Molecular characterization of a Marek's disease virus strain detected in tumour-bearing turkeys

Molecular characterization of the meq oncogene of Marek's disease virus in vaccinated Brazilian poultry farms reveals selective pressure on prevalent strains

Marek's disease virus (MDV) has become an increasingly virulent pathogen in the poultry industry despite vaccination efforts to control it. Brazil has experienced a significant rise of Marek's disease (MD) outbreaks in recent years. Our study aimed to analyze the complete meq gene sequences to understand the molecular epidemiological basis of MD outbreaks in Brazilian vaccinated layer farms. We detected a high incidence rate of visceral MD (67.74%) and multiple circulating MDV strains. The most prevalent and geographically widespread genotype presented several clinical and molecular characteristics of a highly virulent strain and evolving under positive selective pressure. Phylogenetic and phylogeographic analysis revealed a closer relationship with strains from the USA and Japan. This study sheds light on the circulation of MDV strains capable of infecting vaccinated birds. We emphasize the urgency of adopting preventive measures to manage MDV outbreaks threatening the poultry farming industry.

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.

The genome evolution of Marek's disease viruses in chickens and turkeys in China

The virus that causes Marek's disease (MD) is globally ubiquitous in chickens, continuously evolving, and poses a significant threat to the poultry industry. Although vaccines are extensively used, MD still occurs frequently and the virus has evolved increased virulence in China. Here, we report an outbreak of MD in vaccinated chickens and unvaccinated turkeys in a backyard farm in Guangdong province, China, in 2018. Phylogenetic analysis revealed two lineages of MDVs at this farm, with one lineage, containing isolates from two turkeys and five chickens, clustering with virulent Chinese strains and displays a relatively high genetic divergence from the vaccine strains. These new isolates appear to have broken through vaccine immunity, yielding this outbreak of MD in chickens and turkeys. The second lineage included four chicken isolates that clustered with the CVI988 and 814 vaccine strains. The large diversity of MDVs in this single outbreak reveals a complex circulation of MDVs in China. Poor breeding conditions and the weak application of disease prevention and control measures make backyard farms a hotbed for the evolution of viruses that cause infectious diseases. This is especially important in MDV as the MD vaccines do not provide sterilizing immunity, which allows the replication and shedding of virulent field viruses by vaccinated individuals and supporting the continuous evolution of MDVs. Hence, constant monitoring of the evolution of MDVs is necessary to understand the evolution of these field viruses and potential expansions of their host range.

First report of Serotype-1 Marek's disease virus (MDV-1) with oncogenic form in backyard turkeys in Turkey: a molecular analysis study

Background

Marek’s disease (MD) is a lymphoproliferative disease caused by Gallid alphaherpesvirus 2 (GaHV-2, MDV-1), which primarily affects chickens. However, the virus is also able to induce tumors and polyneuritis in turkeys, albeit less frequently than in chickens.

Results

This is the first study in Turkey reporting the molecular characterization of a MDV-1 strain detected in a flock of backyard turkeys exhibiting visceral lymphoma. Here, MEQ, vIL-8, pp38 and 132-bp tandem repeat regions, which are frequently preferred in the pathotyping of MDV-1, were examined. It was determined that the MEQ gene of MDV-1/TR-21/turkey strain obtained in the present study encoded 339 amino acids (1020 nt) and had four proline-rich repeat regions (PPPP). Based on the nucleotide sequence of the MEQ gene of the MDV-1/TR-21/turkey strain, a phylogenetic tree was created using the MEGA-X software with the Maximum Likelihood Method (in 1000 replicates). Our strain was highly identical (> 99.8) to the Italian/Ck/625/16, Polish (Polen5) and some Turkish (Layer-GaHV-2-02-TR-2017, Tr/MDV-1/19) MDV-1 strains. Also, nt and aa sequences of the MEQ gene of our strain were 99.1 and 99.41% identical to another Turkish strain (MDV/Tur/2019) originated from chickens. Sequence analysis of pp38 and vIL-8 genes also supported the above finding. The identity ratios of nucleotide and amino acid sequences of vIL-8 and pp38 genes of MDV-1/TR-21/turkey strain were 99.64–100% and 99.79–100%, respectively, when compared with those of the Polish strain. According to 132-bp tandem repeat PCR results, the MDV-1/TR-21/turkey strain had five copies.

Conclusions

These results suggested that the MDV-1/TR-21/turkey strain obtained from backyard turkeys can be either very virulent or very virulent plus pathotype, though experimental inoculation is required for precise pathotyping.

Gallid Alphaherpesvirus 2 in the Egyptian Turkeys: Molecular Characterization and Establishment of a Universal System for Phylogenetic Classification

INTRODUCTION

Gallid alphaherpesvirus 2 (GaHV-2) is a highly contagious oncogenic virus that causes Marek's disease in chickens and occasionally in turkeys. Among 100 genes identified in GaHV-2 genome, the Meq gene appears to involve viral virulence, oncogenicity, and genetic diversity. Despite the use of Meq gene sequences in phylogenetic classification of GaHV-2 strains circulating in many countries worldwide, no integrated system exists yet.

METHODS

Turkeys from 2 commercial Egyptian farms were presented with signs of dullness, dehydration, and emaciation. Samples prepared from the internal organs were examined by histopathology and immunohistochemistry. Pools of the internal organs were analyzed by PCR for identification of GaHV-2, avian leucosis virus, and reticuloendotheliosis virus. The Meq gene of an Egyptian strain was sequenced and analyzed in comparison to 40 reference strains for generation of a universal system for phylogenetic classification of GaHV-2 strains.

RESULTS

Gross and histopathological examination revealed grayish-white soft masses in the internal organs characterized by diffuse infiltration of pleomorphic neoplastic cells. All lymphoma cells were identified as T-lymphocytes of CD3+ phenotype. Samples of both farms were only positive for GaHV-2 by PCR. Sequence analysis of the Meq gene has classified the current turkey strain as related to the Egyptian strains identified in chicken in 2012. A universal phylogenetic system for classification of GaHV-2 strains into 4 clusters was proposed. The vaccine strains were all grouped in cluster 2, and most of the classical American strains belonged to cluster 4. Cluster 1 was further divided into 3 subclusters (1.1-1.3).

CONCLUSION

GaHV-2 was identified in turkeys for the first time in Africa and the Middle East. Sequence analysis of the Meq gene of the Egyptian strain along with a wide array of the global strains has enabled the construction of a novel phylogenetic classification system.

Out of Sight, but Not Out of Mind: Aspects of the Avian Oncogenic Herpesvirus, Marek's Disease Virus

Marek's disease virus is an economically important avian herpesvirus that causes tumors and immunosuppression in chickens and turkeys. The virus, disease, and vaccines have been known for more than 50 years, but as knowledge gaps still exists, intensive research is still ongoing. The understanding of MDV complexity can provide scientific insight in topics that cannot be experimented in humans, providing a unique model that is dually useful for the benefit of the poultry industry and for studying general herpesvirology. The present review presents the following topics: the MDV biology, the vaccine's and virulent virus' peculiar presence in feathers, protection by vaccination. In addition, two relatively behind the scenes topics are reviewed; first, the meq MDV oncogene and its recent implication in molecular epidemiology and in the MDV virulence determination, and second, the functionality of conformational epitopes of the MDV immunodominant protein, glycoprotein B. Our studies were particular, as they were the only ones describing three-dimensional MDV gB oligomers. MDV gB (glycoprotein B) continuous and discontinuous epitopes were shown to possess distinctive neutralization activities. In contrast, the significance of oligomerization of the viral membrane proteins for the creation of discontinuous epitopes in other herpesviruses was explored extensively.