Molecular comparison of serotype I Marek's disease virus (MDV) in vivo and effect of chicken infectious anemia virus co-infection on MDV pathogenicity

Marek's disease virus (MDV) is a herpesvirus that causes tumors and immunosuppression in chickens. Over the Last few decades MDV strains with increasing virulence have been isolated. The purpose of this thesis was to identify molecular differences that might be responsible for the increase in virulence. Specific-pathogen-free chickens were inoculated at hatch with very virulent (vv) MDV isolates or more recent very virulent plus (vv+) MDV isolates. Tissues were collected at various time points post inoculations, nucleic acids were isolated, and various molecular biology techniques were used to identify differences among the isolates.; Differential display was used to compare RNA expression in spleens of chickens inoculated with a recent vvMDV isolate (TK) or a standard vvMDV isolate (RB1B). Expression of the MDV DNA polymerase gene was more abundant in chickens infected with the TK isolate. Viral DNA and RNA were detected earlier in the spleens of chickens inoculated with the TK isolate, suggesting more efficient viral replication.; The TK isolate used for the first experiments was found to contain both MDV and chicken infectious anemia virus (CIAV). Therefore, an in vivo experiment was conducted to compare infection with RB1B or a vv+MDV isolate (584A), with or without added CIAV. CIAV co-infection increased levels of MDV RNA and DNA detected in the spleens of chickens infected with RB1B, indicating that CIAV co-infection exacerbated RB1B infection. This exacerbation was less evident when birds were co-infected with CIAV and 584A. In addition, levels of MDV DNA and RNA detected were greater in birds inoculated with 584A alone versus RB1B alone.; These data indicated that in vivo viral replication was more efficient with a vv+MDV isolate compared to a vvMDV isolate, however the cause of this difference is not known. During early viral replication, there may be more copies of the viral genome per infected cell, the virus may infect a broader range of cell types, or the virus may be more efficient at evading immune responses. Microarray analysis was evaluated, this technique may provide a good tool for identifying some of the mechanisms for differences in viral replication among serotype 1 MDV strains.