Optimizing protocols for monitoring in vivo replication of a novel chimeric Marek’s disease vaccine with an insertion of the long terminal repeat of reticuloendotheliosis virus in the CVI988 strain genome (CVI-LTR)

ABSTRACT

Monitoring Marek’s disease (MD) vaccination is routinely done by evaluating the load of MD vaccine in the feather pulp (FP) between 7 and 10 days of age. However, attempts in our laboratory to detect a novel CVI-LTR vaccine in the FP samples from commercial flocks failed. The objective of this study was to evaluate the most suitable tissue and age to monitor CVI-LTR vaccination. We used two different commercial CVI988 vaccines as controls. One hundred and sixty 1-day-old commercial brown layers were vaccinated with either CVI-LTR, CVI988-A, CVI988-B or remained unvaccinated. Samples of the spleen, thymus, and bursa were collected at 3, 4, 5, and 6 days of age and samples of FP were collected at 7 and 21 days for DNA isolation. Our results showed that CVI-LTR replicated earlier than CVI988 vaccines in the lymphoid organs but was not detected in the FP at either 7 or at 21 days of age. We also confirmed that either the spleen or thymus collected at 4–6 days was a suitable sample to monitor CVI-LTR vaccination in commercial flocks. Finally, we evaluated the load of oncogenic MDV DNA in five commercial flocks that were vaccinated with either CVI-LTR + rHVT or CVI988-A + rHVT. The load of oncogenic MDV DNA was evaluated at 21 days in the FP in 20 chickens per group. Our results demonstrated that CVI-LTR was more successful in reducing oncogenic MDV DNA at 21 days of age than the CVI988-A strain.

RESEARCH HIGHLIGHTS

• CVI-LTR replicates in the thymus and spleen earlier than CVI988.

• CVI-LTR replicates in lymphoid organs but it cannot be detected in feather pulp.

• CVI-LTR reduced the load of oncogenic MDV DNA more efficiently than CVI988.

KEYWORDS:

• Marek’s disease
• vaccines
• monitoring
• feather pulp
• lymphoid organs
• LTR

Acknowledgements

The authors thank Drs Abdul Rahman Bin Omar and Tan Sheau Wei at the Universiti Putra Malaysia and Dr Sheereen Zulkefly for providing critical data necessary for the publication of this work after the premature death of Dr Nik Faiz.

Animal experiments were conducted following the guidance and under the approval of the University Putra Malaysia Institutional Animal Care & Use Committee (UPM/IACUC/AUP-R032/2021).

Disclosure statement

No potential conflict of interest was reported by the author(s).