GENETIC AND BIOLOGICAL DETERMINANTS OF AVIAN TUMOR VIRUS PATHOGENICITY, TRANSMISSION, AND EVOLUTION
Location: Avian Disease and Oncology Laboratory
Title: Properties of a meq-deleted rMd5 Marek’s disease vaccine: protection against virulent MDV challenge and induction of lymphoid organ atrophy are simultaneously attenuated by serial passage in vitro
Submitted to: Avian Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 12, 2012
Publication Date: N/A
Interpretive Summary: Marek’s disease (MD), a virus-induced cancer-like disease of chickens, is a major disease problem in commercial poultry. The objective of this research was to compare the efficacy of an attenuated genetically engineered MD vaccine termed rMd5 delta-Meq with that of the most effective available vaccine known as CVI988/Rispens in commercial chickens. We have previously shown that deletion of a cancer-inducing gene, Meq, from the very virulent strain of MD virus (MDV), rMd5 virus resulted in a virus that is no longer pathogenic in chickens, and confers 100% protection against challenge with a very virulent MDV. In collaboration with Hy-Line International, we conducted a field trail to test the efficacy of atenuated rMd5 delta-Meq vaccine. The results suggest that although attenuation of this rMd5 delta-Meq vaccine eliminated the negative effects namely virus-induced atrophy of two important lymphoid organs, bursa and thymus, it has no influence on their protective efficacy in maternal antibodies-negative (MAb-) chickens. However, in commercial chickens with MAb (MAb+), the best protection was provided by the original l9th passage and not by the attenmuated high passage vaccine, suggesting that lower passage level of this rMd5 delta-Meq may be useful in commercial chickens, as its protective efficacy is very high and does not induce atrohpy of bursa and thymus in MAb+ chickens.
We have previously shown that deletion of Meq gene from the genome of Cosmid-cloned rMd5 strain of Marek’s disease virus resulted in loss of transformation and oncogenic capacity of the virus. The rMd5 delta-Meq (Meq null) virus has been shown to be an excellent vaccine in maternal antibody positive (MAb+) chickens when challenged with a very virulent plus (vv+) strain of MDV, 648A. The only drawback was that it retained its ability to induce bursa and thymus atrophy (BTA) like that of the parental rMd5 in maternal antibody negative (MAb-) chickens. We recently reported that the attenuated Meq null virus did not induce BTA at 40th cell culture passage onward. Its protective ability against challenge with vv+ MDV, strain 686 was similar to the original virus at 19th passage in MAb- chickens. In this study, we compared the same series of attenuated Meq null viruses in commercial chickens. In commercial chickens with MAb, the attenuated viruses quickly lost protection with increasing cell culture attenuation. These data suggest that although attenuation of these Meq null viruses eliminated BTA, it has no influence on their protective efficacy in MAb- chickens. However, in commercial chickens (MAb+), the best protection was provided by the original l9th passage, the attenuated 40th passage was as good as one of the currently commercial CVI988/Rispens vaccine and it does not induce BTA. Therefore, protection against virulent MDV challenge and induction of lymphoid organ atrophy are simultaneously attenuated by serial passage in vitro.