Marek's disease in turkeys. I. A seven-year survey of commercial flocks and experimental infection using two field isolates

First Report of Marek's Disease Virus in Commercial Turkeys in Slovenia

Marek's disease (MD), caused by Mardivirus gallidalpha 2 (GaAHV-2), also known as MD virus (MDV), is a lymphoproliferative disease that primarily affects chickens. Recently, MDV has been detected in lymphomatous tumors in turkeys in various countries. Between 2021 and 2023, three cases ranging from no to severe clinical disorders (depression, lameness, and increased mortality) occurred in commercial turkey flocks in Slovenia. In all cases, MDV was detected by PCR in DNA samples extracted from organs developing tumor infiltrations. Sequencing and phylogenetic analysis of the meq gene revealed that the GaAHV-2 detected has molecular features of a very virulent pathotype and genetic similarity with GaAHV-2 detected in chickens in Tunisia. This is the first report of MDV in commercial turkeys in Slovenia.

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.

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.

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

Marek's disease (MD) is a lymphoproliferative disease caused by Gallid alphaherpesvirus 2 (GaHV-2), which primarily affects chickens. However, the virus is also able to induce tumours in turkeys, albeit less frequently than in chickens. This study reports the molecular characterization of a GaHV-2 strain detected in a flock of Italian meat-type turkeys exhibiting visceral lymphomas. Sequencing and phylogenetic analysis of the meq gene revealed that the turkey GaHV-2 has molecular features of high virulence and genetic similarity with GaHV-2 strains recently detected in Italian commercial and backyard chickens. GaHV-2 is ubiquitous among chickens despite vaccination, and chicken-to-turkey transmission is hypothesized due to the presence of broilers in neighbouring pens.RESEARCH HIGHLIGHTS A GaHV-2 strain from Italian turkeys was molecularly characterized.The turkey strain presented molecular characteristics of high virulence in its meq gene.The turkey strain was closely related to previously detected chicken strains.

Marek's disease virus associated ocular lymphoma in Roulroul partridges (Rollulus rouloul)

Two 1-year old Roulroul partridges (Rollulus rouloul), one male and one female, were presented because of eye problems and anorexia. Twenty of the 30 Roulroul partridges in the owner's collection had already died. The affected birds stopped eating, became thinner, and eventually died. Antibiotic treatment, which started because of the suspicion of a septicaemic process, was unsuccessful. At clinical examination of the two partridges it was found that in both birds, one eye ball was filled with a whitish yellow amorphous material and the other eye ball of the female showed a distinct corneal opacity. Both presented birds were euthanized. Necropsy revealed no significant abnormalities in addition to the eye lesions. Histology and immunohistochemistry of the female's eye revealed an infiltrate of T-lymphocytes corresponding to ocular lymphoma. Herpesvirus genus-specific PCR, followed by Sanger sequencing confirmed the presumptive diagnosis of Marek's disease in both birds. To our knowledge, this is the first confirmed case of infection with Gallid Herpesvirus 2 (Marek's disease virus) in partridges and the first case in this specific species.

Single-nucleotide polymorphisms in two Marek's disease virus genes (Meq and gD): application to a retrospective molecular epidemiology study (1982–1999) in France

Marek's disease virus (MDV) is a herpesvirus that causes a lymphoproliferative disease in chickens. Vaccines against MDV are available, but the virus is gradually becoming more virulent. A molecular epidemiology study of MDV was carried out by assessing nucleotide variation in two different genes, Meq and gD, in 68 French field isolates circulating from 1982 to 1999, compared with reference strains. Viral DNA was amplified by nested PCR and sequenced directly. Comparison of the nucleotide sequences revealed a high nucleotide sequence identity (98 %). Single-nucleotide polymorphisms were identified, leading to the identification of three gene alleles for gD and six for Meq. Nine combinations of alleles were identified. A majority of French isolates (60·5 %) clustered in the C1 type, which has been present for over 17 years. Waves of non-C1-type isolates appeared when vaccine efficacy decreased. Furthermore, specific discriminating sequences were obtained for the CVI-988 vaccine strain.