Factors influencing the attenuation of serotype 1 Marek's disease virus by serial cell culture passage and evaluation of attenuated strains for protection and replication.

This study was carried out to better understand factors that influenced the process of attenuation of Marek's disease (MD) virus by serial passage in cell cultures. Three virulent (v) pathotype and three very virulent plus (vv+) pathotype strains were passed by three techniques up to 131 times, and the passage level at attenuation was determined. The 18 attenuated or partially attenuated viruses were evaluated for protection against challenge with virulent MD virus, and the virus load (latent infection) in blood lymphocytes at 14-21 days postvaccination was determined.

Evaluation of Marek's disease field isolates by the "best fit" pathotyping assay.

Although determination of the pathotype is central to the study of Marek's disease (MD) field isolates, methods are not standardized and results from different laboratories may not compare well with the original Avian Disease and Oncology Laboratory assay. This study was designed to investigate the validity of the "best fit" pathotyping assay, a simplified method recently described for testing of field isolates of MD virus (MDV). Twenty serotype 1 MDV strains were isolated from 12 breeder and commercial flocks in eight regions of the Russian Federation and were pathotyped by the best fit assay using vaccinated and non-vaccinated chickens from Schelkovo specific pathogen free breeders.