Serotype 1 viruses modified by backpassage or insertional mutagenesis: Approaching the threshold of vaccine efficacy in Marek's disease

Improved vaccines to control Marek's disease (MD) in chickens are desired by the poultry industry but have been difficult to develop. Studies were conducted to evaluate strategies for deriving MD vaccines of high protective efficacy, irrespective of virulence. Candidate viruses from parent strains representing v and vv+ pathotypes were modified by cell culture passage, backpassage in chickens, or insertional mutagenesis following cocultivation with retroviruses. Ten strains considered most likely to exhibit high protective efficacy were selected for further study. The ability of these modified viruses to protect commercial or maternal antibody-positive (ab+) chickens against virulent MD virus (MDV) challenge was compared with that of strain CV1988, the standard commercial MID vaccine. Modified strains were also evaluated for the ability to induce lymphomas or other pathologic changes in ab+ and antibody-negative (ab-) chickens. Two of the 10 modified viruses, strains RMI and CV1988/BP5, provided high levels of protection against highly virulent MDV challenge. The magnitude of protection was greater than that of one laboratory and two commercial preparations of CV1988, bur was approximately equal to that of two other commercial preparations of CV1988 in laboratory and field tests. Three of the strains, including PM I and CV1988/BP5, induced lymphoid organ atrophy in ab-chicks but not in ab+ commercial chicks, a property designated here as L phenotype. Seven strains, including two L+ strains, were mildly oncogenic for ab- chicks, a property designated here as 0 phenotype. Five of these strains caused no tumors in ab+ chickens. The two fully attenuated strains induced neither lymphomas nor lymphoid organ atrophy. The L and 0 phenotypes appeared not to be linked, and both (especially the L phenotype) appeared associated with high levels of protection. These studies also illustrated differences in the protective efficacy of different preparations of CV1988 vaccine, indicating the need to choose carefully the most protective strains as controls for efficacy studies. A new vv+ strain, designated as 686, is described and appears useful as a challenge virus; it is the most virulent of the 48 field isolates of MDV thus far pathotyped in this laboratory. These findings support the conclusion that new virus strains with high levels of protective immunity comparable to that of CV1988 can be developed. However, the question of whether strains can be developed that exceed the efficacy of current CV1988-based vaccines remains unanswered. After more than 30 years of unsuccessful endeavor by many laboratories toward this goal, it now may be useful to consider whether the efficacy of MD vaccines is limited by some type of biologic threshold.