[Protective immunity of a meq-deleted Marek's disease virus against very virulent virus challenge in chickens].

Objective: To evaluate the pathogenicity and immunoprotective effect of a wild strain of Marek's Disease Virus (MDV) after deleting of its meq oncogene.

Methods: Total 150 one-day-old SPF chickens were divided into 5 groups of 30 birds each and kept in 5 isolators with positive pressure-filtered air. At first day, 2000 PFU of meq-deleted GX0101 deltameq were inoculated intra-abdominally (i. a.) into each bird in groups 1 and 5, 2000 PFU of commercial vaccine CVI988/Rispens were injected i. a. for group 2. No viral challenge was made in groups 3 and 4 as controls. Five days later, chickens in groups 1, 2, 3 were challenged i. a. with very virulent (vv) MDV strain rMd5. During 90 days after challenge, all dead birds were recorded and checked for necropsy. The tumor-suspected tissues were examined by histo-sections. In the end, all survival birds were killed for necropsy. The samples of heart, liver and spleen were collected for histo-sections.

Results: Challenge with vv rMd5 caused 87% mortality and 25% of dead birds with gross tumors in non-vaccinated chickens of group 3 but there was no mortality in group 5 inoculated with GX0101 deltameq. Commercial vaccine CVI988/Rispens gave 89% protective index against vv rMd5 challenge and there were 9/42 histo-sections with suspected lymphocyte filtrated nodules in group 2. But vaccination with GX0101 deltameq provided 100% protective index against challenge with vv rMd5 and there was no tumor lesion even in histo-sections in group 1.

Conclusion: The meq-deleted mutant GX0101 deltameq is able to replicate in cell cultures stably, it has no pathogenicity and oncogenicity to chickens and is able to induce better protective immunity against vvMDV challenge than commercial vaccine CVI988/Rispens.