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Marek’s disease is an infectious disease in poultry that usually appears in neural and visceral tumors. This disease is caused by Gallid alphaherpesvirus 2 infection in lymphocytes, and its meq gene is commonly used in virulent studies for coding the key protein functional in oncogenic transformation of the lymphocytes. Although vaccines have been introduced in many countries to control its spread and are proven to be efficient, recent records show a decline of such efficiency due to viral evolution. In this study, we reviewed the outbreak of Marek’s disease in Asia for the last 10 years, together with associated meq sequences, finding a total of 36 studies recording outbreaks with 132 viral strains in 12 countries. The visceral type is the most common (13 in 16 studies) form of Marek’s disease, but additional unobserved neural changes may exist. MD induces liver lymphoma most frequently (11 in 14 studies), and tumors were also found in spleen, kidney, heart, gizzard, skin, intestine, lung, and sciatic nerve. Twelve viral strains distributed in China have been reported to escape the CVI988 vaccine, reaching a mortality rate of more than 30%. Phylogenetic analyses show the internal connection between the Middle East (Turkey, Iraq, Iran, Saudi Arabia), South Asia (India, Indonesia), and East Asia (China and Japan), while external viral communications might occasionally occur. In 18 strains with both sequential and mortality data, amino acid alignment showed several point substitutions that may be related to its virulence. We suggest more behavioral monitoring in Marek’s disease-endemic regions and further studies on strain virulence, together with its Meq protein structural changes.

Marek's disease (MD), one of the most important avian immunosuppressive and neoplastic diseases, causes enormous economic losses in China. In 2023, seven 3-month-old chickens were determined to be infected with Marek's disease virus (MDV) by histopathological examination, polymerase chain reaction (PCR), and gene sequencing. All of them originated from chicken flocks in JuRong (Jiangsu, China) and were immunized with the CVI988/Rispens (CVI988) vaccine. The novel MDV strain identified in these chickens was named JuRong 2023 and was isolated from feather follicle samples. Sequence analysis revealed that the Meq gene was most closely related to the SD-2012 strain, which was detected in Shandong Province. Furthermore, compared with the reference strain CVI988, the amino acid sequence of JuRong 2023 had mutations at positions 71, 80, 115, 176, 217 and 326, which may affect the function of the virus. Constant surveillance of newly isolated field strains in China is essential to evaluate the immune protection efficacy of different MD vaccines.

The aim of this study was to demonstrate the genomic features of Meq gene of Marek's disease virus (MDV) recently circulating in Saudi Arabia (SA). Two poultry flocks suffering from mortalities and visceral tumors were presented to the Veterinary Teaching Hospital, King Faisal University, SA. Subjected to different diagnostic procedures: Case history, clinical signs, and necropsy as well as polymerase chain reaction followed by Meq gene sequence analysis. Case history, clinical signs, and necropsy were suggestive of MDV infection. The Meq gene was successfully detected in liver and spleen of infected chickens. A 1062 bp band including the native Meq ORF in addition to a 939 bp of S-Meq (short isoform of Meq) were amplified from Saudi 01-13 and Saudi 02-13, respectively. The nucleotide and deduced amino acids sequences of the amplified Meq genes of both Saudi isolates showed distinct polymorphism when compared with the standard USA virulent isolates Md5 and GA. The sequence analysis of the S-Meq gene showed a 123 bp deletion representing 41 amino acids between two proline-rich areas without any frameshift. The Meq gene encoded four repeats of proline-rich repeats (PRRs sequences), whereas the S-Meq contains only two PRRs. Interestingly, the phylogenetic analysis revealed that both of SA MDV isolates are closely related to the MDV strains from Poland. The two MDV isolates contain several nucleotide polymorphisms resulting in distinct amino acid substitutions. It is suggested that migratory and wild birds, as well as world trading of poultry and its by-products, have a great contribution in the transmission of MDVs overseas.

Background: Marek’s disease (MD) is a pathology affecting chickens caused by Marek’s disease virus (MDV), an acute transforming alphaherpesvirus of the genus Mardivirus. MD is characterized by paralysis, immune suppression, and the rapid formation of T-cell (primarily CD4+) lymphomas. Over the last 50 years, losses due to MDV infection have been controlled worldwide through vaccination; however, these live-attenuated vaccines are non-sterilizing and potentially contributed to the virulence evolution of MDV field strains. Mutations common to field strains that can overcome vaccine protection were identified in the C-terminal proline-rich repeats of the oncoprotein Meq (Marek’s EcoRI-Q-encoded protein). These mutations in meq have been found to be distinct to their region of origin, with high virulence strains obtained in Europe differing from those having evolved in the US. The present work reports on meq mutations identified in MDV field strains in Nigeria, arising at farms employing different vaccination practices. Materials and Methods: DNA was isolated from FTA cards obtained at 12 farms affected by increased MD in the Plateau State, Nigeria. These sequences included partial whole genomes as well as targeted sequences of the meq oncogenes from these strains. Several of the meq genes were cloned for expression and their localization ability to interact with the chicken NF-IL3 protein, a putative Meq dimerization partner, were assessed. Results: Sequence analysis of the meq genes from these Nigerian field strains revealed an RB1B-like lineage co-circulating with a European Polen5-like lineage, as well as recombinants harboring a combination of these mutations. In a number of these isolates, Meq mutations accumulated in both N-terminal and C-terminal domains. Discussion: Our data, suggest a direct effect of the vaccine strategy on the selection of Meq mutations. Moreover, we posit the evolution of the next higher level of virulence MDVs, a very virulent plus plus pathotype (vv++).

Background Marek’s disease virus (MDV) causes Marek’s disease (MD) in chickens, which is characterized by malignant lymphomas and neurological disorders. Although MD is currently controlled using live vaccines, the virulence of field strains has continuously increased in recent decades. Polymorphisms in the MDV-encoded oncoprotein Meq are shared among field strains according to their virulence. In particular, very virulent MDV strains harbor characteristic amino acid changes in the basic region of Meq at positions 77 and 80; however, the contribution of these polymorphisms to virulence remains unclear. Methods To assess the impact of these polymorphisms on MDV virulence, we generated recombinant MDV (rMDV) based on the very virulent RB-1B strain, harboring K77E and D80Y substitutions in Meq found in low-virulent strains (rRB-1B_Meq77/80). Chickens were challenged with rMDVs, and survival rates and tumor incidence were evaluated. Viral loads in major organs were quantified by quantitative PCR, and the dynamics of MDV-infected cells and T cells were analyzed using flow cytometry. In addition, histopathological analysis was performed to further examine differences in pathogenesis in detail. To elucidate the mechanisms underlying pathogenesis, we conducted reporter assays to assess the effect of these polymorphisms in the basic region on its transcriptional regulatory activity. Results rRB-1B_Meq77/80 exhibited a reduced virulence but unexpectedly caused other clinical signs, including open-mouth breathing, in infected chickens. Quantitative PCR analysis showed consistently lower viral loads across all examined organs in rRB-1B_Meq77/80-infected chickens. Flow cytometric analysis revealed a reduction in MDV-infected cells, accompanied by a notable increase in CD8⁺ T cell populations. Histopathological analysis showed bronchus-associated lymphoid tissue hyperplasia in the lungs. Reporter assays revealed that most amino acid substitutions in the basic region in low-virulence strains reduced transcriptional regulatory activity. Conclusion Our data indicate that polymorphisms at positions 77 and 80 in the Meq of low-virulence strains reduce MDV virulence and Meq-mediated transcription and possibly alter pathogenesis. This study improves our understanding of the mechanisms underlying MDV virulence.

Marek’s disease virus (MDV) infects chickens and is among the most common tumor in animals. The first MDV vaccinations that prevented cancer and reduced losses in the chicken industry were live-attenuated vaccines. Despite the fact that the current gold standard vaccination effectively guards against the disease, because of the Meq gene the virus is continuously mutating to become more virulent. the two vaccine generations previously given protection was able to be overcome by emerging field strains. In the present study collecting the tissue samples from infected, A standardized PCR was used. 132-bp tandem repeat region targeted for serotype-1 MD viruses. in Further, the oncogenes' characterization. PCR and nucleotide sequencing were used to detect Meq gen and viral interleukin 8 (vIL-8) gen. Meq gene sequence analysis for different clinical cases from Iraq revealed 100 % homology with Indian strain, Japan strain, Turkish strain, Germany strain (very virulent). and with (very very virulent) Iranian strains showed 100% identity with the Iraq strain. With the use of more GenBank sequences, a phylogenetic analysis of oncogenes was carried out. Finally, based on nucleotide and phylogenetic analyses of the viruses, we get to the conclusion that MDV strains found in the current outbreaks in Iraq could be classified as virulent or very very virulent pathotypes.

Marek's disease (MD) is an important disease of avian species and a potential threat to the poultry industry worldwide. In this study, 16 dead commercial chickens from flocks with suspected MD were necropsied immediately after death. Pathological findings were compatible with MD, and gallid alphaherpesvirus 2 was identified in PCR of spleen samples. Virus isolation was performed in primary cell culture, and partial sequencing of the meq gene of the isolate revealed >99% nucleotide sequence identity to virulent and very virulent plus strains from a number of European countries, placing it in the same subclade of clade III as two virulent Italian strains and a very virulent plus Polish strain as well as virulent strains of geese and ducks. The data reported here indicate that a virulent strain of Marek’s disease virus is circulating in Turkey and has not been stopped by the current national vaccination programme.

Background Marek’s disease virus (MDV) causes malignant lymphomas in chickens (Marek’s disease, MD). MD is currently controlled by vaccination; however, MDV strains have a tendency to develop increased virulence. Distinct diversity and point mutations are present in the Meq proteins, the oncoproteins of MDV, suggesting that changes in protein function induced by amino acid substitutions might affect MDV virulence. We previously reported that recent MDV isolates in Japan display distinct mutations in Meq proteins from those observed in traditional MDV isolates in Japan, but similar to those in MDV strains isolated from other countries. Methods To further investigate the genetic characteristics in Japanese field strains, we sequenced the whole genome of an MDV strain that was successfully isolated from a chicken with MD in Japan. A phylogenetic analysis of the meq gene was also performed. Results Phylogenetic analysis revealed that the Meq proteins in most of the Japanese isolates were similar to those of Chinese and European strains, and the genomic sequence of the Japanese strain was classified into the Eurasian cluster. Comparison of coding region sequences among the Japanese strain and MDV strains from other countries revealed that the genetic characteristics of the Japanese strain were similar to those of Chinese and European strains. Conclusions The MDV strains distributed in Asian and European countries including Japan seem to be genetically closer to each other than to MDV strains from North America. These findings indicate that the genetic diversities of MDV strains that emerged may have been dependent on the different vaccination-based control approaches.

Marek’s disease virus (MDV) is a highly contagious tumor virus that causes detrimental outbreaks in poultry. Since its initial description, the virus’s virulence and acuteness have progressively increased. During this study, we investigated suspected tumorigenic cases of MDV-1 infection among different avian species (chicken, ducks, and turkey) in various Egyptian governorates, including Al-Sharqia, Gharbia, Dakahlia, Port Said, Damietta, and Fayoum, between 2020 and 2023. A molecular study targeting the virulent oncogenic Meq gene revealed that the tumorigenic masses in chicken and duck tissues were identified as virulent MDV-1, but turkeys with cauliflower-like ovarian tumors showed negative results. The isolated MDV-1 strain of chicken origin was given the designation YLE2021 and the sequence was submitted to GenBank with accession number PQ59985. Sequence analysis revealed a partial Meq open reading frame encoding 296 amino acids and contains seven proline motifs, three of them are interrupted (187 PLQPP 191, 195 PAPP198, 224 PPQPP 228). Experimental infection of one-day-old specific-pathogen-free (SPF) chickens with a strain recovered from a chicken tumor resulted in 40% of infected birds showing the classical neural form of MDV infection. No parenchymal tumors were observed, and the virus could be molecularly detected in the peripheral blood mononuclear cells (PMNCs) of infected and neighboring uninfected SPF birds. In conclusion, this is the first report to identify the presence of MDV-1 in Egyptian ducks. Further investigations are recommended to detect the main cause of the turkeys’ tumor. Continuous molecular monitoring of circulating field viruses is crucial to investigate the mechanisms behind the increase in virus evolution, which could lead to increased virus virulence and allow the virus to evade vaccine protection.

Marek's disease (MD), caused by Marek's disease virus (MDV), a poultry-borne alphaherpesvirus, is a devastating disease of poultry causing an estimated annual loss of one billion dollars to poultry producers, worldwide. Despite decades of control through vaccination, MDV field strains continue to emerge having increased virulence. The evolutionary mechanism driving the emergence of this continuum of strains to increased MDV virulence, however, remains largely enigmatic. Increase in MDV virulence has been associated with specific amino acid changes within the C-terminus domain of Mareks's EcoRI-Q (meq)-encoded oncoprotein. In this study, we sought to determine whether the meq gene has evolved adaptively and whether past vaccination efforts have had any significant effect on the reduction or increase of MDV diversity over time. Our analysis suggests that meq is estimated to be evolving at a much faster rate than most dsDNA viruses, and is comparable with the evolutionary rate of RNA viruses. Interestingly, most of the polymorphisms in meq gene appear to have evolved under positive selection and the time of divergence at the meq locus coincides with the period during which the poultry industry had undergone transitions in management practices including the introduction and widespread use of live attenuated vaccines. Our study has revealed that the decades-long use of vaccines did not reduce MDV diversity, but rather had a stimulating effect on the emergence of field strains with increased genetic diversity until the early 2000s. During the years 2004-2005, there was an abrupt decline in the genetic diversity of field isolates followed by a recovery from this bottleneck in the year 2010. Collectively, these data suggest that vaccination seems to not have had any effect on MDV eradication, but rather had a stimulating effect on MDV emergence through adaptation.