Molecular Characterization of Fowlpox Virus Isolates and Evaluation of Recombinant Vaccines Generated for Protection Against Fowlpox and Reticuloendotheliosis

135 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2002.Fowlpox, an economically important disease of chickens, is prevented by routine immunization of birds in endemic areas with an attenuated live Fowlpox virus (FPV) vaccine. However, despite this precaution, outbreaks of Fowlpox still continue to occur in vaccinated chickens suggesting a lack of adequate protection by current vaccines. To determine the probable causes for these outbreaks, field isolates were compared with the vaccine strains. Antigenic variation amongst the strains was evident in immunoblotting with polyclonal and monoclonal antibodies. Biological differences were detected by the prolonged persistence and appearance of secondary lesions with field strains as well as variation in their protective ability. The contrasting difference noted between field isolates and vaccine strains was, the retention of variable lengths of an avian retrovirus sequences integrated in their genomes. Whereas only remnants of the long terminal repeats (LTR) of reticuloendotheliosis virus (REV) were detected in the genomes of the vaccine strains, either these partial LTR or nearly intact proviral copies of REV were found in the genomes of all field isolates so far examined. Since integrated REV can be released from the FPV genome and cause immuno-suppression, this symbiotic relationship presumably benefits both viruses by maintaining REV as part of FPV and enhancing the pathogenicity of FPV. In this regard, removal of REV from the genome of a field strain of FPV coincided with decreased virulence of the altered virus for the chickens. However, REV associated suppression of lymphocyte response to mitogen stimulation due to REV infection was not detected, suggesting that other immuno-suppressive mechanisms are involved. Since, under natural conditions, chickens become dually infected with both FPV and REV, a FPV recombinant expressing the REV envelop gene was generated. In current study, chickens immunized with the recombinant were protected against challenge both FPV and REV. This is the first study on (i) the molecular analyses of field strains of FPV isolated from outbreaks in previously vaccinated chicken flocks (ii) the role of integrated REV in the virulence of FPV and (iii) generation of the first FPV strain lacking any integrated REV sequences.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD