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

First report on the molecular characterization and phylogenetic analysis of meq oncogene from virulent serotype-1 Marek's disease virus in laying hen flocks and broiler breeders in Algeria

Marek's disease (MD) is a highly contagious viral infection in poultry, caused by Gallid alphaherpesvirus 2 (GaHV-2), better known as serotype 1 of Marek's disease virus (MDV-1). It is one of the most concerning diseases in the poultry industry due to its significant economic impact. The disease mainly affects chickens, although other avian species can also be affected. The objective of this study was to examine the occurrence of Marek's disease virus (MDV) in fourteen poultry farms in Algeria through autopsy, histopathological analysis and molecular characterization of Marek's disease virus. This study represents one of the first investigations combining macroscopic, histopathological, and molecular analyses to assess the prevalence and genetic diversity of Marek's disease virus in Algerian poultry farms. A survey to assess the current occurrence of Marek's disease in poultry farms in Algeria was conducted from June to December 2024. Tissue samples were collected from 70 chickens, including 35 from broiler breeder farms and 35 from layer farms. Macroscopic and histopathological examinations were performed. Macroscopic pathological alterations, such as hepatomegaly, splenomegaly, renal hypertrophy, and sciatic nerve enlargement with follicular atresia and lymphomatous infiltration, were observed. Histopathological analyses revealed diffuse and multifocal whitish infiltration of the spleen, neoplastic infiltration of the liver, intrafollicular lymphoid infiltration in the bursa of Fabricius, and infiltration in the sciatic nerves. The molecular characterization of MDV through PCR amplification and sequencing of the meq gene provides novel insights into the circulating strains in Algeria. The samples were screened by PCR amplification of the meq gene. Additionally, we sequenced the meq gene from five samples per flock, and a phylogenetic tree was constructed using the deduced amino acid sequences of the meq gene. This study is the first to identify and classify MDV strains in Algerian broiler breeder and laying hen farms, revealing the presence of very virulent (vv) MDV pathotypes. Marek's disease remains a major veterinary and economic challenge in the poultry industry, requiring ongoing research to better understand its mechanisms, genetic variations, and control methods.

Onset and long-term duration of immunity provided by a single vaccination with recombinant a Marek's disease virus with REV-LTR insertion

Marek's Disease (MD), caused by Marek's disease virus (MDV), is a highly contagious lymphoproliferative disease in poultry. Despite the fact that MD has been effectively controlled by vaccines, the virulence of field isolates of MDV has continued to evolve, becoming more virulent under the immune pressure of vaccines. Our previous research has confirmed that the recombinant rMDV strain with REV-LTR insertion can be used as a live attenuated vaccine candidate. The aim of this research was to evaluate the onset and duration of immunity of the rMDV strain through two experiments. In both experiments, 1-day-old SPF chickens were vaccinated subcutaneously with the rMDV strain at a dose of 3,000 Plaque Formation Unit (PFU) per chick in 0.2 mL of the MD diluent. Then, in Experimental design 1, the chicks in the groups Vac-3d/CC-3d, Vac-5d/CC-5d, and Vac-7d/CC-7d were challenged separately with 500 PFU vvMDV strain MD5 at 3 days, 5 days, and 7 days after vaccination; in Experimental design 2, the chicks in group Vac-60d/CC-60d, Vac-120d/CC-120d, and Vac-180d/CC-180d were challenged at 60 days, 120 days, and 180 days after vaccination. The clinical symptoms and weight gain of chickens in each group were observed and recorded. The results showed that the rMDV strain with REV-LTR insertion provides protection starting from 3 days of age and achieves good immune effects at 5 days of age after 1-day-old immunization, and the immunization duration can reach for at least 180 days. Given age-related resistance, it can be confirmed that our vaccine can actually provide lifelong immunity. This study provides valuable insights into the onset and duration of immunity of the rMDV strain, which will provide a basis for the development and improvement of MD vaccines.

Application of lentinan in suppression of Marek's disease virus infection

Marek's disease virus (MDV) is an extremely widespread avian immunosuppressive virus. In recent years, many reports have shown that there are still cases of MDV infection and immunosuppression after immunization with the vaccine. Consequently, there is a need to develop alternative complementary approaches for strengthening the efficacy of MDV prevention and control measures. Lentinan (LNT) is a macromolecular compound with immune-enhancing activity extracted from shiitake mushrooms. To explore the value and effectiveness of administering LNT through drinking water in the prevention and control of MDV, this study first observed the effects of high and low doses of LNT on weight gain, organ development, viral replication, and antibody titer of an avian influenza virus subtype H9 (AIV-H9) inactivated vaccine in specific pathogen-free (SPF) chicks infected with the wild strain of MDV. The results showed that both high and low doses of LNT significantly alleviated the weight gain retardation and liver and spleen enlargement caused by MDV infection, and significantly inhibited the replication of MDV in SPF chicks (P < 0.05), compared with the MDV-positive control group, both high and low doses of LNT significantly increased the antibody titer of AIV-H9 after immunization with inactivated AIV-H9 vaccine (P < 0.0001). On this basis, we also observed the effects of a chicken Marek's disease meq gene deletion live vaccine (SC9-1 strain), administered alone or in combination with LNT, on MDV infections of varying virulence in Hy-Line Brown chicks. The results showed that combined administration of LNT and the SC9-1 vaccine resulted in a significant alleviation of weight gain retardation and liver and spleen enlargement due to MDV infection (P < 0.05), as well as a significant inhibition of MDV replication and release in Hy-Line Brown chicks compared to the vaccine alone (P < 0.0001). These findings suggest that LNT not only alleviates the adverse effects of MDV infection in chickens but also enhances the efficacy of MDV vaccination, offering a potential auxiliary measure for controlling MDV infection.

Immunopotentiating effect of lentinan on chicks and its inhibitory effect on Marek's disease virus infection

Marek's disease virus (MDV) is a significant tumorigenic virus that causes severe immunosuppression in chickens. Lentinan (LNT) is an immunomodulator containing β-glucans and is widely used in areas such as antiviral, anticancer, and immune regulation. To investigate the immunomodulatory effects of LNT on specific pathogen-free (SPF) chicks and its potential to inhibit MDV infection, we conducted an MDV challenge experiment and observed the immune-enhancing effect of LNT on SPF chicks. The results showed that LNT promoted the growth and development of SPF chicks and induced the upregulation of cytokines such as Mx protein, interferon-γ (INF-γ), tumor necrosis factor-α (TNF-α), and interleukin-2 (IL-2). The specific gravity of CD4+ T-lymphocytes and CD8+ T-lymphocytes and their ratios were also significantly upregulated. Prophylactic use of LNT inhibited MDV replication in lymphocytes, liver, and spleen. It also alleviated MDV-induced weight loss and hepatosplenomegaly in SPF chicks. The present study confirms that LNT can enhance the levels of innate and cellular immunity in SPF chicks and contributes to the inhibition of MDV replication in vivo and mitigation of immune organ damage in chicks due to MDV infection. This provides an adjunctive measure for better control of MDV infection.

Immune escape of avian oncogenic Marek's disease herpesvirus and antagonistic host immune responses

Marek's disease virus (MDV) is a highly pathogenic and oncogenic alpha herpesvirus that causes Marek's disease (MD), which is one of the most important immunosuppressive and rapid-onset neoplastic diseases in poultry. The onset of MD lymphomas and other clinical diseases can be efficiently prevented by vaccination; these vaccines are heralded as the first demonstration of a successful vaccination strategy against a cancer. However, the persistent evolution of epidemic MDV strains towards greater virulence has recently resulted in frequent outbreaks of MD in vaccinated chicken flocks worldwide. Herein, we provide an overall review focusing on the discovery and identification of the strategies by which MDV evades host immunity and attacks the immune system. We have also highlighted the decrease in the immune efficacy of current MD vaccines. The prospects, strategies and new techniques for the development of efficient MD vaccines, together with the possibilities of antiviral therapy in MD, are also discussed.

Differences in Pathogenicity and Vaccine Resistance Discovered between Two Epidemic Strains of Marek's Disease Virus in China

Despite highly effective vaccines, Marek's disease (MD) causes great economic loss to the poultry industry annually, largely due to the continuous emergence of new MD virus (MDV) strains. To explore the pathogenic characteristics of newly emerged MDV strains, we selected two strains (AH/1807 and DH/18) with clinically different pathotypes. We studied each strain's infection process and pathogenicity and observed differences in immunosuppression and vaccine resistance. Specific pathogen-free chickens, unvaccinated or vaccinated with CVI988, were challenged with AH/1807 or DH/18. Both infections induced MD damage; however, differences were observed in terms of mortality (AH/1807: 77.8%, DH/18: 50%) and tumor rates (AH/1807: 50%, DH/18: 33.3%). The immune protection indices of the vaccine also differed (AH/1807: 94.1, DH/18: 61.1). Additionally, while both strains caused interferon-β and interferon-γ expression to decline, DH/18 infection caused stronger immunosuppression than AH/1807. This inhibition persisted even after vaccination, leading to increased replication of DH/18 that ultimately broke through vaccine immune protection. These results indicate that both strains have different characteristics, and that strains such as DH/18, which cause weaker pathogenic damage but can break through vaccine immune protection, require further attention. Our findings increase the understanding of the differences between epidemic strains and factors underlying MD vaccination failure in China.

Pathogenicity and Pathotype Analysis of Henan Isolates of Marek’s Disease Virus Reveal Long-Term Circulation of Highly Virulent MDV Variant in China

In recent years, outbreaks of Marek’s disease (MD) have been frequently reported in vaccinated chicken flocks in China. Herein, we have demonstrated that four Marek’s disease virus (MDV) isolates, HN502, HN302, HN304, and HN101, are all pathogenic and oncogenic to hosts. Outstandingly, the HN302 strain induced 100% MD incidence, 54.84% mortality, and 87.10% tumor incidence, together with extensive atrophy of immune organs. Pathotyping of HN302 was performed in comparison to a standard very virulent (vv) MDV strain Md5. We found that both CVI988 and HVT vaccines significantly reduced morbidity and mortality induced by HN302 or Md5 strains, but the protection indices (PIs) provided by these two vaccines against HN302 were significantly lower (27.03%) or lower (33.33%) than that against Md5, which showed PIs of 59.89% and 54.29%, respectively. These data suggested that HN302 possesses a significant higher virulence than Md5 and at least could be designated as a vvMDV strain. Together with our previous phylogenetic analysis on MDV-1 meq genes, we have presently suggested HN302 to be a typical highly virulent MDV variant belonging to an independent Chinese branch. To our knowledge, this is the first report to provide convincible evidence to identify a pathogenic MDV variant strain with a higher virulence than Md5 in China, which may have emerged and circulating in poultry farms in China for a long time and involved in the recent MD outbreaks.