Comprehensive Report: Challenges in Marek’s Disease Vaccination

 

Dr. Majed Hemed Al Sayeg / poultry pathologist / Australia

 

Introduction

Marek’s Disease (MD) is a highly impactful viral disease in poultry, characterised by immunosuppression and tumor formation. It causes substantial global economic losses exceeding USD 1 billion annually. Despite widespread vaccine use, outbreaks in vaccinated flocks highlight emerging challenges, particularly with hyper virulent Marek’s Disease Virus (HV-MDV) strains.

 

Recent Findings on Hyper virulent Strains

A 2023 study by Liu et al. investigated seven new MDV strains isolated from tumor-bearing chickens in China. Four of these were identified as HV-MDV strains (SDCW01, HNXZ05, HNSQ05, and HNSQ01). Key findings include:

High Pathogenicity: MD incidence reached 100% in some cases, with significant tumor formation.

Severe Immunosuppression: Marked atrophy in the thymus and bursa, coupled with hepatosplenomegaly.

High Mortality: Over 80% in the most virulent strains.

 

Vaccine Efficacy Analysis

The study compared the efficacy of four commercial MD vaccines (CVI988, HVT, CVI988+HVT, and 814) against the HV-MDV strain SDCW01:

Protection Indices (PI): None of the vaccines provided optimal protection, with PIs ranging from 28% (814) to 50% (CVI988+HVT).

Tumor Incidence: Tumors occurred in birds vaccinated with CVI988 or HVT, with incidences of 7.7% and 11.5%, respectively.

Survival Outcomes: Vaccines improved survival rates but failed to completely prevent disease or infection.

 

Key Challenges

Increased Virulence: Long-term immune pressure from vaccines has driven the evolution of HV-MDV strains, which now breach vaccine-induced immunity.

Limited Efficacy of Classical Vaccines: Traditional vaccines, including CVI988 and HVT, are less effective against emerging HV-MDV variants.

Geographic and Strain Variability: Regional diversity in MDV strains contributes to vaccine failures.

Incomplete Virus Neutralisation: Current vaccines prevent clinical symptoms but do not block infection or viral shedding, promoting virus evolution.

 

Strategic Recommendations

1- Development of Novel Vaccines:

Meq Gene Deletion Vaccines: Vaccines based on the deletion of the oncogenic Meq gene (e.g., SC9-1) show promise for enhanced protection.

Recombinant Vaccines: Leveraging genetic engineering to target conserved viral antigens.

Strain-Specific Vaccines: Tailored vaccines to match local MDV isolates.

 

2- Enhanced Surveillance:

Regular genomic sequencing and phylogenetic analysis to monitor virulence trends. Epidemiological studies to identify and track outbreaks of HV-MDV strains.

 

3- Integrated Management Strategies:

Combining vaccination with robust biosecurity measures to limit virus spread.

Genetic selection for MD-resistant poultry lines.

Global Collaboration:

Establishing international research initiatives to address MDV evolution and share vaccine development strategies.

 

Conclusion

The emergence of HV-MDV strains poses a significant challenge to the poultry industry. While current vaccines have reduced mortality, their limited efficacy against evolving MDV strains underscores the need for innovative approaches. Developing next-generation vaccines, enhancing biosecurity, and adopting integrated management practices are critical for sustainable control of Ma