Genetic evolution of Gallid herpesvirus 2 isolated in China

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.

Detection of a Novel Alphaherpesvirus and Avihepadnavirus in a Plantar Papilloma from a Rainbow Lorikeet (Trichoglosis moluccanus)

Cutaneous plantar papillomas are a relatively common lesion of wild psittacine birds in Australia. Next-generation sequencing technology was used to investigate the potential aetiologic agent(s) for a plantar cutaneous papilloma in a wild rainbow lorikeet (Trichoglosis moluccanus). In the DNA from this lesion, two novel viral sequences were detected. The first was the partial sequence of a herpesvirus with the proposed name, psittacid alphaherpesvirus 6, from the Mardivirus genus of the family alphaherpesviruses. This represents the first mardivirus to be detected in a psittacine bird, the first mardivirus to be detected in a wild bird in Australia, and the second mardivirus to be found in a biopsy of an avian cutaneous papilloma. The second virus sequence was a complete sequence of a hepadnavirus, proposed as parrot hepatitis B genotype H (PHBV-H). PHBV-H is the first hepadnavirus to be detected in a wild psittacine bird in Australia. Whether other similar viruses are circulating in wild birds in Australia and whether either of these viruses play a role in the development of the plantar papilloma will require testing of biopsies from similar lesions and normal skin from other wild psittacine birds.

Emerging Hypervirulent Marek's Disease Virus Variants Significantly Overcome Protection Conferred by Commercial Vaccines

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.

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.

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.

Analysis of the evolution and transmission dynamics of the field MDV in China during the years 1995-2020, indicating the emergence of a unique cluster with the molecular characteristics of vv+ MDV that has become endemic in southern China

Marek's disease (MD) continues to threaten the sustainability of the world poultry industry. In this study, the sequences of the meq gene of 220 MDV strains isolated during the years 1964-2020 were analysed, including 50 from our group plus 170 isolates from the GenBank. Analyses, using phylogenetic trees, amino acid (aa)-mutation screening, evolutionary studies and transmission dynamics were all performed. All strains were divided into two clusters (Clusters 1 and 2), and Cluster 1 includes the mild strains, the vaccine strains and the foreign virulent strains, while Cluster 2 was dominated by the Chinese field strains. Our study identified that the Chinese field strains in Cluster 2 during the years 1995-2020 likely originated in the 1980s from abroad, and the estimated genetic diversity of these strains experienced two growth phases in the years 2005-2007.5 and 2015-2017. Viral phylogeography identified 3 major geographic provincial regions for the Chinese field strains of Cluster 2: the Northeastern Region (Jilin, Liaoning and Heilongjiang), the East-central Region (Henan, Shandong and Jiangsu) and the Southern Region (Guangxi, Guangdong and Yunnan). The spread of Northeastern strains to East-central chicken flocks and the further spread from Guangxi to Guangdong are strongly indicated. The emergence of the mutations A88T and Q93R together in the Southern strains during the years 2017-2020 with molecular characteristics of vv+ MDV were also found later than those in the Northern strains. Overall, the Chinese field strains in Cluster 2 in southern China in recent years have been rapidly evolving. Guangxi Province has become an epicentre for these viruses and the chicken flocks in the Southern region have been facing the adverse effects of the emerging vv+ MDV.

Genetic characterization of a Marek's disease virus strain isolated in Japan

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.

Genomic analysis of a Chinese MDV strain derived from vaccine strain CVI988 through recombination

Disease caused by Marek's disease virus (MDV), a highly oncogenic alpha-herpesvirus, is controlled mainly by vaccination. Since 1990s, CVI988 has been widely used as vaccine strain. However, as an attenuated live vaccine, CVI988 has the potential of virulence enhancement and the risk of recombination that should be considered. In this study, we sequenced the whole genome of a Chinese strain HNLC503 and found the close relationship between HNLC503 and CVI988. Further study indicated that HNLC503 had undergone recombination in US region, the same position as that previously occurred in Eurasian strains isolated from 2010 to 2014. By comparing ORFs, it was found that non-synonymous mutations were introduced in US2, US3, SORF4 and gD genes by recombination, while natural mutations occurred in RLORF1, vIL-8, UL36, VP22 and gE, in HNLC503. In summary, our study revealed the phenomenon of MDV vaccine strain recombination, warning that vaccine strains have the potential to enhance virulence through recombination.

Natural outbreak of Marek's disease in indigenous chicken and Japanese quail (Coturnix coturnix japonica) in Jos, Plateau State, Nigeria

Carcasses of an indigenous adult chicken and Japanese quail from different flocks were presented to a veterinary clinic for postmortem (PM) examination in 2014 in Jos, Plateau State, Nigeria. PM observations revealed cutaneous, hepatic, and splenic tumors in the Indigenous chicken. The quail carcass was emaciated with hepatic tumors. Histopathology revealed severe focally extensive non-encapsulated circumscribed large nodules with pleomorphic population of cells mainly composed of lymphoplasmacytic and mixed neutrophilic polymorphonuclear cells in the chicken. The pleomorphic infiltration of lymphohistioplasmacytic cells mixed with neutrophilic polymorphonuclear cells in the quail was consistent with Marek’s disease virus (MDV) infection. Polymerase chain reaction (PCR) was carried out, and the Meq oncogene of the MDV was amplified in the samples collected from the chicken and quail to confirm the presence of the virulent MDV. The samples were also subjected to PCR for detection of MDV Rispens CVI988 vaccine strain which was detected in both chicken and quail samples. The findings in this study represent the first report of confirmatory diagnosis of MD using histopathology in an indigenous chicken and Japanese quail in Nigeria. It is also the first report of the detection of MDV Rispens CVI988 vaccine strain in unvaccinated chicken and quail in Nigeria.

The Transcriptional Landscape of Marek's Disease Virus in Primary Chicken B Cells Reveals Novel Splice Variants and Genes

Marek's disease virus (MDV) is an oncogenic alphaherpesvirus that infects chickens and poses a serious threat to poultry health. In infected animals, MDV efficiently replicates in B cells in various lymphoid organs. Despite many years of research, the viral transcriptome in primary target cells of MDV remained unknown. In this study, we uncovered the transcriptional landscape of the very virulent RB1B strain and the attenuated CVI988/Rispens vaccine strain in primary chicken B cells using high-throughput RNA-sequencing. Our data confirmed the expression of known genes, but also identified a novel spliced MDV gene in the unique short region of the genome. Furthermore, de novo transcriptome assembly revealed extensive splicing of viral genes resulting in coding and non-coding RNA transcripts. A novel splicing isoform of MDV UL15 could also be confirmed by mass spectrometry and RT-PCR. In addition, we could demonstrate that the associated transcriptional motifs are highly conserved and closely resembled those of the host transcriptional machinery. Taken together, our data allow a comprehensive re-annotation of the MDV genome with novel genes and splice variants that could be targeted in further research on MDV replication and tumorigenesis.

Phylogenetic and molecular epidemiological studies reveal evidence of recombination among Marek's disease viruses

Marek's disease has brought enormous loss in chicken production worldwide and the increasing virulence of Marek's disease virus (MDV) became a severe problem. To better understand the genetic basis underlying, a Chinese MDV strain HNGS101 isolated from immunized chickens was sequenced. Phylogenetic analysis implied that HNGS101 showed more relatedness to Eurasian strains than GaHV-2 circulating in North America. Recombination networks analysis showed the evidence of recombination among MDV strains, and several recombination events in the UL and US region were found. Further analysis indicated that the HNGS101 strain seemed to be generated by the recombination of the earliest Eurasian strains and North American strains in the US region, which may be responsible for the MD outbreaks in China. In summary, this study demonstrates recombination events among MDV strains [corrected], which may shed light on the mechanism of virulence enhancement.

Recombinant Gallid herpesvirus 2 with interrupted meq genes confers safe and efficacious protection against virulent field strains

Gallid herpesvirus 2 (GaHV-2) continuously evolves, which reduces the effectiveness of existing vaccines. To construct new GaHV-2 candidate vaccines, LMS, which is a virulent GaHV-2 field strain isolated from diseased chicken flocks in Southwest China in 2007, was modified such that both copies of its meq oncogene were partially deleted. The resulting virus, i.e., rMSΔmeq, was characterized using PCR and sequencing. To evaluate the safety and protective efficacy of rMSΔmeq, specific pathogen-free (SPF) chickens were inoculated with 2000 plaque forming units (pfu) and 20,000pfu of rMSΔmeq immediately after hatching. All birds grew well during the experimental period, and none of the challenged chickens developed Marek's disease-associated lymphoma. In addition, the rMSΔmeq- and CVI988/Rispens-vaccinated SPF chickens were challenged with 1000 pfu and 5000 pfu of the representative virulent GaHV-2 Md5 strain and 1000 pfu of the variant GaHV-2 strains LCC or LTS. The results showed that the rMSΔmeq strain provided complete protection, which was similar to that provided by the CVI988/Rispens vaccine (protective index (PI) of 95.5) when challenged with a conventional dose of the Md5 strain. However, rMSΔmeq provided a PI of 90.9 when challenged with 5000 pfu of the Md5 strain, which was significantly higher than that provided by the CVI988/Rispens vaccine (54.5). rMSΔmeq provided a PI of 86.4 against LCC, which was equal to that provided by the CVI988/Rispens vaccine (81.8). In addition, rMSΔmeq provided a PI of 100 against LTS, which was significantly higher than that provided by the CVI988/Rispens vaccine (68.2). Altogether, the rMSΔmeq virus provided efficient protection against representative and variant GaHV-2 strains. In conclusion, the rMSΔmeq virus is a safe and effective vaccine candidate for the prevention of Marek's disease and is effective against the Chinese variant GaHV-2 strains.

The first whole genome sequence and pathogenicity characterization of a fowl adenovirus 4 isolated from ducks associated with inclusion body hepatitis and hydropericardium syndrome

In June 2015, an infectious disease with high prevalence causing severe hydropericardium syndrome (HPS) first appeared in duck farms of northeast China. The disease showed high morbidity of 35% and mortality of 15% in a commercial duck farm with 200,000 45-day-old ducks. One strain of hypervirulent fowl adenovirus serotype 4 was identified and designated as HLJDAd15. The whole genome of the duck isolate was sequenced and found to contain the same large deletions as a genotype that has become prevalent in chickens in China recently, indicating that this disease might be transmitted from chickens to ducks. The pathogenicity of HLJDAd15 was evaluated in SPF chickens and ducks. The results showed that chickens were more susceptible to this new genotype of fowl adenovirus, and it was more difficult to infect ducks than chickens with the duck origin virus. Thus, it appears that this severe HPS in ducks is far more likely to have been transmitted from chickens to ducks than from ducks to ducks. Therefore, transmission from chickens to ducks constitutes a threat to the duck farming industry, and this transmission route is a very important consideration for the prevention and control of the new genotype of fowl adenovirus. This is the first whole genome sequence of a FAdV-4 isolated from ducks, and this information is important for understanding the molecular characteristics and evolution of aviadenoviruses. The potential risks of infection with this new hypervirulent FAdV-4 genotype in chickens and ducks urgently require an effective vaccine.

Different Dynamic Distribution in Chickens and Ducks of the Hypervirulent, Novel Genotype Fowl Adenovirus Serotype 4 Recently Emerged in China

A hypervirulent fowl adenovirus serotype 4 (FAdV-4) has caused hepatitis-hydropericardium syndrome (HHS) with mortalities that range from 30 to 80% in outbreaks across China since 2015. The FAdV-4 strain was characterized as a novel genotype based on the specific genome characteristics. However, our understanding of the dynamic distribution, tissue tropism, and pathogenesis of the novel FAdV-4 is incomplete. In this study, a new, sensitive and FAdV-4-specific real-time PCR was developed and applied to detect the dynamic distribution of the duck origin, novel FAdV-4 strain HLJDAd15 in experimentally infected special-pathogen free (SPF) chickens and ducks. Notably, the pathogenicity and replication pattern of HLJDAd15 were completely different between chickens and ducks. Severe hydropericardium and 10% mortality were induced in chickens, whereas no clinical signs were observed in any duck. The virus replicated was detected throughout the study in both chickens and ducks. However, only one replication peak with a high virus concentration appeared in chickens at 5 days post infection (dpi), whereas two peaks with relatively low virus titres appeared in ducks at 7 and 21 dpi. Thus, ducks could be a natural reservoir of the novel FAdV-4 absent of clinical signs, and a new transmission route from ducks shedding FAdV-4 continually to chickens was revealed, which might aggravate the outbreak of HHS in chickens. This study provides the first accurate quantitative data for the replication kinetics of the novel FAdV-4 in different hosts. The different pathogenicity, dynamic distribution and replication pattern in chickens and ducks provide a foundation for further clarification of the pathogenesis of the novel FAdV-4.

Characterization of a Gallid herpesvirus 2 strain with novel reticuloendotheliosis virus long terminal repeat inserts

A bacterial artificial chromosome clone, designated LCY, was constructed from a Gallid herpesvirus 2 (GaHV-2) isolate from a GaHV-2 and reticuloendotheliosis virus co-infected clinical sample. The LCY GaHV-2 insert was sequenced and found to consist of 175,319 nucleotides. LCY GaHV-2 open reading frames (ORFs) had a high sequence identity to those of reference strains. The major difference was that two REV long terminal repeats (LTRs), in the same direction, were inserted at the internal repeat short (IRs)/unique short (Us) and Us/terminal repeat short (TRs) junctions. In addition, the a-like sequence and UL36 were different from other strains. Phylogenetic analysis revealed that LCY was closely related to pandemic strains in China. A pathogenicity study and a vaccination-challenge test were performed on LCY and the reference strain, GA. The results showed that LCY induced gross Marek's disease (MD) lesions and mortality in 71.4 and 7.1% of chickens, respectively, which are lower rates than those observed for the reference strain GA (85.7 and 35.7%). The commercially available CVI988 vaccine provided complete protection against LCY and GA (100%). These results showed that the isolate exhibited lower pathogenicity in SPF chickens. This study revealed that a novel pattern of LTR inserts was found in the strain LCY and that the strain was of low virulence. The present work expands the available genetic information for GaHV-2 and will be useful for the control of MD in China.