Molecular characterization and phylogenetic analysis of a virulent Marek's disease virus field strain in broiler chickens in Japan

Circulating hypervirulent Marek's disease viruses in vaccinated chicken flocks in Taiwan by genetic analysis of meq oncogene

Marek's disease (MD) is an important neoplastic disease caused by serotype 1 Marek's disease virus (MDV-1), which results in severe economic losses worldwide. Despite vaccination practices that have controlled the MD epidemic, current increasing MD-suspected cases indicate the persistent viral infections circulating among vaccinated chicken farms in many countries. However, the lack of available information about phylogeny and molecular characterization of circulating MDV-1 field strains in Taiwan reveals a potential risk in MD outbreaks. This study investigated the genetic characteristics of 18 MDV-1 strains obtained from 17 vaccinated chicken flocks in Taiwan between 2018 and 2020. Based on the sequences of the meq oncogene, the phylogenetic analysis demonstrated that the circulating Taiwanese MDV-1 field strains were predominantly in a single cluster that showed high similarity with strains from countries of the East Asian region. Because the strains were obtained from CVI988/Rispens vaccinated chicken flocks and the molecular characteristics of the Meq oncoprotein showed features like vvMDV and vv+MDV strains, the circulating Taiwanese MDV-1 field strains may have higher virulence compared with vvMDV pathotype. In conclusion, the data presented demonstrates the circulation of hypervirulent MDV-1 strains in Taiwan and highlights the importance of routine surveillance and precaution strategies in response to the emergence of enhanced virulent MDV-1.

Virulence evaluation of Israeli Marek's disease virus isolates from commercial poultry using their meq gene sequence

Fifty-seven Gallid alphaherpesvirus 2 (GaHV-2) isolates, collected during a 30-year period (1990-2019) from commercial poultry flocks affected by Marek's disease (MD), were molecularly characterised. The GaHV-2 meq gene was amplified and sequenced to evaluate the virus virulence, based on the number of PPPPs within the proline-rich repeats (PRRs) of its transactivation domain. The present illustration of virus virulence evaluation on a large scale of field virus isolates by molecular analysis exemplifies the practical benefit and usefulness of the molecular marker in commercial GaVH-2 isolates. The alternative assay of GaVH-2 virulence pathotyping is the classical Gold Standard ADOL method, which is difficult and impossible to employ on a large scale using the Specific Pathogen Free (SPF) chicks of the ADOL strains kept in isolators for two months. The phylogenetic analysis performed in the present study showed that the meq gene amino acid sequences of the 57 Israeli strains divide into 16 phylogenetic branches. The virulence evaluation was performed in comparison with 36 GaHV-2 prototype strains, previously characterised by the in vivo Gold Standard ADOL assay. The results obtained revealed that the GaHV-2 strains circulating in Israel have evolved into a higher virulence potential during the years, as the four-proline stretches number in the meq gene decreased over the investigated period, typically of very virulent virus prototypes. The present study supports the meq gene molecular markers for the assessment of field GaVH-2 strains virulence.

Molecular characterization and phylogenetic analysis of Marek's disease virus in chickens from Ogun State, Nigeria

Marek's disease (MD) is caused by oncogenic MD virus serotype 1 (MDV1) and is characterized by lymphoproliferative lesions resulting in high morbidity and mortality in chickens. Despite being ubiquitous on poultry farms, there is a dearth of information on its molecular characteristics in Nigeria. This study aimed at characterizing three virulence genes (Meq, pp38, and vIL-8) of MDV1 from chickens in Ogun state, Nigeria. Blood, feather quill, and tumour samples of chickens from different commercial poultry farms in Ogun State were pooled, spotted on 107 FTA cards, and screened for MDV1 by polymerase chain reaction (PCR). Phylogenetic analysis was carried out to compare Nigerian MDV1 Meq, pp38, and vIL-8 genes sequences with the published references. Thirteen samples were MDV1-positive and the Meq, as well as pp38, and vIL-8 genes from the different samples were 100% identical. The Meq genes contained 339 amino acids (aa) with three PPPP motifs in the transactivation domain and two interruptions of the PPPP motifs due to proline-to-arginine substitutions at positions 176 and 217 resulting in a 20.88% proline composition. Phylogenetic analysis revealed that the Meq gene clustered with strains from Egypt and very virulent ATE2539 strain from Hungary. Mutations were observed in the pp38 protein (at positions 107 and 109) and vIL-8 protein (at positions 4 and 31). Based on the molecular analysis of the three genes, the results indicate the presence of MDV1 with virulence signatures; therefore, further studies on in vivo pathotyping of Nigerian MDV1 from all states should be performed.RESEARCH HIGHLIGHTS Meq, pp38 and vIL-8 genes were 100% identical between Nigerian MDV strains.Proline content in Nigerian meq gene was 20.88% with two PPPP motifs interruptions.Meq, pp38 and vIL-8 genes of Nigerian MDV were similar to Egyptian and Indian strains.

Phenotypic Characterization of Recombinant Marek's Disease Virus in Live Birds Validates Polymorphisms Associated with Virulence

Marek's disease (MD) is a highly infectious lymphoproliferative disease in chickens with a significant economic impact. Mardivirus gallidalpha 2, also known as Marek's disease virus (MDV), is the causative pathogen and has been categorized based on its virulence rank into four pathotypes: mild (m), virulent (v), very virulent (vv), and very virulent plus (vv+). A prior comparative genomics study suggested that several single-nucleotide polymorphisms (SNPs) and genes in the MDV genome are associated with virulence, including nonsynonymous (ns) SNPs in eight open reading frames (ORF): UL22, UL36, UL37, UL41, UL43, R-LORF8, R-LORF7, and ICP4. To validate the contribution of these nsSNPs to virulence, the vv+MDV strain 686 genome was modified by replacing nucleotides with those observed in the vMDV strains. Pathogenicity studies indicated that these substitutions reduced the MD incidence and increased the survival of challenged birds. Furthermore, using the best-fit pathotyping method to rank the virulence, the modified vv+MDV 686 viruses resulted in a pathotype similar to the vvMDV Md5 strain. Thus, these results support our hypothesis that SNPs in one or more of these ORFs are associated with virulence but, as a group, are not sufficient to result in a vMDV pathotype, suggesting that there are additional variants in the MDV genome associated with virulence, which is not surprising given this complex phenotype and our previous finding of additional variants and SNPs associated with virulence.

Genetic characterization of Marek's disease virus in chickens in Thailand reveals a high genetic diversity of circulating strains

Marek's disease (MD) is a highly contagious lymphoproliferative disease of chickens caused by Gallid alphaherpesvirus 2, commonly known as serotype 1 Marek's disease virus (MDV-1). Despite widespread vaccination, MD-related cases have been frequently observed worldwide, including in Thailand. However, no information is available on the genetic characteristics of MDV-1 field strains circulating in chickens in Thailand. This study investigated the geographic distribution and genetic characteristics of MDV-1 field strains circulating in chickens in Thailand between 2013 and 2021 by analysing the Meq and pp38 genes. Out of a total of the 286 clinical samples obtained from 70 chicken farms located in major chicken raising areas of Thailand, 138 samples (48.25%) from 46 chicken farms (65.71%) tested positive for MDV-1 field strains. Results demonstrated that MDV-1 field strains were extensively distributed in major chicken raising areas. Phylogenetic analyses based on the Meq gene revealed that four clusters of MDV-1 circulated in chickens in Thailand between 2013 and 2021. Among these clusters, cluster 1 was the predominant cluster circulating in chickens in Thailand. Additionally, our findings based on molecular characteristics of Meq and pp38 gene/protein suggested that most of the Thai MDV-1 field strains were potentially highly virulent. In conclusion, our data demonstrated the circulation of different clusters of MDV-1 with virulence characteristics in chickens in Thailand, indicating high genetic diversity of MDV-1 in Thailand. This study highlights the importance of more effective vaccine development and routine MDV-1 surveillance for early detection and control of highly virulent MDV-1.

A Review on the Marek's Disease Outbreak and Its Virulence-Related meq Genovariation in Asia between 2011 and 2021

Simple Summary

Marek’s disease is continuously causing an economic loss in Asia, despite the wide use of vaccines in the last decade. This review aims at summarizing the outbreak, the virulence-related meq gene variation, and the pathological information of Marek’s disease in the last decade in Asia. We found that a total of 132 viral strains emerged in 12 countries with different meq sequences. Among the evidence we have collected, 12 strains found in China were vaccine-resistant, reaching a mortality rate of 30% and above. This evidence requires the related region in China to consider the renewal of its vaccination type; however, more studies regarding the vaccination efficiency in other Asian countries are recommended, as the current information is not enough. The visceral tumor is the most common pathological type (13 in 16 studies) in Asia, while it is possible that a neural type may exist. We suggest that farmers monitor the behavioral changes of chickens to identify this harmful disease at the early stage. The phylogenetic analysis shows interconnection between Middle Eastern, South Asian, and East Asian countries that are geologically connected—poultry trading managers should consider the potential of viral transmitting.

Abstract

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.

A highly pathogenic Marek's disease virus isolate from chickens immunized with a bivalent vaccine in China

Marek's disease virus (MDV) is an important oncogenic poultry pathogen that can generally be controlled by vaccination. However, MDV infections still occur occasionally on vaccinated farms, possibly due to genetic variation among MDV strains or management-related issues. In this study, a novel MDV strain, designated LZ1309, was isolated from a poultry flock that had been vaccinated with the HVT and CVI988 vaccine strains. Animal experiments showed that LZ1309 infection led to high morbidity (100%) and mortality (90%). Moreover, existing vaccines provided only partial protection against LZ1309, with protection rates of 68.4%, 85%, and 90% for HVT, CVI988, and HVT plus CVI988, respectively. This study demonstrates the presence of a more virulent strain of MDV in vaccinated chickens in China that poses a new potential threat to poultry farms. In future studies, the development of new treatment strategies should be of high priority.

Molecular detection and phylogenetic analysis of Marek's disease virus virulence-associated genes from vaccinated flocks in southern India reveals circulation of virulent MDV genotype

Marek's disease (MD) is a re-emerging viral disease of chickens and a serious economic threat to the poultry industry worldwide. Continuous surveillance with molecular investigation is essential to monitor the emergence of virulent Marek's disease virus (MDV) strains and to devise any appropriate vaccination strategy and implement bio-security programmes. In the present study, we investigated the cases of MD outbreaks in vaccinated poultry flocks. The MD outbreak was confirmed through necropsy (mainly visceral tumours), histopathology and viral gene specific PCR. The pathotypes of the field MDV strains were assessed by molecular analysis of three virulence-associated genes, meq, pp38 and vIL-8. The Meq sequence of the field strains analyzed in this study lacked the 59 aa unique to mild strains, indicating that they are potentially virulent strains. Mutation at position 71 and the presence of five proline rich repeats in the transactivation domain, both associated with virulence were observed in these strains; however, the signature sequences specific to very virulent plus strains were absent. Phylogenetic analysis of meq oncogene sequences revealed clustering of the field strains with North Indian strains and with a very virulent plus ATE 2539 strain from Hungary. Analyses of pp38 protein at positions 107 and 109 and vIL-8 protein at positions 4 and 31 showed signatures of virulence. Sequence and phylogenetic analysis of oncogene and virulence-associated genes of field MDVs from vaccinated flock indicated that these strains possessed molecular features of virulent strains.

Genetic evolution of Marek's disease virus in vaccinated poultry farms

Background and Aim:

The Marek’s disease virus (MDV) is a neoplastic disease causing serious economic losses in poultry production. This study aimed to investigate MDV occurrence in poultry flocks in the Lower Egypt during the 2020 breakout and genetically characterized Meq, gL, and ICP4 genes in field strains of MDV.

Materials and Methods:

Forty samples were collected from different breeds from eight Egyptian governorates in 2020. All flocks had received a bivalent vaccine (herpesvirus of turkey FC-126 + Rispens CVI988). However, weight loss, emaciation, reduced egg production, paralysis, and rough/raised feather follicles occurred. Samples were collected from feather follicles, liver, spleen, and nerve tissue for diagnosis by polymerase chain reaction. MDV genetic characterization was then performed by sequencing the Meq, gL, and ICP4 genes of five positive samples representing different governorates and breeds.

Results:

A total of 28 samples were positive for MDV field strains, while two were related to MDV vaccinal strains. All samples tested negative for ALV (A, B, C, D, and J) and REV. Phylogenetic analysis of the Meq gene of sequenced samples revealed that all MDVs were related to the highly virulent European viruses (Gallid herpesvirus 2 ATE and PC12/30) with high amino acid (A.A.) identity 99.2-100%. Alternatively, there was low A.A. identity with the vaccine strains CVI988 and 3004 (up to 82.5%). These results indicate that further investigation of the efficacy of current Egyptian vaccines is required. The Egyptian strains also harbor a specific mutation, allowing clustering into two subgroups (A and B). By mutation analysis of the Meq gene, the Egyptian viruses in our study had R101K, P217A, and E263D mutations present in all Egyptian viruses. Furthermore, R176A and T180A mutations specific to our strains contributed to the high virulence of highly virulent strains. There were no mutations of the gL or ICP4 genes.

Conclusion:

Further studies should evaluate the protection contributed by current vaccines used in Egypt.

Genetic variability of the Avian leukosis virus subgroup J gp85 gene in layer flocks in Lower Egypt

Aim

This study aimed to determine the prevalence of layer flock tumor disease in Lower Egypt during the period of 2018-2019 and to undertake molecular characterization and determine the genetic diversity of all identified viruses.

Materials and Methods

Forty samples were collected from layer chicken located in six governorates of Lower Egypt during the period of 2018-2019. Samples were taken from tumors in different organs. Tumor tissues were identified by histopathological sectioning and then further confirmed by a reverse-transcription polymerase chain reaction. Finally, genetic evolution of Avian leukosis virus (ALV-J) gp85 gene was studied.

Results

All the study samples were negative for Marek's disease virus, reticuloendotheliosis virus, ALV (A,B,C and D) and 20 samples were positive for ALV-J in backyard in six governrates. Sequencing of ALV-J gp85 gene was performed for six representative samples (one from each governorate), and they were found to be genetically related to prototype virus HPRS-1003 (identity percentage: 91.2-91.8%), but they were from a different group that was similar to the AF88-USA strain (first detected in 2000) with specific mutations, and they differed from a strain that was previously isolated in Egypt in 2005, forming two different subgroups (I and II) that had mutations in the hr1domain (V128F, R136A) and hr2 domain (S197G, E202K).

Conclusion

The ALV-J virus was the main cause of neoplastic disease in layer chickens from Lower Egypt in the period of 2018-2019. We found that the genetic evolution of ALV-J gp85 gene was related to prototype virus HPRS-1003 but in a different group with a specific mutation. Further studies are needed to evaluate the antigenicity and pathogenicity of recently detected ALV-J strains.

Natural co-infection with two virulent wild strains of Marek's disease virus in a commercial layer flock

Marek's disease (MD) is a highly contagious lymphoproliferative poultry disease caused by the oncogenic herpesvirus, Marek's disease virus (MDV). MDV strains have shown a continued evolution of virulence leading to immune failure, and MD cases continue to occur. Co-infection of virulent MDV strains is an important factor leading to viral evolution and host immune failure. This study conducted a laboratory diagnosis and analysis of a MDV infected flock. Testing showed that all samples were MDV positive. PCR detection identified a variable 132-base pair repeat (132-bpr) sequence copy number. This indicated that two virulent strains of MDV were co-infecting the flock. Therefore, we performed homology, sequence alignment, and phylogenetic tree analysis of MDV variant genes including meq, pp38, and RLORF4. Two MDV strains had co-infected the flock; one was the 132bpr two-copy characteristic strain (AH2C) and the other was a 132bpr three-copy characteristic strain (AH3C). Specific mutations in AH3C were found, suggesting that it is a new variant strain. Furthermore, the viral load of the two strains in vivo indicated that both strains had high and similar replication ability. There was no significant difference in the proportion of positive samples of the two strains causing disease. In the whole flock, neither strain displayed an obvious advantage. However, there was a dominant strain in individual chickens, with the exception of one sample. This study reported the co-infection regularity of two virulent MDV strains in the same flock, and even in the same chicken in field conditions. In the context of overall epidemiology, this study is a useful reference.

Marek's disease viruses circulating in commercial poultry in Italy in the years 2015-2018 are closely related by their meq gene phylogeny

Marek's disease (MD) is a lymphoproliferative disease important to the poultry industry worldwide; it is caused by Gallid alphaherpesvirus 2 (GaHV-2). The virulence of GaHV-2 isolates has shifted over the years from mild to virulent, very virulent and very virulent +. Nowadays the disease is controlled by vaccination, but field strains of increased virulence are emerging worldwide. Economic losses due to MD are mostly associated with its acute form, characterized by visceral lymphomas. The present study aimed to molecularly classify a group of 13 GaHV-2 strains detected in vaccinated Italian commercial chicken flocks during acute MD outbreaks, and to scrutinize the ability of predicting GaHV-2 virulence, according to the meq gene sequence. The full-length meq genes were amplified, and the obtained amino acid (aa) sequences were analysed, focusing mainly on the number of stretches of four proline molecules (PPPP) within the transactivation domain. Phylogenetic analysis was carried out with the Maximum Likelihood method using the obtained aa sequences, and the sequences of Italian strains detected in backyard flocks and of selected strains retrieved from GenBank. All the analysed strains showed 100% sequence identity in the meq gene, which encodes a Meq protein of 339 aa. The Meq protein includes four PPPP motifs in the transactivation domain and an interruption of a PPPP motif due to a proline-to-serine substitution at position 218. These features are typically encountered in highly virulent isolates. Phylogenetic analysis revealed that the analysed strains belonged to a cluster that includes high-virulence GaHV-2 strains detected in Italian backyard flocks and a hypervirulent Polish strain. Our results support the hypothesis that the virulence of field isolates can be suggested by meq aa sequence analysis.

Molecular characterization of the meq gene of Marek's disease viruses detected in unvaccinated backyard chickens reveals the circulation of low- and high-virulence strains

Marek's disease (MD) is an important lymphoproliferative disease of chickens, caused by Gallid alphaherpesvirus 2 (GaHV-2). Outbreaks are commonly reported in commercial flocks, but also in backyard chickens. Whereas the molecular characteristics of GaHV-2 strains from the commercial poultry sector have been reported, no recent data are available for the rural sector. To fill this gap, 19 GaHV-2 strains detected in 19 Italian backyard chicken flocks during suspected MD outbreaks were molecularly characterized through an analysis of the meq gene, the major GaHV-2 oncogene. The number of four consecutive prolines (PPPP) within the proline-rich repeats of the Meq transactivation domain, the proline content, and the presence of amino acid (aa) substitutions were determined. Phylogenetic analysis was performed using the Maximum Likelihood method. Sequence analysis revealed a heterogeneous population of GaHV-2 strains circulating in Italian backyard flocks. Seven strains, detected from birds affected by classical MD, showed a unique meq isoform of 418 aa with a very high number of PPPP motifs. Molecular and clinical features are suggestive of a low oncogenic potential of these strains. The remaining 12 strains, detected from flocks experiencing acute MD, transient paralysis, or sudden death, had shorter Meq protein isoforms (298 or 339 aa) with a lower number of PPPP motifs and point mutations interrupting PPPP. These features allow us to assert the high virulence of these strains. These findings reveal the circulation of low- and high-virulence GaHV-2 strains in the Italian rural sector.

Marek's Disease Virus Activates the PI3K/Akt Pathway Through Interaction of Its Protein Meq With the P85 Subunit of PI3K to Promote Viral Replication

It is known that viruses can active the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway in host cells to support cell survival and viral replication; however, the role of PI3K/Akt signaling in the pathogenic mechanisms induced by Marek’s disease virus (MDV) which causes a neoplastic Marek’s disease in poultry, remains unknown. In this study, we showed that MDV activated the PI3K/Akt pathway in chicken embryo fibroblasts (CEFs) at the early phase of infection, whereas treatment with a PI3K inhibitor LY294002 prior to MDV infection decreased viral replication and DNA synthesis. Flow cytometry analysis showed that inhibition of the PI3K/Akt pathway could significantly increase apoptosis in MDV-infected host cells, indicating that activation of PI3K/Akt signaling could facilitate viral replication through support of cell survival during infection. Evaluation of the underlying molecular mechanism by co-immunoprecipitation and laser confocal microscopy revealed that a viral protein Meq interacted with both p85α and p85β regulatory subunits of PI3K and could induce PI3K/Akt signaling in Meq-overexpressing chicken fibroblasts. Our results showed, for the first time, that MDV activated PI3K/Akt signaling in host cells through interaction of its Meq protein with the regulatory p85 subunit of PI3K to delay cell apoptosis and promote viral replication. This study provides clues for further studies of the molecular mechanisms underlying MDV infection and pathogenicity for the host.