Cas9 gene-edited viruses, has provided a new efficient approach for future generation of specific mAbs against viral proteins.

Dynamic alteration of the epitranscriptome exerts regulatory effects on the lifecycle of oncogenic viruses in vitro. However, little is known about these effects in vivo because of the general lack of suitable animal infection models of these viruses. Using a model of rapid-onset Marek's disease lymphoma in chickens, we investigated changes in viral and host messenger RNA (mRNA) N6-methyladenosine (m(6)A) modification during Marek's disease virus (MDV) infection in vivo. We found that the expression of major epitranscriptomic proteins varies among viral infection phases, reprogramming both the viral and the host epitranscriptomes. Specifically, the methyltransferase-like 3 (METTL3)/14 complex was suppressed during the lytic and reactivation phases of the MDV lifecycle, whereas its expression was increased during the latent phase and in MDV-induced tumors. METTL3/14 overexpression inhibits, whereas METTL3/14 knockdown enhances, MDV gene expression and replication. These findings reveal the dynamic features of the mRNA m(6)A modification program during viral replication in vivo, especially in relation to key pathways involved in tumorigenesis.

Over the past two decades, numerous non-coding RNAs (ncRNAs) have been identified in different biological systems including virology, especially in large DNA viruses such as herpesviruses. As a representative oncogenic alphaherpesvirus, Marek’s disease virus (MDV) causes an important immunosuppressive and rapid-onset neoplastic disease of poultry, namely Marek’s disease (MD). Vaccinations can efficiently prevent the onset of MD lymphomas and other clinical disease, often heralded as the first successful example of vaccination-based control of cancer. MDV infection is also an excellent model for research into virally-induced tumorigenesis. Recently, great progress has been made in understanding the functions of ncRNAs in MD biology. Herein, we give a review of the discovery and identification of MDV-encoded viral miRNAs, focusing on the genomics, expression profiles, and emerging critical roles of MDV-1 miRNAs as oncogenic miRNAs (oncomiRs) or tumor suppressor genes involved in the induction of MD lymphomas. We also described the involvements of host cellular miRNAs, lincRNAs, and circRNAs participating in MDV life cycle, pathogenesis, and/or tumorigenesis. The prospects, strategies, and new techniques such as the CRISPR/Cas9-based gene editing applicable for further investigation into the ncRNA-mediated regulatory mechanisms in MDV pathogenesis/oncogenesis were also discussed, together with the possibilities of future studies on antiviral therapy and the development of new efficient MD vaccines.

马立克病(MD)是由马立克病病毒(MDV)感染引起的严重危害世界养禽业健康发展的一种重要的家禽免疫抑制病与肿瘤病,是人类历史上第一个可以通过疫苗免疫接种成功预防肿瘤疾病发生的案例.对雏鸡早期接种MD疫苗,可有效控制MD肿瘤的发生.随着MD疫苗的广泛使用和长期免疫压力,MDV流行毒株也在不断进化、毒力增强或变异,部分毒株已突破现有商品疫苗的免疫保护,导致MD疫情在全球频繁暴发.自MD疫苗问世至今,已有多种MD疫苗研制成功,并且二价疫苗、三价疫苗、重组载体疫苗、基因缺失或插入的MD疫苗等也在不断被研究和优化.利用各种新兴的生物学技术,如细菌人工染色体(BAC)、基因同源重组和CRISPR/Cas9 基因编辑等来开展MD基因工程疫苗研究,已成为病毒病疫苗研究领域的最新热点.本文回顾了过去50 年来MD疫苗研究历程及巨大成就,并全面综述了当前国内外MD基因工程疫苗和基因编辑疫苗所取得的最新研究进展,同时也对现有MD商品疫苗免疫预防所面临的新问题、新挑战和未来前景进行了展望,以期为下一代新型高效的MD疫苗创制提供重要参考.

马立克病(MD)是由马立克病病毒(MDV)感染引起的危害最严重的一种家禽免疫抑制病与肿瘤病,制备MDV单克隆抗体可为后续的基因功能、致病机制及诊断技术研究提供关键试剂.本研究利用NE-PER™细胞核和细胞质提取试剂,分别制备了 vvMDV(MDV超强毒株)标准毒株Md5感染的鸡胚成纤维细胞(CEF)核蛋白和浆蛋白,通过切向流超滤浓缩后制备免疫原,分别免疫6～8周龄雌性BALB/c小鼠,常规细胞融合方法制备单克隆抗体杂交瘤细胞.通过间接免疫荧光试验(IFA)筛选,建立了一个容量为31株纯化的MDV单克隆抗体杂交瘤细胞库,包括27株稳定分泌MDV-1特异性抗体和4株稳定分泌MDV-1、MDV-2和火鸡疱疹病毒(HVT)保守抗体的杂交瘤细胞株.经过293T细胞真核表达gB蛋白并结合IFA染色,其中1株单克隆抗体J-1F3-C6被鉴定为MDV糖蛋白gB的特异性单克隆抗体,其单抗腹水IFA效价为1∶25 600,轻链型为Kappa,IgG亚型为IgG2a.进一步IFA染色表明,J-1F3-C6可特异性识别MDV-1、MDV-2和HVT毒株表达的gB蛋白,但在蛋白质免疫印迹(Western blot)分析中J-1F3-C6只与MDV-1和HVT发生特异性反应.综上,本研究建立了一个MDV单克隆抗体杂交瘤细胞库,并从中筛选鉴定了 1株gB蛋白特异性的单克隆抗体,为后续研究奠定了重要基础.

In recent years, outbreaks of Marek’s disease (MD) have been frequently reported in vaccinated chicken flocks in China. Herein, we have demonstrated that four Marek’s disease virus (MDV) isolates, HN502, HN302, HN304, and HN101, are all pathogenic and oncogenic to hosts. Outstandingly, the HN302 strain induced 100% MD incidence, 54.84% mortality, and 87.10% tumor incidence, together with extensive atrophy of immune organs. Pathotyping of HN302 was performed in comparison to a standard very virulent (vv) MDV strain Md5. We found that both CVI988 and HVT vaccines significantly reduced morbidity and mortality induced by HN302 or Md5 strains, but the protection indices (PIs) provided by these two vaccines against HN302 were significantly lower (27.03%) or lower (33.33%) than that against Md5, which showed PIs of 59.89% and 54.29%, respectively. These data suggested that HN302 possesses a significant higher virulence than Md5 and at least could be designated as a vvMDV strain. Together with our previous phylogenetic analysis on MDV-1 meq genes, we have presently suggested HN302 to be a typical highly virulent MDV variant belonging to an independent Chinese branch. To our knowledge, this is the first report to provide convincible evidence to identify a pathogenic MDV variant strain with a higher virulence than Md5 in China, which may have emerged and circulating in poultry farms in China for a long time and involved in the recent MD outbreaks.

马立克病(MD)是由马立克病病毒(MDV)早期感染雏鸡引起的一种重要的家禽免疫抑制病与肿瘤病.MDV以水平传播为主,具有高度接触性和传染性、高致病率以及高致死率等特征,给全球养禽业造成巨大经济损失.该病主要依赖MD疫苗免疫预防,近50年来在MD疫苗广泛使用和持续免疫压力下,MDV毒力不断增强,部分毒株已突破现有商品疫苗的免疫保护.尽早对MD疑似病例进行快速准确的鉴别诊断,可为生产企业和养殖户及时采取措施、减少和挽回部分经济损失提供重要依据.经典的诊断方法已难以满足当下对MD病例快速诊断的需要.本文重点对近十年中新建立的分子生物学技术,如聚合酶链式反应(PCR)、实时荧光定量PCR(qPCR)、环介导等温扩增(LAMP)、重组酶聚合酶扩增(RPA)、多重PCR及液相芯片等在MDV检测和MD诊断中的研究及应用进行了综述,以期为今后研究建立用于MD流行病学调查、临床病例早期诊断和流行毒株分型鉴定的鉴别诊断技术提供参考.

The avian immunosuppressive and neoplastic diseases caused by Marek’s disease virus (MDV), avian leucosis virus (ALV), and reticuloendotheliosis virus (REV) are seriously harmful to the global poultry industry. In recent years, particularly in 2020–2022, outbreaks of such diseases in chicken flocks frequently occurred in China. Herein, we collected live diseased birds from 30 poultry farms, out of 42 farms with tumour-bearing chicken flocks distributed in central China, to investigate the current epidemiology and co-infections of these viruses. The results showed that in individual diseased birds, the positive infection rates of MDV, ALV, and REV were 69.5% (203/292), 14.4% (42/292), and 4.7% (13/277), respectively, while for the flocks, the positive infection rates were 96.7% (29/30), 36.7% (11/30), and 20% (6/30), respectively. For chicken flocks, monoinfection of MDV, ALV, or REV was 53.3% (16/30), 3.3% (1/30), and 0% (0/30), respectively, but a total of 43.3% (13/30) co-infections was observed, which includes 23.3% (7/30) of MDV+ALV, 10.0% (3/30) of MDV+REV, and 10.0% (3/30) of MDV+ALV+REV co-infections. Interestingly, no ALV+REV co-infection or REV monoinfection was observed in the selected poultry farms. Our data indicate that the prevalence of virulent MDV strains, partially accompanied with ALV and/or REV co-infections, is the main reason for current outbreaks of avian neoplastic diseases in central China, providing an important reference for the future control of disease.

基于CRISPR/Cas9系统的基因编辑是最新一代的基因组编辑技术,在向导RNA(gRNA)的介导下几乎可以靶向编辑任何一种基因,实现基因组的定点突变、敲除或插入.近年来将CRISPR/Cas9基因编辑技术应用于大基因组DNA病毒的研究,尤其是用于疱疹病毒的基因编辑已成为病毒学研究领域的最新国际热点.自2016年首次报道利用CRISPR/Cas9系统改造家禽疱疹病毒如马立克病病毒(MDV)基因组以来,短短5年时间已全面应用于家禽疱疹病毒的蛋白编码基因和非编码RNA基因的编辑、基因缺失疫苗和重组疫苗研发、抗病毒治疗以及抗病育种等领域.本文详细综述了当前CRISPR/Cas9基因编辑技术在家禽疱疹病毒中的应用进展和最新成果,并对其面临的问题和前景进行了展望,以期为后续研究提供重要参考.

Marek's disease (MD) is a neoplastic disease of chickens caused by an avian alphaherpesvirus, Marek's disease virus (MDV, also known as Gallid alphaherpesvirus 2 [GaHV2]). A total of 14 microRNA (miRNA) precursors and 26 mature miRNAs have been identified in MDV genome, which were grouped in three distinct clusters. In recent years, our studies revealed the role of MDV encoded cluster 3 miRNAs (or miR-M8-M10) and the specific function of its three members, miR-M6, miR-M7 and miR-M10, in regulating MDV replication and pathogenesis. In this study, we characterized the unique function of the other two members, miR-M8 and miR-M13, in cluster 3 miRNAs. Our results show that miR-M8 and miR-M13 are not important for MDV plaque formation and genome replication in vitro. Animal experiment results show that deletion of miR-M8-5p and miR-M13-5p eliminates the bursa atrophy, but not thymus atrophy, of MDV inoculated chickens. In addition, we found that the survival curve and MD incidences were not affected by disruption of miR-M8 and miR-M13. Taken together, this study uncovers the unique role of miR-M8 and miR-M13 in MDV replication and pathogenesis, which filled the gap in the research of MDV encoded miRNAs.